261 lines
9.3 KiB
Plaintext
261 lines
9.3 KiB
Plaintext
[[indices-flush]]
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=== Flush
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Flushing an index is the process of making sure that any data that is currently
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only stored in the <<index-modules-translog,transaction log>> is also
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permanently stored in the Lucene index. When restarting, {es} replays any
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unflushed operations from the transaction log into the Lucene index to bring it
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back into the state that it was in before the restart. {es} automatically
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triggers flushes as needed, using heuristics that trade off the size of the
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unflushed transaction log against the cost of performing each flush.
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Once each operation has been flushed it is permanently stored in the Lucene
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index. This may mean that there is no need to maintain an additional copy of it
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in the transaction log, unless <<index-modules-translog-retention,it is retained
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for some other reason>>. The transaction log is made up of multiple files,
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called _generations_, and {es} will delete any generation files once they are no
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longer needed, freeing up disk space.
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It is also possible to trigger a flush on one or more indices using the flush
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API, although it is rare for users to need to call this API directly. If you
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call the flush API after indexing some documents then a successful response
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indicates that {es} has flushed all the documents that were indexed before the
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flush API was called.
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[source,console]
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--------------------------------------------------
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POST twitter/_flush
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--------------------------------------------------
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// TEST[setup:twitter]
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[float]
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[[flush-parameters]]
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==== Request Parameters
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The flush API accepts the following request parameters:
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[horizontal]
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`wait_if_ongoing`:: If set to `true` the flush operation will block until the
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flush can be executed if another flush operation is already executing. If set to
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`false` then an exception will be thrown on the shard level if another flush
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operation is already running. Defaults to `true`.
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`force`:: Whether a flush should be forced even if it is not necessarily needed
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i.e. if no changes will be committed to the index. This can be used to force
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the generation number of the transaction log to be incremented even if no
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uncommitted changes are present. This parameter should be considered internal.
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[float]
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[[flush-multi-index]]
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==== Multi Index
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The flush API can be applied to more than one index with a single call, or even
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on `_all` the indices.
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[source,console]
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--------------------------------------------------
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POST kimchy,elasticsearch/_flush
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POST _flush
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--------------------------------------------------
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// TEST[s/^/PUT kimchy\nPUT elasticsearch\n/]
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[[synced-flush-api]]
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==== Synced Flush
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{es} keeps track of which shards have received indexing activity recently, and
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considers shards that have not received any indexing operations for 5 minutes to
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be inactive. When a shard becomes inactive {es} performs a special kind of flush
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known as a _synced flush_. A synced flush performs a normal
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<<indices-flush,flush>> on each copy of the shard, and then adds a marker known
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as the `sync_id` to each copy to indicate that these copies have identical
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Lucene indices. Comparing the `sync_id` markers of the two copies is a very
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efficient way to check whether they have identical contents.
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When allocating shard copies, {es} must ensure that each replica contains the
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same data as the primary. If the shard copies have been synced-flushed and the
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replica shares a `sync_id` with the primary then {es} knows that the two copies
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have identical contents. This means there is no need to copy any segment files
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from the primary to the replica, which saves a good deal of time during
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recoveries and restarts.
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This is particularly useful for clusters having lots of indices which are very
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rarely updated, such as with time-based indices. Without the synced flush
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marker, recovery of this kind of cluster would be much slower.
