73 lines
4.1 KiB
Plaintext
73 lines
4.1 KiB
Plaintext
[[index-modules-history-retention]]
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== History retention
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{es} sometimes needs to replay some of the operations that were performed on a
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shard. For instance, if a replica is briefly offline then it may be much more
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efficient to replay the few operations it missed while it was offline than to
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rebuild it from scratch. Similarly, {ccr} works by performing operations on the
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leader cluster and then replaying those operations on the follower cluster.
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At the Lucene level there are really only two write operations that {es}
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performs on an index: a new document may be indexed, or an existing document may
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be deleted. Updates are implemented by atomically deleting the old document and
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then indexing the new document. A document indexed into Lucene already contains
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all the information needed to replay that indexing operation, but this is not
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true of document deletions. To solve this, {es} uses a feature called _soft
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deletes_ to preserve recent deletions in the Lucene index so that they can be
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replayed.
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{es} only preserves certain recently-deleted documents in the index because a
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soft-deleted document still takes up some space. Eventually {es} will fully
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discard these soft-deleted documents to free up that space so that the index
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does not grow larger and larger over time. Fortunately {es} does not need to be
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able to replay every operation that has ever been performed on a shard, because
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it is always possible to make a full copy of a shard on a remote node. However,
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copying the whole shard may take much longer than replaying a few missing
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operations, so {es} tries to retain all of the operations it expects to need to
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replay in future.
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{es} keeps track of the operations it expects to need to replay in future using
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a mechanism called _shard history retention leases_. Each shard copy that might
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need operations to be replayed must first create a shard history retention lease
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for itself. For example, this shard copy might be a replica of a shard or it
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might be a shard of a follower index when using {ccr}. Each retention lease
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keeps track of the sequence number of the first operation that the corresponding
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shard copy has not received. As the shard copy receives new operations, it
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increases the sequence number contained in its retention lease to indicate that
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it will not need to replay those operations in future. {es} discards
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soft-deleted operations once they are not being held by any retention lease.
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If a shard copy fails then it stops updating its shard history retention lease,
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which means that {es} will preserve all new operations so they can be replayed
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when the failed shard copy recovers. However, retention leases only last for a
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limited amount of time. If the shard copy does not recover quickly enough then
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its retention lease may expire. This protects {es} from retaining history
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forever if a shard copy fails permanently, because once a retention lease has
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expired {es} can start to discard history again. If a shard copy recovers after
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its retention lease has expired then {es} will fall back to copying the whole
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index since it can no longer simply replay the missing history. The expiry time
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of a retention lease defaults to `12h` which should be long enough for most
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reasonable recovery scenarios.
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Soft deletes are enabled by default on indices created in recent versions, but
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they can be explicitly enabled or disabled at index creation time. If soft
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deletes are disabled then peer recoveries can still sometimes take place by
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copying just the missing operations from the translog
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<<index-modules-translog-retention,as long as those operations are retained
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there>>. {ccr-cap} will not function if soft deletes are disabled.
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[float]
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=== History retention settings
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`index.soft_deletes.enabled`::
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Whether or not soft deletes are enabled on the index. Soft deletes can only be
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configured at index creation and only on indices created on or after 6.5.0.
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The default value is `true`.
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`index.soft_deletes.retention_lease.period`::
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The maximum length of time to retain a shard history retention lease before
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it expires and the history that it retains can be discarded. The default
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value is `12h`.
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