[ML-DataFrame] add a wait_for_completion option to the stop data frame api

[hendrikmuhs]

Default the stop data frame job API is async, this change adds an option to block until the job has really stopped. This is useful for testing and a prerequisite for a UI. wait_for_completion follows similar namings in rollup and reindex.

[elasticmachine]

Pinging @elastic/ml-core

[benwtrent]

Is there any mechanism for the end user to see the status of the request? Or to see when it is completed?

From what I can tell, since we do not return a task ID for the action, there is no way for the end user to check the completion of the task. I may be wrong about this as the way this async/sync request handling is done is different than others I have seen in the code base.

[benwtrent]

timeout already exists with the ActionRequest of which this is a descendant.
Is there a reason for not using that field?

[hendrikmuhs]

I changed it to reuse the timeout value from the base class. As discussed it is a bit ugly, but I hope I found a good compromise.

[hendrikmuhs]

@benwtrent

The stats API enables the user to check the state of a data frame:

GET _data_frame/jobs/my_job/_stats

example output:

{
  "count" : 1,
  "jobs" : [
    {
      "id" : "my_job",
      "state" : {
        "job_state" : "started",
        "current_position" : {
          "reviewer" : "XFEDSvjzlLR3Q7wsUw"
        },
        "generation" : 1
      },
      "stats" : {
...

So the task id is basically the job. Right now there is a 1:1 relationship between a job and a task, but that might change. With other words, a user interacts with the high level concept job, the underlying task is an implementation detail.

Coming back to this PR. When issuing a stop, the job_state above changes from started to stopped, that happens async per default, but sync if you use wait_for_completion.

This is API-wise no different than rollup(#34811), but different to reindex, because in reindex you interact with tasks.

I hope this makes it more clear.

[hendrikmuhs]

run gradle build tests 2