I am struggling with testing a method that uploads documents to Amazon S3, but I think this question applies to any non-trivial API/external dependecy. I've only come up with three potential solutions but none seem satisfactory:

  1. Do run the code, actually upload the document, check with AWS's API that it has been uploaded and delete it at the end of the test. This will make the test very slow, will cost money every time the test is run and won't alway return the same result.

  2. Mock S3. This is super hairy because I have no idea about that object's internals and it feels wrong because it's way too complicated.

  3. Just make sure that MyObject.upload() is called with the right arguments and trust that I am using the S3 object correctly. This bothers me because there is no way to know for sure I used the S3 API correctly from the tests alone.

I checked how Amazon tests their own SDK and they do mock everything. They have a 200 lines helper that does the mocking. I don't feel it's practical for me to do the same.

How do I solve this?

  • Not an answer, but in practice we do use the three approaches you've exposed.
    – jlhonora
    Commented Oct 7, 2015 at 14:30

4 Answers 4


There are two issues we have to look at here.

The first is that you seem to be looking at all of your tests from the unit test perspective. Unit tests are extremely valuable, but are not the only kinds of tests. Tests can actually be divided into several different layers, from very fast unit tests to less fast integration tests to even slower acceptance tests. (There can be even more layers broken out, like functional tests.)

The second is that you are mixing together calls to third-party code with your business logic, creating testing challenges and possibly making your code more brittle.

Unit tests should be fast and should be run often. Mocking dependencies helps to keep these tests running fast, but can potentially introduce holes in coverage if the dependency changes and the mock doesn't. Your code could be broken while your tests still run green. Some mocking libraries will alert you if the dependency's interface changes, others cannot.

Integration tests, on the other hand, are designed to test the interactions between components, including third-party libraries. Mocks should not be used at this level of testing because we want to see how the actual object interact together. Because we are using real objects, these tests will be slower, and we will not run them nearly as often as our unit tests.

Acceptance tests look at an even higher level, testing that the requirements for the software are met. These tests run against the entire, complete system that would get deployed. Once again, no mocking should be used.

One guideline people have found valuable regarding mocks is to not mock types you don't own. Amazon owns the API to S3 so they can make sure it doesn't change beneath them. You, on the other hand, do not have these assurances. Therefore, if you mock out the S3 API in your tests, it could change and break your code, while your tests all show green. So how do we unit test code that uses third-party libraries?

Well, we don't. If we follow the guideline, we can't mock objects we don't own. But… if we own our direct dependencies, we can mock them out. But how? We create our own wrapper for the S3 API. We can make it look a lot like the S3 API, or we can make it fit our needs more closely (preferred). We can even make it a little more abstract, say a PersistenceService rather than an AmazonS3Bucket. PersistenceService would be an interface with methods like #save(Thing) and #fetch(ThingId), the types of methods we might like to see (these are examples, you might actually want different methods). We can now implement a PersistenceService around the S3 API (say a S3PersistenceService), encapsulating it away from our calling code.

Now to the code that calls the S3 API. We need to replace those calls with calls to a PersistenceService object. We use dependency injection to pass our PersistenceService into the object. It's important not to ask for a S3PersistenceService, but to ask for a PersistenceService. This allows us to swap out the implementation during our tests.

All the code that used to use the S3 API directly now uses our PersistenceService, and our S3PersistenceService now makes all the calls to the S3 API. In our tests, we can mock out PersistenceService, since we own it, and use the mock to make sure that our code makes the correct calls. But now that leaves how to test S3PersistenceService. It has the same problem as before: we can't unit test it without calling to the external service. So… we don't unit test it. We could mock out the S3 API dependencies, but this would give us little-to-no additional confidence. Instead, we have to test it at a higher level: integration tests.

This may sound a little troubling saying that we shouldn't unit test a part of our code, but let's look at what we accomplished. We had a bunch of code all over the place we couldn't unit test that now can be unit tested through the PersistenceService. We have our third-party library mess confined to a single implementation class. That class should provide the necessary functionality to use the API, but does not have any external business logic attached to it. Therefore, once it is written, it should be very stable and should not change very much. We can rely on slower tests that we don't run that often because the code is stable.

The next step is to write the integration tests for S3PersistenceService. These should be separated out by name or folder so we can run them separately from our fast unit tests. Integration tests can often use the same testing frameworks as unit tests if the code is sufficiently informative, so we don't need to learn a new tool. The actual code to the integration test is what you would write for your Option 1.

  • the question is how do you run the integration or rather e2e tests for API that you expose. You cannot mock the PersistenceService for obvious reasons. Either I misunderstood something, or adding another layer in between the application API and AWS API, gives you nothing more than having easier time doing unit tests
    – Yerken
    Commented Feb 17, 2017 at 13:41
  • @Yerken As I am thinking about it, I'm pretty sure I could fill another long answer to that question. That might even be worthwhile for you because you might get more than just my response.
    – cbojar
    Commented Feb 17, 2017 at 13:54

You need to do both.

Running, uploading and deleting is an integration test. It interfaces with an external system and can therefore be expected to run slow. It should probably not be part of every single build you do locally, but it should be part of a CI build or nightly build. That offsets the slowness of those tests and still provides the value of having it tested automatically.

You also need unittests that run more quickly. Since it is generally smart to not hard-depend on an external system too much (so you can swap out implementations or switch over) you should probably try and write a simple interface over S3 that you can code against. Mock that interface in unittests so you can have quick-running unittests.

The first tests check that your code actually works with S3, the second tests that your code correctly calls the code that talks to S3.


I would say that it depends on the complexity of your use of the API.

  1. You definitely need to do at least some testing that actually invokes the S3 API and confirms that it worked from end to end.

  2. You also definitely need to do additional testing that doesn't actually call the API, so you can test your own software adequately without invoking the API all of the time.

The question that remains is: do you need to mock the API?

And I think that depends on how much you do with it. If you are only performing one or two simple actions, I don't think you need to go to all the trouble of a mock-up. I would be satisfied with just checking my use of the functions and doing some live testing.

However, if your use of it is more complex, with different scenarios and different variables that could affect the results, you probably need to mock it up to do more thorough testing.


Adding to the previous answers, the main question is whether (and how) you want to mock the S3 API for your tests.

Instead of manually mocking individual S3 responses, you can take advantage of some very sophisticated existing mocking frameworks. For instance moto provides functionality that is very similar to the actual S3 API.

You could also take a look at LocalStack, a framework which combines existing tools and provides a fully functional local cloud environment (including S3) that facilitates integration testing.

Although some of these tools are written in other languages (Python), it should be easy to spin up the test environment in an external process from your tests in, say, Java/JUnit.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.