feat: allow to specify read-only replica for SELECTs

[vmihailenco]

There are some minor API changes. Most notable of them is the removal of GetConn() IConn method, because the internal query conn IConn must be initialized to nil so Bun can resolve to a random read-only connect.

Overwriting the connection using query.Conn(conn) still works.

The usage should be like this:

resolver := bun.NewReadWriteConnResolver(
  bun.WithReadOnlyReplica(follower1),
  bun.WithReadOnlyReplica(follower2),
)

db := bun.NewDB(sqldb, dialect, bun.WithConnResolver(resover))

Closes #1037

[Tiscs]

Awesome.

I think variadic function is a good way to allow multiple replicas to be set at once.

db := bun.NewDB(sqldb, dialect,
  bun.WithReadOnlyReplicas(sqldb2),
  bun.WithReadOnlyReplicas(sqldb3, sqldb4),
  bun.WithReadOnlyReplicas(replicas...))

[j2gg0s]

When we want to use the master for a specific query to avoid master-slave replication delay, how should we proceed?

[vmihailenco]

When we want to use the master for a specific query to avoid master-slave replication delay, how should we proceed?

There is a Conn() method on each query that allow to specify the connection that will be used. So you can just:

db.NewSelect().Conn(db)

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. If there is no update within the next 7 days, this pr will be closed. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!

[vmihailenco]

This is ready for review. Please speak up if there are any concerns.

[Aoang]

Is it necessary to declare two slices and always use the one not utilized by r.healthyReplicas to avoid memory allocation?

[vmihailenco]

It sounds like you suggest to declare 2 slices and re-cycle them in the monitor.

It is not totally safe since we don't exactly know when the slice becomes free, but more importantly I believe it is not worth it since a slice allocation every 3-5 seconds is not going to change much.