In kubernetes, pods have 2 places where you can specify containers to run:
initContainers and containers.
initContainers start in the order they're specified
and only after the previous has finished
(unless it's a 1.28 sidecar with restartPolicy: Always).
Once that's done,
containers start in the order they're specified (is this an implementation detail leaking?)
without depending on each other for lifecycle.
So if you're creating a task runner
and have an ordered list of steps to execute,
you might think: oh i'll just put it all in initContainers.
Which might be fine...
until you decide that what you actually need is an graph (DAG) of steps,
rather than a linear list.
This is where something like the tekton project's
entrypoint
come in.
An executor command is copied / mounted into every step,
and wraps the actual command.
It writes to a specified file when the actual command exits,
and waits for files from previous steps to be present before starting its command.
This does have a slight disadvantage:
after the first step is complete,
the container exits (remember to set restartPolicy: Never),
and the pod enters into NotReady state,
even though for our purposes,
it's executing as expected.
Usage is slightly verbose:
1apiVersion: v1
2kind: Pod
3metadata:
4 name: tekton-ordered-0
5 namespace: default
6 labels:
7 experiment: ordered
8spec:
9 restartPolicy: Never
10 initContainers:
11 # copy itself to a volume that can be mounted in every container
12 - name: copy-entry
13 image: gcr.io/tekton-releases/github.com/tektoncd/pipeline/cmd/entrypoint:v0.52.0
14 command:
15 - /ko-app/entrypoint
16 - cp
17 - /ko-app/entrypoint
18 - /ordered/bin/entrypoint
19 volumeMounts:
20 - name: bin # shared volume with executor
21 mountPath: /ordered/bin
22 containers:
23 - name: one
24 image: alpine
25 command:
26 - /ordered/bin/entrypoint # our entrypoint command
27 - -post_file=/ordered/steps/one # write to here on exit
28 - -termination_path=/ordered/steps/termintaion # termination log, nicer k8s status
29 - -entrypoint=sh # actual thing to be executed
30 - -- # everything after this is passed to the subprocess
31 - -c
32 - |
33 echo step one
34 echo hello world
35 terminationMessagePath: /ordered/steps/termination # termination log
36 terminationMessagePolicy: File
37 volumeMounts:
38 - name: bin # shared volume with executor
39 mountPath: /ordered/bin
40 - name: steps # shared volume for step status
41 mountPath: /ordered/stepsered/steps
42
43 - name: two
44 image: alpine
45 command:
46 - /ordered/bin/entrypoint
47 - -wait_file=/ordered/steps/one # wait for file from previous step to be present before proceeding
48 - -post_file=/ordered/steps/two
49 - -termination_path=/ordered/steps/termintaion
50 - -entrypoint=sh
51 - --
52 - -c
53 - |
54 echo step two
55 echo fizz buzz
56 terminationMessagePath: /ordered/steps/termination
57 terminationMessagePolicy: File
58 volumeMounts:
59 - name: bin
60 mountPath: /ordered/bin
61 - name: steps
62 mountPath: /ord
63
64 - name: three
65 image: alpine
66 command:
67 - /ordered/bin/entrypoint
68 - -wait_file=/ordered/steps/two
69 - -post_file=/ordered/steps/three
70 - -termination_path=/ordered/steps/termintaion
71 - -entrypoint=sh
72 - --
73 - -c
74 - |
75 echo step three
76 echo lorem ipsum
77 terminationMessagePath: /ordered/steps/termination
78 terminationMessagePolicy: File
79 volumeMounts:
80 - name: bin
81 mountPath: /ordered/bin
82 - name: steps
83 mountPath: /ordered/steps
84 volumes:
85 - name: bin
86 emptyDir: {}
87 - name: steps
88 emptyDir: {}