Transaction mode

Transaction mode allows PgDog to share just a few of PostgreSQL server connections with thousands of clients. This is required for at-scale production deployments where the number of clients is much higher than the number of available connections to the database.

How it works

All queries served by PostgreSQL run inside transactions. Transactions can be started manually by executing a BEGIN command, or automatically by running individual statements.

PgDog takes advantage of this behavior and can split up transactions inside client connections and send them, individually and in order, to the first available PostgreSQL server in the connection pool.

In practice, this allows thousands of client connections to re-use just one PostgreSQL server connection. Most pools will have several server connections, so 100,000s of clients can use the pooler to execute queries without exceeding the database connection limit.

Enabling transaction mode

Transaction mode is enabled by default. This is controllable via configuration, at the global, user and database levels:

pgdog.toml (global)pgdog.toml (database)users.toml

[general]
pooler_mode = "transaction"

[[databases]]
name = "prod"
host = "127.0.0.1"
pooler_mode = "transaction"

[[users]]
name = "alice"
database = "prod"
pooler_mode = "transaction"

Session-level state

Clients can set session-level variables, e.g., by passing them in connection parameters or using the SET command. This works fine when connecting to Postgres directly, but transaction poolers share server connections between multiple clients.

To avoid session-level state leaking between clients, PgDog tracks connection parameters for each client and updates connection settings before giving a connection to each client.

This is performed efficiently, and server parameters are updated only if they differ from the ones set on the client.

Specifying connection parameters

Most Postgres connection drivers support passing parameters in the connection URL. Using the special options setting, each parameter is set using the -c flag, for example:

postgres://user@host:6432/db?options=-c%20statement_timeout%3D3s

This sets the statement_timeout setting to 3s (3 seconds). Each time this client executes a transaction, the pooler will check the value for statement_timeout on the server connection, and if it differs, issue a command to Postgres to update it:

SET statement_timeout TO '3s';

Tracking SET commands

If the client manually changes server settings, i.e., by issuing SET commands, the server will send the updated setting in a ParameterStatus message. The pooler will see this message and update client connection parameters accordingly, so as to avoid issuing unnecessary SET statements on subsequent transactions.

Impact on latency

PgDog keeps a real-time mapping of servers and their parameters, so checking the current value for any parameter doesn't require the pooler to talk to the database. Additionally, it's typically expected that applications have similar connection parameters, so the pooler won't have to synchronize parameters frequently.