MsBackend
classesPackage: Spectra
Authors: RforMassSpectrometry Package Maintainer [cre],
Laurent Gatto [aut] (https://orcid.org/0000-0002-1520-2268), Johannes Rainer
[aut] (https://orcid.org/0000-0002-6977-7147), Sebastian Gibb
[aut] (https://orcid.org/0000-0001-7406-4443), Philippine
Louail [aut] (https://orcid.org/0009-0007-5429-6846), Jan Stanstrup
[ctb] (https://orcid.org/0000-0003-0541-7369), Nir Shahaf
[ctb], Mar Garcia-Aloy [ctb] (https://orcid.org/0000-0002-1330-6610)
Last modified: 2024-12-19 03:32:45.445034
Compiled: Thu Dec 19 03:35:07 2024
This vignette briefly describes the MsBackend class
which is used by the Spectra package to represent
and provide Mass Spectrometry (MS) data and illustrates how a new such
backend class can be created and tested for validity.
Contributions to this vignette (content or correction of typos) or requests for additional details and information are highly welcome (ideally via pull requests or github issues).
MsBackend?The Spectra package separates the code for the analysis
of MS data from the code needed to import, represent and provide the
data. The former is implemented for the Spectra class which
is the main object users will use for their analyses. The
Spectra object relies on a so-called backend to
provide the MS data. The MsBackend virtual class defines
the API that new backend classes need to implement in order to
be used with the Spectra object. Each Spectra
object contains an implementation of such a MsBackend
within its @backend slot which provides the MS data to the
Spectra object. All data management is thus hidden from the
user. In addition this separation allows to define new, alternative,
data representations and integrate them seamlessly into a
Spectra-based data analysis workflow.
This concept is an extension of the of in-memory and on-disk data representations from the MSnbase package (Gatto, Gibb, and Rainer 2020).
General conventions for MS data of a Spectra are:
Spectra object is supposed to contain MS (spectral)
data of multiple MS spectra.NA) for m/z values are not
supported.coreSpectraVariables() function.dataStorage and dataOrigin are two special
spectra variables that define for each spectrum where the data is stored
and from where the data derived (or was loaded, such as the data
origin). Both are expected to be of typecharacter and need
to be defined by the backend (i.e., they can not be empty or
missing).MsBackend implementations can also represent purely
read-only data resources. In this case only data accessor
methods need to be implemented but not data replacement methods. Whether
a backend is read-only can be set with the @readonly slot
of the virtual MsBackend class (the
isReadOnly() function can be used to retrieve the value for
this slot). The default is @readonly = FALSE and thus all
data replacement method listed in section Data replacement
methods have to be implemented. For read-only backends
(@readonly = TRUE) only the methods in section Required
methods need to be implemented. Backends can also be
partially read-only, such as the MsBackendMzR.
This backend allows for example to change spectra variables, but not the
peaks data (i.e. the m/z and intensity values). Also, backends for
purely read-only resources could extend the MsBackendCached
from the Spectra
package to enable support for modifying (or adding) spectra variables.
Any changes to spectra variables will be internally cached by the
MsBackendCached without the need of them being propagating
to the underlying data resource (see for example the
MsBackendMassbankSql from the MsBackendMassbank
package).For parallel processing, Spectra splits the backend
based on a defined factor and processes each in parallel
(or in serial if a SerialParam is used). The
splitting factor can be defined for Spectra by
setting the parameter processingChunkSize. Alternatively,
through the backendParallelFactor() function the backend
can also suggest a factor that should/could be
used for splitting and parallel processing. The default implementation
for backendParallelFactor() is to return an empty
factor (factor()) hence not suggesting any
preferred splitting. backendParallelFactor() for
MsBackendMzR on the other hand returns a
factor based on the data files the data is stored in
(i.e. based on the dataStorage of the MS data).
Besides parallel processing, this chunk-wise processing can also reduce the memory demand for operations, because only the peak data of the current chunk needs to be realized in memory.
The MsBackend class defines core methods that have to be
implemented by a MS backend as well as optional
methods with default implementations that might be implemented for a new
backend but don’t necessarily have to. These functions are described in
sections Required methods and Optional methods,
respectively.
To create a new backend a class extending the virtual
MsBackend needs to be implemented. In the example below we
create thus a simple class with a data.frame to contain
general spectral properties (spectra variables) and two slots
for m/z and intensity values. These are stored as
NumericList objects since both m/z and intensity values are
expected to be of type numeric and to allow to store data
from multiple spectra into a single backend object. We also define a
simple constructor function that returns an empty instance of our new
class.
library(Spectra)
library(IRanges)
setClass("MsBackendTest",
contains = "MsBackend",
slots = c(
spectraVars = "data.frame",
mz = "NumericList",
intensity = "NumericList"
),
prototype = prototype(
spectraVars = data.frame(),
mz = NumericList(compress = FALSE),
intensity = NumericList(compress = FALSE)
))
MsBackendTest <- function() {
new("MsBackendTest")
}The 3 slots spectraVars, mz and
intensity will be used to store our MS data, each row in
spectraVars being data for one spectrum with the columns
being the different spectra variables (i.e. additional
properties of a spectrum such as its retention time or MS level) and
each element in mz and intensity being a
numeric with the m/z and intensity values of the respective
spectrum.
We should ideally also add some basic validity function that ensures
the data to be OK. The function below simply checks that the number of
rows of the spectraVars slot matches the length of the
mz and intensity slot.
setValidity("MsBackendTest", function(object) {
if (length(object@mz) != length(object@intensity) ||
length(object@mz) != nrow(object@spectraVars))
return("length of 'mz' and 'intensity' has to match the number of ",
"rows of 'spectraVars'")
NULL
})## Class "MsBackendTest" [in ".GlobalEnv"]
##
## Slots:
##
## Name: spectraVars mz intensity readonly version
## Class: data.frame NumericList NumericList logical character
##
## Extends: "MsBackend"We can now create an instance of our new class with the
MsBackendTest() function.
## An object of class "MsBackendTest"
## Slot "spectraVars":
## data frame with 0 columns and 0 rows
##
## Slot "mz":
## NumericList of length 0
##
## Slot "intensity":
## NumericList of length 0
##
## Slot "readonly":
## [1] FALSE
##
## Slot "version":
## [1] "0.1"Note that a backend class does not necessarily need to
contain all the data like the one from our example. Backends such as the
MsBackendMzR for example retrieve the data on the fly from
the raw MS data files or the MsBackendSql from the MsBackendSql
a SQL database.
Methods listed in this section must be implemented for a new class
extending MsBackend. Methods should ideally also
implemented in the order they are listed here. Also, it is strongly
advised to write dedicated unit tests for each newly implemented method
or function already during the development.
dataStorage()The dataStorage spectra variable of a spectrum provides
some information how or where the data is stored. The
dataStorage() method should therefor return a
character vector with length equal to the number of spectra
of a backend object with that information. For most backends the data
storage information can be a simple string such as "memory"
or "database" to specify that the data of a spectrum is
stored within the object itself or in a database, respectively.
Backend classes that keep only a subset of the MS data in memory and
need to load data from data files upon request will use this spectra
variable to store and keep track of the original data file for each
spectrum. An example is the MsBackendMzR backend that
retrieves the MS data on-the-fly from the original data file(s) whenever
m/z or intensity values are requested from the backend. Calling
dataStorage() on an MsBackendMzR returns thus
the names from the originating files.
For our example backend we define a simple dataStorage()
method that simply returns the column "dataStorage" from
the @svars (as a character).
length()length() is expected to return a single
integer with the total number of spectra that are available
through the backend class. For our example backend we simply return the
number of rows of the data.frame stored in the
@spectraVars slot.
backendInitialize()The backendInitialize() method is expected to be called
after creating an instance of the backend class and should prepare
(initialize) the backend which in most cases means that MS data is
loaded. This method can take any parameters needed by the backend to get
loaded/initialized with data (which can be file names from which to load
the data, a database connection or object(s) containing the data).
During backendInitialize() usually also the special spectra
variables dataStorage and dataOrigin are
set.
