To make this discussion more concrete, imagine that you have an API that accepts a JSON blob over HTTP/POST. The blob is expected to be something like: {

To make this discussion more concrete, imagine that you have an API that accepts a JSON blob over HTTP/POST. The blob is expected to be something like: { "name": "bob", "age": 42 } Your code needs to handle the case where the fields aren't there, or they have invalid values (eg the wrong type, or a negative age). You at least need to ignore this input, but if you want this to be a nice API to work with, you probably also want to return error messages that explain what the client did wrong. You can hand-code this logic (please excuse the pseudo-code): if (!input.hasKey("name")) print "The 'name' key is required." if (typeof input["name"] != "string") print("The 'name' key is the wrong type. Expected 'string' " + "but got %s" % typeof input["name"]) if (!input.hasKey("age")) ... ... but I think it's clear that this quickly gets tedious and error-prone. JSON schema is one possible solution: // schema { "type": "object", "properties": { "name": { "type": "string", "minLength": 1 }, "age": { "type": "integer", "minimum": 0 } } } // handler validator = new JSONSchemaValidator(schema) if (validator.validate(instance)) // handle the request else print validator.errors.join("\n")