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api-highlevel.rst

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High-level API

.. versionadded:: 0.7.1

The high-level API is a wrapper over the mid-level connection API, combined with a connection pool.

Pooling

The high-level API provides a connection pool that automatically maintains a certain amount of idle connections to the PostgreSQL server. As only one query can be issued to a server at any one time, this provides an effective way of performinng multiple queries concurrently.

Warning

These connections are PERSISTENT connections. The pool does not reap connections for idle connections. If you use something ala pgbouncer, that automatically closes idle connections, and your application is relatively low activity, there will be a cascading failure as broken connections are checked out of the pool and aren't reconnected until an obvious error happens upon trying to query on a disconnected connection.

Connecting

To create a connection pool, use :meth:`.open_pool`.

.. automethod:: pg_purepy.pool.open_pool

Warning

Your connection count should be relatively low. The default is a very good idea for nearly all applications. Don't change it unless you have the benchmarks to prove it's a good idea. See: https://github.com/brettwooldridge/HikariCP/wiki/About-Pool-Sizing

.. autoclass:: pg_purepy.pool.PooledDatabaseInterface
    :members: max_connections, idle_connections, waiting_tasks

Querying

The connection pool object has a similar high-level query API to the mid-level API.

.. automethod:: pg_purepy.PooledDatabaseInterface.execute


.. automethod:: pg_purepy.PooledDatabaseInterface.fetch


.. automethod:: pg_purepy.PooledDatabaseInterface.fetch_one

Transactions

As two subsequent queries may not be on the same connection, transactions get tricky. For that end, the pool has a special :meth:`.PooledDatabaseInterface.checkout_in_transaction` method which checks out a connection for exclusive usage in a transaction block.

.. automethod:: pg_purepy.PooledDatabaseInterface.checkout_in_transaction


async with pool.checkout_in_transaction() as conn:
    await conn.fetch("insert into ...")

The transaction will be automatically committed or rolled back as appropriate at the end of the async with block, and the connection will not be reused until the checkout is done.

Converters

You can add converters like the other two APIs using :meth:`.PooledDatabaseInterface.add_converter`. This will add it to all open connections, as well as any future connections that may be opened.

.. automethod:: pg_purepy.PooledDatabaseInterface.add_converter

If you wish to automatically add the array converter for converting PostgreSQL arrays of a custom type that is converted, use :meth:`.PooledDatabaseInterface.add_converter_with_array`.

.. automethod:: pg_purepy.PooledDatabaseInterface.add_converter_with_array

Cancellation

Application-level cancellation is supported automatically. If a query is cancelled via a cancel scope, a cancellation request will be issued to the server to avoid having to drain more events from the server if possible.

# timeout block will automatically cancel the query after a while
# and it will be returned to the pool for use (hopefully) immediately
with anyio.move_on_after(timeout):
    async for result in pool.fetch(really_long_query):
        await do_long_running_operation(result)

This also works automatically in transactions - the insertion will be cancelled and the transaction will be rolled back.