Below we define a backendInitialize() method that takes
as arguments a data.frame with spectra variables and two
lists with the m/z and intensity values for each
spectrum.
setMethod(
"backendInitialize", "MsBackendTest",
function(object, svars, mz, intensity) {
if (!is.data.frame(svars))
stop("'svars' needs to be a 'data.frame' with spectra variables")
if (is.null(svars$dataStorage))
svars$dataStorage <- "<memory>"
if (is.null(svars$dataOrigin))
svars$dataOrigin <- "<user provided>"
object@spectraVars <- svars
object@mz <- NumericList(mz, compress = FALSE)
object@intensity <- NumericList(intensity, compress = FALSE)
validObject(object)
object
})In addition to adding the data to object, the function also defined
the dataStorage and dataOrigin spectra
variables. The purpose of these two variables is to provide some
information on where the data is stored (in memory as in our
example) and from where the data is originating. The
dataOrigin would for example allow to specify from which
original data files individual spectra derive.
We can now create an instance of our backend class and fill it with
data. We thus first define our MS data and pass this to the
backendInitialize() method.
## A data.frame with spectra variables.
svars <- data.frame(msLevel = c(1L, 2L, 2L),
rtime = c(1.2, 1.3, 1.4))
## m/z values for each spectrum.
mzs <- list(c(12.3, 13.5, 16.5, 17.5),
c(45.1, 45.2),
c(64.4, 123.1, 124.1))
## intensity values for each spectrum.
ints <- list(c(123.3, 153.6, 2354.3, 243.4),
c(100, 80.1),
c(12.3, 35.2, 100))
## Create and initialize the backend
be <- backendInitialize(MsBackendTest(),
svars = svars, mz = mzs, intensity = ints)
be## An object of class "MsBackendTest"
## Slot "spectraVars":
## msLevel rtime dataStorage dataOrigin
## 1 1 1.2 <memory> <user provided>
## 2 2 1.3 <memory> <user provided>
## 3 2 1.4 <memory> <user provided>
##
## Slot "mz":
## NumericList of length 3
## [[1]] 12.3 13.5 16.5 17.5
## [[2]] 45.1 45.2
## [[3]] 64.4 123.1 124.1
##
## Slot "intensity":
## NumericList of length 3
## [[1]] 123.3 153.6 2354.3 243.4
## [[2]] 100 80.1
## [[3]] 12.3 35.2 100
##
## Slot "readonly":
## [1] FALSE
##
## Slot "version":
## [1] "0.1"While this method works and is compliant with the
MsBackend API (because there is no requirement on the input
parameters for the backendInitialize() method), it would be
good practice for backends that are supposed to support replacing data,
to add an optional additional parameter data that would
allow passing the complete MS data (including m/z and intensity
values) to the function as a DataFrame. This would simplify
the implementation of some replacement methods and would in addition
also allow to change the backend of a Spectra using the
setBackend() to our new backend. We thus re-implement the
backendInitialize() method supporting also to initialize
the backend with such a data frame and we also implement a helper
function that checks spectra variables for the correct data type.
#' Helper function to check if core spectra variables have the correct
#' data type.
#'
#' @param x `data.frame` with the data for spectra variables.
#'
#' @param name `character` defining the column names (spectra variables) of `x`
#' for which the correct data type should be evaluated.
.sv_valid_data_type <- function(x, name = colnames(x)) {
sv <- coreSpectraVariables()[names(coreSpectraVariables()) %in% name]
for (i in seq_along(sv)) {
if (!is(x[, names(sv[i])], sv[i]))
stop("Spectra variabe \"", names(sv[i]), "\" is not of type ",
sv[i], call. = FALSE)
}
TRUE
}This function is then used to check the input data in our new
backendInitialize() method.
setMethod(
"backendInitialize", "MsBackendTest",
function(object, svars, mz, intensity, data) {
if (!missing(data)) {
svars <- as.data.frame(
data[, !colnames(data) %in% c("mz", "intensity")])
if (any(colnames(data) == "mz"))
mz <- data$mz
if (any(colnames(data) == "intensity"))
intensity <- data$intensity
}
if (!is.data.frame(svars))
stop("'svars' needs to be a 'data.frame' with spectra variables")
if (is.null(svars$dataStorage))
svars$dataStorage <- "<memory>"
if (is.null(svars$dataOrigin))
svars$dataOrigin <- "<user provided>"
.sv_valid_data_type(svars)
object@spectraVars <- svars
object@mz <- NumericList(mz, compress = FALSE)
object@intensity <- NumericList(intensity, compress = FALSE)
validObject(object)
object
})We below create the backend again with the updated
backendInitialize().
## Create and initialize the backend
be <- backendInitialize(MsBackendTest(),
svars = svars, mz = mzs, intensity = ints)
be## An object of class "MsBackendTest"
## Slot "spectraVars":
## msLevel rtime dataStorage dataOrigin
## 1 1 1.2 <memory> <user provided>
## 2 2 1.3 <memory> <user provided>
## 3 2 1.4 <memory> <user provided>
##
## Slot "mz":
## NumericList of length 3
## [[1]] 12.3 13.5 16.5 17.5
## [[2]] 45.1 45.2
## [[3]] 64.4 123.1 124.1
##
## Slot "intensity":
## NumericList of length 3
## [[1]] 123.3 153.6 2354.3 243.4
## [[2]] 100 80.1
## [[3]] 12.3 35.2 100
##
## Slot "readonly":
## [1] FALSE
##
## Slot "version":
## [1] "0.1"The backendInitialize() method that we implemented for
our backend class expects the user to provide the full MS data. This
does however not always have to be the case. The
backendInitialize() method of the MsBackendMzR
backend takes for example the file names of the raw mzML, mzXML or CDF
files as input and initializes the backend by importing part of the data
from these. Also the backends defined by the MsBackendMgf
or r Biocpkg("MsBackendMsp") packages work in the same way
and thus allow to import MS data from these specific file formats. The
backendInitialize() method of the backend defined in the
MsBackendSql
on the other hand takes only the connection to a database containing the
data as input and performs some sanity checks on the data but does not
load the data into the backend. Any subsequent data access is handled by
the methods of the backend class through SQL calls to the database.
The purpose of the backendInitialize() method is to
initialize and prepare the data in a way that it can be
accessed by a Spectra object (through the initialized
backend class). Whether the data is loaded by the
backendInitialize() method into memory or simply referenced
to within the backend class does not matter as long as the backend is
able to provide the data with its accessor methods.
Note also that a backendInitialize() function should
ideally also perform some data sanity checks (e.g. whether spectra
variables have the correct data type etc).
spectraVariables()The spectraVariables() method should return a
character vector with the names of all available spectra
variables of the backend. While a backend class should support defining
and providing their own spectra variables, each MsBackend
class must provide also the core spectra
variables (in the correct data type). Since not all data file
formats provide values for all these spectra variables they can however
also be NA (with the exception of the spectra variable
"dataStorage").
The coreSpectraVariables() function returns the full
list of mandatory spectra variables along with their expected data
type.
## msLevel rtime acquisitionNum
## "integer" "numeric" "integer"
## scanIndex mz intensity
## "integer" "NumericList" "NumericList"
## dataStorage dataOrigin centroided
## "character" "character" "logical"
## smoothed polarity precScanNum
## "logical" "integer" "integer"
## precursorMz precursorIntensity precursorCharge
## "numeric" "numeric" "integer"
## collisionEnergy isolationWindowLowerMz isolationWindowTargetMz
## "numeric" "numeric" "numeric"
## isolationWindowUpperMz
## "numeric"A typical spectraVariables() method for a
MsBackend class will thus be implemented similarly to the
one for our MsBackendTest test backend: it will return the
union of the core spectra variables and the names for all available
spectra variables within the backend object.
setMethod("spectraVariables", "MsBackendTest", function(object) {
union(names(coreSpectraVariables()), colnames(object@spectraVars))
})
spectraVariables(be)## [1] "msLevel" "rtime"
## [3] "acquisitionNum" "scanIndex"
## [5] "mz" "intensity"
## [7] "dataStorage" "dataOrigin"
## [9] "centroided" "smoothed"
## [11] "polarity" "precScanNum"
## [13] "precursorMz" "precursorIntensity"
## [15] "precursorCharge" "collisionEnergy"
## [17] "isolationWindowLowerMz" "isolationWindowTargetMz"
## [19] "isolationWindowUpperMz"spectraData()The spectraData() method should return the
full spectra data within a backend as a
DataFrame object (defined in the S4Vectors
package). The second parameter columns allows to define the
names of the spectra variables that should be returned in the
DataFrame. Each row in this data frame should represent one
spectrum, each column a spectra variable. Columns "mz" and
"intensity" (if requested) have to contain each a
NumericList with the m/z and intensity values of the
spectra. The DataFrame must provide values
(even if they are NA) for all requested
spectra variables of the backend (including the core
spectra variables).
