mirror of
https://github.com/honeymoose/OpenSearch.git
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29450de7b5
Today Cross Cluster Search requires at least one node in each remote cluster to be up once the cross cluster search is run. Otherwise the whole search request fails despite some of the data (either local and/or remote) is available. This happens when performing the _search/shards calls to find out which remote shards the query has to be executed on. This scenario is different from shard failures that may happen later on when the query is actually executed, in case e.g. remote shards are missing, which is not going to fail the whole request but rather yield partial results, and the _shards section in the response will indicate that.
This commit introduces a boolean setting per cluster called search.remote.$cluster_alias.skip_if_disconnected, set to false by default, which allows to skip certain clusters if they are down when trying to reach them through a cross cluster search requests. By default all clusters are mandatory.
Scroll requests support such setting too when they are first initiated (first search request with scroll parameter), but subsequent scroll rounds (_search/scroll endpoint) will fail if some of the remote clusters went down meanwhile.
The search API response contains now a new _clusters section, similar to the _shards section, that gets returned whenever one or more clusters were disconnected and got skipped:
"_clusters" : {
"total" : 3,
"successful" : 2,
"skipped" : 1
}
Such section won't be part of the response if no clusters have been skipped.
The per cluster skip_unavailable setting value has also been added to the output of the remote/info API.
433 lines
12 KiB
Plaintext
433 lines
12 KiB
Plaintext
[[modules-cross-cluster-search]]
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== Cross Cluster Search
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The _cross cluster search_ feature allows any node to act as a federated client across
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multiple clusters. In contrast to the <<modules-tribe,tribe node>> feature, a cross cluster search node won't
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join the remote cluster, instead it connects to a remote cluster in a light fashion in order to execute
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federated search requests.
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Cross cluster search works by configuring a remote cluster in the cluster state and connecting only to a
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limited number of nodes in the remote cluster. Each remote cluster is referenced by a name and a list of seed nodes.
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When a remote cluster is registered, its cluster state is retrieved from one of the seed nodes so that up to 3
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_gateway nodes_ are selected to be connected to as part of upcoming cross cluster search requests.
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Cross cluster search requests consist of uni-directional connections from the coordinating node to the previously
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selected remote nodes only. It is possible to tag which nodes should be selected through
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node attributes (see <<cross-cluster-search-settings>>).
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Each node in a cluster that has remote clusters configured connects to one or more _gateway nodes_ and uses
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them to federate search requests to the remote cluster.
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[float]
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=== Configuring Cross Cluster Search
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Remote clusters can be specified globally using <<cluster-update-settings,cluster settings>>
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(which can be updated dynamically), or local to individual nodes using the
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`elasticsearch.yml` file.
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If a remote cluster is configured via `elasticsearch.yml` only the nodes with
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that configuration will be able to connect to the remote cluster. In other
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words, federated search requests will have to be sent specifically to those
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nodes. Remote clusters set via the <<cluster-update-settings,cluster settings API>>
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will be available on every node in the cluster.
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[WARNING]
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This feature was added as Beta in Elasticsearch `v5.3` with further improvements made in 5.4 and 5.5. It requires gateway eligible nodes to be on `v5.5` onwards.
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The `elasticsearch.yml` config file for a _cross cluster search_ node just needs to list the
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remote clusters that should be connected to, for instance:
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[source,yaml]
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--------------------------------
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search:
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remote:
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cluster_one: <1>
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seeds: 127.0.0.1:9300
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cluster_two: <1>
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seeds: 127.0.0.1:9301
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--------------------------------
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<1> `cluster_one` and `cluster_two` are arbitrary cluster aliases representing the connection to each cluster.
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These names are subsequently used to distinguish between local and remote indices.
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The equivalent example using the <<cluster-update-settings,cluster settings API>>
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to add remote clusters to all nodes in the cluster would look like the
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following:
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[source,js]
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--------------------------------
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PUT _cluster/settings
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{
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"persistent": {
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"search": {
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"remote": {
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"cluster_one": {
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"seeds": [
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"127.0.0.1:9300"
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]
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},
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"cluster_two": {
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"seeds": [
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"127.0.0.1:9301"
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]
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},
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"cluster_three": {
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"seeds": [
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"127.0.0.1:9302"
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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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// CONSOLE
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// TEST[setup:host]
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// TEST[s/127.0.0.1:9300/\${transport_host}/]
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//////////////////////////
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We want to be sure that settings have been updated,
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because we'll use them later.
