WSO2 ESB - Design for testability


Testability is one of the underestimated qualities of software. This is also the case for WSO2 ESB projects. However it is important to design the integrations for testability and this starts with the way you setup the proxies. This blog gives some guidelines which you can use to design for testability.


Sequences are a way in WSO2 to group Mediators. A proxy has by default an in-Sequence, out-sequence and optionally an error-sequence. These sequences can be split up in sub-sequences and this is a good way for reuse, but also a way to split up the design. A proxy usually contains the following functional parts:
  • Validation
  • Transformation
  • Sending to an endpoint

These parts can be put in separate sequences. This has the advantage that the part scan be reused in other proxies. This is also the way to enable testing for these parts. 

Design for testability

The sequences are the basis for the testability of WSO2 ESB proxies. The following design guidelines and steps can be used for testing your proxies.
  1. Configure separate testable parts within a separate sequence
    A good guideline in splitting is that the parts should be as independant as possible from other parts. Good examples are: input validation, data transformation, sending a message to an endpoint, a step within an iteration.

  2. Define a separate developer studio project for the test proxies
    This will be the proxies that will contain a testable sequence and can be called from soapUI for example. This way also the test package can be deployed separately. This way the test project is not deployed on production.

  3. Define a soapUI project for testing the component
    It is wise to define a separate soapUI project for each component (WSO2 proxy) you want to test. Note that this can also be used within a Continuous Integration environment for automatic testing.


The following is an example in which a data transformation is tested.
Step 1 – Define data transformation in a separate sequence

Step 2 – Define separate ESB project with test proxy
Note that the sequence is referenced and the result of the sequence is returned with respond mediator.
This result can be checked within soapUI using Asserts later on.


Step 3 - Define a soapUI project with test cases
In the request message put a soap message as expected by the sequence. Note that the ESB uses soap as its common message format within the mediation. The response can be checked with assertions.



The sequence split can be used to configure testable parts within a WSO2 ESB implementation.

This may not always be easy todo but hopefully you can use it to test your ESB implementations.
Feel free to comment on this blog ! All feedback is welcome


  1. Splitting into named sequences and creating a separate ESB project for testing sounds like a good concept. I would like to see examples of how you would set up tests for more complex scenarios like splitting an incoming message into multiple new messages.

    1. Hello Wouter,
      Thx for the feedback.

      W.r.t the split, i assume you mean an Iterator? I would put the handling of each iterator within a sequence and test the sequence. So that you test that at least each step will work.

    2. Yes, the iterator is the mediator i was thinking about. To be more specific: i have an iterator that splits the original message in multiple messages. For each sub-message i will apply an Xslt transformation and then a filter mediator is used to put some of the messages on a queue for further processing.

    3. I put XSLT in sequence, filter in sequence, and putting the message on Queue in a separate sequence.
      PS I do not know the context and functionality but why letting the "publisher" do the filtering?

    4. That still will not test the most essential part of the whole sequence: splitting the original message. I can test XSLT-transformations with regular unit testing frameworks like XUnit and putting a message on a queue is part of the infrastructure.

      Good point about the filtering. I should negotiate with the receiving party to do the filtering themselves.

    5. Ok but what is the essential part of the splitting? What is complex about the splitting?

    6. Splitting is not complex, it's just an xpath based iterator. Point is, if there is an error in the xpath, i will miss messages. When a transformation if wrong, it will result in an error and it will be logged. When the iterator select to few messages because of a faulty xpath it will not be logged. That's why it is essential to test the iterator.

      The way i should approach it, is to create a test proxy that executes the sequence containing the iterator and route all calls to a mock service. The mock will need to keep track of the number of calls, so in the end i can verify if the number off call equals the expected count.

    7. When a transformation is wrong w.r.t. (correct) xpath it will also not result in an error b.t.w. ;-)

      With respect to testing: it of course also depends on how detailed and complete you want to test. Maybe in case of an XPath you could just check with XSLT editor.