This is now a first problem for our toy backend class, since we keep
the spectra variable data in a simple data.frame without
any constraints such as required columns etc. A simple solution to this
(which is also used by all backend classes in the Spectra
package) is to fill missing spectra variables on-the-fly into
the returned DataFrame. We thus define below a simple
helper function that adds columns with missing values (of the correct
data type) for core spectra variables that are not available within the
backend to the result.
#' @description Add columns with missing core spectra variables.
#'
#' @param x `data.frame` or `DataFrame` with some spectra variables.
#'
#' @param core_vars `character` with core spectra variable names that should
#' be added to `x` if not already present.
#'
.fill_core_variables <- function(x, core_vars = names(coreSpectraVariables())) {
fill_vars <- setdiff(core_vars, colnames(x))
core_type <- coreSpectraVariables()
n <- nrow(x)
if (length(fill_vars)) {
fill <- lapply(fill_vars, function(z) {
rep(as(NA, core_type[z]), n)
})
names(fill) <- fill_vars
x <- cbind(x, as.data.frame(fill))
}
x
}We next implement the spectraData() method that uses
this helper function to fill eventually missing core spectra variables.
Note also that this function should return a DataFrame even
for a single column.
setMethod(
"spectraData", "MsBackendTest",
function(object, columns = spectraVariables(object)) {
if (!all(columns %in% spectraVariables(object)))
stop("Some of the requested spectra variables are not available")
## Add m/z and intensity values to the result
res <- DataFrame(object@spectraVars)
res$mz <- object@mz
res$intensity <- object@intensity
## Fill with eventually missing core variables
res <- .fill_core_variables(
res, intersect(columns, names(coreSpectraVariables())))
res[, columns, drop = FALSE]
})As an alternative, we could also initialize the
@spectraVars data frame within the
backendInitialize() method adding columns for spectra
variables that are not provided by the user and require that this data
frame always contains all core spectra variables. Extracting spectra
data (single spectra variables or the full data) might thus be more
efficient then the on-the-fly initialization with eventual missing
spectra variables, but the backend class would also have a larger memory
footprint because even spectra variables with only missing values for
all spectra need to be stored within the object.
We can now use spectraData() to either extract the full
spectra data from the backend, or only the data for selected spectra
variables.
## DataFrame with 3 rows and 19 columns
## msLevel rtime acquisitionNum scanIndex mz
## <integer> <numeric> <integer> <integer> <NumericList>
## 1 1 1.2 NA NA 12.3,13.5,16.5,...
## 2 2 1.3 NA NA 45.1,45.2
## 3 2 1.4 NA NA 64.4,123.1,124.1
## intensity dataStorage dataOrigin centroided smoothed
## <NumericList> <character> <character> <logical> <logical>
## 1 123.3, 153.6,2354.3,... <memory> <user provided> NA NA
## 2 100.0, 80.1 <memory> <user provided> NA NA
## 3 12.3, 35.2,100.0 <memory> <user provided> NA NA
## polarity precScanNum precursorMz precursorIntensity precursorCharge
## <integer> <integer> <numeric> <numeric> <integer>
## 1 NA NA NA NA NA
## 2 NA NA NA NA NA
## 3 NA NA NA NA NA
## collisionEnergy isolationWindowLowerMz isolationWindowTargetMz
## <numeric> <numeric> <numeric>
## 1 NA NA NA
## 2 NA NA NA
## 3 NA NA NA
## isolationWindowUpperMz
## <numeric>
## 1 NA
## 2 NA
## 3 NA## DataFrame with 3 rows and 3 columns
## rtime mz centroided
## <numeric> <NumericList> <logical>
## 1 1.2 12.3,13.5,16.5,... NA
## 2 1.3 45.1,45.2 NA
## 3 1.4 64.4,123.1,124.1 NA## DataFrame with 3 rows and 2 columns
## centroided polarity
## <logical> <integer>
## 1 NA NA
## 2 NA NA
## 3 NA NApeaksData()The peaksData() method extracts the MS peaks data from a
backend, which includes the m/z and intensity values of each MS peak of
a spectrum. These are expected to be returned as a List of
numerical matrices with columns in each matrix being the
requested peaks variables (with the default being
"mz" and "intensity") of one spectrum.
Backends must provide at least these two peaks variables.
Below we implement the peaksData() method for our
backend. We need to loop over the @mz and
@intensity slots to merge the m/z and intensity of each
spectrum into a matrix. Also, for simplicity reasons, we
accept only c("mz", "intensity") for the
columns parameter. This is the expected default behavior
for a MsBackend, but in general the columns
parameter is thought to allow the user to specify which peaks variables
should be returned in each matrix.
setMethod(
"peaksData", "MsBackendTest",
function(object, columns = c("mz", "intensity")) {
if (length(columns) != 2 && columns != c("mz", "intensity"))
stop("'columns' supports only \"mz\" and \"intensity\"")
mapply(mz = object@mz, intensity = object@intensity,
FUN = cbind, SIMPLIFY = FALSE, USE.NAMES = FALSE)
})And with this method we can now extract the peaks data from our backend.
## [[1]]
## mz intensity
## [1,] 12.3 123.3
## [2,] 13.5 153.6
## [3,] 16.5 2354.3
## [4,] 17.5 243.4
##
## [[2]]
## mz intensity
## [1,] 45.1 100.0
## [2,] 45.2 80.1
##
## [[3]]
## mz intensity
## [1,] 64.4 12.3
## [2,] 123.1 35.2
## [3,] 124.1 100.0The peaksData() method is used in many data analysis
functions of the Spectra object to extract the MS data,
thus ideally this method should be implemented in an efficient way. For
our backend we need to loop over the lists of m/z and intensity values
which is obviously not ideal. Thus, storing the m/z and intensity values
in separate slots as done in this backend might not be ideal. The
MsBackendMemory backend for example stores the MS data
already as a list of matrices which results in a more
efficient peaksData() method (but comes also with a larger
overhead when adding, replacing or checking MS data).
Note also that while a backend needs to provide m/z and intensity
values, additional peak variables would also be supported. The
MsBackendMemory class for example allows to store and
provide additional peak variables that can then be added as additional
columns to each returned matrix. In this case the default
peaksVariables() method should also be overwritten to list
the additionally available variables and the columns
parameter of the peaksData() method should allow selection
of these additional peaks variables (in addition to the required
"mz" and "intensity" variables).
extractByIndex() and [The extractByIndex() and [ methods allows
to subset MsBackend objects. This operation is expected to
reduce a MsBackend object to the selected spectra. These
methods must also support duplication (e.g. [c(1, 1, 1)]
and extraction in any arbitrary order
(e.g. [c(3, 1, 5, 3)]). While both methods subset the
object, extractByIndex() only supports to subset with an
integer index, while [, to be compliant with
the base R implementation, should support to subset by indices or
logical vectors. An error should be thrown if indices are out of bounds,
but the method should also support returning an empty backend with
[integer()]. Note that the
MsCoreUtils::i2index function can be used to check for
correct input (and convert the input to an integer
index).
The extractByIndex() method is used by the data
operation and analysis methods on Spectra objects, while
the [ is intended to be used by the end user (if needed).
Below we implement extractByIndex() for our backend:
setMethod("extractByIndex", c("MsBackendTest", "ANY"), function(object, i) {
object@spectraVars <- object@spectraVars[i, ]
object@mz <- object@mz[i]
object@intensity <- object@intensity[i]
object
})The [ does not need to be defined because a default
implementation for the base MsBackend exists.
