| Title: | Distributed Storage and List |
|---|---|
| Description: | An abstract DList class helps storing large list-type objects in a distributed manner. Corresponding high-level functions and methods for handling distributed storage (DStorage) and lists allows for processing such DLists on distributed systems efficiently. In doing so it uses a well defined storage backend implemented based on the DStorage class. |
| Authors: | Ingo Feinerer [aut], Stefan Theussl [aut, cre], Christian Buchta [ctb] |
| Maintainer: | Stefan Theussl <[email protected]> |
| License: | GPL-3 |
| Version: | 0.1-7 |
| Built: | 2024-08-22 06:14:08 UTC |
| Source: | CRAN |
Retrieves "DList" data distributed as chunks.
DGather( x, keys = FALSE, n = -1L, names = TRUE )DGather( x, keys = FALSE, n = -1L, names = TRUE )
x |
a |
keys |
logical; should only keys be retrieved from chunks?
Default: |
n |
an integer specifying the number of chunks to be read. |
names |
logical; should the return value be a named list?
Default: |
DGather() is similar to an MPI_GATHER (see
http://www.mpi-forum.org/docs/mpi-3.1/mpi31-report/node103.htm#Node103)
where: “[...] each process (root process included) sends the
contents of its send buffer to the root process. The root process
receives the messages and stores them in rank order.” For
"DList" objects DGather() will gather data
contained in chunks possibly distributed on a cluster of workstations
and store it in a (possibly named) list. Note that
depending of the size of the data, the resulting list may not fit into
memory.
A (named) list.
dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) DGather( dl ) ## retrieve keys unlist(DGather( dl, keys = TRUE, names = FALSE )) ## remove DList and garbage collect it rm( dl ) gc()dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) DGather( dl ) ## retrieve keys unlist(DGather( dl, keys = TRUE, names = FALSE )) ## remove DList and garbage collect it rm( dl ) gc()
Functions to construct, coerce, check for, and interact with storage
of objects of class "DList".
DList( ... ) as.DList( x, DStorage = NULL, ... ) is.DList( x ) DL_storage( x ) `DL_storage<-`( x, value )DList( ... ) as.DList( x, DStorage = NULL, ... ) is.DList( x ) DL_storage( x ) `DL_storage<-`( x, value )
... |
objects, possibly named. |
x |
an object. |
DStorage |
an object representing the virtual (distributed)
storage for storing data. See class |
value |
the new storage of class |
An object of class "DList" or, in case of DL_storage(),
an object of class "DStorage".
## coerce to 'DList' object using a default virtual storage l <- list( cow = "girl", bull = "boy" ) dl <- as.DList( l ) is.DList( dl ) DL_storage(dl) ## remove DList and garbage collect it rm(dl) gc()## coerce to 'DList' object using a default virtual storage l <- list( cow = "girl", bull = "boy" ) dl <- as.DList( l ) is.DList( dl ) DL_storage(dl) ## remove DList and garbage collect it rm(dl) gc()
When using class DList the underlying
‘virtual’ storage plays an
important role. It defines how to use the given storage (read/write
methods, etc.), where the data is to be stored (i.e., the base
directory on the file system), and how DMap as well as
DReduce have to be applied.
DStorage(type = c("LFS", "HDFS"), base_dir, chunksize = 1024^2) is.DStorage( ds )DStorage(type = c("LFS", "HDFS"), base_dir, chunksize = 1024^2) is.DStorage( ds )
type |
the type of the storage to be created. Currently only
|
base_dir |
specifies the base directory where data is to be stored. |
chunksize |
defines the size of each chunk written to the virtual storage. |
ds |
a virtual possibly distributed storage. |
An object which inherits from class DStorage, or, in case of
is.DStorage() a logical indicating whether it inherits from
"DStorage" or not.
## creating a new virtual storage using 50MB chunks ds <- DStorage(type = "LFS", base_dir = tempdir(), chunksize = 50 * 1024^2) is.DStorage( ds )## creating a new virtual storage using 50MB chunks ds <- DStorage(type = "LFS", base_dir = tempdir(), chunksize = 50 * 1024^2) is.DStorage( ds )
Key/value pairs in "DList" objects.
DKeys( x )DKeys( x )
x |
a |
A character vector representing all keys of the key/value pairs stored
in chunks by "DList" objects.
## create a 2 elements DList dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) ## retrieve keys DKeys( dl ) ## remove DList and garbage collect it rm( dl ) gc()## create a 2 elements DList dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) ## retrieve keys DKeys( dl ) ## remove DList and garbage collect it rm( dl ) gc()
"DList" ObjectsInterface to apply functions on elements of "DList" objects.
