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HDF5 REST VOL plugin

HDF5 REST VOL plugin version 1.0.0 - currently under development

Table of Contents:

I. Introduction
II. Installation
    A. Prerequisites
        i. External Libraries
        ii. HDF5 REST API server access
    B. Building the REST VOL plugin
        i. Obtaining the Source
        ii. One-Step Build
            a. Build Script Options
        iii. Manual Build
            a. Options for `configure`
            b. Options for CMake
        iv. Build Results
    C. Testing the REST VOL plugin installation
III. Using the REST VOL plugin
    A. Writing HDF5 REST VOL plugin applications
        i. Skeleton Example
    B. Building HDF5 REST VOL plugin applications
    C. Running HDF5 REST VOL plugin applications
        i. Runtime Environment
        ii. Example applications
IV. Feature Support
    A. Unsupported HDF5 API calls
    B. Unsupported HDF5 Features
    C. Problematic HDF5 Features
V. More Information

I. Introduction

The REST VOL plugin is a plugin for HDF5 designed with the goal of allowing HDF5 applications, both existing and future, to utilize web-based storage systems by translating HDF5 API calls into HTTP-based REST calls, as defined by the HDF5 REST API (See section V. for more information on RESTful HDF5).

Using a VOL plugin allows an existing HDF5 application to interface with different storage systems with minimal changes necessary. The plugin accomplishes this by utilizing the HDF5 Virtual Object Layer in order to re-route HDF5's public API calls to specific callbacks in the plugin which handle all of the usual HDF5 operations. The HDF5 Virtual Object Layer is an abstraction layer that sits directly between HDF5's public API and the underlying storage system. In this manner of operation, the mental data model of an HDF5 application can be preserved and transparently mapped onto storage systems that differ from a native filesystem, such as Amazon's S3.

The REST VOL plugin is under active development, and details given here may change.

II. Installation

Notes and instructions related to obtaining, building and installing the REST VOL plugin and accompanying HDF5 library.

II.A. Prerequisites

Before building and using the HDF5 REST VOL plugin, a few requirements must be met.

II.A.i. External Libraries

To build the REST VOL plugin, the following libraries are required:

Compiled libraries must either exist in the system's library paths or must be supplied to the REST VOL plugin's build scripts. Refer to section II.B.ii. below for more information.

II.A.ii. HDF5 REST API server access

Additionally, the HDF5 REST VOL plugin requires access to a server which implements the HDF5 REST API.

For more information on The HDF Group's officially supported service, please see https://www.hdfgroup.org/hdf-kita.

II.B. Building the REST VOL plugin

II.B.i. Obtaining the Source

The latest and most up-to-date REST VOL plugin code can be viewed at:

https://bitbucket.hdfgroup.org/projects/HDF5VOL/repos/rest/browse

and can directly be obtained from:

git clone https://[email protected]/scm/hdf5vol/rest.git

A source distribution of HDF5 has been included in the REST VOL plugin source in the /hdf5 directory. This version of HDF5 has been modified to support the REST VOL plugin.

II.B.ii. One-step Build

Use one of the supplied Autotools or CMake build scripts, depending on preference or system support.

  • Autotools

    Run build_vol_autotools.sh. See section II.B.ii.a for configuration options.

  • CMake

    Run build_vol_cmake.sh (Linux or OS X) or build_vol_cmake.bat (Windows). See section II.B.ii.a for configuration options.

By default, all of these build scripts will compile and link with the provided HDF5 source distribution. However, if you wish to use a manually built version of the HDF5 library, include the flag -H <dir> where dir is the path to the HDF5 install prefix. Refer to the documentation in hdf5/release_docs (where hdf5 is the HDF5 distribution root directory) for more information on building HDF5 manually. Note that if you wish to use a manually built version of the HDF5 library, it must be a version which contains the VOL abstraction layer; otherwise, the REST VOL plugin will not function correctly.

