Federated Advanced Global Search for Self-Managed

Problem to solve

Related to #6468 (closed) - specifically teams may have code that sits across both public and private instances and want a unified search experience for those.

Further details

GitHub has a product called GitHub Connect which supports unified search enabling searching from self-managed instances to include results of public and private projects on GitHub.com.

Search support is one-way with GitHub in that only .com results can be searched on-prem. On-prem results can't be searched from .com instances.

Proposal

Some potential ideas have been suggested here for search.

Introduce cross server search, by having one instance hit another instance's global search API (Elasticsearch): https://docs.gitlab.com/ee/api/search.html#global-search-api. Initially this could use a single defined user account to perform the search, with the desired level of access (likely minimal), to avoid having to dealing with per-user cross server authentication up front.

Permissions and Security

It would be important to understand permissions of users across multiple instances so that results are returned properly.

Starting with public only may be a sane default here to prevent needing to comprehensively understand the permissions model in two places.

Documentation

Documentation would need to be updated to specify limitations of search across multiple instances (i.e. public/private results, one-way search, etc...).