We can now subset our backend to the last two spectra.
## DataFrame with 2 rows and 19 columns
## msLevel rtime acquisitionNum scanIndex mz
## <integer> <numeric> <integer> <integer> <NumericList>
## 1 2 1.3 NA NA 45.1,45.2
## 2 2 1.4 NA NA 64.4,123.1,124.1
## intensity dataStorage dataOrigin centroided smoothed polarity
## <NumericList> <character> <character> <logical> <logical> <integer>
## 1 100.0, 80.1 <memory> <user provided> NA NA NA
## 2 12.3, 35.2,100.0 <memory> <user provided> NA NA NA
## precScanNum precursorMz precursorIntensity precursorCharge collisionEnergy
## <integer> <numeric> <numeric> <integer> <numeric>
## 1 NA NA NA NA NA
## 2 NA NA NA NA NA
## isolationWindowLowerMz isolationWindowTargetMz isolationWindowUpperMz
## <numeric> <numeric> <numeric>
## 1 NA NA NA
## 2 NA NA NAOr extracting the second spectrum multiple times.
## DataFrame with 3 rows and 19 columns
## msLevel rtime acquisitionNum scanIndex mz intensity
## <integer> <numeric> <integer> <integer> <NumericList> <NumericList>
## 2 2 1.3 NA NA 45.1,45.2 100.0, 80.1
## 2.1 2 1.3 NA NA 45.1,45.2 100.0, 80.1
## 2.2 2 1.3 NA NA 45.1,45.2 100.0, 80.1
## dataStorage dataOrigin centroided smoothed polarity precScanNum
## <character> <character> <logical> <logical> <integer> <integer>
## 2 <memory> <user provided> NA NA NA NA
## 2.1 <memory> <user provided> NA NA NA NA
## 2.2 <memory> <user provided> NA NA NA NA
## precursorMz precursorIntensity precursorCharge collisionEnergy
## <numeric> <numeric> <integer> <numeric>
## 2 NA NA NA NA
## 2.1 NA NA NA NA
## 2.2 NA NA NA NA
## isolationWindowLowerMz isolationWindowTargetMz isolationWindowUpperMz
## <numeric> <numeric> <numeric>
## 2 NA NA NA
## 2.1 NA NA NA
## 2.2 NA NA NAbackendMerge()The backendMerge() method merges (combines)
MsBackend objects (of the same type!) into a single
instance. For our test backend we thus need to combine the values in the
@spectraVars, @mz and @intensity
slots. To support also merging of data.frames with
different set of columns we use the
MsCoreUtils::rbindFill() function instead of a simple
rbind() (this function joins data frames making an union of
all available columns).
setMethod("backendMerge", "MsBackendTest", function(object, ...) {
res <- object
object <- unname(c(object, ...))
res@mz <- do.call(c, lapply(object, function(z) z@mz))
res@intensity <- do.call(c, lapply(object, function(z) z@intensity))
res@spectraVars <- do.call(MsCoreUtils::rbindFill,
lapply(object, function(z) z@spectraVars))
validObject(res)
res
})Again, this implementation which requires 3 loops might not be the
most efficient - but it allows to merge backends of the type
MsBackendTest.
## An object of class "MsBackendTest"
## Slot "spectraVars":
## msLevel rtime dataStorage dataOrigin
## 1 1 1.2 <memory> <user provided>
## 2 2 1.3 <memory> <user provided>
## 3 2 1.4 <memory> <user provided>
## 21 2 1.3 <memory> <user provided>
## 11 1 1.2 <memory> <user provided>
## 22 2 1.3 <memory> <user provided>
## 31 2 1.4 <memory> <user provided>
##
## Slot "mz":
## NumericList of length 7
## [[1]] 12.3 13.5 16.5 17.5
## [[2]] 45.1 45.2
## [[3]] 64.4 123.1 124.1
## [[4]] 45.1 45.2
## [[5]] 12.3 13.5 16.5 17.5
## [[6]] 45.1 45.2
## [[7]] 64.4 123.1 124.1
##
## Slot "intensity":
## NumericList of length 7
## [[1]] 123.3 153.6 2354.3 243.4
## [[2]] 100 80.1
## [[3]] 12.3 35.2 100
## [[4]] 100 80.1
## [[5]] 123.3 153.6 2354.3 243.4
## [[6]] 100 80.1
## [[7]] 12.3 35.2 100
##
## Slot "readonly":
## [1] FALSE
##
## Slot "version":
## [1] "0.1"$The $ method is expected to extract a single spectra
variable from a backend. Parameter name should allow to
name the spectra variable to return. Each MsBackend
must support extracting the core spectra variables with
this method (even if no data might be available for that variable). In
our example implementation below we make use of the
spectraData() method, but more efficient implementations
might be available as well (that would not require to first
subset/create a DataFrame with the full data and to then
subset that again). Also, the $ method should check if the
requested spectra variable is available and should throw an error
otherwise.
With this we can now extract the MS levels
## [1] 1 2 2or a core spectra variable that is not available within the backend.
## [1] NA NA NAor also the m/z values
## NumericList of length 3
## [[1]] 12.3 13.5 16.5 17.5
## [[2]] 45.1 45.2
## [[3]] 64.4 123.1 124.1lengths()The lengths() method is expected to return an
integer vector (same length as the number of spectra in the
backend) with the total number of peaks per spectrum.
For our MsBackendTest we can simply use the
lengths() method on the m/z or intensity values for
that.
setMethod("lengths", "MsBackendTest", function(x, use.names = FALSE) {
lengths(x@mz, use.names = use.names)
})And we can now get the peaks count per spectrum:
## [1] 4 2 3isEmpty()The isEmpty() method is expected to return for each
spectrum the information whether it is empty, i.e. does not
contain any MS peaks (and hence m/z or intensity values). The result of
the method has to be a logical of length equal to the
number of spectra represented by the backend with TRUE
indicating whether a spectrum is empty and FALSE otherwise.
For our implementation of the isEmpty() method we use the
lenghts() method defined above that returns the number of
MS peaks per spectrum.
## [1] FALSE FALSE FALSEacquisitionNum()Extract the acquisitionNum core spectra variable. The
method is expected to return an integer vector with the
same length as there are spectra represented by the backend. For our
backend we simply re-use the spectraData() method.
setMethod("acquisitionNum", "MsBackendTest", function(object) {
spectraData(object, "acquisitionNum")[, 1L]
})
acquisitionNum(be)## [1] NA NA NAcentroided()Extract for each spectrum the information whether it contains
centroided data. The method is expected to return a
logical vector with the same length as there are spectra
represented by the backend.
setMethod("centroided", "MsBackendTest", function(object) {
spectraData(object, "centroided")[, 1L]
})
centroided(be)## [1] NA NA NAcollisionEnergy()Extract for each spectrum the collision energy applied to generate
the fragment spectrum. The method is expected to return a
numeric vector with the same length as there are spectra
represented by the backend (with NA_real_ for spectra for
which this information is not available, such as MS1 spectra).
setMethod("collisionEnergy", "MsBackendTest", function(object) {
spectraData(object, "collisionEnergy")[, 1L]
})
collisionEnergy(be)## [1] NA NA NAdataOrigin()Extract the data origin spectra variable for each spectrum.
This spectra variable can be used to store the origin of each spectra.
The method is expected to return a character vector of
length equal to the number of spectra represented by the backend.
setMethod("dataOrigin", "MsBackendTest", function(object) {
spectraData(object, "dataOrigin")[, 1L]
})
dataOrigin(be)## [1] "<user provided>" "<user provided>" "<user provided>"intensity()Extract the intensity values for each spectrum in the backend. The
result is expected to be a NumericList of length equal to
the number of spectra represented by the backend. For our test backend
we can simply return the @intensity slot since the data is
already stored within a NumericList.
