| Load balancing of read queries | :white_check_mark: | Using random between replicas. Primary is included when `primary_reads_enabled` is enabled (default). |
| Sharding | :white_check_mark: | Transactions are sharded using `SET SHARD TO` and `SET SHARDING KEY TO` syntax extensions; see examples below. |
| Failover | :white_check_mark: | Replicas are tested with a health check. If a health check fails, remaining replicas are attempted; see below for algorithm description and examples. |
| Statistics | :white_check_mark: | Statistics available in the admin database (`pgcat` and `pgbouncer`) with `SHOW STATS`, `SHOW POOLS` and others. |
| Live configuration reloading | :white_check_mark: | Reload supported settings with a `SIGHUP` to the process, e.g. `kill -s SIGHUP $(pgrep pgcat)` or `RELOAD` query issued to the admin database. |
| Client authentication | :white_check_mark: :wrench: | MD5 password authentication is supported, SCRAM is on the roadmap; one user is used to connect to Postgres with both SCRAM and MD5 supported. |
| Admin database | :white_check_mark: | The admin database, similar to PgBouncer's, allows to query for statistics and reload the configuration. |
See `Dockerfile` for example deployment using Docker. The pooler is configured to spawn 4 workers so 4 CPUs are recommended for optimal performance. That setting can be adjusted to spawn as many (or as little) workers as needed.
For quick local example, use the Docker Compose environment provided:
| `host` | The pooler will run on this host, 0.0.0.0 means accessible from everywhere. | `0.0.0.0` |
| `port` | The pooler will run on this port. | `6432` |
| `enable_prometheus_exporter` | Enable prometheus exporter which will export metrics in prometheus exposition format. | `true` |
| `prometheus_exporter_port` | Port at which prometheus exporter listens on. | `9930` |
| `pool_size` | Maximum allowed server connections per pool. Pools are separated for each user/shard/server role. The connections are allocated as needed. | `15` |
| `pool_mode` | The pool mode to use, i.e. `session` or `transaction`. | `transaction` |
| `connect_timeout` | Maximum time to establish a connection to a server (milliseconds). If reached, the server is banned and the next target is attempted. | `5000` |
| `healthcheck_timeout` | Maximum time to pass a health check (`SELECT 1`, milliseconds). If reached, the server is banned and the next target is attempted. | `1000` |
| `shutdown_timeout` | Maximum time to give clients during shutdown before forcibly killing client connections (ms). | `60000` |
| `healthcheck_delay` | How long to keep connection available for immediate re-use, without running a healthcheck query on it | `30000` |
| `ban_time` | Ban time for a server (seconds). It won't be allowed to serve transactions until the ban expires; failover targets will be used instead. | `60` |
| `autoreload` | Enable auto-reload of config after fixed time-interval. | `false` |
| | | |
| **`user`** | | |
| `name` | The user name. | `sharding_user` |
| `password` | The user password in plaintext. | `hunter2` |
| **`shards`** | Shards are numerically numbered starting from 0; the order in the config is preserved by the pooler to route queries accordingly. | `[shards.0]` |
| `servers` | List of servers to connect to and their roles. A server is: `[host, port, role]`, where `role` is either `primary` or `replica`. | `["127.0.0.1", 5432, "primary"]` |
| `database` | The name of the database to connect to. This is the same on all servers that are part of one shard. | |
| `default_role` | Traffic is routed to this role by default (random), unless the client specifies otherwise. Default is `any`, for any role available. | `any`, `primary`, `replica` |
| Load balancing | :white_check_mark: | :white_check_mark: | We could test this by emitting statistics for each replica and compare them. |
| Failover | :white_check_mark: | :white_check_mark: | Misconfigure a replica in `pgcat.toml` and watch it forward queries to spares. CI testing is using Toxiproxy. |
In session mode, a client talks to one server for the duration of the connection. Prepared statements, `SET`, and advisory locks are supported. In terms of supported features, there is very little if any difference between session mode and talking directly to the server.
To use session mode, change `pool_mode = "session"`.
In transaction mode, a client talks to one server for the duration of a single transaction; once it's over, the server is returned to the pool. Prepared statements, `SET`, and advisory locks are not supported; alternatives are to use `SET LOCAL` and `pg_advisory_xact_lock` which are scoped to the transaction.
All queries are load balanced against the configured servers using the random algorithm. The most straight forward configuration example would be to put this pooler in front of several replicas and let it load balance all queries.
If the configuration includes a primary and replicas, the queries can be separated with the built-in query parser. The query parser will interpret the query and route all `SELECT` queries to a replica, while all other queries including explicit transactions will be routed to the primary.
The query parser will do its best to determine where the query should go, but sometimes that's not possible. In that case, the client can select which server it wants using this custom SQL syntax:
All servers are checked with a `SELECT 1` query before being given to a client. If the server is not reachable, it will be banned and cannot serve any more transactions for the duration of the ban. The queries are routed to the remaining servers. If all servers become banned, the ban list is cleared: this is a safety precaution against false positives. The primary can never be banned.
The ban time can be changed with `ban_time`. The default is 60 seconds.
Failover behavior can get pretty interesting (read complex) when multiple configurations and factors are involved. The table below will try to explain what PgCat does in each scenario:
| **Query** | **`SET SERVER ROLE TO`** | **`query_parser_enabled`** | **`primary_reads_enabled`** | **Target state** | **Outcome** |
| Read query | unset (any) | false | false | down | First instance is banned for reads. Next target in the random loop is attempted. |
| Read query | unset (any) | true | false | down | First replica instance is banned. Next replica instance is attempted in the random loop. |
| Read query | unset (any) | true | true | down | First instance (even if primary) is banned for reads. Next instance is attempted in the random loop. |
| Read query | replica | false | false | down | First replica instance is banned. Next replica instance is attempted in the random loop. |
| Write query e.g. `INSERT` | unset (any) | false | false | up | The query is attempted against the first available instance in the random loop. If the instance is a replica, the query fails and the client receives an error. |
We use the `PARTITION BY HASH` hashing function, the same as used by Postgres for declarative partitioning. This allows to shard the database using Postgres partitions and place the partitions on different servers (shards). Both read and write queries can be routed to the shards using this pooler.
To route queries to a particular shard, we use this custom SQL syntax:
The stats are very similar to what Pgbouncer reports and the names are kept to be comparable. They are accessible by querying the admin database `pgcat`, and `pgbouncer` for compatibility.
The config can be reloaded by sending a `kill -s SIGHUP` to the process or by querying `RELOAD` to the admin database. Not all settings are currently supported by live reload:
