[[transform-chain]]
=== Chain Transform

A <<transform, Transform>> that executes an ordered list of configured transforms
in a chain, where the output of one transform serves as the input of the next
transform in the chain. The payload that is accepted by this transform serves as
the input of the first transform in the chain and the output of the last transform
in the chain is the output of the `chain` transform as a whole.

You can use chain transforms to build more complex transforms out of the other
available transforms. For example, you can combine a <<transform-search, `search`>>
transform and a <<transform-script, `script`>> transform, as shown in the
following snippet:

[source,js]
--------------------------------------------------
"transform" : {
  "chain" : [ <1>
    {
      "search" : {  <2>
        "indices" : [ "logstash-*" ],
        "body" : {
          "size" : 0,
          "query" : {
            "match" : { "priority" : "error" }
          }
        }
      }
    },
    {
      "script" : "return [ error_count : ctx.payload.hits.total ]"  <3>
    }
  ]
}
--------------------------------------------------
<1> The `chain` transform definition
<2> The first transform in the chain (in this case, a `search` transform)
<3> The second and final transform in the chain (in this case, a `script`
    transform)

This example executes a `count` search on the cluster to look for `error` events.
The search results are then passed to the second `script` transform. The `script`
transform extracts the total hit count and assigns it to the `error_count` field
in a newly-generated payload. This new payload is the output of the `chain`
transform and replaces the payload in the watch execution context.
