Introduce least-outstanding-connections load balancing #282
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drdrsh
commented
Jan 14, 2023
Comment on lines
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| candidates.sort_by(|a, b| { | ||
| self.busy_connection_count(b) | ||
| .partial_cmp(&self.busy_connection_count(a)) | ||
| .unwrap() |
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This is actually, the least_checked_out_connections which can be skewed by clients that are idle in transactions, I think we should be fine though.
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This is really cool, can't wait. This is also called |
drdrsh
changed the title
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I created a load balancer simulator that runs in the browser. Had fun tinkering with that |
levkk
reviewed
Jan 17, 2023
src/pool.rs
Outdated
| @@ -565,6 +579,20 @@ impl ConnectionPool { | |||
| pub fn server_info(&self) -> BytesMut { | |||
| self.server_info.clone() | |||
| } | |||
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| fn busy_connection_count(&self, address: &Address) -> u32 { | |||
| let state = self.databases[address.shard][address.address_index].state(); | |||
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Least outstanding connections load balancing can improve the load distribution between instances but for Pgcat it may also improve handling slow replicas that don't go completely down. With LoC, traffic will quickly move away from the slow replica without waiting for the replica to be banned.
If all replicas slow down equally (due to a bad query that is hitting all replicas), the algorithm will degenerate to Random Load Balancing (which is what we had in Pgcat until today).
This may also allow Pgcat to accommodate pools with differently-sized replicas.