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OCFL Java Usage

OCFL Repository Configuration

Use the OcflRepositoryBuilder to construct OcflRepository instances. You should only create a single OcflRepository instance per OCFL repository, and you should not reuse the same OcflRepositoryBuilder to create multiple OcflRepository instances.

The OcflRepositoryBuilder will initialize a new OCFL repository if it's pointed at an empty directory, or open an existing repository if it's pointed at an existing OCFL storage root.

Use OcflRepositoryBuilder.build() to construct standard OCFL repository and OcflRepositoryBuilder.buildMutable() to construct an OCFL repository that supports the mutable HEAD extension.

Required Properties

  • storage: Sets the storage layer implementation that the OCFL repository should use. Use OcflStorageBuilder.builder() to create an implementation.
  • workDir: Sets the path to the directory that is used to assemble OCFL versions. If you are using filesystem storage, it is critical that this directory is located on the same volume as the OCFL storage root.
  • defaultLayoutConfig: Configures the default storage layout the OCFL repository uses. The storage layout is used to map OCFL object IDs to object root directories within the repository. The layout configuration must be set when creating new OCFL repositories, but is not required when opening an existing repository. Storage layouts must be defined in by an OCFL extension. Currently, the following extensions are implemented:

Optional Properties

  • ocflConfig: Sets the following default values that are used for creating new OCFL objects: OCFL version, digest algorithm, version zero-padding width, and content directory. The defaults are 1.1, sha512, 0, and content.
  • verifyStaging: Determines whether the contents of staged versions should be verified immediately prior to installing them. This is enabled by default, but can be safely disabled if you are concerned about performance on particularly slow filesystems.
  • prettyPrintJson: Enables pretty print JSON in newly written inventory files. By default, pretty printing is disabled to reduce inventory file size.
  • contentPathConstraints: Configures what file name constraints are enforced on OCFL content paths. By default, there are no special constraints applied. Used ContentPathConstraints for a selection of pre-configured defaults. You may want to apply constraints if you are concerned about portability between filesystems. For example, disallowing : and \ characters.
  • logicalPathMapper: LogicalPathMapper implementations are used to map logical paths to safe content paths. By default, logical paths are mapped directly to content paths without making any changes. See LogicalPathMappers for more pre-configured options, such as LogicalPathMappers.percentEncodingWindowsMapper(), which percent-encodes a handful of characters that are problematic on Windows.
  • unsupportedExtensionBehavior: By default set to FAIL, which means that repositories and objects that contain unsupported extensions will not be allowed. May be set to WARN to all unsupported extensions.
  • ignoreUnsupportedExtensions: A set of unsupported extension names that should be allowed either without causing a failure, if unsupportedExtensionBehavior is set to FAIL, or not logging, if set to WARN
  • inventoryCache: By default, an in-memory Caffeine cache is used to cache deserialized inventories.
  • objectLock: Set the lock implementation that's used to lock objects for writing. By default, it is an in-memory lock with a 10 second wait to acquire. Use ObjectLockBuilder construct an alternate lock. When more than one processes may be concurrently writing to an OCFL repository, a different implementation, such as DbObjectLock, should be used.
  • objectDetailsDb: Configures a database to use to store OCFL object metadata. By default, this feature is not used. It is intended to be used when using cloud storage, and caches a copy of the most recent inventory file. In addition to speeding cloud operations up a little, it also addresses the eventual consistency problem, though most cloud storage, including S3, is now strongly consistent. Use ObjectDetailsDatabaseBuilder to construct an ObjectDetailsDatabase.
  • fileLockTimeoutDuration: Configures the max amount of time to wait for a file lock when updating an object from multiple threads. This only matters if you concurrently write files to the same object, and can otherwise be ignored. The default timeout is 1 minute.

Storage Implementations

Filesystem

The basic OCFL repository implementation stores objects under an OCFL storage root on a locally attached filesystem. The client constructs new object versions in a work directory before attempting to move them into the object root. Ideally, this move operation can be executed as an atomic rename, and, as such, the work directory configured on the ocfl-java client should be located on the same mount as the OCFL storage root.

Configuration

Use OcflStorageBuilder.builder() to create and configure an OcflStorage instance.