## NumericList of length 3
## [[1]] 123.3 153.6 2354.3 243.4
## [[2]] 100 80.1
## [[3]] 12.3 35.2 100isolationWindowLowerMz()Extract the core spectra variable isolationWindowLowerMz
from the backend. This information is usually provided for each spectrum
in the raw mzML files. The method is expected to return a
numeric vector of length equal to the number of spectra
represented by the backend.
setMethod("isolationWindowLowerMz", "MsBackendTest", function(object) {
spectraData(object, "isolationWindowLowerMz")[, 1L]
})
isolationWindowLowerMz(be)## [1] NA NA NAisolationWindowTargetMz()Extract the core spectra variable
isolationWindowTargetMz from the backend. This information
is usually provided for each spectrum in the raw mzML files. The method
is expected to return a numeric vector of length equal to
the number of spectra represented by the backend.
setMethod("isolationWindowTargetMz", "MsBackendTest", function(object) {
spectraData(object, "isolationWindowTargetMz")[, 1L]
})
isolationWindowTargetMz(be)## [1] NA NA NAisolationWindowUpperMz()Extract the core spectra variable isolationWindowUpperMz
from the backend. This information is usually provided for each spectrum
in the raw mzML files. The method is expected to return a
numeric vector of length equal to the number of spectra
represented by the backend.
setMethod("isolationWindowUpperMz", "MsBackendTest", function(object) {
spectraData(object, "isolationWindowUpperMz")[, 1L]
})
isolationWindowUpperMz(be)## [1] NA NA NAmsLevel()Extract the MS level for each spectrum in the backend. This method is
expected to return an integer of length equal to the number
of spectra represented by the backend.
setMethod("msLevel", "MsBackendTest", function(object) {
spectraData(object, "msLevel")[, 1L]
})
msLevel(be)## [1] 1 2 2mz()Extract the m/z values for each spectrum in the backend. The result
is expected to be a NumericList of length equal to the
number of spectra represented by the backend. Also, the m/z values are
expected to be ordered increasingly for each element (spectrum).
## NumericList of length 3
## [[1]] 12.3 13.5 16.5 17.5
## [[2]] 45.1 45.2
## [[3]] 64.4 123.1 124.1polarity()Extract the polarity core spectra variable for each
spectrum in the backend. This method is expected to return an
integer of length equal to the number of spectra
represented by the backend. Negative and positive polarity are expected
to be encoded by 0L and 1L, respectively.
setMethod("polarity", "MsBackendTest", function(object) {
spectraData(object, "polarity")[, 1L]
})
polarity(be)## [1] NA NA NAprecScanNum()Extract the acquisition number of the precursor for each spectrum.
This method is expected to return an integer of length
equal to the number of spectra represented by the backend. For MS1
spectra (or if the acquisition number of the precursor is not provided)
NA_integer_ has to be returned.
setMethod("precScanNum", "MsBackendTest", function(object) {
spectraData(object, "precScanNum")[, 1L]
})
precScanNum(be)## [1] NA NA NAprecursorCharge()Extract the charge of the precursor for each spectrum. This method is
expected to return an integer of length equal to the number
of spectra represented by the backend. For MS1 spectra (or if the charge
of the precursor is not provided) NA_integer_ has to be
returned.
setMethod("precursorCharge", "MsBackendTest", function(object) {
spectraData(object, "precursorCharge")[, 1L]
})
precursorCharge(be)## [1] NA NA NAprecursorIntensity()Extract the intensity of the precursor for each spectrum. This method
is expected to return an numeric of length equal to the
number of spectra represented by the backend. For MS1 spectra (or if the
precursor intensity for a fragment spectrum is not provided)
NA_real_ has to be returned.
setMethod("precursorIntensity", "MsBackendTest", function(object) {
spectraData(object, "precursorIntensity")[, 1L]
})
precursorIntensity(be)## [1] NA NA NAprecursorMz()Extract the precursor m/z for each spectrum. This method is expected
to return an numeric of length equal to the number of
spectra represented by the backend. For MS1 spectra (or if the precursor
m/z for a fragment spectrum is not provided) NA_real_ has
to be returned.
setMethod("precursorMz", "MsBackendTest", function(object) {
spectraData(object, "precursorMz")[, 1L]
})
precursorMz(be)## [1] NA NA NArtime()Extract the retention time of each spectrum. This method is expected
to return a numeric of length equal to the number of
spectra represented by the backend.
setMethod("rtime", "MsBackendTest", function(object) {
spectraData(object, "rtime")[, 1L]
})
rtime(be)## [1] 1.2 1.3 1.4scanIndex()Extract the scan index core spectra variable. The scan index
represents the relative index of the spectrum within the respective raw
data file and can be different than the acquisitionNum
(which is the index of a spectrum as recorded by the MS instrument).
This method is expected to return a integer of length equal
to the number of spectra represented by the backend.
setMethod("scanIndex", "MsBackendTest", function(object) {
spectraData(object, "scanIndex")[, 1L]
})
scanIndex(be)## [1] NA NA NAsmoothed()Extract the smoothed core spectra variable that
indicates whether a spectrum was smoothed. This variable is
supported for backward compatibility but seldomly used. The method is
expected to return a logical with length equal to the
number of spectra represented by the backend.
setMethod("smoothed", "MsBackendTest", function(object) {
spectraData(object, "smoothed")[, 1L]
})
smoothed(be)## [1] NA NA NAspectraNames()The spectraNames() can be used to extract (optional)
names (or IDs) for individual spectra of a backend, or NULL
if not set. For our test backend we can use the rownames of
the @spectraVars slot to store spectra names.
setMethod("spectraNames", "MsBackendTest", function(object) {
rownames(object@spectraVars)
})
spectraNames(be)## [1] "1" "2" "3"These are all the methods that need to be implemented for a valid
read-only MsBackend class and running a test on
such an object as described in section Testing the validity of the
backend should not produce any errors. For backends that support
also data replacement also the methods listed in the next section need
to be implemented.
tic()The tic() method should return the total ion count
(i.e. the sum of intensities) for each spectrum. This information is
usually also provided by the raw MS data files, but can also be
calculated on the fly from the data. The parameter initial
(which is by default TRUE) allows to define whether the
provided original tic should be returned (for
initial = TRUE) or whether the tic should be calculated on
the actual data (initial = FALSE). The original tic values
are usually provided by a spectra variable "totIonCurrent".
Thus, for initial = TRUE, in our implementation below we
return the value of such a spectra variable it it is avaialble or
NA if it is not.
setMethod("tic", "MsBackendTest", function(object, initial = TRUE) {
if (initial) {
if (any(spectraVariables(object) == "totIonCurrent"))
spectraData(object, "totIonCurrent")[, 1L]
else rep(NA_real_, length(object))
} else vapply(intensity(object), sum, numeric(1), na.rm = TRUE)
})We can now either return the original (initial) TIC (which is not available).
## [1] NA NA NAOr calculate the TIC based on the actual intensity values.
## [1] 2874.6 180.1 147.5As stated in the general description, MsBackend
implementations can also be purely read-only resources allowing
to just access data, but not to replace the data. Thus, it is not
strictly required to implement these methods, but for a fully functional
backend it is suggested (as much as possible). A backend for a purely
read-only MS data resource might even extend the
MsBackendCached backend defined in the Spectra
package that provides a mechanism to cache (spectra variable) data in a
data.frame within the object. The
MsBackendMassbankSql implemented in the MsBackendMassbank
package extends for example this backend and thus allows modifying some
spectra variables without changing the original data in the MassBank SQL
database.
spectraData<-The spectraData<- method should allow to replace the
data within a backend. The method should take a DataFrame
with the full data as input value and is expected to replace the
full data within the backend, i.e. all spectra
variables as well as peak data. Also, importantly, the number of spectra
before and after calling the spectraData<- method on an
object has to be the same. For our implementation we can make use of the
optional parameter data that we added to the
backendInitialize() method and that allows to fill a
MsBackendTest object with the full data.
setReplaceMethod("spectraData", "MsBackendTest", function(object, value) {
if (!inherits(value, "DataFrame"))
stop("'value' is expected to be a 'DataFrame'")
if (length(object) && length(object) != nrow(value))
stop("'value' has to be a 'DataFrame' with ", length(object), " rows")
object <- backendInitialize(MsBackendTest(), data = value)
object
})To test this new method we extract the full spectra data, add an additional column (spectra variable) and replace the data again.
