# PgCat [![CircleCI](https://circleci.com/gh/levkk/pgcat/tree/main.svg?style=svg)](https://circleci.com/gh/levkk/pgcat/tree/main) ![PgCat](./pgcat3.png) Meow. PgBouncer rewritten in Rust, with sharding, load balancing and failover support. **Alpha**: don't use in production just yet. ## Local development 1. Install Rust (latest stable is fine). 2. `cargo run --release` (to get better benchmarks). 3. Change the config in `pgcat.toml` to fit your setup. 4. Install Postgres and create a user and a DB, e.g. `CREATE ROLE lev ENCRYPTED PASSWORD 'lev' LOGIN;` and `createdb lev`. ### Tests You can just PgBench to test your changes: ``` pgbench -i -h 127.0.0.1 -p 6432 && \ pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol simple && \ pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol extended ``` ## Features 1. Session mode. 2. Transaction mode. 3. `COPY` protocol support. 4. Query cancellation. 5. Round-robin load balancing of replicas. 6. Banlist & failover ### Session mode Each client owns its own server for the duration of the session. Commands like `SET` are allowed. This is identical to PgBouncer session mode. ### Transaction mode The connection is attached to the server for the duration of the transaction. `SET` will pollute the connection, but `SET LOCAL` works great. Identical to PgBouncer transaction mode. ### COPY protocol That one isn't particularly special, but good to mention that you can `COPY` data in and from the server using this pooler. ### Query cancellation Okay, this is just basic stuff, but we support cancelling queries. If you know the Postgres protocol, this might be relevant given than this is a transactional pooler but if you're new to Pg, don't worry about it, it works. ### Round-robin load balancing This is the novel part. PgBouncer doesn't support it and suggests we use DNS or a TCP proxy instead. We prefer to have everything as part of one package; arguably, it's easier to understand and optimize. This pooler will round-robin between multiple replicas keeping load reasonably even. ### Banlist & failover This is where it gets even more interesting. If we fail to connect to one of the replicas or it fails a health check, we add it to a ban list. No more new transactions will be served by that replica for, in our case, 60 seconds. This gives it the opportunity to recover while clients are happily served by the remaining replicas. This decreases error rates substantially! Worth noting here that on busy systems, if the replicas are running too hot, failing over could bring even more load and tip over the remaining healthy-ish replicas. In this case, a decision should be made: either lose 1/x of your traffic or risk losing it all eventually. Ideally you overprovision your system, so you don't necessarily need to make this choice :-). ### Sharding We're implemeting Postgres' `PARTITION BY HASH` sharding function for `BIGINT` fields. This works well for tables that use `BIGSERIAL` primary key which I think is common enough these days. We can also add many more functions here, but this is a good start. See `src/sharding.rs` and `tests/sharding/setup.sql` for more details on the implementation. The biggest advantage of using this sharding function is that anyone can shard the dataset using Postgres partitions while also access it for both reads and writes using this pooler. No custom obscure sharding function is needed and database sharding can be done entirely in Postgres. To select the shard we want to talk to, we introduced special syntax: ```sql SET SHARDING KEY TO '1234'; ``` This sharding key will be hashed and the pooler will select a shard to use for the next transaction. If the pooler is in session mode, this sharding key will be used until it's set again or the client disconnects. ## Missing 1. Authentication, ehem, this proxy is letting anyone in at the moment. ## Benchmarks You can setup PgBench locally through PgCat: ``` pgbench -h 127.0.0.1 -p 6432 -i ``` Coincidenly, this uses `COPY` so you can test if that works. ### PgBouncer ``` $ pgbench -i -h 127.0.0.1 -p 6432 && pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol simple && pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol extended dropping old tables... creating tables... generating data... 100000 of 100000 tuples (100%) done (elapsed 0.01 s, remaining 0.00 s) vacuuming... creating primary keys... done. starting vacuum...end. transaction type: scaling factor: 1 query mode: simple number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 1.089 ms tps = 918.687098 (including connections establishing) tps = 918.847790 (excluding connections establishing) starting vacuum...end. transaction type: scaling factor: 1 query mode: extended number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 1.136 ms tps = 880.622009 (including connections establishing) tps = 880.769550 (excluding connections establishing) ``` ### PgCat ``` $ pgbench -i -h 127.0.0.1 -p 6432 && pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol simple && pgbench -t 1000 -p 6432 -h 127.0.0.1 --protocol extended dropping old tables... creating tables... generating data... 100000 of 100000 tuples (100%) done (elapsed 0.01 s, remaining 0.00 s) vacuuming... creating primary keys... done. starting vacuum...end. transaction type: scaling factor: 1 query mode: simple number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 1.142 ms tps = 875.645437 (including connections establishing) tps = 875.799995 (excluding connections establishing) starting vacuum...end. transaction type: scaling factor: 1 query mode: extended number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 1.181 ms tps = 846.539176 (including connections establishing) tps = 846.713636 (excluding connections establishing) ``` ### Direct Postgres ``` $ pgbench -i -h 127.0.0.1 -p 5432 && pgbench -t 1000 -p 5432 -h 127.0.0.1 --protocol simple && pgbench -t 1000 -p 5432 -h 127.0.0.1 --protocol extended Password: dropping old tables... creating tables... generating data... 100000 of 100000 tuples (100%) done (elapsed 0.01 s, remaining 0.00 s) vacuuming... creating primary keys... done. Password: starting vacuum...end. transaction type: scaling factor: 1 query mode: simple number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 0.902 ms tps = 1109.014867 (including connections establishing) tps = 1112.318595 (excluding connections establishing) Password: starting vacuum...end. transaction type: scaling factor: 1 query mode: extended number of clients: 1 number of threads: 1 number of transactions per client: 1000 number of transactions actually processed: 1000/1000 latency average = 0.931 ms tps = 1074.017747 (including connections establishing) tps = 1077.121752 (excluding connections establishing) ```