# 1 Motivation

One of the advantages of using HDF5 is that data stored on disk can be compressed, reducing both the space required to store them and the time needed to read those data. This data compression is applied as part of the HDF5 “filter pipeline” that modifies data during I/O operations. HDF5 includes several filter algorithms as standard, and the version of the HDF5 library found in Rhdf5lib is additionally compiled with support for the deflate and szip compression filters which rely on third-party compression libraries. Collectively HDF5 refer to these as the “internal” filters. It is possible to use any combination of these (including none) when writing data using rhdf5. The default filter pipeline is shown in Figure 1.

This pipeline approach has been designed so that filters can be chained together (as in the diagram above) or easily substituted for alternative filters. This allows tailoring the compression approach to best suit the data or application.

It may be case that for a specific usecase an alternative, third-party, compression algorithm would be the most appropriate to use. Such filters, which are not part of the standard HDF5 distribution, are referred to as “external” filters. In order to allow their use without requiring either the HDF5 library or applications to be built with support for all possible filters HDF5 is able to use dynamically loaded filters. These are compiled independently from the HDF5 library, but are available to an application at run time.

This package currently distributes external HDF5 filters employing bzip2 and the Blosc meta-compressor. In total rhdf5filters provides access to eight1 zlib compression is almost always available in a standard HDF5 installation, but is also available via Blosc. compression filters than can be applied to HDF5 datasets. The full list of filters currently provided by the package is:

• bzip2
• blosclz
• lz4
• lz4hc
• snappy
• zstd
• zlib
• lzf

# 2 Usage

## 2.1 With rhdf5

rhdf5filters is principally designed to be used via the rhdf5 package, where several functions are able to utilise the compression filters. For completeness those functions are described here and are also documented in the rhdf5 vignette.

### 2.1.1 Writing data

The function h5createDataset() within rhdf5 takes the argument filter which specifies which compression filter should be used when a new dataset is created.

Also available in rhdf5 are the functions H5Pset_bzip2(), H5Pset_lzf() and H5Pset_blosc(). These are not part of the standard HDF5 interface, but are modelled on the H5Pset_deflate() function and allow the bzip2, lzf and blosc filters to be set on dataset create property lists.

As long as rhdf5filters is installed, rhdf5 will be able to transparently read data compressed using any of the filters available in the package without requiring any action on your part.

## 2.2 With external applications

The dynamic loading design of the HDF5 compression filters means that you can use the versions distributed with rhdf5filters with other applications, including other R packages that interface HDF5 as well as external applications not written in R e.g. HDFVIEW. The function hdf5_plugin_path() will return the location of in your packages library where the compiled plugins are stored. You can the set the environment variable HDF5_PLUGIN_PATH and other applications will be able to dynamically load the compression plugins found there if needed.

rhdf5filters::hdf5_plugin_path()
## [1] "/tmp/RtmpissNTz/Rinst24c17d7bc7efe6/rhdf5filters/lib"

### 2.2.1h5dump example

The next example demonstrates how the filters distributed by rhdf5filters can be used by external applications to decompress data. Do do this we’ll use the version of h5dump installed on the system2 If h5dump is not found on your system these example will fail. and a file distributed with this package that has been compressed using the blosc filter. Since rhdf5filters sets the HDF5_PLUGIN_PATH environment variable in an R session, we will manually unset it to demonstrate the typical behaviour.

## blosc compressed file
blosc_file <- system.file("h5examples/h5ex_d_blosc.h5",
package = "rhdf5filters")
## unset environment variable
Sys.setenv("HDF5_PLUGIN_PATH" = "")

Now we use system2() to call the system version of h5dump and capture the output, which is then printed below. The most important parts to note are the FILTERS section, which shows the dataset was indeed compressed with blosc, and DATA, where the error shows that h5dump is currently unable to read the dataset.

h5dump_out <- system2('h5dump',
args = c('-p', '-d /dset', blosc_file),
stdout = TRUE, stderr = TRUE)
cat(h5dump_out, sep = "\n")
## HDF5 "rhdf5filters/h5examples/h5ex_d_blosc.h5" {
## DATASET "/dset" {
##    DATATYPE  H5T_IEEE_F32LE
##    DATASPACE  SIMPLE { ( 30, 10, 20 ) / ( 30, 10, 20 ) }
##    STORAGE_LAYOUT {
##       CHUNKED ( 10, 10, 20 )
##       SIZE 3347 (7.171:1 COMPRESSION)
##    }
##    FILTERS {
##       USER_DEFINED_FILTER {
##          FILTER_ID 32001
##          COMMENT blosc
##          PARAMS { 2 2 4 8000 4 1 0 }
##       }
##    }
##    FILLVALUE {
##       FILL_TIME H5D_FILL_TIME_IFSET
##       VALUE  H5D_FILL_VALUE_DEFAULT
##    }
##    ALLOCATION_TIME {
##       H5D_ALLOC_TIME_INCR
##    }
##    DATA {h5dump error: unable to print data
##
##    }
## }
## }

Next we set HDF5_PLUGIN_PATH to the location where rhdf5filters has stored the filters and re-run the call to h5dump. Printing the output3 The dataset is quite large, so we only show a few lines here. no longer returns an error in the DATA section, indicating that the blosc filter plugin was found and used by h5dump.

## set environment variable to hdf5filter location
Sys.setenv("HDF5_PLUGIN_PATH" = rhdf5filters::hdf5_plugin_path())
h5dump_out <- system2('h5dump',
args = c('-p', '-d /dset', '-w 50', blosc_file),
stdout = TRUE,  stderr = TRUE)

## find the data entry and print the first few lines
DATA_line <- grep(h5dump_out, pattern = "DATA \\{")
cat( h5dump_out[ (DATA_line):(DATA_line+2) ], sep = "\n" )
##   DATA {
##    (0,0,0): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
##    (0,0,11): 11, 12, 13, 14, 15, 16, 17, 18,

# Session info

## R version 4.2.1 (2022-06-23)
## Platform: x86_64-pc-linux-gnu (64-bit)
## Running under: Ubuntu 20.04.5 LTS
##
## Matrix products: default
## BLAS:   /home/biocbuild/bbs-3.16-bioc/R/lib/libRblas.so
## LAPACK: /home/biocbuild/bbs-3.16-bioc/R/lib/libRlapack.so
##
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
##  [3] LC_TIME=en_GB              LC_COLLATE=C
##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
##
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base
##
## other attached packages:
## [1] BiocStyle_2.26.0
##
## loaded via a namespace (and not attached):
##  [1] bookdown_0.29       digest_0.6.30       rhdf5filters_1.10.0
##  [4] R6_2.5.1            jsonlite_1.8.3      magrittr_2.0.3
##  [7] evaluate_0.17       highr_0.9           stringi_1.7.8
## [10] cachem_1.0.6        rlang_1.0.6         cli_3.4.1
## [13] jquerylib_0.1.4     bslib_0.4.0         rmarkdown_2.17
## [16] tools_4.2.1         stringr_1.4.1       xfun_0.34
## [19] yaml_2.3.6          fastmap_1.1.0       compiler_4.2.1
## [22] BiocManager_1.30.19 htmltools_0.5.3     knitr_1.40
## [25] sass_0.4.2`