How to write integration tests?

Question

My team develops a Web API 2 application using Entity framework 6 ORM.

Our application acts as an email, and it's main purpose and action is to send a letter from one client to another.

Since it's the application's main purpose, we'd all like to have an integration test that will assure us that a letter is sent correctly. That is because we don't want to accidentally insert code that will break that.

When sending a letter the main actions that happen and that we'd like to test are-

• A letter is received through HTTP Post method and is validated on server
• The letter is saved to the Database .
• The letter is serialized.
• The serialized letter is sent to the specified destinations through an external module.
• The letter is deserialized on reciever side.
• The letter is added to recieved client's database.

I'm asking this because I'm clueless about how to approach this, yet I really want that sort of test which I can rely on.

How should a test for this be set? Where do I start? How to achieve an actual letter sending through a test and make sure it works?

It doesn't have to be an end to end test.

Thanks

Edit:

I don't mind tests to take long time, they can be run at night, but I want them to verify as much as possible that the application main parts are working as expected.

Answer 1

I would set up multiple, non-actual code tests to verify your logic:

1. Reception of the letter, and it's logic
2. Verify that the save method to the database is invoked only when it should
3. Verify the serialization/deserialization bits of code
4. Verify that the destination picker module is working correctly.

Once you have these "non-invoking" tests, you know that all the bits of your system are working correctly individually. You can then set up a unit-test with some kind of marking (ie TestCategory("SUPERSLOW")) and then have that run manually against a mocked set of recipients for verification during integration test runs, NOT as part of your CI.

Keywords here are:

• SOLID principles
• Separation of Concerns
• Mocking
• Continuous Integration
• Inversion of Control