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Two‐Phase Commit in Python’s DB‐API

Marc uploaded a new revision of the Python DB-API 2.0 Specification yesterday that documents the new two phase commit extension that I helped develop on the db-sig mailing list.

My interest in this started from the desire to support two phase commit in Storm – without that feature there are far fewer occasions where its ability to talk to multiple databases can be put to use. As I was doing some work on psycopg2 for Launchpad, I initially put together a PostgreSQL specific patch, which was (rightly) rejected by Federico.

He suggested that it would be better to try and standardise on an API on the db-sig list, so that’s what I did. I looked over the API exposed by other database adapters that supported 2PC, and the 2PC APIs of the major free databases that did not have support in their Python adapters (MySQL and PostgreSQL). The resulting API is a bit more complicated than my original PostgreSQL-only but has the advantage of being implementable on other databases such as MySQL.

Below is a simple example of using the API directly (missing some of the error handling):

# begin transactions for each database connection
conn1.tpc_begin(conn1.xid(42, 'transaction ID', 'connection 1'))
conn2.tpc_begin(conn2.xid(42, 'transaction ID', 'connection 2'))
# Do stuff with both connections
...
try:
    conn1.tpc_prepare()
    conn2.tpc_prepare()
except DatabaseError:
    conn1.tpc_rollback()
    conn2.tpc_rollback()
else:
    conn1.tpc_commit()
    conn2.tpc_commit()

Or alternatively, if you’ve got one connection supporting 2PC and the other only supporting one-phase commit, it could be structured as follows:

# begin transactions for each database connection
conn1.tpc_begin(conn1.xid(42, 'transaction ID', 'connection 1'))
# Do stuff with both connections
...
try:
    conn1.tpc_prepare()
    conn2.commit()
except DatabaseError:
    conn1.tpc_rollback()
    conn2.rollback()
else:
    conn1.tpc_commit()

While it is possible to use the 2PC API directly, it is expected that most applications will rely on a transaction manager to coordinate global transactions, such as Zope’s transaction module.

The hope is that by offering a consistent API, Python application frameworks will be more likely to bother supporting this feature of databases. Hopefully you’ll be able to use the API with PostgreSQL and Storm soon.

Zeroconf Branch Sharing with Bazaar

Bazaar logoAt Canonical, one of the approaches taken to accelerate development is to hold coding sprints (otherwise known as hackathons, hackfests or similar). Certain things get done a lot quicker face to face compared to mailing lists, IRC or VoIP.

When collaborating with someone at one of these sprints the usual way to let others look at my work would be to commit the changes so that they could be pulled or merged by others. With legacy version control systems like CVS or Subversion, this would generally result in me uploading all my changes to a server in another country only for them to be downloaded back to the sprint location by others.

In contrast, with a modern VCS like Bazaar we should be able to avoid this since the full history of the branch is available locally – enough information to let others pull or merge the changes. That said, we’ve often ended up using a server on the internet to exchange changes despite this. This is the same work flow we use when working from home, so I guess the pain of switching to a new work flow outweighs the potential productivity gains.

The Solution

Bazaar makes it easy to run a read only server locally:

bzr serve [--directory=DIR]

However, there is still the issue of others finding the branch. They’d need to know the IP address assigned to my computer at the sprint, and the path to the branch on the server. Ideally they’d just need to know the name of the my branch. As it happens, we’ve got the technology to fix this.

Avahi logoAvahi makes it trivial to advertise and browse for services on the local network without having to worry about what IP addresses have been assigned or what people name their computer. So the solution is to hook Avahi and Bazaar together. This was fairly easy due to Avahi’s DBus interface and the dbus-python bindings.

The result is my bzr-avahi plugin. You can either download tarballs or install the latest version directly with from Bazaar:

bzr branch lp:bzr-avahi ~/.bazaar/plugins/avahi

To use the plugin, you must have at least version 1.1 of Bazaar, the Python bindings for DBus and Avahi, and a working Avahi setup. Once the plugin is installed, it hooks into the standard “bzr serve” command to do the following:

  • scan the directory being served for branches that the user has asked to advertise.
  • ask Avahi to advertise said branches

You can ask to advertise a branch using the new “bzr advertise” command:

bzr advertise [BRANCH-NAME]

If no name is specified, the branch’s nickname is used. The advertise command sends a signal over the session bus to tell any running servers about the change, so there is no need to restart “bzr serve” to see the change.

At this point, the advertised branches should be visible with a service browser like avahi-discover, so that’s half the problem solved. From the client side two things are provided: a special redirecting transport and a command to list all advertised branches on the local network.

The transport allows you to access the branch by its advertised name with most Bazaar commands. For example, merging a branch is as simple as:

$ bzr merge local:BRANCH-NAME
local:BRANCH-NAME is redirected to bzr://hostname.local:4155/path/to/branch
...
All changes applied successfully.
$

If you want to get a list of all advertised branches on the network, the “bzr browse” command will print out a list of branch names and the URLs they translate to.

I believe using these tools together should offer a low enough overhead for direct sharing of branches at sprints that people would actually bother using it. It should be quite useful at the next sprint I go to.

Client Side OpenID

The following article discusses ideas that I wouldn’t even class as vapourware, as I am not proposing to implement them myself. That said, the ideas should still be implementable if anyone is interested.

One well known security weakness in OpenID is its weakness to phishing attacks. An OpenID authentication request is initiated by the user entering their identifier into the Relying Party, which then hands control to the user’s OpenID Provider through an HTTP redirect or form post. A malicious RP may instead forward the user to a site that looks like the user’s OP and record any information they enter. As the user provided their identifier, the RP knows exactly what site to forge.

