Container networking with Dagger

One of the biggest limitations I saw with Dagger up until now was that you couldn’t connect containers on a network level. This prevented use-cases like testing against database containers (e.g. for end-to-end testing scenarios).

Dagger 0.4 now supports what they call “services”. You can create a container, define exposed ports, and then add it as a dependency to another container so that it can connect via TCP to it. I’ve used that so far for verifying that a binary I’m building for gograz-meetup actually starts a working HTTP server:

// Create a backend container that exposes its service on
// port 8080:
backendContainer := goContainer.WithExposedPort(8080).
    WithExec([]string{"./gograz-meetup", "--addr", ""})

// Bind the backendContainer as service with the hostname
// "backend" to a new container:
code, err := goContainer.
	WithServiceBinding("backend", backendContainer).
	WithExec([]string{"curl", "--fail", "http://backend:8080/alive"}).

if err != nil {
	return err

This is just the simplest application of services. You can learn more (especially about the life-cycle of containers and best-practices on persisting data) in the official documentation!