DLapply( x, FUN, parallel, ..., keep = FALSE ) DMap( x, MAP, parallel, keep = FALSE ) DReduce( x, REDUCE, parallel, ... )DLapply( x, FUN, parallel, ..., keep = FALSE ) DMap( x, MAP, parallel, keep = FALSE ) DReduce( x, REDUCE, parallel, ... )
x |
a |
FUN |
the function to be applied to each element (i.e., the
values) of |
MAP |
the function to be applied to each key/value pair in
|
REDUCE |
the function to be applied to each key/value pair in
|
... |
optional arguments to |
parallel |
logical; should the provided functions applied in
parallel? Default: |
keep |
logical; should the current data be kept as a separate
revision for further processing later? Default: |
The MapReduce programming model as defined by Dean and Ghemawat (2008)
is as follows: the computation takes a set of input key/value pairs, and produces a
set of output key/value pairs. The user expresses the computation as two
functions: Map and Reduce. The Map function takes an input pair and produces a set of
intermediate key/value pairs. The Reduce function accepts an
intermediate key and a set of values for that key (possibly
grouped by the MapReduce library). It merges these values
together to form a possibly smaller set of values. Typically, just zero or
one output value is produced per reduce invocation. Furthermore, data is
usually stored on a (distributed) file system which is recognized by the
MapReduce library. This allows such a framework to handle lists of
values (here objects of class "DList") that are too
large to fit in main memory (i.e., RAM).
A "DList".
J. Dean and S. Ghemawat (2008). MapReduce: Simplified Data Processing on Large Clusters. Communications of the ACM, 51, 107–113.
dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) res <- DLapply( dl, function(x) unlist(strsplit(x, " ")) ) as.list( res ) foo <- function( keypair ) list( key = paste("next_", keypair$key, sep = ""), value = gsub("first", "mapped", keypair$value) ) dlm <- DMap( x = dl, MAP = foo) ## retrieve keys unlist(DGather(dlm, keys = TRUE, names = FALSE)) ## retrieve values as.list( dlm ) ## simple wordcount based on two files: dir(system.file("examples", package = "DSL")) ## first force 1 chunk per file (set max chunk size to 1 byte): ds <- DStorage("LFS", tempdir(), chunksize = 1L) ## make "DList" from files, i.e., read contents and store in chunks dl <- as.DList(system.file("examples", package = "DSL"), DStorage = ds) ## read files dl <- DMap(dl, function( keypair ){ list( key = keypair$key, value = tryCatch(readLines(keypair$value), error = function(x) NA) ) }) ## split into terms splitwords <- function( keypair ){ keys <- unlist(strsplit(keypair$value, " ")) mapply( function(key, value) list( key = key, value = value), keys, rep(1L, length(keys)), SIMPLIFY = FALSE, USE.NAMES = FALSE ) } res <- DMap( dl, splitwords ) as.list(res) ## now aggregate by term res <- DReduce( res, sum ) as.list( res )dl <- DList( line1 = "This is the first line.", line2 = "Now, the second line." ) res <- DLapply( dl, function(x) unlist(strsplit(x, " ")) ) as.list( res ) foo <- function( keypair ) list( key = paste("next_", keypair$key, sep = ""), value = gsub("first", "mapped", keypair$value) ) dlm <- DMap( x = dl, MAP = foo) ## retrieve keys unlist(DGather(dlm, keys = TRUE, names = FALSE)) ## retrieve values as.list( dlm ) ## simple wordcount based on two files: dir(system.file("examples", package = "DSL")) ## first force 1 chunk per file (set max chunk size to 1 byte): ds <- DStorage("LFS", tempdir(), chunksize = 1L) ## make "DList" from files, i.e., read contents and store in chunks dl <- as.DList(system.file("examples", package = "DSL"), DStorage = ds) ## read files dl <- DMap(dl, function( keypair ){ list( key = keypair$key, value = tryCatch(readLines(keypair$value), error = function(x) NA) ) }) ## split into terms splitwords <- function( keypair ){ keys <- unlist(strsplit(keypair$value, " ")) mapply( function(key, value) list( key = key, value = value), keys, rep(1L, length(keys)), SIMPLIFY = FALSE, USE.NAMES = FALSE ) } res <- DMap( dl, splitwords ) as.list(res) ## now aggregate by term res <- DReduce( res, sum ) as.list( res )