zero-downtime Elasticsearch re-indexing

Allow for versioned Elasticsearch indexes:

After https://gitlab.com/gitlab-org/gitlab-ee/issues/328, #12548 (closed) are done, we should be able to add more than one Elasticsearch schema/search code, so we're ready to allow zero-downtime reindexing. The tasks involve:

  • #12555 (closed) This is when we add the table es_indexes(name, is_active, progress) On every data updates we would update every active index. While the last index is in the process of building we could use the next-oldest active one. The idea would be for only two indexes to ever be active, the current one and the one being migrated to.

  • https://gitlab.com/gitlab-org/gitlab-ee/issues/12622 UI control for this

  • We can also consider taking advantage of index aliases to specify which index we use for searches, but we still need to send updates to all active indexes

  • Document how the zero-downtime indexing operation works

Edited by Mark Chao