Index Shard Allocation

Allows to control the allocation of indices on nodes based on include/exclude filters. The filters can be set both on the index level and on the cluster level. Lets start with an example of setting it on the cluster level:

Lets say we have 4 nodes, each has specific attribute called tag associated with it (the name of the attribute can be any name). Each node has a specific value associated with tag. Node 1 has a setting node.tag: value1, Node 2 a setting of node.tag: value2, and so on.

We can create an index that will only deploy on nodes that have tag set to value1 and value2 by setting index.routing.allocation.include.tag to value1,value2. For example:

curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.include.tag" : "value1,value2"
}'

On the other hand, we can create an index that will be deployed on all nodes except for nodes with a tag of value value3 by setting index.routing.allocation.exclude.tag to value3. For example:

curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.exclude.tag" : "value3"
}'

index.routing.allocation.require.* can be used to specify a number of rules, all of which MUST match in order for a shard to be allocated to a node. This is in contrast to include which will include a node if ANY rule matches.

The include, exclude and require values can have generic simple matching wildcards, for example, value1*. Additonally, special attribute names called _ip, _name, _id and _host can be used to match by node ip address, name, id or host name, respectively.

Obviously a node can have several attributes associated with it, and both the attribute name and value are controlled in the setting. For example, here is a sample of several node configurations:

node.group1: group1_value1
node.group2: group2_value4

In the same manner, include, exclude and require can work against several attributes, for example:

curl -XPUT localhost:9200/test/_settings -d '{
    "index.routing.allocation.include.group1" : "xxx"
    "index.routing.allocation.include.group2" : "yyy",
    "index.routing.allocation.exclude.group3" : "zzz",
    "index.routing.allocation.require.group4" : "aaa",
}'

The provided settings can also be updated in real time using the update settings API, allowing to "move" indices (shards) around in realtime.

Cluster wide filtering can also be defined, and be updated in real time using the cluster update settings API. This setting can come in handy for things like decommissioning nodes (even if the replica count is set to 0). Here is a sample of how to decommission a node based on _ip address:

curl -XPUT localhost:9200/_cluster/settings -d '{
    "transient" : {
        "cluster.routing.allocation.exclude._ip" : "10.0.0.1"
    }
}'

The index.routing.allocation.total_shards_per_node setting allows to control how many total shards (replicas and primaries) for an index will be allocated per node. It can be dynamically set on a live index using the update index settings API.

Elasticsearch can be configured to prevent shard allocation on nodes depending on disk usage for the node. This functionality is enabled by default, and can be changed either in the configuration file, or dynamically using:

curl -XPUT localhost:9200/_cluster/settings -d '{
    "transient" : {
        "cluster.routing.allocation.disk.threshold_enabled" : false
    }
}'

Once enabled, Elasticsearch uses two watermarks to decide whether shards should be allocated or can remain on the node.

cluster.routing.allocation.disk.watermark.low controls the low watermark for disk usage. It defaults to 85%, meaning ES will not allocate new shards to nodes once they have more than 85% disk used. It can also be set to an absolute byte value (like 500mb) to prevent ES from allocating shards if less than the configured amount of space is available.

cluster.routing.allocation.disk.watermark.high controls the high watermark. It defaults to 90%, meaning ES will attempt to relocate shards to another node if the node disk usage rises above 90%. It can also be set to an absolute byte value (similar to the low watermark) to relocate shards once less than the configured amount of space is available on the node.

Both watermark settings can be changed dynamically using the cluster settings API. By default, Elasticsearch will retrieve information about the disk usage of the nodes every 30 seconds. This can also be changed by setting the cluster.info.update.interval setting.