Posting data with an alternate primary key

I have a REST service that posts a lot of data to a table in Appian (136k+ rows).  The problem is the primary key in the service is not the same primary key that Appian is using.  I cannot match the keys because, the service's key is unpredictable.  Government data, yea! 

 For my REST post I simply need it to find the record with a matching value (if it exists) then update it, or else create a new record.

a!localVariables(
  local!value: cast(
    'recordType!{b44d0eeb-090e-4c8f-b858-1f684423adbc}DCQ Weekly Extract',
    a!fromJson(http!request.body)
  ),
  a!writeRecords(
    records: local!value,
    /*
    * Construct an HTTP response to return to the caller
    */
    onSuccess: a!httpResponse(
      statusCode: 200,
      /*
      * Set an HTTP header that tells the client that the body of the response will be in JSON format
      */
      headers: {
        a!httpHeader(name: "Content-Type", value: "application/json")
      },
      /*
      *In the response body, return the records created or updated
      */
      body: a!toJson(fv!recordsUpdated)
    ),
    onError: a!httpResponse(
      statusCode: 500,
      headers: {
        a!httpHeader(name: "Content-Type", value: "application/json")
      },
      body: a!toJson(
        a!map(
          message: "Write request has failed",
          error: fv!error
        )
      )
    )
  )
)