Thomas Mueller created OAK-762:
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Summary: MongoMK: automatic unique cluster id with few bits
Key: OAK-762
URL: https://issues.apache.org/jira/browse/OAK-762
Project: Jackrabbit Oak
Issue Type: Improvement
Reporter: Thomas Mueller
Currently, the cluster id of a MongoMK instance is configurable. It needs to be
set when constructing the MongoMK instance, or by setting a system property.
Both solutions are not nice. Instead, the cluster id should be automatically
assigned unless explicitly set, and if explicitly set, the MongoMK should have
a mechanism to ensure the same cluster id is not used multiple times
concurrently.
As the revision contains the cluster id, the cluster id should be a low number
(ideally, the first cluster id of a cluster should be 0, the next 1, and so
on). This would keep the size of the revision numbers low. To simplify support,
it would be nice if the same repository uses the same cluster id after a
restart, even thought this isn't strictly necessary.
We could use the same or a similar algorithm as MongoDB uses for the machine id
part of ObjectIDs.
We could also use a persistent mapping between unique repository identifier and
cluster id. The mapping itself could be stored in a MongoDB collection. The
unique repository identifier (the cluster node id for Jackrabbit 2.x) could be
the combination of the MAC address of the first network interface of the
machine, and a guaranteed unique value within the given machine. The unique
value could be the "repository home" directory (that would be nice as it
survives restarts), or an incrementing number assigned by a service running on
the given machine (so the first repository on this machine would get id "0",
the second id "1", and so on - this would survive restarts as well in the
common case where there is only one cluster node per machine).
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