[jira] [Created] (SLING-2892) Add simple paranoia check to detect duplicate sling.id in a cluster

["Stefan Egli (JIRA)" <ji...@apache.org>]

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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