Introduction
Behavioral Headers MuleSoft play a critical role in design-specific development by enabling effective API mocking during early integration stages. When teams build new applications or implement new functionality that consumes APIs still under development, consumers such as front-end developers cannot wait for teams to fully complete those APIs. Instead, they rely on mocking services that provide sample success and error responses so teams can continue development in parallel.
While mocking services make it easy to validate successful responses, teams often find it more challenging to validate error scenarios. For example, returning a specific HTTP 500 error without a live backend server requires additional control mechanisms. Behavioral Headers in MuleSoft address this challenge by allowing developers to dynamically alter mocking service behavior during testing.
In this article, we explain how Behavioral Headers in MuleSoft work and how teams can use them effectively when testing API mocking services.
What Are Behavioral Headers in MuleSoft?
Behavioral Headers in MuleSoft enable developers to change how a mocking service responds by passing specific key-value pairs in the request headers. These headers allow you to simulate different behaviors, such as error responses, response delays, random data generation, and skipping RAML validation.
MuleSoft provides a defined set of Behavioral Headers that teams use to test expected behaviors without modifying the underlying API specification. As a result, Behavioral Headers prove especially useful during early-stage API development and integration testing.
Before testing Behavioral Headers, you need two prerequisites:
- A valid RAML specification
- A published Mocking Service URL
Once teams publish the RAML and make it public, MuleSoft generates a mocking URL that teams can invoke using any API client. In this example, the article uses Postman to demonstrate the behavior.
Organizations that rely on MuleSoft Managed Services often use Behavioral Headers to streamline API testing, reduce integration risks, and accelerate delivery across distributed development teams.
MS2-Status-Code Behavioral Header
The MS2-Status-Code Behavioral Header forces the mocking service to return a specific HTTP status code. This is particularly useful when testing error-handling logic in consuming applications.
For example, if you want the mocking service to return a 500 Internal Server Error, you can include the following header in the request:
- Key: MS2-Status-Code
- Value: 500
When this header is passed, the mocking service returns the response defined in the RAML for that status code.
Important note:
If you pass a status code that is not defined in the RAML specification, the mocking service returns an error indicating that the status code does not match any implemented response. For example, passing 404 when no 404 response exists in the RAML will result in a validation error.
MS2-Delay Behavioral Header
The MS2-Delay Behavioral Header is used to simulate delayed API responses. This is particularly valuable when front-end applications need to handle asynchronous behavior, loading states, or timeout scenarios.
To introduce a delay, include the following header:
- Key: MS2-Delay
- Value: Time in milliseconds
For example, passing a value of 5000 introduces a 5-second delay before the response is returned.
Limitation:
The maximum supported delay is 10,000 milliseconds. Any value beyond this limit is not accepted by the mocking service.
MS2-Randomize Behavioral Header
The MS2-Randomize Behavioral Header is useful when the RAML specification defines only data types but does not include example responses. In such cases, consumers may still want to receive realistic responses for testing purposes.
By setting this header to true, MuleSoft generates randomized responses based on the defined data types in the RAML.
- Key: MS2-Randomize
- Value: true
This allows developers to quickly test integrations without manually defining response examples, saving time during early development phases.
MS2-Strict-Model-Validation Behavioral Header
By default, MuleSoft validates the entire RAML specification before allowing any mocking service invocation. If there are validation errors in the RAML, the mocking service cannot be tested.
The MS2-Strict-Model-Validation Behavioral Header allows you to bypass this restriction.
- Key: MS2-Strict-Model-Validation
- Value: false
This header is especially helpful when certain endpoints contain errors, but you still want to test other valid endpoints within the same API specification.
Conclusion
Behavioral Headers in MuleSoft provide powerful capabilities for testing API mocking services more effectively. By using these headers, developers can:
- Simulate specific HTTP error responses
- Introduce artificial response delays
- Generate randomized response payloads
- Bypass strict RAML validation during testing
These features make Behavioral Headers an essential tool for parallel development, front-end testing, and early integration validation within MuleSoft-based API programs.