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[source,js]
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--------------------------------------------------
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{
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"acknowledged" : true,
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"persistent": {
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"search": {
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"remote": {
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"cluster_one": {
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"seeds": [
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"127.0.0.1:9300"
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]
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},
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"cluster_two": {
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"seeds": [
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"127.0.0.1:9301"
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]
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},
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"cluster_three": {
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"seeds": [
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"127.0.0.1:9302"
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]
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}
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}
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}
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},
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"transient" : {}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/127.0.0.1:9300/\${transport_host}/]
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//////////////////////////
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A remote cluster can be deleted from the cluster settings by setting its seeds to `null`:
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[source,js]
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--------------------------------
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PUT _cluster/settings
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{
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"persistent": {
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"search": {
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"remote": {
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"cluster_three": {
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"seeds": null <1>
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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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// CONSOLE
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// TEST[continued]
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<1> `cluster_three` would be removed from the cluster settings, leaving `cluster_one` and `cluster_two` intact.
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//////////////////////////
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We want to be sure that settings have been updated,
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because we'll use them later.
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[source,js]
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--------------------------------------------------
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{
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"acknowledged" : true,
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"persistent" : {},
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"transient" : {}
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}
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--------------------------------------------------
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// TESTRESPONSE
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//////////////////////////
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[float]
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=== Using cross cluster search
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To search the `twitter` index on remote cluster `cluster_one` the index name
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must be prefixed with the cluster alias separated by a `:` character:
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[source,js]
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--------------------------------------------------
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GET /cluster_one:twitter/tweet/_search
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{
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"query": {
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"match": {
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"user": "kimchy"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[continued]
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// TEST[setup:twitter]
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[source,js]
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--------------------------------------------------
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{
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"took": 150,
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"timed_out": false,
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"_shards": {
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"total": 1,
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"successful": 1,
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"failed": 0,
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"skipped": 0
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},
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"_clusters": {
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"total": 1,
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"successful": 1,
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"skipped": 0
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},
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"hits": {
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"total": 1,
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"max_score": 1,
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"hits": [
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{
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"_index": "cluster_one:twitter",
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"_type": "tweet",
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"_id": "0",
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"_score": 1,
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"_source": {
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"user": "kimchy",
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"date": "2009-11-15T14:12:12",
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"message": "trying out Elasticsearch",
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"likes": 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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// TESTRESPONSE[s/"took": 150/"took": "$body.took"/]
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// TESTRESPONSE[s/"max_score": 1/"max_score": "$body.hits.max_score"/]
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// TESTRESPONSE[s/"_score": 1/"_score": "$body.hits.hits.0._score"/]
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In contrast to the `tribe` feature cross cluster search can also search indices with the same name on different
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clusters:
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[source,js]
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--------------------------------------------------
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GET /cluster_one:twitter,twitter/tweet/_search
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{
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"query": {
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"match": {
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"user": "kimchy"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[continued]
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Search results are disambiguated the same way as the indices are disambiguated in the request. Even if index names are
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identical these indices will be treated as different indices when results are merged. All results retrieved from a
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remote index
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will be prefixed with their remote cluster name:
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[source,js]
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--------------------------------------------------
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{
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"took": 150,
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"timed_out": false,
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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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"skipped": 0
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},
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"_clusters": {
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"total": 2,
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"successful": 2,
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"skipped": 0
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},
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"hits": {
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"total": 2,
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"max_score": 1,
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"hits": [
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{
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"_index": "cluster_one:twitter",
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"_type": "tweet",
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"_id": "0",
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"_score": 1,
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"_source": {
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"user": "kimchy",
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"date": "2009-11-15T14:12:12",
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"message": "trying out Elasticsearch",
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"likes": 0
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}
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},
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{
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"_index": "twitter",
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"_type": "tweet",
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"_id": "0",
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"_score": 2,
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"_source": {
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"user": "kimchy",
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"date": "2009-11-15T14:12:12",
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"message": "trying out Elasticsearch",
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"likes": 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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// TESTRESPONSE[s/"took": 150/"took": "$body.took"/]
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// TESTRESPONSE[s/"max_score": 1/"max_score": "$body.hits.max_score"/]
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// TESTRESPONSE[s/"_score": 1/"_score": "$body.hits.hits.0._score"/]
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// TESTRESPONSE[s/"_score": 2/"_score": "$body.hits.hits.1._score"/]
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[float]
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=== Skipping disconnected clusters
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By default all remote clusters that are searched via Cross Cluster Search need to be available when
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the search request is executed, otherwise the whole request fails and no search results are returned
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despite some of the clusters are available. Remote clusters can be made optional through the
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boolean `skip_unavailable` setting, set to `false` by default.
