TDD on a client-server application

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I’m having problem to grasp how to do TDD when building a client-server system. The simple katas (Fizzbuzz etc) are easy to understand, but when my client needs to send the server a file using TCP sockets and get a response back from the server I’m getting confused regarding how to test that.

I had a project building a file-sync system. The client will monitor a folder and every time a change happens (new file, file deletion etc.) the server should update automatically. The client can have many devices, for example I can have a copy of the folder in two different computers and they all should sync perfectly.

I started the project with tests, but once I reached the part of speaking with the server I got stuck and didn’t understand how to implement tests.

Most of the things I find regarding TDD are the simple stuff. I would love your advice on this slightly more complex application.

client-server testing mocking tdd unit-testing
2 Answers

I’m having problem to grasp how to do TDD when building a client-server system.

The reference you want is Growing Object Oriented Software, Guided by Tests

I started the project with tests, but once I reached the part of speaking with the server I got stuck and didn’t understand how to implement tests.

Basic idea: you are trying to work towards a design where you can separate the complicated code from the code that is hard/expensive to test.

This often means three "modules"

  • A really simple module that knows how to talk to the network
  • A complicated module that knows how to prepare messages for the network, and how to interpret the responses (and timeouts)
  • A module that can coordinate the interaction of the two modules above.

The first module, you "test" using code review, acceptance testing, and taking advantage of the fact that it doesn't change very often (because it is so simple).

The second module, you use lots of programmer tests to make sure that the logic correctly handles all of the different messages that can pass through it

The third module, you concentrate on testing the protocol. Here, we'll often use a substitute implementation (aka a mock or some other flavor of test double) for one or both of the first two modules.

In a language with types like Java or C#, the need for substitutes will often mean that the first two modules will need to implement some interface, and the third module will depend on those interfaces rather than having direct dependencies on the implementations.

You'll likely also need some code in your composition root that wires together the actual implementations.

For a good take on separating the networking client from the networking logic/protocol, see Cory Benfield 2016.

It might also be useful to review:

user_547340

Put the client code that is working with the socket into a separate class that can be injected in the "business code". For your tests inject a mock instead, verifying that the API of the "client socket adapter" is called in the apropriate way. Mocking libraries make this easy.

Put the server code that is working with the socket into a separate class and design an internal API for the "business code" that the "server socket adapter" is calling. Ignore the adapter in your tests and call the API of the business code directly.

You might want to read about the Ports & Adapter architecture (sometimes also called the "Hexagonal Model").

EricSchaefer_89760
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