## [1] "a" "b" "c"intensity<-The intensity<- method should allow to replace the
intensity values of all spectra in a backend. This method is expected to
only replace the values of the intensities, but must not change
the number of intensities (and hence peaks) of a spectrum (that could be
done with the peaksData<- method that allows to replace
intensity and m/z values at the same time). The
value for the method should ideally be a
NumericList to ensure that all intensity values are indeed
numeric. In addition to the method we implement also a
simple helper function that checks for the correct length of
value. Each data replacement method needs to check for that
and this function thus reduces code duplication.
.match_length <- function(x, y) {
if (length(x) != length(y))
stop("Length of 'value' has to match the length of 'object'")
}
setReplaceMethod("intensity", "MsBackendTest", function(object, value) {
.match_length(object, value)
if (!(is.list(value) || inherits(value, "NumericList")))
stop("'value' has to be a list or NumericList")
if (!all(lengths(value) == lengths(mz(object))))
stop("lengths of 'value' has to match the number of peaks per spectrum")
if (!inherits(value, "NumericList"))
value <- NumericList(value, compress = FALSE)
object@intensity <- value
object
})We could now use this method to replace the intensities in our backend with modified intensities.
## NumericList of length 3
## [[1]] 123.3 153.6 2354.3 243.4
## [[2]] 100 80.1
## [[3]] 12.3 35.2 100## NumericList of length 3
## [[1]] 113.3 143.6 2344.3 233.4
## [[2]] 90 70.1
## [[3]] 2.3 25.2 90mz<-The mz<- method should allow to replace the m/z
values of all spectra in a backend. The implementation can be the same
as for the intensity<- method. m/z values within each
spectrum need to be increasingly ordered. We thus also check that this
is the case for the provided m/z values. We take here the advantage that
a efficient is.unsorted() implementation for
NumericList is already available, which is faster than
e.g. calling vapply(mz(be), is.unsorted, logical(1)).
setReplaceMethod("mz", "MsBackendTest", function(object, value) {
.match_length(object, value)
if (!(is.list(value) || inherits(value, "NumericList")))
stop("'value' has to be a list or NumericList")
if (!all(lengths(value) == lengths(mz(object))))
stop("lengths of 'value' has to match the number of peaks per spectrum")
if (!inherits(value, "NumericList"))
value <- NumericList(value, compress = FALSE)
if (any(is.unsorted(value)))
stop("m/z values need to be increasingly sorted within each spectrum")
object@mz <- value
object
})peaksData<-The peaksData<- should allow to replace the peaks
data (m/z and intensity values) of all spectra in a backend. In contrast
to the mz<- and intensity<- methods this
method should also support changing the number of peaks per spectrum
(e.g. due to filtering). Parameter value has to be a
list of matrix objects with columns
"mz" and "intensity". The length of this list
has to match the number of spectra in the backend. In the implementation
for our backend class we need to loop over this list to extract the m/z
and intensity values and assign them to the @mz and
@intensity slots.
setReplaceMethod("peaksData", "MsBackendTest", function(object, value) {
if (!(is.list(value) || inherits(value, "SimpleList")))
stop("'value' has to be a list-like object")
.match_length(object, value)
object@mz <- NumericList(lapply(value, "[", , "mz"), compress = FALSE)
object@intensity <- NumericList(lapply(value, "[", , "intensity"),
compress = FALSE)
validObject(object)
object
})Using the peaksData<- method we can now also for
example remove peaks.
pd <- peaksData(be)
## Remove the first peak from the first spectrum
pd[[1L]] <- pd[[1L]][-1L, ]
lengths(be)## [1] 4 2 3## [1] 3 2 3$<-The $<- method should allow to replace values for
spectra variables or also to add additional spectra variables to the
backend. As with all replacement methods, the length() of
value has to match the number of spectra represented by the
backend.
setReplaceMethod("$", "MsBackendTest", function(x, name, value) {
.match_length(x, value)
if (name == "mz") {
mz(x) <- value
} else if (name == "intensity") {
intensity(x) <- value
} else {
x@spectraVars[[name]] <- value
}
.sv_valid_data_type(x@spectraVars, name)
x
})We can now replace for example existing spectra variables:
## [1] 1 2 2## [1] 2 1 2Or even add new spectra variables.
## [1] "a" "b" "c"Replacement methods for all core spectra variables can be implemented similarly.
selectSpectraVariables()The selectSpectraVariables() function should allow to
reduce the information within the backend (parameter
object) to the selected spectra variables (parameter
spectraVariables). This is equivalent to a subset by
columns/variables. For core spectra variables, if not specified by
parameter spectraVariables, only their values are expected
to be removed (since core spectra variables are expected to be available
even if they are not defined within a backend). The implementation for
our backend will remove any columns in the @spectraVars
data frame not defined in the spectraVariables parameter.
Special care is given to the "mz" and
"intensity" spectra variables: if they are not selected,
the @mz and @intensity slots are initialized
with empty NumericList (of length matching the number of
spectra). Note also that some backends might throw an error if a spectra
variable required for the backend is removed (such as
"dataStorage" for a MsBackendMzR backend,
which is required by the backend to allow retrieval of m/z and intensity
values).
setMethod(
"selectSpectraVariables", "MsBackendTest",
function(object, spectraVariables = spectraVariables(object)) {
keep <- colnames(object@spectraVars) %in% spectraVariables
object@spectraVars <- object@spectraVars[, keep, drop = FALSE]
if (!any(spectraVariables == "mz"))
object@mz <- NumericList(vector("list", length(object)),
compress = FALSE)
if (!any(spectraVariables == "intensity"))
object@intensity <- NumericList(vector("list", length(object)),
compress = FALSE)
validObject(object)
object
})We can now use selectSpectraVariables() to remove for
example the spectra variable "new_var" added above.
be2 <- be
be2 <- selectSpectraVariables(be2, c("msLevel", "rtime", "mz",
"intensity", "dataStorage"))
spectraVariables(be2)## [1] "msLevel" "rtime"
## [3] "acquisitionNum" "scanIndex"
## [5] "mz" "intensity"
## [7] "dataStorage" "dataOrigin"
## [9] "centroided" "smoothed"
## [11] "polarity" "precScanNum"
## [13] "precursorMz" "precursorIntensity"
## [15] "precursorCharge" "collisionEnergy"
## [17] "isolationWindowLowerMz" "isolationWindowTargetMz"
## [19] "isolationWindowUpperMz"The spectra variable "new_var" is now no longer be
available. Note however that still all core spectra
variables are listed, even if they were not selected with the
spectraVariables parameter. While these variables (such as
"dataOrigin") are still listed by
spectraVariables(be2), their actual values have been
removed:
## [1] "<user provided>" "<user provided>" "<user provided>"## [1] NA NA NAIf "mz" and "intensitity" are not selected,
the m/z and intensity values get removed.
## NumericList of length 3
## [[1]] numeric(0)
## [[2]] numeric(0)
## [[3]] numeric(0)## NumericList of length 3
## [[1]] numeric(0)
## [[2]] numeric(0)
## [[3]] numeric(0)centroided<-Replace the value for the centroided core spectra variable.
The provided data type must be a logical. We re-use the
function .sv_valid_data_type which was defined above for
backendInitialize() to check for the correct data type of
core spectra variables.
setReplaceMethod("centroided", "MsBackendTest", function(object, value) {
object@spectraVars[["centroided"]] <- value
.sv_valid_data_type(object@spectraVars, "centroided")
object
})Alternatively, we could also simply re-use the $<-
replacement method above. This would make the whole code base for our
backend cleaner as replacing or adding spectra variables would be
handled in a single central function.
setReplaceMethod("centroided", "MsBackendTest", function(object, value) {
object$centroided <- value
object
})## [1] TRUE FALSE TRUEcollisionEnergy<-Replace the values for the collision energy. Parameter
value has to be of type numeric.
setReplaceMethod("collisionEnergy", "MsBackendTest", function(object, value) {
object$collisionEnergy <- value
object
})## [1] NA 20 20dataOrigin<-Replace the values for the data origin spectra variable. Parameter
value has to be of type character.