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[source,js]
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--------------------------------
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PUT _cluster/settings
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{
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"persistent": {
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"search.remote.cluster_two.skip_unavailable": true <1>
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}
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}
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--------------------------------
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// CONSOLE
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// TEST[continued]
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<1> `cluster_two` is made optional
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[source,js]
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--------------------------------------------------
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GET /cluster_one:twitter,cluster_two:twitter,twitter/tweet/_search <1>
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{
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"query": {
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"match": {
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"user": "kimchy"
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}
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}
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}
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--------------------------------------------------
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// CONSOLE
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// TEST[continued]
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<1> Search against the `twitter` index in `cluster_one`, `cluster_two` and also locally
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[source,js]
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--------------------------------------------------
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{
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"took": 150,
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"timed_out": false,
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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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"skipped": 0
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},
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"_clusters": { <1>
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"total": 3,
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"successful": 2,
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"skipped": 1
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},
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"hits": {
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"total": 2,
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"max_score": 1,
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"hits": [
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{
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"_index": "cluster_one:twitter",
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"_type": "tweet",
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"_id": "0",
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"_score": 1,
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"_source": {
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"user": "kimchy",
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"date": "2009-11-15T14:12:12",
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"message": "trying out Elasticsearch",
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"likes": 0
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}
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},
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{
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"_index": "twitter",
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"_type": "tweet",
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"_id": "0",
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"_score": 2,
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"_source": {
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"user": "kimchy",
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"date": "2009-11-15T14:12:12",
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"message": "trying out Elasticsearch",
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"likes": 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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// TESTRESPONSE[s/"took": 150/"took": "$body.took"/]
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// TESTRESPONSE[s/"max_score": 1/"max_score": "$body.hits.max_score"/]
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// TESTRESPONSE[s/"_score": 1/"_score": "$body.hits.hits.0._score"/]
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// TESTRESPONSE[s/"_score": 2/"_score": "$body.hits.hits.1._score"/]
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<1> The `clusters` section indicates that one cluster was unavailable and got skipped
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[float]
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[[cross-cluster-search-settings]]
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=== Cross cluster search settings
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`search.remote.connections_per_cluster`::
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The number of nodes to connect to per remote cluster. The default is `3`.
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`search.remote.initial_connect_timeout`::
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The time to wait for remote connections to be established when the node starts. The default is `30s`.
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`search.remote.node.attr`::
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A node attribute to filter out nodes that are eligible as a gateway node in
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the remote cluster. For instance a node can have a node attribute
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`node.attr.gateway: true` such that only nodes with this attribute will be
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connected to if `search.remote.node.attr` is set to `gateway`.
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`search.remote.connect`::
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By default, any node in the cluster can act as a cross-cluster client and
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connect to remote clusters. The `search.remote.connect` setting can be set
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to `false` (defaults to `true`) to prevent certain nodes from connecting to
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remote clusters. Cross-cluster search requests must be sent to a node that
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is allowed to act as a cross-cluster client.
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`search.remote.${cluster_alias}.skip_unavailable`::
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Per cluster boolean setting that allows to skip specific clusters when no
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nodes belonging to them are available and they are searched as part of a
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cross cluster search request. Default is `false`, meaning that all clusters
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are mandatory by default, but they can selectively be made optional by
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setting this setting to `true`.
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[float]
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[[retrieve-remote-clusters-info]]
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=== Retrieving remote clusters info
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The <<cluster-remote-info, Remote Cluster Info API>> allows to retrieve
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information about the configured remote clusters, as well as the remote
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nodes that the Cross Cluster Search node is connected to.
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