  • fileSystem: Required, path to the OCFL storage root directory.
  • verifyInventoryDigest: Whether to verify inventory digests on read. Default: true.

Example

var repo = new OcflRepositoryBuilder()
        .defaultLayoutConfig(new HashedTruncatedNTupleConfig())
        .storage(storage -> storage.fileSystem(repoDir))
        .workDir(workDir)
        .build();

Amazon S3

The Amazon S3 storage implementation stores OCFL objects directly in an Amazon S3 bucket. Optionally, a key prefix can be used to partition the repository to use only a portion of its bucket, allowing you to store multiple OCFL repositories in the same bucket or non-OCFL content.

At the minimum, the client needs permissions to the following actions:

  • s3:PutObject
  • s3:GetObject
  • s3:DeleteObject
  • s3:ListBucket
  • s3:AbortMultipartUpload

If it is possible that multiple applications may be writing to the OCFL repository, then it is essential that a distributed lock is used to ensure that only one process is updating an object at a time. ocfl-java provides a builtin database based locking mechanism that can be used for these purposes. This is configured by setting the objectLock on the OcflRepositoryBuilder as shown in the example below.

Additionally, another database table may be optionally used to cache details about the objects in the repository. This allows ocfl-java to retrieve object details without needing to read inventories from S3. It also addresses the problem of eventually consistent writes. However, Amazon S3 is now strongly consistent, so it is no longer critical to use this feature. If you do want to use it, configure the objectDetailsDb on the OcflRepositoryBuilder as shown in the example below.

Currently, the only supported databases are PostgreSQL, MariaDB, and H2. The ocfl-java client populates the object details database on demand. There is no need to pre-populate it, and the table can safely be wiped anytime.

Note, the Amazon S3 storage implementation is significantly slower than the file system implementation. It will likely not perform well on large files or objects with lots of files. Additionally, it does not cache any object files locally, requiring them to be retrieved from S3 on every access.

S3 Transfer Manager

ocfl-java uses the new S3 Transfer Manager to upload files larger than 8MB to S3. You can configure the transfer manager to target a specific throughput, based on the needs of your application. Consult the official documentation for details. Note that it is crucial that you configure the transfer manager to use the new CRT S3 client.

Additionally, if you are using a 3rd party S3 implementation, you will likely need to disable object integrity checks on the client that is used by the transfer manager. This is because most/all 3rd party implementations do not support it, and it causes the requests to fail. Object integrity checks are disabled when constructing the client as follows:

S3AsyncClient.crtBuilder().checksumValidationEnabled(false).build();

S3 Client

In addition to the CRT client that's created for the transfer manager as described above, you also need to create a second non-CRT client. The reason for this is that the CRT client is optimized to work with large files and does not perform well with small files.

You will likely want to set connectionAcquisitionTimeout, writeTimeout, readTimeout, and maxConcurrency on this client. This is critical because ocfl-java queues concurrent writes, and the client needs to be configured to handle your application's load. An example configuration looks something like:

S3AsyncClient.builder()
        .region(Region.US_EAST_2)
        .httpClientBuilder(NettyNioAsyncHttpClient.builder()
                .connectionAcquisitionTimeout(Duration.ofSeconds(60))
                .writeTimeout(Duration.ofSeconds(120))
                .readTimeout(Duration.ofSeconds(60))
                .maxConcurrency(100))
        .build();

If you see failures related to acquiring a connection from the pool, then you either need to increase the concurrency, increase the acquisition timeout, or both.

Configuration

Use OcflStorageBuilder.builder() to create and configure an OcflStorage instance.

  • cloud: Required, sets the CloudClient implementation to use. For Amazon S3, use OcflS3Client.builder().
  • verifyInventoryDigest: Whether to verify inventory digests on read. Default: true.

Example

var repo = new OcflRepositoryBuilder()
        .defaultLayoutConfig(new HashedNTupleLayoutConfig())
        .contentPathConstraints(ContentPathConstraints.cloud())
        .objectLock(lock -> lock.dataSource(dataSource))
        .objectDetailsDb(db -> db.dataSource(dataSource))
        .storage(storage -> storage
                .cloud(OcflS3Client.builder()
                        .s3Client(s3Client)
                        .transferManager(transferManager)
                        .bucket(name)
                        .repoPrefix(prefix)
                        .build()))
        .workDir(workDir)
        .build();

Database

If you use a database backed object lock or the object details database, then you'll need to setup a database for the client to connect to. Currently, PostgreSQL >= 9.3, MariaDB >= 10.2, and H2 are supported. The client automatically creates the tables that it needs.

