Validating JSON payloads when the payload is sent as a query parameter in WSO2 ESB

In cases where we want to validate a JSON payload against a particular schema that is being sent by the client before sending it to the backend, we can use the Validate mediator of WSO2 ESB. This support has been added from WSO2 ESB v5.0.0 onward. The samples given in the WSO2 ESB documentation are for scenarios where the JSON payload is sent as a message body.

However, if the JSON payload is being sent as a query parameter, the configuration given in the samples will not work and we will have to tweak the configuration to support this. Given below  is an example scenario which explains this in detail.

I have an API deployed in WSO2 ESB which does a GET call by passing the JSON message payload as a query parameter.

 http://localhost:8280/jsonAPI/jsonapi?jsonPayload={"getQuote": {"request": {"symbol": "WSO2"}}}

The schema (StockQuoteSchema.json) used to validate the incoming payload is as below. Note that this schema is saved in the registry under the path /_system/config/schema
 
{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "getQuote": {
      "type": "object",
      "properties": {
        "request": {
          "type": "object",
          "properties": {
            "symbol": {
              "type": "string"
            }
          },
          "required": [
            "symbol"
          ]
        }
      },
      "required": [
        "request"
      ]
    }
  },
  "required": [
    "getQuote"
  ]
}

To validate the JSON object passed as a query parameter in the URL from the parameter jsonPayload the following API configuration should be used.

    <api context="/jsonAPI" name="jsonAPI">
        <resource methods="GET" protocol="http" uri-template="/jsonapi">
            <inSequence>
                <property expression="$url:jsonPayload"
                    name="jsonKeyValue" scope="default" type="STRING"/>
                <payloadFactory media-type="json">
                    <format>$1</format>
                    <args>
                        <arg evaluator="xml" expression="get-property('jsonKeyValue')"/>
                    </args>
                </payloadFactory>
                <validate>
                    <schema key="conf:/schema/StockQuoteSchema.json"/>
                    <on-fail>
                        <payloadFactory media-type="json">
                            <format>{"Error":"$1"}</format>
                            <args>
                                <arg evaluator="xml" expression="$ctx:ERROR_MESSAGE"/>
                            </args>
                        </payloadFactory>
                        <respond/>
                    </on-fail>
                </validate>
                <respond/>
            </inSequence>
        </resource>
    </api>


With this Synapse configuration in place, the validation should happen flawlessly.