OpenAPI has rich tools to describe inbound parameters, the request body and API responses. FLAT can validate all incoming and outgoing HTTP messages against these schemas.

Code and document tend to divert quickly. Therefore, it is strongly recommended to activate validation. It helps spotting errors early during development. In production, the error logs help noticing and diagnosing validation errors.

The tutorial has a chapter on validation.


Overall validation is configured in the top-level extension object x-flat-validate:

swagger: "2.0"
description: "…"
version: "1.0.0"
title: "My API"
basePath: "/v1"
request: true
response: true

The request property activates validation of inbound parameters and request bodies. If the path operation is not defined or the HTTP message does not adhere to the schema, the request is terminated with a system error and status 400. In that case, the configured flow is not executed.

The response property activates validation of the outgoing HTTP response before it is sent to the client. If the response status code is not defined for the path or the response body does not follow the schema, the request is terminated with a system error and status 500.

Whenever a schema error has been detected, a meaningful message is logged.

Possible values for the validation properties are:

  • true activates validation and enforces the schema

  • false disables Swagger validation (however, the paths must still be defined)

  • report-only performs validation and logs a validation error without terminating the request

The default value is false.

You can also set x-flat-validate more specifically for certain paths and methods, for example:

swagger: "2.0"
request: report-only
# Reports invalid requests (top-level fallback).
# Does not validate responses (default).
# Validates all incoming requests.
# Validates responses for GET.
# Reports invalid responses for POST.
request: true
response: report-only
response: true
# No validation whatsoever (overrides top-level)
request: false
response: false

If the validation configuration has to be dynamic (e.g. read an environment variable to active it), you can use conf/config.xml instead of x-flat-validate.

System Error Document

FLAT uses an error document for all validation errors. It looks like this:

"error": {
"message": "Input Validation Failed",
"status": 400,
"requestID": "W8chVblhSDR5pk2xwBMUzwAAAA0",
"info": [
"Required constraint violated in body for name: The property userName is required."