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Overview
To aid in ingest or to provide services for external content, Fedora 4 has the ability to expose that content as if it were included in the repository. Federation may be useful for migrating content into Fedora 4 or serving large files already on disk.
Modeshape's federation overview provides more background on how federation works and the underlying concepts.
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Federated content is accessible through the REST APi – however, some features will not work:
- Filesystem federation is currently read-only, currently.
- Access control can currently only be applied to an entire federated filesystem, not different access controls for different sub-directories/files within the resource.
- Transactions are not available.
- Versioning is not available.
- Querying using the internal search functionality (including calculating object and byte counts in the repository root) results in every file and directory being accessed each time the caching timeout expires (this is set with the
cacheTtlSecondsparameter). For federations with large numbers of files, this can take a long time and result in "Too many open files" errors as files are opened faster than Java garbage-collects them.
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Filesystem federation maps a node in the repository to a directory on disk. This allows files on disk to be served and updated by Fedora 4 as though they were in the repository. Filesystem federation avoids having circumvents the need to transfer files using HTTP – and with larger file sizes (or with larger numbers a large number of files being processed), this feature can improve performance significantly. If you are ingesting a large number of multi-gigabyte files, we recommend you consider filesystem federation.
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directoryPath- base directory for all files shared with the repository- propertiesDirectoryPath - (optional) a path, that if specified causes computed properties to be stored in an external file structure
projections -lists one or more mappings from the repository to the filesystem. The format is "{workspace}:{repository path} => {path relative todirectoryPath}". See the section "Multiple Directories" below for how to handle multiple mappings.contentBasedSha1 -controls how internal identifiers are computed for files. By default (contentBaseSha1= true), Modeshape computes the SHA-1 checksum of a file's content every time the file is accessed. For small files this creates a modest overhead. For large files, however, this dramatically reduces performance, since generating the checksum can take several seconds per gigabyte of data. For this reason, we recommend settingcontentBasedSha1to false when serving files larger than 100MBreadonly -controls whether the contents of the filesbase directory for all files shared with the repository are read-only (
currently read-only is the only supported mode)extraPropertiesStorage- sets the format for storing "extra" properties (properties that can't be set using filesystem attributes). Recommended values are "json" for the current JSON properties format, or "none" for disabling extra properties. This property is ignored if propertiesDirectoryPath is set, since an external properties store will be used. (A warning or notice will appear in the logs indicating that the asserted preference here is overridden.)cacheTtlSeconds- the maximum time that cached entries are held before being refreshed. Setting to a low value will make changes to the filesystem (like adding new files) show up more quickly in the REST API. Setting to a higher value will improve performance for files that don't change often.
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If you want to map multiple directories, the first entry in the projections array should map the parent directory (i.e, the directory in directoryPath). Subsequent entries can map subdirectories to other repository paths. For example, if you have a directory /pub/ that that contains two directories (/pub/project1/ and /pub/project2/) which you want to map the project1 and project2 directories to the top level of the repository:
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