Black Lives Matter. Support the Equal Justice Initiative.

Managing connections

For the vast majority of programs, you needn’t adjust the sql.DB connection pool defaults. But for some advanced programs, you might need to tune the connection pool parameters or work with connections explicitly. This topic explains how.

The sql.DB database handle is safe for concurrent use by multiple goroutines (meaning the handle is what other languages might call “thread-safe”). Some other database access libraries are based on connections that can only be used for one operation at a time. To bridge that gap, each sql.DB manages a pool of active connections to the underlying database, creating new ones as needed for parallelism in your Go program.

The connection pool is suitable for most data access needs. When you call an sql.DB Query or Exec method, the sql.DB implementation retrieves an available connection from the pool or, if needed, creates one. The package returns the connection to the pool when it’s no longer needed. This supports a high level of parallelism for database access.

Setting connection pool properties

You can set properties that guide how the sql package manages a connection pool. To get statistics about the effects of these properties, use DB.Stats.

Setting the maximum number of open connections

DB.SetMaxOpenConns imposes a limit on the number of open connections. Past this limit, new database operations will wait for an existing operation to finish, at which time sql.DB will create another connection. By default, sql.DB creates a new connection any time all the existing connections are in use when a connection is needed.

Keep in mind that setting a limit makes database usage similar to acquiring a lock or semaphore, with the result that your application can deadlock waiting for a new database connection.

Setting the maximum number of idle connections

DB.SetMaxIdleConns changes the limit on the maximum number of idle connections sql.DB maintains.

When an SQL operation finishes on a given database connection, it is not typically shut down immediately: the application may need one again soon, and keeping the open connection around avoids having to reconnect to the database for the next operation. By default an sql.DB keeps two idle connections at any given moment. Raising the limit can avoid frequent reconnects in programs with significant parallelism.

Setting the maximum amount a time a connection can be idle

DB.SetConnMaxIdleTime sets the maximum length of time a connection can be idle before it is closed. This causes the sql.DB to close connections that have been idle for longer than the given duration.

By default, when an idle connection is added to the connection pool, it remains there until it is needed again. When using DB.SetMaxIdleConns to increase the number of allowed idle connections during bursts of parallel activity, also using DB.SetConnMaxIdleTime can arrange to release those connections later when the system is quiet.

Setting the maximum lifetime of connections

Using DB.SetConnMaxLifetime sets the maximum length of time a connection can be held open before it is closed.

By default, a connection can be used and reused for an arbitrarily long amount of time, subject to the limits described above. In some systems, such as those using a load-balanced database server, it can be helpful to ensure that the application never uses a particular connection for too long without reconnecting.

Using dedicated connections

The database/sql package includes functions you can use when a database may assign implicit meaning to a sequence of operations executed on a particular connection.

The most common example is transactions, which typically start with a BEGIN command, end with a COMMIT or ROLLBACK command, and include all the commands issued on the connection between those commands in the overall transaction. For this use case, use the sql package’s transaction support. See Executing transactions.

For other use cases where a sequence of individual operations must all execute on the same connection, the sql package provides dedicated connections. DB.Conn obtains a dedicated connection, an sql.Conn. The sql.Conn has methods BeginTx, ExecContext, PingContext, PrepareContext, QueryContext, and QueryRowContext that behave like the equivalent methods on DB but only use the dedicated connection. When finished with the dedicated connection, your code must release it using Conn.Close.