[[input-chain]]
=== Chain Input

Use the `chain` input to load data from multiple sources into the watch 
execution context when the watch is triggered. The inputs in a chain
are processed in order and the data loaded by an input can be accessed by the
subsequent inputs in the chain. 

The `chain` input enables you perform actions based on data from multiple 
sources. You can also use the data collected by one input to load data
from another source.

For example, the following chain input loads data from an HTTP server using the
path set by a `simple` input:

[source,js]
--------------------------------------------------
"input" : {
  "chain" : {
    "inputs" : [ <1>
      {
        "first" : {
          "simple" : { "path" : "/_search" }
        }
      },
      {
        "second" : {
          "http" : {
            "request" : {
              "host" : "localhost",
              "port" : 9200,
              "path" : "{{ctx.payload.first.path}}" <2>
            }
          }
        }
      }
    ]
  }
}
--------------------------------------------------
<1> The inputs in a chain are specified as an array to guarantee the order in
    which the inputs are processed. (JSON does not guarantee the order of
    arbitrary objects.)
<2> Loads the `path` set by the `first` input.

==== Accessing Chained Input Data

To reference data loaded by a particular input, you use the input's name,
`ctx.payload.<input-name>.<value>`.

==== Transforming Chained Input Data

In certain use-cases the output of the first input should be used as input
in a subsequent input. This requires you to do a transform, before you pass
the data on to the next input.

In order to achieve this you can use a transform input between the two
specified inputs, see the following example. Note, that the first input will
still be available in its original form in `ctx.payload.first`.

[source,js]
--------------------------------------------------
"input" : {
  "chain" : {
    "inputs" : [ <1>
      {
        "first" : {
          "simple" : { "path" : "/_search" }
        }
      },
      {
        "second" : {
          "transform" : {
            "script" : "return [ 'path' : 'ctx.payload.first.path' + '/' ]"
          }
        }
      },
      {
        "third" : {
          "http" : {
            "request" : {
              "host" : "localhost",
              "port" : 9200,
              "path" : "{{ctx.payload.second.path}}" <2>
            }
          }
        }
      }
    ]
  }
}
--------------------------------------------------
