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PgBouncer FAQ

PgBouncer FAQ

How to connect to PgBouncer?

PgBouncer acts as a Postgres server, so simply point your client to the PgBouncer port.

How to load-balance queries between several servers?

PgBouncer does not have an internal multi-host configuration. It is possible via external tools:

  1. DNS round-robin. Use several IPs behind one DNS name. PgBouncer does not look up DNS each time a new connection is launched. Instead, it caches all IPs and does round-robin internally. Note: if there are more than 8 IPs behind one name, the DNS backend must support the EDNS0 protocol. See README for details.

  2. Use a TCP connection load-balancer. Either LVS or HAProxy seem to be good choices. On the PgBouncer side it may be a good idea to make server_lifetime smaller and also turn server_round_robin on: by default, idle connections are reused by a LIFO algorithm, which may work not so well when load-balancing is needed.

How to failover

PgBouncer does not have internal failover-host configuration nor detection. It is possible with external tools:

  1. DNS reconfiguration: When the IP address behind a DNS name is reconfigured, PgBouncer will reconnect to the new server. This behaviour can be tuned by two configuration parameters: dns_max_ttl tunes the lifetime for one host name, and dns_zone_check_period tunes how often a zone SOA will be queried for changes. If a zone SOA record has changed, PgBouncer will re-query all host names under that zone.

  2. Write a new host to the configuration and let PgBouncer reload it: send SIGHUP or use the RELOAD command on the console. PgBouncer will detect a changed host configuration and reconnect to the new server.

  3. Use the RECONNECT command. This is meant for situations where neither of the two options above are applicable, for example when you use the aforementioned HAProxy to route connections downstream from PgBouncer. RECONNECT simply causes all server connections to be reopened. So run that after that other component has changed its connection routing information.

How to use prepared statements with session pooling?

In session pooling mode, the reset query must clean old prepared statements. This can be achieved by server_reset_query = DISCARD ALL; or at least to DEALLOCATE ALL;

How to use prepared statements with transaction pooling?

Since version 1.21.0 PgBouncer can track prepared statements in transaction pooling mode and make sure they get prepared on-the-fly on the linked server connection. To enable this feature, max_prepared_statements needs to be set to a non-zero value. See the docs for max_prepared_statements for more details.

If you use PHP/PDO, depending on its version it might be incompatible with PgBouncer its prepared statement support (#991). PHP/PDO is only compatible when PHP 8.4+ and libpq 17 are used. So for setups with older versions it's recommended to upgrade, or to to disable prepared statements on the client side.

Disabling prepared statements in JDBC

The proper way to do it for JDBC is adding the prepareThreshold=0 parameter to the connection string.

Disabling prepared statements in PHP/PDO

To disable use of server-side prepared statements, the PDO attribute PDO::ATTR_EMULATE_PREPARES must be set to true. Either at connect-time:

$db = new PDO("dsn", "user", "pass", array(PDO::ATTR_EMULATE_PREPARES => true));

or later:

$db->setAttribute(PDO::ATTR_EMULATE_PREPARES, true);

How to upgrade PgBouncer without dropping connections?

You can use a rolling restart by following the procedure described in the section of the docs for SHUTDOWN WAIT_FOR_CLIENTS

How to know which client is on which server connection?

Use the SHOW CLIENTS and SHOW SERVERS commands on the console.

  1. Use ptr and link to map local client connection to server connection.

  2. Use addr and port of client connection to identify TCP connection from client.

  3. Use local_addr and local_port to identify TCP connection to server.

Should PgBouncer be installed on the web server or database server?

It depends.

Installing PgBouncer on the web server is good when short-lived connections are used. Then the connection setup latency is minimised. (TCP requires a couple of packet roundtrips before a connection is usable.) Installing PgBouncer on the database server is good when there are many different hosts (e.g., web servers) connecting to it. Then their connections can be optimised together.

It is also possible to install PgBouncer on both web server and database server. One negative aspect of that is that each PgBouncer hop adds a small amount of latency to each query.

In the end, you will need to test which model works best for your performance needs. You should also consider how installing PgBouncer will affect the failover of your applications in the event of a web server vs. database server going away.