NOTE: For those who are capable of using both build systems, the autotools build currently does not support out-of-tree builds. If the REST VOL source directory is used for an autotools build, it is important not to re-use the source directory for a later build using CMake. This will causes build conflicts and result in strange and unexpected behavior.

II.B.ii.a. Build Script Options

The following configuration options are available to all of the build scripts:

-h      Prints out a help message indicating script usage and available options.

-d      Enables debugging information printouts within the REST VOL plugin.

-c      Enables debugging information printouts from cURL within the REST VOL plugin.

-m      Enables memory usage tracking within the REST VOL plugin. This option is
        mostly useful in helping to diagnose any possible memory leaks or other
        memory errors within the plugin.

-t      Build the HDF5 tools with REST VOL plugin support.
        WARNING: This option is experimental and should not currently be used.

-P DIR  Specifies where the REST VOL plugin should be installed. The default
        installation prefix is `rest_vol_build` inside the REST VOL plugin source
        root directory.

-H DIR  Prevents building of the provided HDF5 source. Instead, uses the compiled
        library found at directory `DIR`, where `DIR` is the path used as the
        installation prefix when building HDF5 manually.

-C DIR  Specifies the top-level directory where cURL is installed. Used if cURL is
        not installed to a system path or used to override

-Y DIR  Specifies the top-level directory where YAJL is installed. Used if YAJL is
        not installed to a system path or used to override

Additionally, the CMake build scripts have the following configuration option:

-B DIR  Specifies the directory that CMake should use as the build tree location.
        The default build tree location is `rest_vol_cmake_build_files` inside the
        REST VOL plugin source root directory. Note that the REST VOL does not
        support in-source CMake builds.

-G DIR  Specifies the CMake Generator to use when generating the build files
        for the project. On Unix systems, the default is "Unix Makefiles" and if
        this is not changed, the build script will automatically attempt to build
        the project after generating the Makefiles. If the generator is changed, the
        build script will only generate the build files and the build command to
        build the project will have to be run manually.

II.B.iii. Manual Build

In general, the process for building the REST VOL plugin involves either obtaining a VOL-enabled HDF5 distribution or building one from source. Then, the REST VOL plugin is built using that HDF5 distribution by including the appropriate header files and linking against the HDF5 library.

Once you have a VOL-enabled HDF5 distribution available, follow the instructions below for your respective build system in order to build the REST VOL plugin against the HDF5 distribution.

Autotools

$ autogen.sh
$ configure --with-hdf5[=DIR] [options]
$ make
$ make check (requires HDF5 REST API server access -- see section II.A.ii.)
$ make install

CMake

$ mkdir builddir
$ cd builddir
$ cmake -G "CMake Generator (Unix Makefiles, etc.)" -DPREBUILT_HDF5_DIR=built_hdf5_dir [options] rest_vol_src_dir
$ build command (e.g. `make && make install` for CMake Generator "Unix Makefiles")

and optionally:

$ cpack

II.B.iii.a. Options for configure

When building the REST VOL plugin manually using Autotools, the following options are available to configure.

The options in the supplied Autotools build script are mapped to the corresponding options here:

-h, --help      Prints out a help message indicating script usage and available
                options.

--prefix=DIR    Specifies the location for the resulting files. The default location
                is `rest_vol_build` in the same directory as configure.

--enable-build-mode=(production|debug)
                Sets the build mode to be used.
                Debug - enable debugging printouts within the REST VOL plugin.
                Production - Focus more on optimization.

--enable-curl-debug
                Enables debugging information printouts from cURL within the
                REST VOL plugin.

--enable-mem-tracking
                Enables memory tracking within the REST VOL plugin. This option is
                mostly useful in helping to diagnose any possible memory leaks or
                other memory errors within the plugin.

--enable-tests
                Enables/Disables building of the REST VOL plugin tests.

--enable-examples
                Enables/Disables building of the REST VOL HDF5 examples.

--enable-tools
                Enables/Disables building of the HDF5 tools with REST VOL plugin
                support. (Currently experimental and should not be used)

--with-hdf5=DIR Used to specify the directory where an HDF5 distribution that uses
                the VOL layer has already been built. This is to help the REST VOL
                plugin locate the HDF5 header files that it needs to include.

--with-curl=DIR Used to specify the top-level directory where cURL is installed, if
                cURL is not installed to a system path.

--with-yajl=DIR Used to specify the top-level directory where YAJL is installed, if
                YAJL is not installed to a system path.

II.B.iii.b. Options for CMake

When building the REST VOL plugin manually using CMake, the following options are available.

Some of the options in the supplied CMake build script are mapped to the corresponding options here:

CMAKE_INSTALL_PREFIX (Default: `rest_vol_build` in REST VOL plugin source root
                      directory)
                Specifies the directory where CMake will install the resulting
                files to.

PREBUILT_HDF5_DIR (Default: empty)
                Specifies a directory which contains a pre-built HDF5
                distribution which uses the VOL abstraction layer. By default,
                the REST VOL plugin's CMake build will attempt to build the
                included HDF5 source distribution, then use that to build the
                plugin itself. However, if a VOL-enabled HDF5 distribution
                is already available, this option can be set to point to the
                directory of the HDF5 distribution. In this case, CMake will
                use that HDF5 distribution to build the REST VOL plugin and
                will not attempt to build HDF5 again.

BUILD_SHARED_LIBS (Default: ON)
                Enables/Disables building of the REST VOL as a shared library.

BUILD_STATIC_EXECS (Default: OFF)
                Enables/Disables building of REST VOL executables as static
                executables.

REST_VOL_ENABLE_DEBUG (Default: OFF)
                Enables/Disables debugging printouts within the REST VOL plugin.

REST_VOL_ENABLE_CURL_DEBUG (Default: OFF)
                Enables/Disables debugging information printouts from cURL within
                the REST VOL plugin.

REST_VOL_ENABLE_MEM_TRACKING (Default: OFF)
                Enables/Disables memory tracking withing the REST VOL plugin. This
                options is mostly useful in helping to diagnose any possible memory
                leaks or other memory errors within the plugin.

REST_VOL_ENABLE_EXAMPLES (Default: ON)
                Enables/Disables building of the REST VOL HDF5 examples.

BUILD_TESTING (Default: ON)
                Enables/Disables building of the REST VOL tests.

II.B.iv. Build Results

If the build is successful, files are written into an installation directory. By default, these files are placed in rest_vol_build in the REST VOL plugin source root directory. For Autotools, this default can be overridden with build_vol_autotools.sh -P DIR (when using the build script) or configure --prefix=<DIR> (when building manually). For CMake, the equivalent for overriding this default is build_vol_cmake.sh/.bat -P DIR (when using the build script) or -DCMAKE_INSTALL_PREFIX=DIR (when building manually).

If the REST VOL plugin was built using one of the included build scripts, all of the usual files from an HDF5 source build should appear in the respective bin, include, lib and share directories in the install directory. Notable among these is bin/h5cc (when built with Autotools), a special-purpose compiler wrapper script that streamlines the process of building HDF5 applications.

II.C. Testing the REST VOL plugin installation

The REST VOL plugin tests require access to an HDF5 REST API-aware server -- see section II.A.ii and III.C.

After building the REST VOL plugin and obtaining access to a server which implements the HDF5 REST API according to the above reference, it is highly advised that you run make check (for Autotools builds) or ctest . (for CMake builds) to verify that the HDF5 library and REST VOL plugin are functioning correctly.

Each of these commands will run the test_rest_vol executable, which is built by each of the REST VOL plugin's build systems and contains a set of tests to cover a moderate amount of the HDF5 public API. Alternatively, this executable can simply be run directly.

III. Using the REST VOL plugin

This section outlines the unique aspects of writing, building and running HDF5 applications with the REST VOL plugin.

III.A. Writing HDF5 REST VOL plugin applications

Any HDF5 application using the REST VOL plugin must:

  • Include rest_vol_public.h, found in the include directory of the REST VOL plugin installation directory.

  • Link against librestvol.a (or similar), found in the lib directory of the REST VOL plugin installation directory.

An HDF5 REST VOL plugin application requires three new function calls in addition to those for an equivalent HDF5 application:

  • RVinit() - Initializes the REST VOL plugin

    Called upon application startup, before any file is accessed.

  • H5Pset_fapl_rest_vol() - Set REST VOL plugin access on File Access Property List.

    Called to prepare a FAPL to open a file through the REST VOL plugin. See https://support.hdfgroup.org/HDF5/Tutor/property.html#fa for more information about File Access Property Lists.

  • RVterm() - Cleanly shutdown the REST VOL plugin

    Called on application shutdown, after all files have been closed.

III.A.i. Skeleton Example

Below is a no-op application that opens and closes a file using the REST VOL plugin. For clarity, no error-checking is performed.

#include "hdf5.h"
#include "rest_vol_public.h"

int main(void)
{
    hid_t fapl_id;
    hid_t file_id;

    RVinit();

    fapl_id = H5Pcreate(H5P_FILE_ACCESS);
    H5Pset_fapl_rest_vol(fapl_id);
    file_id = H5Fopen("my/file.h5");

    /* operate on file */

    H5Pclose(fapl_id);
    H5Fclose(file_id);

    RVterm();

    return 0;
}

III.B. Building HDF5 REST VOL plugin applications

Assuming an application has been written following the above instructions, the application must be built prior to running. In general, the application should be built as normal for any other HDF5 application.

To link in the required libraries, the compiler will likely require the additional linker flags: -lrestvol -lcurl -lyajl

However, these may vary depending on platform, compiler and installation location of the REST VOL plugin.

If the REST VOL plugin was built using Autotools, it is highly recommended that compilation of HDF5 REST VOL plugin applications be done using the supplied h5cc script, as it will manage linking with the HDF5 library. h5cc may be found in the /bin directory of the installation. The above notice about additional libraries applies to usage of h5cc. For example:

h5cc -lrestvol -lcurl -lyajl my_restvol_application.c -o my_executable

III.C. Running HDF5 REST VOL plugin applications

Running applications that use the HDF5 REST VOL plugin requires access to a server which implements the HDF5 REST API; see section II.A.ii.

III.C.i. Runtime Environment

For the REST VOL plugin to correctly interact with an HDF5 REST API-aware server instance, there are three environment variables (two optional and one required) which should first be set. These are:

  • HSDS_USERNAME - (optional) The username to use for authentication

  • HSDS_PASSWORD - (optional) The password to use for authentication

  • HSDS_ENDPOINT - The base URL of the instance (e.g. http://hsdshdflab.hdfgroup.org)

Note that there are cases where authentication may not be required, such as when simply retrieving the information about a publicly-accessible HDF5 dataset or similar. In these cases, it may not be necessary to supply any authentication information and just the HSDS_ENDPOINT environment variable should be set.

III.C.ii. Example applications

The file test/test_rest_vol.c, in addition to being the source for the REST VOL plugin test suite, serves double purpose with each test function being an example application in miniature, focused on a particular behavior. This application tests a moderate amount of HDF5's public API functionality with the REST VOL plugin and should be a good indicator of whether the REST VOL plugin is working correctly in conjunction with a running HDF5 REST API-aware instance.

In addition to this file, some of the example C applications included with HDF5 distributions have been adapted to work with the REST VOL plugin and are included under the top-level examples directory in the REST VOL plugin source root directory.

IV. Feature Support

Not all aspects of HDF5 are implemented by or are applicable to the REST VOL plugin.

