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PGBouncer Primary Database Pool Service

Logging

Section titled “Logging”

PgBouncer is a connection pooler for PostgreSQL, allowing many frontend connections to re-use existing PostgreSQL backend connections. For example, you can map 1024 PgBouncer connections to 100 PostgreSQL connections.

For more information refer to PgBouncer’s website.

Pooling Mode

Section titled “Pooling Mode”

PgBouncer has three pooling “aggressiveness” settings that uses to determine how it manages its pooled connections:

  • Session Pooling: When a (postgres) client connects, a server connection will be assigned to it until it disconnects. All Postgres features are available.
  • Transaction Pooling: A server connection is assigned to a client only during a transaction. Session based-features like SET statement_timeout = 0 cannot be relied on in this mode.
  • Statement Pooling: A server connection per statement. This means that only single-statement (i.e. “autocommit”) transactions are allowed.

Given that our postgres clients (sidekiq nodes, web nodes, etc) use long-lived connections to execute transactions from different requests spread over time, session pooling is inefficient for our purposes. And evidently our application logic doesn’t work in autocommit mode. Therefore, we use Transaction Pooling.

For more details, see https://www.pgbouncer.org/features.html

PgBouncer Hosts

Section titled “PgBouncer Hosts”

architecture overview of pgbouncer

  • Primary (read-write) has 3 dedicated hosts in front of the database host, since it serving more traffic.
  • Replica (read-only) had 3 PgBouncer processes running on the same host that is running the PostgreSQL process.

PgBouncer is configured via omnibus via these config options. The PgBouncer configuration files are located in /var/opt/gitlab/pgbouncer, including a database.ini file from consul, the port PgBouncer listens on is 6432.

PgBouncer Commands

Section titled “PgBouncer Commands”

PgBouncer is controlled using systemd (systemctl). Note that restarting PgBouncer will terminate existing connections immediately, possibly leading to application errors.

It is also possible to connect directly to PgBouncer:

  • sudo pgb-console

You can also control and show statistics for PgBouncer when connected to it using its own set of commands. See http://pgbouncer.github.io/usage.html#admin-console for more information.

Applying Changes

Section titled “Applying Changes”

Almost all settings of PgBouncer can be managed by editing the relevant Chef roles:

  • roles/[env]-base-db-pgbouncer-common.json
  • roles/[env]-base-db-pgbouncer-pool.json
  • roles/[env]-base-db-pgbouncer-sidekiq.json
  • roles/[env]-base-db-pgbouncer-sidekiq-ci.json
  • roles/[env]-base-db-pgbouncer.json
  • roles/[env]-base-db-pgbouncer-ci.json

Most settings only require a reload of pgbouncer and will not cause an interruption of service. To manually reload, run sudo systemctl reload pgbouncer

To manually restart, run sudo systemctl restart pgbouncer. Note: This will cause an interruption to existing connections.

Healthcheck

Section titled “Healthcheck”

In gprd and gstg, clients access pgbouncer via an internal load balancer (ILB) named ENV-pgbouncer-regional (for primary traffic) and ENV-pgbouncer-sidekiq-regional for sidekiq.

Before Dec 2019 there was an HTTP-based healthcheck (with consul used to limit active nodes to N-1) called pgbouncer-leader. If you’re looking for that, it has been removed.

The healthcheck now is a simple TCP check to the pgbouncer port. This causes pgbouncer logs about connections to ‘nodb’ by ‘nouser’; do not be alarmed by these.