setReplaceMethod("dataOrigin", "MsBackendTest", function(object, value) {
object$dataOrigin <- value
object
})## [1] "<user provided>" "<user provided>" "<user provided>"## [1] "unknown" "file a" "file b"dataStorage<-Replace the values for the data storage spectra variable. Parameter
value has to be of type character. Since our
backend does not really make any use of this spectra variable, we can
accept any character value. For other backends, that for example need to
load data on-the-fly from data files, this spectra variable could be
used to store the name of the data files and hence we would need to
perform some additional checks within this replacement function.
setReplaceMethod("dataStorage", "MsBackendTest", function(object, value) {
object$dataStorage <- value
object
})## [1] "<memory>" "<memory>" "<memory>"## [1] "" "" ""isolationWindowLowerMz<-Replace the values for the isolation window lower m/z
spectra variable. Parameter value has to be of type
numeric (NA_real_ missing values are
supported, e.g. for MS1 spectra).
setReplaceMethod(
"isolationWindowLowerMz", "MsBackendTest", function(object, value) {
object$isolationWindowLowerMz <- value
object
})## [1] NA 245.3 NAisolationWindowTargetMz<-Replace the values for the isolation window target m/z
spectra variable. Parameter value has to be of type
numeric (NA_real_ missing values are
supported, e.g. for MS1 spectra).
setReplaceMethod(
"isolationWindowTargetMz", "MsBackendTest", function(object, value) {
object$isolationWindowTargetMz <- value
object
})## [1] NA 245.4 NAisolationWindowUpperMz<-Replace the values for the isolation window upper m/z
spectra variable. Parameter value has to be of type
numeric (NA_real_ missing values are
supported, e.g. for MS1 spectra).
setReplaceMethod(
"isolationWindowUpperMz", "MsBackendTest", function(object, value) {
object$isolationWindowUpperMz <- value
object
})## [1] NA 245.5 NAmsLevel<-Replace the MS level of spectra in a backend. Parameter
value has to be of type integer. Missing
values (NA_integer_) are supported.
setReplaceMethod("msLevel", "MsBackendTest", function(object, value) {
object$msLevel <- value
object
})## [1] 2 1 2## [1] 1 1 2polarity<-Replace the values for the polarity spectra variables. Parameter
value has to be of type integer and should
ideally also use the standard encoding 0L and
1L for negative and positive polarity (and
NA_integer for missing). Thus, in our implementation we
also make sure the input parameter contains the expected values
(although this is not a strictly required).
setReplaceMethod("polarity", "MsBackendTest", function(object, value) {
if (!all(value %in% c(0, 1, NA)))
stop("'polarity' should be encoded as 0L (negative), 1L (positive) ",
"with missing values being NA_integer_")
object$polarity <- value
object
})## [1] 0 0 0rtime<-Replace the retention times for the spectra represented by the
backend. Parameter value must be of type
numeric. Also, although it is not a strict requirement,
retention times should ideally be ordered increasingly per
sample and their unit should be seconds.
setReplaceMethod("rtime", "MsBackendTest", function(object, value) {
object$rtime <- value
object
})## [1] 1.2 1.3 1.4## [1] 3.2 3.3 3.4smoothed<-Replace the spectra variable smoothed that indicates whether
some data smoothing operation was performed on the spectra. Parameter
value must be of type logical.
setReplaceMethod("smoothed", "MsBackendTest", function(object, value) {
object$smoothed <- value
object
})## [1] TRUE TRUE TRUEspectraNames<-Replace the names of individual spectras within the backend. Same as
for names, colnames or rownames,
spectraNames are expected to be of type
character. In our backend implementation we store the
spectra names into the rownames of the
@spectraVars data frame.
setReplaceMethod("spectraNames", "MsBackendTest", function(object, value) {
rownames(object@spectraVars) <- value
object
})## [1] "a" "b" "c"Default implementations for these methods are available for
MsBackend classes, thus these methods don’t have to be
implemented for each new backend. For some backends, depending on how
the data is represented or accessed within it, different implementations
might however be more efficient.
backendBpparam()The backendBpparam() method is supposed to evaluate
whether a provided (or the default) parallel processing setup is
supported by the backend. Backends that do not support parallel
processing should return SerialParam() instead.
The default implementation is shown below.
backendParallelFactor()The backendParallelFactor() allows a backend to suggest
a preferred way how the backend could be split for parallel processing.
See also the notes on parallel processing above for more information.
The default implementation returns factor() (i.e. a
factor of length 0) hence not suggesting any splitting:
backendRequiredSpectraVariables()The backendRequiredSpectraVariables() method can be
implemented if a backend needs specific spectra variables to work. The
default implementation is:
The implementation for MsBackendMzR returns
c("dataStorage", "scanIndex") as the backend needs these
two spectra variables to load the MS data on-the-fly from the original
data files.
dropNaSpectraVariables()The dropNaSpectraVariables() is supposed to allow
removing all spectra variables from a data set (storage) that contain
only missing values (i.e. where the value of a spectra variable for each
spectrum is NA). This function is intended to reduce memory
requirements of backends such as the MsBackendMzR that load
values from all core spectra variables from the original data files,
even if their values are only NA. Removing these missing
values from the backend can hence reduce the size in memory of a backend
without data loss (because methods extracting core spectra variables are
supposed to always return NA values even if no data is
available for them - in such cases the NA values are
supposed to be created on-the-fly.
The default implementation is shown below.
setMethod("dropNaSpectraVariables", "MsBackend", function(object) {
svs <- spectraVariables(object)
svs <- svs[!(svs %in% c("mz", "intensity"))]
spd <- spectraData(object, columns = svs)
keep <- !vapply1l(spd, function(z) all(is.na(z)))
selectSpectraVariables(object, c(svs[keep], "mz", "intensity"))
})isReadOnly()isReadOnly() is expected to return a
logical(1) with either TRUE or
FALSE indicating whether the backend supports replacing
data or not. The default implementation is shown below.
peaksVariables()The peaksVariables() is expected to return a
character vector with the names of the peaks
variables (i.e. information and properties of individual mass
peaks) available in the backend. The default implementation for
MsBackend returns by default
c("mz", "intensity"). This method should only be
implemented for backends that (eventually) also provide additional peaks
variables. The default implementation is shown below.
uniqueMsLevels()This method should return the unique MS level(s) of all spectra within the backend. The default implementation is shown below.
This method thus retrieves first the MS levels of all spectra and
then calls unique() on them. Database-based backends might
avoid such an eventually heavy operation by selecting the unique MS
levels directly using an SQL call.
precursorMz<-Replace the values for the precursor m/z spectra variable.
Parameter value has to be of type numeric
(NA_real_ missing values are supported, e.g. for MS1
spectra). The default implementation uses the $<-
method:
ionCount()The ionCount() method should return a
numeric (length equal to the number of spectra represented
by the backend) with the sum of all intensities within each spectrum.
For empty spectra NA_real_ should be returned. The method
below is the default implementation of the method.
isCentroided()This method should return the information for each spectrum whether
it is centroided. In contrast to the centroided()
method a heuristic approach is used. The default implementation is shown
below.
reset()This is a special method that backends may implement or support, but
don’t necessary have to. This method will be called by the
reset,Spectra method and is supposed to restore the data to
its original state. The default implementation for
MsBackend shown below simply returns the backend as-is. The
MsBackendSql backend from the MsBackendSql
package in contrast re-initializes the data using the data from the
database.
export()This method should export the data from a
MsBackend. The method is called by the
export,Spectra method that passes itself as a second
argument to the function. The export,MsBackend
implementation is thus expected to take a Spectra object as
second argument from which all data should be taken and exported.
