Consumer Schema

Consumer Schema is a tool to enable Consumer Driven Contracts (https://martinfowler.com/articles/consumerDrivenContracts.html) in C# by generating JSON Schemas (https://json-schema.org/) against POCOs, and then asserting those schemas against the actual classes generated by a producer.

As it is only POCOs and JSON, it can be used to check the shape of any data being sent, and can run within an IDE or on a build server.

There are two parts: the schema generator and the schema checker.

Schema Generator

A consumer will first generate the schemas for the messages they are using, passing in the consumer name, the types of the messages and a folder to generate the schemas to.

  var consumerName = "The Consumer Name";
  var folderToCreateSchemasTo = "C:/temp";
  var messagesIConsume = new[] { typeof(MessageOne), typeof(MessageTwo) };
  var schemaGenerator = new SchemaGenerator();
  schemaGenerator.GenerateSchemaDefinitionsToFolder(folderToCreateSchemasTo, consumerName, messagesIConsume);

This will generate a .json.schema file for each of the consumed messages in the folder specified. This should then be sent to the producer of the messages.

Nuget package: https://www.nuget.org/packages/ConsumerSchema.Generator/

Schema Checker

Once the producer has received the schema files, they can be used in a test to assert that the shape of the actual messages being produced hasn't changed from the consumers expectations. Here's an example (using NUnit):

  [Test]
  public void CheckSchemaDefinitions(){
    var messagesIProduce = new[] { typeof(MessageOne), typeof(MessageTwo) };
    var schemaChecker = new SchemaChecker();
    var schemaCheck = schemaChecker.CheckSchemasFromFolder("C:/PathToSchemas", messagesIProduce);

    if (schemaCheck.HasErrors())
    {
      Assert.Fail(schemaCheck.GetErrorsSummary());
    }
    else
    {
      Assert.Pass(); 
    }
  }

If the message produced doesn't match the schema definition, the errors summary will display what doesn't match, for example:

"Class Name: MessageOne. Consumer: The Consumer Name. Errors: Required properties are missing from object: PropertyOne. Path ''."

Nuget package: https://www.nuget.org/packages/ConsumerSchema.Checker/