Connection pooling in Aiven for PostgreSQL services allows you to maintain very large numbers of connections to a database while minimizing the consumption of server resources.
Aiven for PostgreSQL connection pooling uses PgBouncer to manage the database connection. Each pool can handle up to 5000 database client connections. Unlike when you connect directly to the PostgreSQL server, each client connection does not require a separate backend process on the server. PgBouncer automatically inserts the client queries and only uses a limited number of actual backend connections, leading to lower resource usage on the server and better total performance.
Why connection pooling?¶
A high number of backend connections can become a problem with PostgreSQL, as the resource cost per connection is quite high due to how PostgreSQL manages client connections. PostgreSQL creates a separate backend process for each connection, and the unnecessary memory usage caused by the processes will start affecting the total throughput of the system at some point. Moreover, if each connection is very active, the performance can be affected by the high number of parallel executing tasks.
It makes sense to have enough connections so that each CPU core on the server has something to do (each connection can only utilise a single CPU core), but a hundred connections per CPU core may be too much. All this is workload-specific, but often a good number of connections to have is roughly 3-5 times the CPU core count. Aiven enforces connection limits to avoid overloading the PostgreSQL database.
Since 9.6, PostgreSQL offers parallelization support enabling to run queries in parallel on multiple CPU cores.
Without a connection pooler, the database connections are handled directly by PostgreSQL backend processes, with one process per connection:
Adding a PgBouncer pooler that utilizes fewer backend connections frees up server resources for more important uses, such as disk caching:
Instead of having dedicated connections per client, now PgBouncer manages the connections assignment optimising them based on client request and settings like the Connection pooling modes.
Many frameworks and libraries (ORMs, Django, Rails, etc.) support client-side pooling, which solves much the same problem. However, when there are many distributed applications or devices accessing the same database, a server-side solution is a better approach.
Connection pooling modes¶
Aiven for PostgreSQL supports three different operational pool modes:
The default and recommended setting option is
transactionpooling mode allows each client connection to take their turn in using a backend connection for the duration of a single transaction. After the transaction is committed, the backend connection is returned back into the pool and the next waiting client connection gets to reuse the same connection immediately. In practice, this provides quick response times for queries as long as the typical execution times for transactions are not excessively long. This is the most commonly used PgBouncer mode and also the default pooling mode in Aiven for PostgreSQL.
Several PostgreSQL features, described in the official PgBouncer features page, are known to be broken by the default transaction-based pooling and must not be used by the application when in this mode.
You must carefully consider the design of the client applications connecting to PgBouncer, otherwise the application may not work as expected.
sessionpooling mode means that once a client connection is granted access to a PostgreSQL server-side connection, it can hold it until the client disconnects from the pooler. After this, the server connection is added back onto the connection pooler’s free connection list to wait for its next client connection. Client connections are accepted (at TCP level), but their queries only proceed once another client disconnects and frees up its backend connection back into the pool. This mode can be helpful in some cases for providing a wait queue for incoming connections while keeping the server memory usage low, but is of limited use under most common scenarios due to the slow recycling of the backend connections.
statementoperational pooling mode, similar to the
transactionpool mode, except that instead of allowing a full transaction to run, it cycles the server-side connections after each and every database statement (
DELETEstatements, etc.). Transactions containing multiple SQL statements are not allowed in this mode. This mode is sometimes used, for example when running specialised sharding frontend proxies.