Implementation of this method is optional. The implementation of the
method for the MsBackendMzR backend is shown below.
setMethod("export", "MsBackendMzR", function(object, x, file = tempfile(),
format = c("mzML", "mzXML"),
copy = FALSE,
BPPARAM = bpparam()) {
l <- length(x)
file <- sanitize_file_name(file)
if (length(file) == 1)
file <- rep_len(file, l)
if (length(file) != l)
stop("Parameter 'file' has to be either of length 1 or ",
length(x), ", i.e. 'length(x)'.", call. = FALSE)
f <- factor(file, levels = unique(file))
tmp <- bpmapply(.write_ms_data_mzR, split(x, f), levels(f),
MoreArgs = list(format = format, copy = copy),
BPPARAM = BPPARAM)
})See alternatively also the MsBackendMgf
package for an implementation for the MsBackendMgf
backend.
split()The split() method should allow to split a
MsBackend into a list of
MsBackend objects. The default implementation is shown
below.
supportsSetBackend()Whether a MsBackend supports the
setBackend() method that allows to change the backend of a
Spectra object from one to another backend. To support
setBackend() the backend needs to have a parameter
data in its backendInitialize() method that
allows it to be initialized with a DataFrame containing the
full spectra data. The default implementation is shown below.
filterDataOrigin()The filter* methods are expected to take a
MsBackend and to subset it based on some criteria. While
also the [ method could be used to perform such subset
operation, these methods might allow more efficient ways to subset the
data e.g. by performing the operation within a database with a dedicated
SQL call. A default implementation is available for every filter
function and thus a method needs only to be implemented if the data
storage/representation within a backend would allow a more efficient
operation.
All filter methods are expected to return the subset backend (i.e. an instance of the same backend class with the same, or less spectra).
The filterDataOrigin() should subset the backend to
spectra with their dataOrigin spectra variable matching the
values provided with the dataOrigin parameter. The default
implementation for MsBackend is shown below.
filterDataStorage()Similar to the filterDataOrigin(), the
filterDataStorage() should subset a backend to spectra with
their dataStorage spectra variable matching the values
provided with the dataStorage parameter.
filterEmptySpectra()The filterEmptySpectra() should remove all
empty spectra (i.e. spectra without any mass peaks) from the
backend. The method is expected to return the subset backend. The
default implementation for MsBackend is shown below.
filterIsolationWindow()The filterIsolationWindow() filters the backend to
spectra with the provided mz value being within their
isolationWindowLowerMz() and
isolationWindowUpperMz(). The parameter mz
defining this target m/z is expected to be a numeric of
length 1. The default implementation for MsBackend is shown
below.
filterMsLevel()The filterMsLevel() method is expected to reduce the
backend to spectra with the provided MS level(s). Parameter
msLevel has to be an integer (any length). The
default implementation for MsBackend is shown below.
filterPolarity()The filterPolarity() method is expected to subset the
backend to spectra matching the provided polarity (or polarities).
Parameter polarity has to be an integer (of
any length). The default implementation for MsBackend is
shown below.
filterPrecursorMzRange()The filterPrecursorMzRange() method filters the backend
to spectra with their precursorMz being between the
provided m/z range (parameter mz). This method was
previously named filterPrecursorMz(). Parameter
mz is expected to be a numeric of length 2
defining the lower and upper limit of this precursor m/z range. The
default implementation for MsBackend is shown below.
##
## Attaching package: 'MsCoreUtils'## The following objects are masked from 'package:Spectra':
##
## bin, entropy, smooth## The following object is masked from 'package:stats':
##
## smoothfilterPrecursorMzValues()The filterPrecursorMzValues() method filters the backend
to spectra with their m/z values matching to the provided m/z value(s).
Parameters ppm and tolerance (both expected to
be numeric of length 1) allow to define the conditions for
the relaxed matching. Parameter mz has to be a
numeric (of any length). The default implementation for
MsBackend is shown below.
setMethod("filterPrecursorMzValues", "MsBackend",
function(object, mz = numeric(), ppm = 20, tolerance = 0) {
if (length(mz)) {
object[.values_match_mz(precursorMz(object), mz = mz,
ppm = ppm, tolerance = tolerance)]
} else object
})The .values_match_mz function used by this function is
defined as:
filterPrecursorCharge()The filterPrecursorCharge() method filters the backend
to spectra with matching precursor charge. Parameter z
defining the requested precursor charge has to be an
integer (of any length). The default implementation for
MsBackend is shown below.
filterPrecursorScan()The filterPrecursorScan() method filters the backend to
parent (e.g. MS1) and children scans (e.g. MS2) of acquisition number
acquisitionNum. Parameter f defines how the
backend should be split (by default by original data file) to avoid
selecting spectra from different samples/files. The default
implementation for MsBackend is shown below.
setMethod("filterPrecursorScan", "MsBackend",
function(object, acquisitionNum = integer(), f = dataOrigin(object)) {
if (length(acquisitionNum) && length(f)) {
if (!is.factor(f))
f <- factor(f, exclude = character())
keep <- unsplit(lapply(split(object, f = f), function(z, an) {
.filterSpectraHierarchy(acquisitionNum(z),
precScanNum(z),
an)
}, an = acquisitionNum), f = f)
object[keep]
} else object
})filterRt()The filterRt() method filters the backend to spectra
with their retention time being between the provided rt range. Parameter
rt is expected to be a numeric of length 2
defining the lower and upper bound of this range. Parameter
msLevel. (note the . in the name of the
parameter!) can be optionally used to restrict the filtering to the
selected MS levels (i.e. the RT filter is only applied to spectra of the
selected MS levels and all spectra with a different MS level are
returned as well). The default implementation for MsBackend
is shown below.
In this tutorial we implemented a simple in-memory
MsBackend from scratch. For many real-life situation it
might however be better to extend some of the pre-defined backend
classes from the Spectra package to avoid duplicating
functionality. A good starting point might be the
MsBackendMemory backend for any in-memory data
representation, or the MsBackendCached for backends that
retrieve data from inherently read-only resources (such as database
connection or raw data files) but still would need to support adding
spectra variables or changing values of spectra variables. Similarly, if
the only purpose of a backend is to import or export data in a specific
format, the MsBackendMemory might be extended and a single
method (backendInitialize()) would need to be implemented
for the new class: this new backendInitialize() would then
call the code to import the data from the new file format and store it
within the available slots of the MsBackendMemory object.
Examples would be the backends provided by the MsBackendMgf
and MsBackendMsp
classes.
The Spectra package provides a set of unit tests that
allow to check a backend for compliance with MsBackend.
Below we load this test suite and call the tests. The tests will be
performed on a variable be in the current workspace (which
in our case is an instance of our MsBackendTest class).
## R version 4.4.2 (2024-10-31)
## Platform: x86_64-pc-linux-gnu
## Running under: Ubuntu 24.04.1 LTS
##
## Matrix products: default
## BLAS: /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
## LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so; LAPACK version 3.12.0
##
## locale:
## [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C
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## [5] LC_MONETARY=en_US.UTF-8 LC_MESSAGES=en_US.UTF-8
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## [9] LC_ADDRESS=C LC_TELEPHONE=C
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
##
## time zone: Etc/UTC
## tzcode source: system (glibc)
##
## attached base packages:
## [1] stats4 stats graphics grDevices utils datasets methods
## [8] base
##
## other attached packages:
## [1] MsCoreUtils_1.19.0 IRanges_2.41.2 Spectra_1.17.4
## [4] BiocParallel_1.41.0 S4Vectors_0.45.2 BiocGenerics_0.53.3
## [7] generics_0.1.3 BiocStyle_2.35.0
##
## loaded via a namespace (and not attached):
## [1] cli_3.6.3 knitr_1.49 rlang_1.1.4
## [4] xfun_0.49 ProtGenerics_1.39.1 clue_0.3-66
## [7] jsonlite_1.8.9 buildtools_1.0.0 htmltools_0.5.8.1
## [10] maketools_1.3.1 sys_3.4.3 sass_0.4.9
## [13] rmarkdown_2.29 evaluate_1.0.1 jquerylib_0.1.4
## [16] MASS_7.3-61 fastmap_1.2.0 yaml_2.3.10
## [19] lifecycle_1.0.4 BiocManager_1.30.25 cluster_2.1.8
## [22] compiler_4.4.2 codetools_0.2-20 fs_1.6.5
## [25] MetaboCoreUtils_1.15.0 digest_0.6.37 R6_2.5.1
## [28] parallel_4.4.2 bslib_0.8.0 tools_4.4.2
## [31] cachem_1.1.0