About validators - AWS AppConfig

About validators

When you create a configuration and configuration profile, you can specify up to two validators. A validator ensures that your configuration data is syntactically and semantically correct. You can create validators in either JSON Schema or as an AWS Lambda function.


Configuration data stored in SSM documents must validate against an associated JSON Schema before you can add the configuration to the system. SSM parameters do not require a validation method, but we recommend that you create a validation check for new or updated SSM parameter configurations by using AWS Lambda.

JSON Schema Validators

If you create a configuration in an SSM document, then you must specify or create a JSON Schema for that configuration. A JSON Schema defines the allowable properties for each application configuration setting. The JSON Schema functions like a set of rules to ensure that new or updated configuration settings conform to the best practices required by your application. Here is an example.

{ "$schema": "http://json-schema.org/draft-04/schema#", "title": "$id$", "description": "BasicFeatureToggle-1", "type": "object", "additionalProperties": false, "patternProperties": { "[^\\s]+$": { "type": "boolean" } }, "minProperties": 1 }

When you create the configuration by using the procedure in this topic, AWS AppConfig verifies that the configuration conforms to the schema requirements. If it doesn't, Systems Manager returns a validation error.


AWS AppConfig supports JSON Schema version 4.X for inline schema. If your application configuration requires a different version of JSON Schema, then you must create a Lambda validator.

AWS Lambda Validators

Lambda function validators must be configured with the following event schema. AWS AppConfig uses this schema to invoke the Lambda function. The content is a base64-encoded string, and the URI is a string.

{ "content":Base64EncodedByteString, "uri":"The uri of the configuration" }

AWS AppConfig verifies that the Lambda X-Amz-Function-Error header is set in the response. Lambda sets this header if the function throws an exception. For more information about X-Amz-Function-Error, see Error Handling and Automatic Retries in AWS Lambda in the AWS Lambda Developer Guide.

Here is a simple example of a Lambda response code for a successful validation.

import json def handler(event, context): #Add your validation logic here print("We passed!")

Here is a simple example of a Lambda response code for an unsuccessful validation.

def handler(event, context): #Add your validation logic here raise Exception("Failure!")

Here is another example that validates only if the configuration parameter is a prime number.

function isPrime(value) { if (value < 2) { return false; } for (i = 2; i < value; i++) { if (value % i === 0) { return false; } } return true; } exports.handler = async function(event, context) { console.log('EVENT: ' + JSON.stringify(event, null, 2)); const input = parseInt(Buffer.from(event.content, 'base64').toString('ascii')); const prime = isPrime(input); console.log('RESULT: ' + input + (prime ? ' is' : ' is not') + ' prime'); if (!prime) { throw input + "is not prime"; } }

AWS AppConfig calls your validation Lambda when calling the StartDeployment and ValidateConfigurationActivity API actions. You must provide appconfig.amazonaws.com permissions to invoke your Lambda. For more information, see Granting Function Access to AWS Services.