Read Flow

Reading /a/b at revision r1 would happen like below

Key points here

• Secondary NodeStore can be configured to only stored content under certain paths
• Read would be first attempted from any configured secondary NodeStore.
  1. Secondary NodeStore would check if it stores content for that path. Note that it can be configured with path inclusions
  2. It then checks whether its root revision is later than revision at which read is being requested
  3. If NodeState at given path and revision is found then its returned
• If read is not possible from secondary NodeStore then read is done from in memory which may in turn read from remote Mongo in case of cache miss
• If read is successful from Secondary NodeStore which is based on SegmentNodeStore then further child read from that path would be directly handled by SegmentNodeStore by passing DocumentNodeStore altogether. So if /a/b@r1 is found in secondary then read for /a/b/c@r1 would be directly handled by secondary store

Note that even if root revision of secondary store is lagging behind current head its possible that read for /a/b can be handled by secondary store if /a has not been modified recently. So those parts of repo which have not been recently modified would most likely be served from Secondary NodeStore and avoid remote calls to Mongo.

Write Flow

Updates to secondary store happen in 3 ways

Key points here

• Writes done by NodeStore caller i.e. JCR layer are always done on primary store i.e. Mongo
• Secondary NodeStore is updated via Oak Observation support and NodeState diff
• Secondary NodeStore can be configured with path filter and then it would only be interested in changes for configured paths

Setup

For enabling this feature following OSGi configurations have to be done

1. Configure SegmentNodeStore in secondary role

Create an OSGi config file org.apache.jackrabbit.oak.segment.SegmentNodeStoreFactory-secondary.config with following content

role="secondary"

This would create a SegmentNodeStore in secondary role and uses default segmentstore-secondary directory to store the segment files. Refer to config options for more details. Note all the options for SegmentNodeStoreService are applicable for SegmentNodeStoreFactory

2. Configure SecondaryStoreCacheService (optional)

By default secondary NodeStore would be activated based on previous config only. However it can be tweaked further by creating an OSGi config file org.apache.jackrabbit.oak.plugins.document.secondary.SecondaryStoreCacheService.config

includedPaths=[ \
  "/libs", \
  "/apps", \
  "/content"
  ]

Above config would enable secondary NodeStore for paths ‘/libs, /apps and /content’

Setup Considerations

While enabling secondary NodeStore feature following aspects needs to be considered

• SegmentNodeStore used as secondary NodeStore would compete with system resource like memory along with in memory caches of DocumentNodeStore and Lucene index files. So system must have sufficient memory to for all these 3 components
• SegmentNodeStore can be copied from any existing cluster node to a new node.
• If this is being enabled for existing setup then initial sync would take some time. So take that into account while planning to enable this feature
• For best performance include those paths of the repository which are accessed by end user. Specially those content paths where read to write ratio is high.

Administration