zc.zodbwsgi 0.1.0

WSGI Middleware for Managing ZODB Database Conections

WSGI Middleware for Managing ZODB Database Conections

The zc.zodbwsgi provides middleware for managing connections to a ZODB database. It combines several features into a single middleware component:

• database configuration

• database initialization

• connection management

• optional transaction management

• optional request retry on conflict errors (using repoze.retry)

It is designed to work with paste deployment and provides a “filter_app_factory” entry point, named “main”.

A number of configuration options are provided. Option values are strings.

configuration

A required ZConfig formatted ZODB database configuration

If multiple databases are defined, they will define a multi-database. Connections will be to the first defined database.

initializer

An optional database initialization function of the form module:expression

key

An optional name of a WSGI environment key for database connections

This defaults to “zodb.connection”.

transaction_management

An optional flag (either “true” or “false”) indicating whether the middleware manages transactions

Transaction management is enabled by default.

transaction_key

An optional name of a WSGI environment key for transaction managers

This defaults to “transaction.manager”. The key will only be present if transaction management is enabled.

retry

An optional retry count

The default is “3”, indicating that requests will be retried up to 3 times. Use “0” to disable retries.

Note that when retry is not “0”, request bodies will be buffered.

Let’s look at some examples.

First we define an demonstration “application” that we can pass to our factory:

import transaction, ZODB.POSException
from sys import stdout

class demo_app:
    def __init__(self, default):
        pass
    def __call__(self, environ, start_response):
        start_response('200 OK', [('content-type', 'text/html')])
        root = environ['zodb.connection'].root()
        path = environ['PATH_INFO']
        if path == '/inc':
            root['x'] = root.get('x', 0) + 1
            if 'transaction.manager' in environ:
                environ['transaction.manager'].get().note('path: %r' % path)
            else:
                transaction.commit() # We have to commit our own!
        elif path == '/conflict':
            print >>stdout, 'Conflict!'
            raise ZODB.POSException.ConflictError

        return [repr(root)]

Now, we’ll define our application factory using a paste deployment configuration:

[app:main]
paste.app_factory = zc.zodbwsgi.tests:demo_app
filter-with = zodb

[filter:zodb]
use = egg:zc.zodbwsgi
configuration =
   <zodb>
     <demostorage>
     </demostorage>
   </zodb>

Here, for demonstration purposes, we used an in-memory demo storage.

Now, we’ll create an application with paste:

>>> import paste.deploy, os
>>> app = paste.deploy.loadapp('config:'+os.path.abspath('paste.ini'))

The resulting applications has a database attribute (mainly for testing) with the created database. Being newly initialized, the database is empty:

>>> conn = app.database.open()
>>> conn.root()
{}

Let’s do an “increment” request.

>>> import webtest
>>> testapp = webtest.TestApp(app)
>>> testapp.get('/inc')
<200 OK text/html body="{'x': 1}">

Now, if we look at the database, we see that there’s now data in the root object:

>>> conn.sync()
>>> conn.root()
{'x': 1}

We can supply a database initialization function using the initializer option. Let’s define an initialization function:

import transaction

def initialize_demo_db(db):
    conn = db.open()
    conn.root()['x'] = 100
    transaction.commit()
    conn.close()

and update our paste configuration to use it:

[app:main]
paste.app_factory = zc.zodbwsgi.tests:demo_app
filter-with = zodb

[filter:zodb]
use = egg:zc.zodbwsgi
configuration =
   <zodb>
     <demostorage>
     </demostorage>
   </zodb>

initializer = zc.zodbwsgi.tests:initialize_demo_db

Now, when we use the application, we see the impact of the initializer:

>>> app = paste.deploy.loadapp('config:'+os.path.abspath('paste.ini'))
>>> testapp = webtest.TestApp(app)
>>> testapp.get('/inc')
<200 OK text/html body="{'x': 101}">

Sometimes, you may not want the middleware to control transactions. You might do this if your application used multiple databases, including non-ZODB databases [1]. You can suppress transaction management by supplying a value of “false” for the transaction_management option:

[app:main]
paste.app_factory = zc.zodbwsgi.tests:demo_app
filter-with = zodb

[filter:zodb]
use = egg:zc.zodbwsgi
configuration =
   <zodb>
     <demostorage>
     </demostorage>
   </zodb>

initializer = zc.zodbwsgi.tests:initialize_demo_db
transaction_management = false

By default, zc.zodbwsgi adds repoze.retry middleware to retry requests when there are conflict errors:

>>> import ZODB.POSException
>>> app = paste.deploy.loadapp('config:'+os.path.abspath('paste.ini'))
>>> testapp = webtest.TestApp(app)
>>> try: testapp.get('/conflict')
... except ZODB.POSException.ConflictError: pass
... else: print 'oops'
Conflict!
Conflict!
Conflict!
Conflict!

Here we can see that the request was retried 3 times.

We can suppress this by supplying a value of “0” for the retry option:

[app:main]
paste.app_factory = zc.zodbwsgi.tests:demo_app
filter-with = zodb

[filter:zodb]
use = egg:zc.zodbwsgi
configuration =
   <zodb>
     <demostorage>
     </demostorage>
   </zodb>

retry = 0

Now, if we run the app, the request won’t be retried:

>>> app = paste.deploy.loadapp('config:'+os.path.abspath('paste.ini'))
>>> testapp = webtest.TestApp(app)
>>> try: testapp.get('/conflict')
... except ZODB.POSException.ConflictError: pass
... else: print 'oops'
Conflict!

Changes

0.1.0 (2010-02-16)

Initial release

[1]

If you want to use multiple ZODB databases, you can simply define them in your configuration option. Just make sure to give them names. When you want to access a database, use the get_connection method on the connection in the environment:

foo_conn = environ['zodb.connection'].get_connection('foo')