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To check whether a shard has a `sync_id` marker or not, look for the `commit`
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section of the shard stats returned by the <<indices-stats,indices stats>> API:
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[source,console]
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--------------------------------------------------
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GET twitter/_stats?filter_path=**.commit&level=shards <1>
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--------------------------------------------------
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// TEST[s/^/PUT twitter\nPOST twitter\/_flush\/synced\n/]
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<1> `filter_path` is used to reduce the verbosity of the response, but is entirely optional
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which returns something similar to:
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[source,console-result]
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--------------------------------------------------
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{
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"indices": {
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"twitter": {
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"shards": {
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"0": [
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{
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"commit" : {
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"id" : "3M3zkw2GHMo2Y4h4/KFKCg==",
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"generation" : 3,
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"user_data" : {
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"translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA",
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"history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ",
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"local_checkpoint" : "-1",
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"translog_generation" : "2",
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"max_seq_no" : "-1",
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"sync_id" : "AVvFY-071siAOuFGEO9P", <1>
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"max_unsafe_auto_id_timestamp" : "-1",
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"min_retained_seq_no" : "0"
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},
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"num_docs" : 0
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}
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}
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]
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}
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}
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"id" : "3M3zkw2GHMo2Y4h4\/KFKCg=="/"id": $body.indices.twitter.shards.0.0.commit.id/]
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// TESTRESPONSE[s/"translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA"/"translog_uuid": $body.indices.twitter.shards.0.0.commit.user_data.translog_uuid/]
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// TESTRESPONSE[s/"history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ"/"history_uuid": $body.indices.twitter.shards.0.0.commit.user_data.history_uuid/]
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// TESTRESPONSE[s/"sync_id" : "AVvFY-071siAOuFGEO9P"/"sync_id": $body.indices.twitter.shards.0.0.commit.user_data.sync_id/]
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<1> the `sync id` marker
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NOTE: The `sync_id` marker is removed as soon as the shard is flushed again, and
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{es} may trigger an automatic flush of a shard at any time if there are
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unflushed operations in the shard's translog. In practice this means that one
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should consider any indexing operation on an index as having removed its
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`sync_id` markers.
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[float]
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==== Synced Flush API
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The Synced Flush API allows an administrator to initiate a synced flush
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manually. This can be particularly useful for a planned cluster restart where
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you can stop indexing but don't want to wait for 5 minutes until all indices
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are marked as inactive and automatically sync-flushed.
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You can request a synced flush even if there is ongoing indexing activity, and
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{es} will perform the synced flush on a "best-effort" basis: shards that do not
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have any ongoing indexing activity will be successfully sync-flushed, and other
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shards will fail to sync-flush. The successfully sync-flushed shards will have
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faster recovery times as long as the `sync_id` marker is not removed by a
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subsequent flush.
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[source,console]
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--------------------------------------------------
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POST twitter/_flush/synced
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--------------------------------------------------
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// TEST[setup:twitter]
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The response contains details about how many shards were successfully
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sync-flushed and information about any failure.
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Here is what it looks like when all shards of a two shards and one replica
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index successfully sync-flushed:
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[source,console-result]
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--------------------------------------------------
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{
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"_shards": {
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"total": 2,
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"successful": 2,
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"failed": 0
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},
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"twitter": {
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"total": 2,
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"successful": 2,
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"failed": 0
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"successful": 2/"successful": 1/]
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Here is what it looks like when one shard group failed due to pending
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operations:
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[source,js]
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--------------------------------------------------
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{
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"_shards": {
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"total": 4,
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"successful": 2,
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"failed": 2
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},
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"twitter": {
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"total": 4,
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"successful": 2,
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"failed": 2,
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"failures": [
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{
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"shard": 1,
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"reason": "[2] ongoing operations on primary"
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}
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]
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}
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}
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--------------------------------------------------
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// NOTCONSOLE
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NOTE: The above error is shown when the synced flush fails due to concurrent
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indexing operations. The HTTP status code in that case will be `409 Conflict`.
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Sometimes the failures are specific to a shard copy. The copies that failed
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will not be eligible for fast recovery but those that succeeded still will be.
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This case is reported as follows:
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[source,js]
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--------------------------------------------------
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{
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"_shards": {
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"total": 4,
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"successful": 1,
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"failed": 1
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},
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"twitter": {
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"total": 4,
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"successful": 3,
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"failed": 1,
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"failures": [
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{
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"shard": 1,
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"reason": "unexpected error",
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"routing": {
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"state": "STARTED",
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"primary": false,
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"node": "SZNr2J_ORxKTLUCydGX4zA",
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"relocating_node": null,
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"shard": 1,
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"index": "twitter"
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}
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}
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]
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}
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}
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--------------------------------------------------
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// NOTCONSOLE
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NOTE: When a shard copy fails to sync-flush, the HTTP status code returned will
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be `409 Conflict`.
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The synced flush API can be applied to more than one index with a single call,
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or even on `_all` the indices.
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[source,console]
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--------------------------------------------------
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POST kimchy,elasticsearch/_flush/synced
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POST _flush/synced
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--------------------------------------------------
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