IV.A. Unsupported HDF5 API calls

Due to a combination of lack of server support and the complexity in implementing them, or due to a particular call not making sense from the server's perspective, the following HDF5 API calls are currently unsupported:

  • H5A interface

    • H5Aopen_by_idx
    • H5Aget_info_by_idx
    • H5Aget_name_by_idx
    • H5Aget_storage_size
    • H5Adelete_by_idx
    • H5Arename
    • H5Arename_by_name

  • H5D interface

    • H5Dget_offset
    • H5Dget_space_status
    • H5Dget_storage_size
    • H5Dset_extent

  • H5F interface

    • H5Fget_obj_count
    • H5Fget_obj_ids
    • H5Fflush
    • H5Fis_accessible
    • H5Fmount
    • H5Funmount
    • H5Fclear_elink_file_cache
    • H5Fget_file_image
    • H5Fget_free_sections
    • H5Fget_freespace
    • H5Fget_mdc_config
    • H5Fget_mdc_hit_rate
    • H5Fget_mdc_size
    • H5Fget_filesize
    • H5Fget_vfd_handle
    • H5Freset_mdc_hit_rate_stats
    • H5Fset_mdc_config

  • H5G interface

    • H5Gget_info_by_idx

  • H5L interface

    • H5Lget_info_by_idx
    • H5Lget_name_by_idx
    • H5Lget_val_by_idx
    • H5Ldelete_by_idx
    • H5Lcopy
    • H5Lmove

  • H5O interface

    • H5Oopen_by_idx
    • H5Oopen_by_addr
    • H5Oget_info_by_idx
    • H5Oincr_refcount
    • H5Odecr_refcount
    • H5Oexists_by_name
    • H5Ovisit
    • H5Ovisit_by_name
    • H5Ocopy

  • H5R interface

    • H5Rget_name
    • H5Rget_region/Region references

IV.B. Unsupported HDF5 Features

The following other features are currently unsupported:

  • Dataset Fill Values

  • Virtual Dataset layouts

  • External Storage for contiguous dataset layouts

  • Non-predefined integer and floating-point datatypes

  • Variable-length, Opaque, Bitfield and Time datatypes

  • Character sets other than H5T_CSET_ASCII for string datatypes

  • String padding values other than H5T_STR_NULLPAD for fixed-length strings

  • String padding values other than H5T_STR_NULLTERM for variable-length strings (Note that variable-length string datatypes are currently unsupported by the REST VOL plugin, but a dataset can still be created with a variable-length string type)

  • Non-regular hyperslab selections

  • Non-contiguous hyperslab selections

  • User-defined links

  • External links

  • H5Pset_create_intermediate_group property (the plugin will not currently create intermediate groups in a path if they do not exist)

IV.C. Problematic HDF5 Features

Due to underlying implementation details, the following circumstances are known to be problematic for the REST VOL plugin and will likely cause issues for the application if not avoided or taken into account:

  • Trying to open an object in the file by using a pathname where one or more components of the path on the way to the object in question are soft links. For example, trying to open a dataset by the pathname /group/subgroup/soft_link_to_dataset should work. However, trying to open a dataset using a pathname like /group/soft_link_to_group/soft_link_to_dataset will generally fail.

  • Due to a simple basename function implementation which follows the GNU behavior, using a trailing / on path names will likely confuse the plugin and cause incorrect behavior.

  • The use of point selections for dataset writes will generally incur an additional memory overhead of approximately 4/3 the size of the original buffer used for the H5Dwrite call. This is due to the fact that a temporary copy of the buffer must be made and then base64-encoded for the server transfer and base64-encoding generally imposes a 33% overhead.

  • Due to the HDF5 public API call H5Pset_external using the off_t type, it is possible that compilation of the REST VOL plugin on non-POSIX-compliant systems may fail.

V. More Information

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HDF5 REST VOL Connector

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