jpath.md

JSON Path Expressions

JSON Path expressions (or JPath expressions) are used to select data from incoming JSON events and for inserting data into outgoing JSON events. In addition to event transformation, JPath expressions also play a key role in matching events to handlers and in filtering of events.

Simple Structural Transformations

For example, the following excerpt from a transformation descriptor selects everything under /data from the incoming event (right hand side) and inserts it under /event/data into the outgoing event (left hand side).

{
  "{{/event/data}}" : "{{/data}}"
}

Input event:

{
  "data" : {
    "status" : "ok",
    "deviceId": 123
  }
}

Output event:

{
  "event" : {
    "data" : {
      "status" : "ok",
      "deviceId": 123
    }
  }
}

JPath expressions can be used to select simple data fields (string, float, boolean) or complex ones with nested elements (map, array).

Array Path Selectors

You can select from arrays by index or by key.

Input event:

{
  "foo" : [
    { "name" : "status" , "value" : "online" },
    { "name" : "temperature" , "value" : 61 }
  ]
}

Example 1:

{{/foo[0]/value}}

Result is "online".

Example 2:

{{/foo[name=temperature]/value}}

Result is 61.

Functions

JPath expressions can also include function calls for more complex tasks, such as injecting a timestamp or injecting JSON results from an external web service call. See the complete function reference here. Function calls can be nested.

{
  "{{/data/timestamp}}" : "{{time()}}"
}

Results of function calls can also be concatenated.

{
  "{{/data/uniqueId}}" : "{{time()}}-{{uuid()}}"
}

Escape Characters

Use the $ sign to escape {{ or }} to prevent EEL from interpreting the content inside as a JPath expression. This is useful when a downstream service requires mustache notation for further processing.

Example:

"{{/message}}":"User email is ${{email$}}"

Output event:

{ "message" : "User email is {{email}}" }

To escape individual characters such as single quote inside of a function parameter, use the backslash character.

Example:

{{ident('this wasn\'t working in earlier versions')}}