Stefan Egli created SLING-2892:
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Summary: Add simple paranoia check to detect duplicate sling.id in
a cluster
Key: SLING-2892
URL: https://issues.apache.org/jira/browse/SLING-2892
Project: Sling
Issue Type: Improvement
Components: Extensions
Affects Versions: Discovery Impl 1.0.0
Reporter: Stefan Egli
Assignee: Stefan Egli
Add a paranoia check to discovery.impl which detects a situation where more
than one instance in a cluster is running with the same sling.id. Such a
situation can for example occur, when the underlying repository-directory
(which includes a sling.id.file) is copied to add an additional cluster slave.
The suggested paranoia check works as follows:
Each time a heartbeat is written to the repository, the last value of the
heartbeat is checked to see if it matches the value written the last time. If
there should be another instance in the same cluster using the same sling.id,
it would have written the very same jcr-property but presumably with a slightly
different timestamp. Should the value not match to the value last written, then
a log.error is issued (with a reference to this bug and a possible fix for the
situation).
Note: this paranoia check is not 100% fool-proof, as it could be that the two
instances try to write the very same heartbeat time (same millisecond). Yet it
is considered at least a simple, stable mechanism to detect 99.9% of such cases.
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