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CoordinatorTests now support restarts when GatewayMetaState is used
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[[optimistic-concurrency-control]] | ||
== Optimistic concurrency control | ||
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Elasticsearch is distributed. When documents are created, updated, or deleted, | ||
the new version of the document has to be replicated to other nodes in the cluster. | ||
Elasticsearch is also asynchronous and concurrent, meaning that these replication | ||
requests are sent in parallel, and may arrive at their destination out of sequence. | ||
Elasticsearch needs a way of ensuring that an older version of a document never | ||
overwrites a newer version. | ||
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To ensure an older version of a document doesn't overwrite a newer version, every | ||
operation performed to a document is assigned a sequence number by the primary | ||
shard that coordinates that change. The sequence number is increased with each | ||
operation and thus newer operations are guaranteed to have a higher sequence | ||
number than older operations. Elasticsearch can then use the sequence number of | ||
operations to make sure a newer document version is never overridden by | ||
a change that has a smaller sequence number assigned to it. | ||
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||
For example, the following indexing command will create a document and assign it | ||
an initial sequence number and primary term: | ||
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[source,js] | ||
-------------------------------------------------- | ||
PUT products/_doc/1567 | ||
{ | ||
"product" : "r2d2", | ||
"details" : "A resourceful astromech droid" | ||
} | ||
-------------------------------------------------- | ||
// CONSOLE | ||
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You can see the assigned sequence number and primary term in the | ||
the `_seq_no` and `_primary_term` fields of the response: | ||
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[source,js] | ||
-------------------------------------------------- | ||
{ | ||
"_shards" : { | ||
"total" : 2, | ||
"failed" : 0, | ||
"successful" : 1 | ||
}, | ||
"_index" : "products", | ||
"_type" : "_doc", | ||
"_id" : "1567", | ||
"_version" : 1, | ||
"_seq_no" : 362, | ||
"_primary_term" : 2, | ||
"result" : "created" | ||
} | ||
-------------------------------------------------- | ||
// TESTRESPONSE[s/"_seq_no" : \d+/"_seq_no" : $body._seq_no/ s/"_primary_term" : 2/"_primary_term" : $body._primary_term/] | ||
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Elasticsearch keeps tracks of the sequence number and primary of the last | ||
operation to have changed each of the document it stores. The sequence number | ||
and primary term are returned in the `_seq_no` and `_primary_term` fields in | ||
the response of the <<docs-get,GET API>>: | ||
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[source,js] | ||
-------------------------------------------------- | ||
GET products/_doc/1567 | ||
-------------------------------------------------- | ||
// CONSOLE | ||
// TEST[continued] | ||
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returns: | ||
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[source,js] | ||
-------------------------------------------------- | ||
{ | ||
"_index" : "products", | ||
"_type" : "_doc", | ||
"_id" : "1567", | ||
"_version" : 1, | ||
"_seq_no" : 362, | ||
"_primary_term" : 2, | ||
"found": true, | ||
"_source" : { | ||
"product" : "r2d2", | ||
"details" : "A resourceful astromech droid" | ||
} | ||
} | ||
-------------------------------------------------- | ||
// TESTRESPONSE[s/"_seq_no" : \d+/"_seq_no" : $body._seq_no/ s/"_primary_term" : 2/"_primary_term" : $body._primary_term/] | ||
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Note: The <<search-search,Search API>> can return the `_seq_no` and `_primary_term` | ||
for each search hit by requesting the `_seq_no` and `_primary_term` <<search-request-docvalue-fields,Doc Value Fields>>. | ||
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The sequence number and the primary term uniquely identify a change. By noting down | ||
the sequence number and primary term returned, you can make sure to only change the | ||
document if no other change was made to it since you retrieved it. This | ||
is done by setting the `if_seq_no` and `if_primary_term` parameters of either the | ||
<<docs-index_,Index API>> or the <<docs-delete,Delete API>>. | ||
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For example, the following indexing call will make sure to add a tag to the | ||
document without losing any potential change to the description or an addition | ||
of another tag by another API: | ||
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[source,js] | ||
-------------------------------------------------- | ||
PUT products/_doc/1567?if_seq_no=362&if_primary_term=2 | ||
{ | ||
"product" : "r2d2", | ||
"details" : "A resourceful astromech droid", | ||
"tags": ["droid"] | ||
} | ||
-------------------------------------------------- | ||
// CONSOLE | ||
// TEST[continued] | ||
// TEST[catch: conflict] | ||
|
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