Module pool (3.4.0)

Pools managing shared Session objects.

Specifies required API for concrete session pool implementations.

Concrete session pool implementation:

• "Pings" existing sessions via session.exists before returning them.

• Creates a new session, rather than blocking, when get is called on an empty pool.

• Discards the returned session, rather than blocking, when put is called on a full pool.

Concrete session pool implementation:

• Pre-allocates / creates a fixed number of sessions.

• "Pings" existing sessions via session.exists before returning them, and replaces expired sessions.

• Blocks, with a timeout, when get is called on an empty pool. Raises after timing out.

• Raises when put is called on a full pool. That error is never expected in normal practice, as users should be calling get followed by put whenever in need of a session.

Concrete session pool implementation:

• Pre-allocates / creates a fixed number of sessions.

• Sessions are used in "round-robin" order (LRU first).

• "Pings" existing sessions in the background after a specified interval via an API call (session.ping()).

• Blocks, with a timeout, when get is called on an empty pool. Raises after timing out.

• Raises when put is called on a full pool. That error is never expected in normal practice, as users should be calling get followed by put whenever in need of a session.

The application is responsible for calling ping at appropriate times, e.g. from a background thread.

Context manager: hold session checked out from a pool.

Concrete session pool implementation:

In addition to the features of PingingPool, this class creates and begins a transaction for each of its sessions at startup.

When a session is returned to the pool, if its transaction has been committed or rolled back, the pool creates a new transaction for the session and pushes the transaction onto a separate queue of "transactions to begin." The application is responsible for flushing this queue as appropriate via the pool's begin_pending_transactions method.