How does Tekton control the step sequence in a task?

Question

As I know, containers in a pod don't have a sequence for start or stop. But how does Tekton achieve this? I can search the result in the doc "The order in which the Steps appear in this list is the order in which they will execute." Anyone can tell how it works?

Answer 1

When a user writes a Task, he declares the steps with image and command. These commands are overwritten by Tekton to use a Tekton specific entrypoint binary. The entrypoint binary has signalling functinality and communicate with the other steps, such that they are executed in order.

The Entrypoint binary does:

1. Overwrites the user's specified command + args
2. The original command + args are passed as input to the Entrypoint binary
3. Wait for signal to start and then run users command + args
4. When its done, signal the next step.

The presentation Extending Containers with Nested Processes explains how this works for Tekton.