Making a MockKelsey Shannahan

Making a Mock

•Reasons to use a mock

•How we built a mock

•Use cases and limitations of a mock

Why Use a Mock?

•This leads us to a more important question…

•What is a mock?

What is a mock?•A mock is stand-in for another system that you either do not have control over or do not want to setup during the course of testing.

Why use a mock?•Mocks allow us to test a part of the system in isolation from outside factors.•Mocks can also replace applications we cannot run in testing• Internal applications owned by other teams• Applications that cannot be setup in testing• Systems that are completely out of your control (third-party)•Gives us control over the app so we can modify how it interacts with us

Should something be mocked?

•Does it fulfill these criteria?•Required by the system under test

BUT•Is not actually under test

A critical part of our process is out of our control

System A sends a message to System B

System B goes and does things with message

System B returns amessage to System A

How do we test end-to-end?

•Rely on unit tests?

•Reach out to the partner every time you need to test?• How do you deploy quickly then?• What if your change is agnostic to your partner’s process?

•Drop it into production and hope for the best?

How do we test end-to-end?

•Rely on unit tests?

•Reach out to the partner every time you need to test?• How do you deploy quickly then?• What if your change is agnostic to your partner’s process?

•Drop it into production and hope for the best?

•Hook it up to their QA endpoint?

The partner isn’t expecting a message

We want to load test

Build a stand-in system

Build a stand-in system

Build a stand-in system

Build a stand-in system

Why use a mock• Allows us to test our code in isolation• Replaces a system we cannot use in testing• Gives us control over the app so we can adjust to

our needs

How we built our mock

How do we build mocks?• Ruby has a number of libraries that help mock out

code for unit tests• Rspec (okay there’s other stuff but who cares)

If you want to know more about non-end to end mocks…

Go to Bradley Temple’s “Testing Microservices”!

This was not planned it just sort of happened.

End To End Testing

• What the mock looks like will vary based on what you need it to do.• Speaking generally, you can build an shell

application to stand-in for your mocked service.

• This requires a bit of technical and a lot of business knowledge.• Technical to create the mock and point other apps at it• Business to know what is being sent, what is expected in

response, what those should look like, and any ”invisible” actions happening inside the mocked process

Our Requirements

•Synchronous and asynchronous communication•Message validation using .xsd and internal CMM gems•Ability to modify what responses get sent back•Support for HTTP, SFTP, and SOAP•Compatible with our internal credential security•Ability to add features as needed• Logging• Feature flipping• Error conditions

Tools and Tech

• While building it, ask yourself…• Do I want to sink a lot of time into maintaining this?•Ruby on Rails

• because we were already using it•Sidekiq

• because we were already using it•Capistrano

• because we were already using it

Now my entire team uses and maintains it!

•Created a Rails project

•Yes, it includes specs•Left out what wasn’t necessary• No front-end• No database

•The controller does only a few things• Verifies the request from the app is

formed correctly• Returns a status• Queues up a job to later return the

response

•The controller does only a few things• Verifies the request from the app is formed

correctly• Returns a status• Queues up a job to later return the response

•What was with the plan_processing_time method?•It just delays for a few seconds to mimic actual work being done on the request. •It is also how we can mimic scenarios like slow response times

•Helper methods within the control are there to only process the request and extract data we need•No other handling of the request is done – we simply do not care

•The worker job handles authenticating back to our app•It accepts a block of xml which it posts to our app

•The worker job handles authenticating back to our app•It accepts a block of xml which it posts to our app

•The response to the app is stored as xml•We swap in whatever data we need and use a generic template for everything else

•That’s all that we do to send a response!

How we found our templates• Some of our templates were defined by business

requirements• Others were used by unit tests• For the rest, we inspected logs

How to build a mock

• Keep it simple• Use tools that are either easy or well-understood• Only build what you need

Uses and Limitations

Mocks don’t solve everything

•Mocks do not replicate the behavior of the mocked system• This is especially true when the system is third-party and you

don’t know exactly what it does

•The system being mocked may not do what you expect, or it may change•In short: this isn’t a replacement for integration testing

They do give you a lot of flexibility

•Here’s how we’ve used it…

•Obviously, we use it for end-to-end testing.•Our mock can now replicate responses for every integration that is outside of our control.

•We can now hand off an almost finished change for integration testing• Fix bugs early, without using up your partner’s testing time!

Use Cases•We had a scenario where we wanted to test how our system handles bad responses from the other application•Tweaked the endpoint to return 500 responses instead of a message•Deployed that branch of code to our test server

Use Cases

•Debugging.

Use Cases•Load testing or stress testing the system

•The advantage of a mock is that it does almost no processing behind the scenes

•This allows it to keep up with load that may reveal weaknesses in the system under test

Preserving Functionality

•Flipper•Data based decisions

Flipper

Data based decisions

•Our mock only has the information we give it from the calling app•We use this to decide what sort of response to send

Uses and Limitations

•Cannot fully replace integration testing•Allows for early testing during development•Can be used to create specialized scenarios

Making a Mock

•Reasons to use a mock

•How we built a mock

•Use cases and limitations of a mock