Make read_write_split a per database configuration option.

[mijoharas]

Hey Lev,

We're doing some testing, and shifting things around. We have a db that is primary_only (just has one primary), one that is replica_only (just has one replica) and one that has both primaries, and replicas.

In our testing, we want to use the last option, but we only want queries to go to the replica. Now, because read_write_split is a general configuration option, we can't have it use exclude_primary (that makes our primary_only break.

I've got two ideas, and actually, the second one seems easier to implement:

• make read_write_split a database configuration option, so we can override it to exclude_primary just for the last db.
• create a new read_write_split option that's something like exclude_primary_if_replicas_available that will not exclude primary, if there is only one primary for the db.

Let me know what you think makes sense?

[levkk]

Hey Michael!

Makes sense to me. I think this is a bug in exclude_primary actually: it should not do that if there are no replicas. Do you want to put up a PR? Should be pretty straightforward (see lb/mod.rs, fn get_internal(), line 280). Basically, instead of just returning false, make it check that there are replicas present in the candidates set.

[mijoharas]

cool, yeah that makes sense to me. 👌 Will get a PR in.