# Introduction to Twisted Enterprise

## Abstract

Twisted is an asynchronous networking framework, but most database API implementations unfortunately have blocking interfaces -- for this reason, `twisted.enterprise.adbapi` was created. It is a non-blocking interface to the standardized 'dbapi' module, which allows you to access a number of different RDBMSes.

## What you should already know

- Python :-)
- How to write a simple Twisted Server (see this tutorial to learn how)
- Familiarity with using database interfaces (see the documentation for DBAPI 2.0)

## Quick Overview

Twisted is an asynchronous framework. This means standard database modules cannot be used directly, as they typically work something like:

        # Create connection...
        db = dbmodule.connect('mydb', 'andrew', 'password')
        # ...which blocks for an unknown amount of time

        # Create a cursor
        cursor = db.cursor()

        # Do a query...
        resultset = cursor.query('SELECT * FROM table WHERE ...')
        # ...which could take a long time, perhaps even minutes.

Those delays are unacceptable when using an asynchronous framework such as Twisted. For this reason, twisted provides `twisted.enterprise.adbapi`, an asynchronous wrapper for any DB-API 2.0-compliant module.

`enterprise.adbapi` will do blocking database operations in seperate threads, which trigger callbacks in the originating thread when they complete. In the meantime, the original thread can continue doing normal work, like servicing other requests.

## How do I use adbapi?

Rather than creating a database connection directly, use the `adbapi.ConnectionPool` class to manage a connections for you. This allows `enterprise.adbapi` to use multiple connections, one per thread. This is easy:

        # Using the "dbmodule" from the previous example, create a ConnectionPool
        from twisted.enterprise import adbapi
        dbpool = adbapi.ConnectionPool("dbmodule", 'mydb', 'andrew', 'password')

Things to note about doing this:

- There is no need to import dbmodule directly. You just pass the name to `adbapi.ConnectionPool`'s constructor.
- The parameters you would pass to dbmodule.connect are passed as extra arguments to `adbapi.ConnectionPool`'s constructor. Keyword parameters work as well.

So, now you need to be able to dispatch queries to your ConnectionPool. We do this by subclassing `adbapi.Augmentation`. Here's an example:

        class AgeDatabase(adbapi.Augmentation):
            """A simple example that can retrieve an age from the database"""
            def getAge(self, name):
                # Define the query
                sql = """SELECT Age FROM People WHERE name = ?"""
                # Run the query, and return a Deferred to the caller to add
                # callbacks to.
                return self.runOperation(sql, name)

        def gotAge(resultlist, name):
            """Callback for handling the result of the query"""
            age = resultlist[0][0]          # First field of first record
            print "%s is %d years old" % (name, age)

        db = MyDatabase(dbpool)

        # These will *not* block.  Hooray!
        db.getAge("Andrew").addCallbacks(gotAge, db.operationError,
                                         callbackArgs=name).arm()
        db.getAge("Glyph").addCallbacks(gotAge, db.operationError,
                                        callbackArgs=name).arm()

This is straightforward, except perhaps for the return value of `getAge`. It returns a `twisted.python.defer.Deferred`, which allows arbitrary callbacks to be called upon completion (or upon failure).

Also worth noting is that I'm assuming that dbmodule uses the "qmarks" paramstyle in my SQL query.

## And that's it!

That's all you need to know to use a database from within Twisted. You probably should read the adbapi module's documentation to get an idea of the other functions it has, but hopefully this document presents the core ideas.

## Credits

This tutorial was written by Andrew Bennetts (andrew at puzzling dot org). I would like to thank the Twisted crew for their fun (and useful!) framework, and also dash in #python for explaining the database stuff to me in the first place. The final example was improved with the aid of suggestion from Glyph.

Suggestions to improve this document are welcomed.

Last modified: Sat Mar 16 06:29:01 EST 2002