Port/Adapter/Simulator and error conditions


An excellent question on an internal alias came up today, and I wanted to share my response more widely.

The question is around simulating error conditions when doing Port/Adapter/Simulator.

For example, if my production adapter is talking to a database, the database might be unreachable and the real adapter would throw a timeout exception. How can we get the simulator to do that, so we can write a test that verifies that our code behaves correctly in that scenario?

Before I answer, I need to credit Arlo, who taught me at least part of this technique…

Implement common behaviors across all adapters

The first thing to do is to see if we can figure out how to make the simulator behavior mirror the behavior of the real adapter.

If we are implementing some sort of store, the real adapter might throw an “ItemNotFound” exception if the item isn’t there, and we can just make the simulator detect the same situation and throw the same exception. And we will – of course – write a test that we can use to verify that the behavior matches.

Or, if there is a restriction on the names in a store (say one of our adapters stores items in a file, and the name is just the filename), then all of the adapters much implement that restriction (though I’d consider whether I wanted to do that or use an encoding approach to get rid of the restriction for the file adapter).

Those are the simple cases, but the question was specifically about timeouts. Timeouts are random and not-deterministic, right?

Yes, they are non-deterministic in actual use, but there might be a scenario that will always throw a timeout. What does the real adapter do if we pass in a database server that does not exist – something like “DatabaseThatDoesNotExist”? If we can figure out a developer/configuration error that sets up the scenario we want, then we can implement the same behavior in all of our adapters, and our world will be simple.

However, the world is not always that simple…

Cheat

I’ll note here that Arlo did not teach me this technique, so any stupidity belongs to me…

If I can’t find out a deterministic way to get a scenario to happen, then I need to implement a back door. I do this by adding a method to the simulator (not the adapter) that looks something like this:

public void SimulateTimeoutOnLoad();

Note that it is named with “Simulate”<scenario><method-name>, so that it’s easy to know that it isn’t part of the adapter interface and what it does. I will write a unit test to verify that the simulator does this correctly, but – alas – I cannot run this test against the real adapter because the method is not part of the adapter. That means it’s a decent idea to put these tests in a different file from the ones that target the adapter interface.

Now that I have the new method, the test is pretty simple:

Fetcher fetcher = new FetcherSimulator();

ObjectToTest ott = new ObjectToTest(fetcher);

fetcher.SimulateTimeoutOnLoad();

ott.Load();
Assert.Whatever(…);

I’m just using the method to reach into the simulator and tell it to do something specific in a specific scenario.

My goal is to do this as little as possible because it reduces the benefit we get from P/A/S, so I try to look very hard to find a way to not cheat.

I should also note that if you are using P/A/S on the UI side, you are pretty much stuck using this technique, because the things that you are simulating are user actions, and none of them can be triggered through using the real adapter.

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