[Question] Use existing container if it exists

[lonix1]

I'm using this library to setup containers for integration tests for postgres. It works very well.

However it is slow to start and stop, as to be expected. So what I'm trying to achieve:

• On my dev machine, I have another instance of postgres running (almost) all the time. If it is running then I'd like the library to use it.
• On the test server, no containers would be running, so I'd like the library to start/stop throwaway containers as usual.

Is this possible somehow?

(I could add boilerplate code to detect if the container is running, etc,, but I'm hoping this library has functionality to do that?)

[HofmeisterAn]

Right now, .NET Testcontainers does not support container reuse. Although, some Docker resources like network and volume can be reused. I think reusing a container does not match the concept. Starting and stopping should not take that much time (with cached images). There are probably other / better ways to optimize the performance (development experience).

[lonix1]

Understood, thanks.

For anyone who also needs this, what I did was:

• use Docker.DotNet library to determine if container is running (1)
• if not, create new container using this library (2)
• grab connection string from (1) or (2)

[PSanetra]

@HofmeisterAn I am currently also thinking about the container reuse concept. We have some containers that take a while to start (not just pulling the image, but the starting procedure itself). I have measured about 30 seconds in our case. This makes the local development and test feedback cycle very slow. Reusing containers could really improve this.

I have seen that such a reuse feature is already supported for Java Testcontainers, although it is not documented: testcontainers/testcontainers-java#1781

Do you think working on such a contribution for .NET Testcontainers makes sense?

[HofmeisterAn]

I still think reusing containers (resources) does not fit very well into the concept of throwaway instances, especially on CI pipelines. But still, keeping containers is useful for development. For example, I use multiple test sessions (one that starts the container, others to execute the tests), but that requires manual work too.

I'm just concerned that features, like this, will mess up with the tests and develop will use resources that do not belong to their tests. I haven't looked much into the Java implementation, but it looks like they're using a much better approach to identify resources. My first idea, was just the container name.

All we need to do is, to set the container field in TestcontainersContainer to the identified resource (we need to think about other resources too):

https://github.com/HofmeisterAn/dotnet-testcontainers/blob/de4c4a251152ee2cfdcd4061c8b1bcc0621e692f/src/DotNet.Testcontainers/Containers/TestcontainersContainer.cs#L165

Do you think working on such a contribution for .NET Testcontainers makes sense?

Of course, contribution is always welcome.

[swissarmykirpan]

@PSanetra could the tests perhaps be run in parallel, so that the 30 second cost is only felt once?

[PSanetra]

@swissarmykirpan I think the possibility to run the tests in parallel if the test containers are reused, will depend on the container. I guess in many cases it should be possible to run tests in parallel even if the containers are reused, but it is necessary to look at specific cases.

[swissarmykirpan]

@PSanetra sorry I don't think I made myself understood correctly. What I meant to say was that if test execution is serial (one at a time), then yes every test spins up a container, once the previous test has completed. However, if the test execution is parallel, then all the containers of the parallel tests will be started at the same time, and so the impact of having to wait 30 seconds multiple times goes away, without having to reuse containers.

Please tell me if I am making any sense here