Usage Considerations

Running multiple instances

If you intend to write to an OCFL repository from multiple different instances, you should use a database based object lock rather than the default in-memory lock. Additionally, you may want to either adjust or disable inventory caching, or hook up a distributed cache implementation.

Improving write performance

If your objects have a lot of files, then you might get better performance by parallelizing file reads and writes. Parallel writes are only supported as of ocfl-java 2.1.0 or later. ocfl-java does not do this for you automatically, but the following is some example code of one possible way that you could implement parallel writes to an object:

repo.updateObject(ObjectVersionId.head(objectId), null, updater -> {
    List<Future<?>> futures;

    try (var files = Files.find(sourceDir, Integer.MAX_VALUE, (file, attrs) -> attrs.isRegularFile())) {
        futures = files.map(file -> executor.submit(() -> updater.addPath(
                        file, sourceDir.relativize(file).toString())))
                .collect(Collectors.toList());
    } catch (IOException e) {
        throw new UncheckedIOException(e);
    }

    futures.forEach(future -> {
        try {
            future.get();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    });
});

The key bit here is that you use an ExecutorService to add multiple files to the object at the same. You would likely want to use one thread pool per object. Additionally, note that this technique will likely make writes slower if you are not writing a lot of files.

Inventory size

OCFL inventory files can grow quite large when an object has lots of files and/or lots of versions. This problem is compounded by the fact that a copy of the inventory must be persisted in every object version directory. There are three things you can do to attempt to control inventory bloat:

  • Do not generate an excessive number of versions of an object
  • Do not pretty print the inventory files (pretty printing is disabled by default)
  • Use sha256 instead of sha512 for inventory content addressing. sha512 is the default and the spec recommended algorithm. On some systems, sha512 is faster than sha256, however, it requires twice as much space to store. If you are concerned about space, you can change the algorithm by setting OcflRepositoryBuilder.ocflConfig(config -> config.setDefaultDigestAlgorithm(DigestAlgorithm.sha256)). Note, this only changes the digest algorithm used for new OCFL objects. It is not possible to modify existing objects.

Upgrading OCFL Repositories

An existing OCFL repository can be upgraded to a later OCFL spec version by specifying the desired version when initializing the repository. For example:

var repo = new OcflRepositoryBuilder()
        .ocflConfig(config -> config.setOcflVersion(OcflVersion.OCFL_1_1)
                                    .setUpgradeObjectsOnWrite(true))
        .storage(storage -> storage.fileSystem(repoDir))
        .workDir(workDir)
        .build();

If the repository in the above example was an existing 1.0 repository, then, it would be upgraded to 1.1 and all new objects would be created as 1.1 objects. Additionally, anytime an existing 1.0 object was written to, it would be upgraded to 1.1. If upgradeObjectsOnWrite was set to false, then existing objects would remain on version 1.0.

APIs

OcflRepository

See the Javadoc in OcflRepository for more detailed information.

  • putObject: Stores a fully composed object in the repository. The object's previous state is not carried forward. Only the files that are present in the given path are considered to be part of the new version. However, the files are still dedupped against previous versions.
  • updateObject: Unlike putObject, updateObject carries forward the most recent object state, and allows you to make one-off changes (adding, removing, moving, etc files) to an object without having the entire object on hand.
  • getObject: There are two different getObject implementations. The first writes a complete copy of an object at a specified version to a directory outside of the OCFL repository. The second returns an object with lazy-loading references to all of the files that are part of the specified object version.
  • describeObject: Returns metadata about an object and all of its versions.
  • describeVersion: Returns metadata about a specific version of an object.
  • fileChangeHistory: Returns the change history for a specific file within an object. This is useful for identifying at what point specific files were changed.
  • containsObject: Indicates whether the OCFL repository contains an object with the given id.
  • validateObject: Validates an object against the OCFL 1.0 spec and returns a list of any errors or warnings found.
  • purgeObject: Permanently removes an object from the repository. The object is NOT recoverable.
  • listObjectIds: Returns a stream containing the ids of all of the objects in the repository. This API may be slow.
  • exportVersion: Copies the entire contents of an OCFL object version directory to a location outside of the repository.
  • exportObject: Copies the entire contents of an OCFL object directory to a location outside of the repository.
  • importVersion: Imports an OCFL object version into the repository.
  • importObject: Imports an entire OCFL object into the repository.
  • close: Closes the repository, releasing its resources.

