The Build

19 March 2012


A Recipe for Django Transactions on PostgreSQL

As noted before, Django has a lot of facilities for handling transactions, and it’s not at all clear how to use them. In an attempt to cut through the confusion, here’s a recipe for handling transactions sensibly in Django applications on PostgreSQL.

The goals are:

The bits of the recipe are:

The quick reasons behind each step:

This recipe a few other nice features:

xact() also supports the using parameter for multiple databases.

Of course, a few caveats:

To use, just drop the source (one class definition, one function) into a file somewhere in your Django project (such as the omni-present utils application every Django project seems to have), and include it.


from utils.transaction import xact

def my_view_function1(request):
   # Everything here will be in a transaction.
   # It'll roll back if an exception escapes, commits otherwise.

def my_view_function2(request):
   # This stuff won't be in a transaction, so don't modify the database here.
   with xact():
      # This stuff will be, and will commit on normal completion, roll back on a exception

def my_view_function3(request):
   with xact():
      # Modify the database here (let's call it "part 1").
         with xact():
            # Let's call this "part 2."
            # This stuff will be in its own savepoint, and can commit or
            # roll back without losing the whole transaction.
         # Part 2 will be rolled back, but part 1 will still be available to
         # be committed or rolled back.  Of course, if an exception
         # inside the "part 2" block is not caught, both part 2 and
         # part 1 will be rolled back.

The source is available on GitHub. It’s licensed under the PostgreSQL License.

24 January 2012


PostgreSQL Performance When It’s Not Your Job

My presentation from SCALE 10x, “PostgreSQL Performance When It’s Not Your Job” is now available for download.

30 September 2011


“Sharding & IDs at Instagram”

I’d like to recommend an interesting post, “Sharding & IDs at Instagram”, about sharding using Postgres.

27 September 2011


Cleaning up after your Bucardo goats

If you are not familiar with it already, Bucardo is a nifty multi-master replication system for PostgreSQL, written by Greg Sabino Mullane. Written in Perl, it is great if you need replication that doesn’t have the restrictions associated with PG 9’s streaming replication.

To keep your Bucardo installation clean and tidy, a few regular cron jobs are required. One of them cleans up the archived replicated data (stored in a separate database by Bucardo) once you know you are done with it.

The Bucardo page above has a recommended script using all sorts of bashing, but I wanted something a bit more pure-PostgreSQL; it also doesn’t purge more than one old table at a time. So, I whipped up the following PL/pgSQL function.

(Note that this is for Bucardo 4.4. I haven’t played with the forthcoming Bucardo 5, so I’m not sure if this is still required.)

CREATE OR REPLACE FUNCTION bucardo.purge_frozen_child_qs(far_back interval)
    t TEXT;
    qt TEXT;

    IF far_back IS NULL THEN
        RAISE EXCEPTION 'Interval cannot be null.'
            USING HINT = 'So, do not do that.';
    END IF;

    IF (now() + far_back) > now() THEN
        RAISE EXCEPTION 'Interval must be negative.'
            USING HINT = 'Consider using the "ago" form of intervals.';
    END IF;

    FOR t IN 
        SELECT tablename 
            FROM pg_tables
            WHERE schemaname='freezer' 
                  AND tablename like 'child_q_%' 
                  AND (replace(tablename, 'child_q_', '')::timestamp with time zone) < now() + far_back::interval
            ORDER BY tablename
        qt := 'freezer.' || t;
        EXECUTE 'DROP TABLE ' || qt;
        RETURN NEXT qt;

    DELETE FROM bucardo.q 
        WHERE (started < now() + far_back::interval 
                OR ended < now() + far_back::interval 
                OR aborted < now() + far_back::interval 
                OR cdate < now() + far_back::interval) 
              AND (ended IS NULL OR aborted IS NULL);


LANGUAGE plpgsql

To use it, just call it repeatedly from a cron job with the appropriate argument, along the lines of:

SELECT * FROM bucardo.purge_frozen_child_qs('7 days ago'::interval);

It returns the names of the tables it deleted.

This particular function doesn’t need to be run more often than once a day. And it keeps your Bucardo goats nice and clean.

(A “bucardo” is a now-extinct species of goat. For why Bucardo is goat-related, ask Greg.)

26 July 2011


Unbreaking Your Django Application

My tutorial at OSCON 2011, Unbreaking Your Django Application, is now available for download.

18 May 2011


Life with Object-Relational Mappers

The slides from my talk at PGCon 2011 are now available.

28 March 2011


Not Convenient.

DjangoCon Europe and the Apple WWDC are at the exact same time. This is going to be a tough call.

Update: Well, that was quick. WWDC sold out in 10 hours, while I was dithering.

18 March 2011


What It Means to Be In Business

To bring everyone up to date:

Sadly, I find his last post as incoherent as his first one is vitriolic.

Rather than go through it point by point, the crux of his argument is:

Building a business around maximizing your individual happiness is not particularly useful or admirable. That is my position, and I’m well aware that it may be unpopular with some.

I am pleased to report, then, that Mr Payne has absolutely nothing to worry about, because no business that is built around the happiness of the owner as a primary goal has a hope of every getting anywhere, unless the business consists of the owner taking money out of one pocket and putting it in the other. Any business, unless it is operating in a grotesquely distorted marketplace, is primarily about pleasing its customers in exchange for their money.

I’m really not sure what these vaguely masturbatory companies Mr Payne is talking about do for a living, but every (successful) micro-business I know of is insanely, intensely focused on pleasing its customers. They have to be, because they don’t have an installed base, government-granted advantages, or (yes) piles of venture capital in the bank to fall back on if they fail to do so.

Mr Payne wants to run a big company. I wish him all the best. He seems to have his young heart in the right place. I have to say, though, that his emotional overreaction to the idea that someone might want to run a micro-business instead strikes me as the Puritan reacting to the idea that someone, somewhere, might be happy.

13 March 2011


Transaction-Level Advisory Locks in PostgreSQL 9.1

Advisory locks are one of the cool unsung features of PostgreSQL. In 9.1, they are getting even cooler with transaction level locks. Many details here.

9 March 2011


Concern Troll is Concerned: Verifone vs Square

Suppose a major manufacturer of computer keyboards announced a very serious security problem with a specific competitor’s keyboard: Someone could plug this keyboard into a computer running a malicious app, and cause a user to enter sensitive information. Thus, the manufacturer demands that their competitor recall all of these “insecure” keyboards.

Anyone with the technical sense of a rock would pause for a moment, and then burst out in laughter at the utter absurdity of this proclamation. No one would ever attempt to make such a ludicrous and obviously self-serving claim, would they?

Verifone would. Verifone is very very concerned about Square’s iPhone card scanner, because someone could run a malicious app on the iPhone and collect card data using it. The fact that Square just announced new pricing undercutting Verifone’s is, of course, completely coincidental.

Where to begin?

In short, Verifone is bashing a competitor because the competitor’s pricing is more consumer-friendly than Verifone’s. Their technical arguments are nonsense, and they should be ashamed of launching a FUD campaign that plays on credit card security paranoia.

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