Out Of Band Authorisation

One way around this is for the OP to authenticate the user and get authorisation out of band — just because the authentication message begins and ends with HTTP requests does not mean that the actual authentication/authorisation need be done through the web browser.

Possibilities include performing the authorisation via a Jabber message or SMS, or some special purpose protocol. Once authorisation is granted, the OP would need to send the OpenID response. Two ways for the web browser to detect this would be polling via AJAX, or using a server-push technique like Comet.

Using a Browser Extension

While the above method adds security it takes the user outside of their web browser, which could be disconcerting. We should be able to provide an improved user experience by using a web browser extension. So what is the best way for the extension to know when to do its thing?

One answer is whenever the user visits the server URL of their OP. Reading through the specification there are no other times when the user is required to visit that URL. So if the web browser extension can intercept GET and POST requests to a particular URL, it should be able to reliably detect when an authentication request is being initiated.

At this point, the extension can take over up to the point where it redirects the user back to the RP. It will need to communicate with the OP in some way to get the response signed, but we have the option of using some previously established back channel.

Moving the OP Client Side

Using the browser extension from the previous section as a starting point, we’ve moved some of the processing to the client side. We might now ask how much work can be moved to the client, and how much work needs to remain on the server?

From the specification, there are three points at which the RP needs to make a direct connection to the OP (or a related server):

  1. When performing discover, the RP needs to be able to read an HTML or XRDS file off some server.
  2. The associate request, used to generate an association that lets the RP verify authentication responses.
  3. The check_authentication request, used to verify a response in the case where an association was not provided in the request (or the OP said the association was invalid).

In all other cases, communication is mediated through the user’s browser (so are being intercepted by the browser extension). Furthermore, these three cases should only occur after the user initiates an OpenID authentication request. This means that the browser extension should be active and talking to the server.

So one option would be to radically simplify the server side so that it simply proxies the associate and check_authentication requests to the browser extension via a secure channel. This way pretty much the entire OP implementation resides in the browser extension with no state being handled by the server.

Conclusion

So it certainly looks like it is possible to migrate almost everything to the client side. That still leaves open the question of whether you’d actually want to do this, since it effectively makes your identity unavailable when away from a computer with the extension installed (a similar problem to use of self asserted infocards with Microsoft’s CardSpace).

Perhaps the intermediate form that still performs most of the OP processing on the server is more useful, providing a level of phishing resistance that would be difficult to fake (not only does it prevent rogue RPs from capturing credentials, the “proxied OP” attack will fail to activate the extension all together).

Re: Python factory-like type instances

Nicolas: Your metaclass example is a good example of when not to use metaclasses. I wouldn’t be surprised if it is executed slightly different to how you expect. Let’s look at how Foo is evaluated, starting with what’s written:

class Foo:
    __metaclass__ = FooMeta

This is equivalent to the following assignment:

Foo = FooMeta('Foo', (), {...})

As FooMeta has an __new__() method, the attempt to instantiate FooMeta will result in it being called. As the return value of __new__() is not a FooMeta instance, there is no attempt to call FooMeta.__init__(). So we could further simplify the code to:

Foo = {
    'linux2': LinuxFoo,
    'win32': WindowsFoo,
}.get(PLATFORM, None)
if not Foo:
    # XXX: this should _really_ raise something other than Exception
    raise Exception, 'Platform not supported'

So the factory function is gone completely here, and it is clear that the decision about which class to use is being made at module import time rather than class instantiation time.

Now this isn’t to say that metaclasses are useless. In both implementations, the code responsible for selecting the class has knowledge of all implementations. To add a new implementation (e.g. for Solaris or MacOS X), the factory function needs to be updated. A better solution would be to provide a way for new implementations to register themselves with the factory. A metaclass could be used to make the registration automatic:

class FooMeta(type):
    def __init__(self, name, bases, attrs):
        cls = super(FooMeta, self).__init__(name, bases, attrs)
        if cls.platform is not None:
            register_foo_implementation(klass.platform, cls)
        return cls

class Foo:
    __metaclass__ = FooMeta
    platform = None
    ...

class LinuxFoo(Foo):
    platform = 'linux2'

Now the simple act of defining a SolarisFoo class would be enough to have it registered and ready to use.

Allocated Seating at Greater Union

On the weekend, I had my first encounter with allocated seating at the Greater Union Innaloo cinemas.

As usual, we’d bought tickets separately. It wasn’t until going in to the actual cinema that a staff member said that we were expected to sit in seats scattered around the cinema (one of which was on the very edge).

As the cinema wasn’t completely full, we did the only sensible thing: ignore the allocations and pick some seats next to each other. Looking around the cinema, it looked like a number of other people were ignoring the allocations (the seat I’d been allocated was taken by someone else in a group of about 5 people).

As far as I can understand, the reason for introducing this was to make the internet booking more compelling by letting you pick your seat. I guess they felt the need to do something, since the current system has never seemed worth it:

  • They charge an extra dollar per ticket for internet sales. This is despite the fact that they get the money earlier, and you might not even turn up (the tickets are sold on a no returns basis).
  • While there is a special queue for picking up internet sales tickets, there often isn’t anyone staffing it. I’ve only seen people in the queue a few times, and they needed to wait until one of the other ticket sellers was free.

Maybe they thought screwing with the majority of their customers’ experience would make the extra dollar worth it.

I sent a complaint to Greater Union, and in future plan to treat their seating allocations as a suggestion. It is a shame that so many other cinemas have been closing down over the years 🙁