Task manager to have migrations

[mikecote]

Problem

The .kibana_task_manager index doesn't have a way to change the mapping or to migrate data. I want to add new fields to the index like retryAt and startedAt (see #39829).

Proposed solution

The solution I'm thinking of is to convert the index to be managed under saved objects but still as its own index. This will solve the problem of upgrading mappings and migrating the data by using the common migrator.

Solution problems

• We'll need a way to migrate and transfer the data from the existing index to a new index created under saved objects
• We may need to enhance saved objects client or still use callCluster to match current operations the task store does to Elasticsearch

[elasticmachine]

Pinging @elastic/kibana-stack-services

[mikecote]

@epixa @elastic/kibana-operations Any idea how we could transfer the data from the existing index to a new index created by the saved objects client?

[mikecote]

After speaking with @chrisdavies and @tylersmalley, we have come up with 4 options:

1. When the migrator converts an index to an alias. As it moves the documents to a new index with _1 suffix, we'll allow a script to be provided that can convert the task manager documents from {id} to {type}:{id} and use the regular migrator for the rest.
2. We can relax the rules of isRawSavedObject to not look for a prefix in the id. Use the regular migrator to change ids and the rest.
3. Have some logic when isRawSavedObject returns false for documents to be claimed by a saved object type and run a migration function before the regular migrations to change the id and other fields if necessary.
4. Write custom startup code to migrate the documents before the task manager starts up.

I will be exploring option 1 at this time.