Container Runtime Interface streaming explained

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2024-05-10 02:30:02

The Kubernetes Container Runtime Interface (CRI) acts as the main connection between the kubelet and the Container Runtime. Those runtimes have to provide a gRPC server which has to fulfill a Kubernetes defined Protocol Buffer interface. This API definition evolves over time, for example when contributors add new features or fields are going to become deprecated.

In this blog post, I'd like to dive into the functionality and history of three extraordinary Remote Procedure Calls (RPCs), which are truly outstanding in terms of how they work: Exec, Attach and PortForward.

Exec can be used to run dedicated commands within the container and stream the output to a client like kubectl or crictl. It also allows interaction with that process using standard input (stdin), for example if users want to run a new shell instance within an existing workload.

Attach streams the output of the currently running process via standard I/O from the container to the client and also allows interaction with them. This is particularly useful if users want to see what is going on in the container and be able to interact with the process.