OcflObjectUpdater

See the Javadoc in OcflObjectUpdater for more detailed information.

  • addPath: Adds a file or directory to the object.
  • writeFile: Adds a file to the object, using an InputStream as the source of the file.
  • removeFile: Removes the file at the logical path from the object. The file is not removed from storage and can be reinstated later.
  • renameFile: Renames a file at an existing logical path to a new logical path.
  • reinstateFile: Restores a file that existed in a previous version of the object.
  • clearVersionState: By default, updateObject carries forward the current object state, calling clearVersionState clears everything out of the new version, so that it behaves the same as putObject.
  • addFileFixity: Adds an entry to the object's fixity block.
  • clearFixityBlock: Removes all of the entries from the object's fixity block.

OcflOption

A number of the APIs accept optional OcflOption arguments.

  • OVERWRITE: By default, ocfl-java will not overwrite files that already exist within an object. If you want to overwrite a file, you must specify OcflOption.OVERWRITE in the operation.
  • MOVE_SOURCE: By default, ocfl-java copies source files into an internal staging directory where it builds the new object version before moving the version into the repository. Specifying OcflOption.MOVE_SOURCE instructs ocfl-java to move the source files into the staging directory instead of copying them.
  • NO_VALIDATION: By default, ocfl-java will run validations on objects and versions that are imported and exported from the repository. This flag instructs it not to do these validations.

Extensions

OCFL extensions are additional features that the community has specified that are outside of the scope of the OCFL spec.

Storage Layout Extensions

Storage layout extensions describe how OCFL object IDs should be mapped to paths within the OCFL storage root. ocfl-java includes built-in implementations of registered extensions, but, you can override these implementations or add custom layout extensions.

The following is a list of currently supported storage layout extensions:

Custom Storage Layout Extensions

Custom storage layout extensions are supported by implementing OcflStorageLayoutExtension and OcflExtensionConfig. Reference the built-in extensions for an example of what this looks like.

After defining the extension classes, the extension must be registered with ocfl-java before initializing your OCFL repository. It would look something like this:

OcflExtensionRegistry.register(NewLayoutExtension.EXTENSION_NAME, NewLayoutExtension.class);
var repo = new OcflRepositoryBuilder().defaultLayoutConfig(new NewExtensionConfig())...

If you would like ocfl-java to write a copy of your extension's specification to the OCFL storage root, then include it as a Markdown file inside the jar your extension is defined in. The file should be at ocfl-specs/EXTENSION_NAME.md.

Mutable HEAD Extension

The mutable HEAD extension enables an OCFL object to have a mutable HEAD version that is stored inside of the object root. This version is not an official OCFL version, and it is not recognized by clients that do not implement this extension. This extension allows you to iteratively make changes to an object without every change producing a new OCFL version. When you are satisfied with the state of the object, the mutable HEAD version should be committed, which moves it into an immutable OCFL version that is recognized by all OCFL clients.

To enable this extension, call OcflRepositoryBuilder.buildMutable(). Note, you do not need to enable the extension for reading. ocfl-java will automatically read mutable HEAD versions that already exist. However, it will not allow you to write to an object with a mutable HEAD unless the extension is enabled.

APIs

See the Javadoc in MutableOcflRepository for more detailed information.

  • stageChanges: This method works the same as updateObject, but, instead of making a new version, it updates or creates a mutable HEAD version.
  • commitStagedChanges: Converts a mutable HEAD version into an immutable OCFL version.
  • purgeStagedChanges: Purges a mutable HEAD version without creating a new OCFL version.
  • hasStagedChanges: Indicates if an object has a mutable HEAD.