postgresql when it's not your job

09:37

Two very cool things.

First, read this essay about the disaster that is PHP. Every word is correct.

Then, view this photo set.

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23:55

Blah, Blah, Blah, First Half of 2012 Edition

I’ll be speaking at the following conferences through July:

• PGCon, Ottawa, Ontario, Canada, May 17-18.
• DjangoCon Europe, Zurich, Switzerland, June 4-6.
• Southwest LinuxFest, Charlotte, North Carolina, June 8-10.
• EuroPython, Florence, Italy, July 2-8.
• OSCON, Portland, Oregon, USA, July 16-20.

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08:00

The Elements of postgresql.conf Style

… or, inexcusable things I am tired of seeing in postgresql.conf files.

Do not mix ‘n’ match override styles.

There are two valid styles for overriding the default values in postgresql.conf: Putting your changes as a cluster at the end, or uncommenting the defaults and overriding in place. Both have advantages and disadvantages. Having some settings one way and some another is pure disadvantage. Do not do this.

Use units.

Quick, what is log_min_duration_statement set to here?

log_min_statement_duration = 2000

Now, what is it set to here?

log_min_statement_duration = 2s

Always use units with numeric values if a unit is available.

Do not remove the default settings.

If you strip out all of the defaults, it becomes impossible to tell what a particular value is set to. Leave the defaults in place, and if you comment out a setting, reset the value to the default (or at least include comments that make it clear what is going on).

Do not leave junk postgresql.conf files scattered around.

If you need to move postgresql.conf (and the other configuration files) to a different location from where the package for your system puts it, don’t leave the old, dead postgresql.conf lying around. Delete any trace of the old installation hierarchy.

Thank you.

7 comments

12:09

Instagram’s Technology Stack

Instagram has been in the news lately. In this really great post on Tumblr, Instagram talks about its technology stack.

I have some acquaintance with the Instagram people, and they are among the smartest technologists I’ve met. Really nice, too. (Of course, they mention this blog in the post, so I’m biased.)

1 comment

12:33

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:

• Database operations that do not modify the database aren’t wrapped in a transactions at all.
• Database operations that modify the database are wrapped in a transactions.
• We have a lot of fine-grained control over sections that modify the databse vs those that don’t.

The bits of the recipe are:

• Use the autocommit option in your database configuration.
• Do not use the transaction middleware.
• Wrap the sections of code which modify the database in the xact() decorator / context manager below, using it like you would the commit_on_success() decorator.
• Profit!

The quick reasons behind each step:

• Turning on autocommit prevents psycopg2 from automatically starting a new transaction on the first database operation on each connection; this means that the transaction only starts when we want it to.
• Similarly, the transaction middleware will set the connection state to “managed,” which will defeat the autocommit option above, so we leave it out.
• The xact() decorator will set up the connection so that a transaction is started in the relevant block, which is what we want for database-modifying operations.

This recipe a few other nice features:

• xact() operates like commit_on_success(), in that it will issue a rollback if an exception escapes from the block or function it is wrapping.
• xact() ignores the dirty flag on the Django connection. Since we’re deliberately wrapping stuff that modifies the database with it, the chance of it being dirty is near 100%, and a commit on a transaction that did not modify the database is no more expensive in PostgreSQL than a rollback. It also means you can do raw SQL inside an xact() block without the foot-gun of forgetting to call set_dirty.
• Like the built-in Django transaction decorators, it can be used either as a decorator or as a context manager with the with statement.
• xact() can be nested, giving us nested transactions! If it sees that there is already a transaction open when it starts a new block, it will use a savepoint to set up a nested transaction block. (PostgreSQL does not have nested transactions as such, but you can use savepoints to get 99.9% of the way there.)
• By not wrapping operations that do not modify the database, we get better behavior when using pgPool II (more on that in a future post).
• xact() works around an outstanding bug in Django’s transaction handling on psycopg2.

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

Of course, a few caveats:

• xact() requires the postgresql_psycopg2 backend, and PostgreSQL 8.2 or higher. It’s possible it can be hacked to work on other backends that support savepoints.
• xact() works just the way you want if it is nested inside a commit_on_success() block (it will properly create a savepoint insted of a new transaction). However, a commit_on_success() block nested inside of xact() will commit or rollback the entire transaction, somewhat defeating the outer xact(). To the extent possible, use only xact() in code you write.
• Be sure you catch exceptions outside of the xact() block; otherwise, the automatic rollback will be defeated. Allow the exception to escape the xact() block, and then catch it. (Of course, if the intention is to always commit and to defeat the rollback, by all means catch the exception inside the block.)

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.

Examples:

from utils.transaction import xact

@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").
      try:
         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.
      except:
         # 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.

11 comments

23:03

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.

3 comments

22:39

“Sharding & IDs at Instagram”

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

1 comment

22:25

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)
    RETURNS SETOF TEXT AS
$purge_frozen_child_qs$
DECLARE
    t TEXT;
    qt TEXT;
BEGIN

    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
    LOOP
        qt := 'freezer.' || t;
        EXECUTE 'DROP TABLE ' || qt;
        RETURN NEXT qt;
    END LOOP;

    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);

    RETURN;

END
$purge_frozen_child_qs$
LANGUAGE plpgsql
    VOLATILE;

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.)

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14:14

Unbreaking Your Django Application

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

11 comments

11:31

Life with Object-Relational Mappers

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

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