Many years ago I wrote a simple perl+mysql polling script for work. It collected votes for multiple-choice polls, stored them in a database, and returned a results page. It was so simple and easy to use, we’ve continued using it for YEARS. Every so often we ask a question that is HIGHLY controversial (I’m looking at you NOW) and we are suddenly inundated with votes, causing much of the site to slow down or return errors.

The last time this happened I made some changes to the code that made it more difficult to vote more than once. Part of that involved storing the current vote totals in Memcached instead of asking the database each time. But even with that change, we’re still running a steady 3 or 4 votes per second and would be vulnerable to legitimate spikes in traffic from on-air promotion or something similar.

So today, I’ve reworked the script to run ALL traffic through memcached and NEVER hit the database at all unless memcached is completely unavailable. The basis for this is BroddlIT’s “memcached as simple message queue” at least as far as I can figure from his rough sketch.

The general idea is, there’s a lock, a pointer to the next slot in the queue, and the queue entries themselves.

Getting the lock

This waits up to a second to get the lock by sleeping for 1/1000th of a second between attempts. You can implement multiple queues by varying the $key_prefix. If the loop exits without getting the lock, you have to decide what to do.

  my $key_prefix = "poll-queue";
  my $total_sleep = 1000000;
  while($total_sleep > 0) {
    my $lock_key = $memd->incr("$key_prefix-lock");

    if(!defined $lock_key) { # No key in cache.
      $memd->set("$key_prefix-lock", $lock_key = 1);
      $memd->set("$key_prefix-curr", 0);
      last;
    } elsif($lock_key == 1) { # We got the lock.
      last;
    }

    # Someone else has the lock, so wait 1/1000th of a second.
    $total_sleep -= usleep(1000);
  }
  if($total_sleep <= 0) {
    # You didn't get the lock. Figure something out.
  }

If you make sure you normally hold the lock for a very small amount of time, you can assume that waiting significantly longer than that without success gives you reasonable cause to take over the lock. The only trick there, as you might expect, is that you need to then release it before another process makes the same assumption.

Get Next Key Pointer

The $key_prefix-curr memcached key contains the index of the last entry in the queue. By calling the $memd->incr() method we get the next one and update the memcache at the same time. If for some reason the pointer doesn’t exist, it is created. The $key variable becomes the memcached key for this message.

  my $next_key = $memd->incr("$key_prefix-curr");
  if(!defined($next_key)) { $memd->set("$key_prefix-curr", 1); }
  my $key = sprintf("$key_prefix-key-%d", $next_key);

Store the Message in the Queue

Simple. Run this and the previous step in a loop to insert multiple values. But be careful because the lock is active, so the longer you take the longer you block other requests.

  $memd->set($key, $message);

Release the Lock

This releases the lock, which allows another process to insert its messages.

  $memd->set("$key_prefix-lock", 0);

Reading and Flushing the Queue

Obtain the lock in the same way as above, then get the $key_prefix-curr value and loop from 1. Just make sure you process the values after you release the lock so you don’t block new entries.

  my $last_key = $memd->get("$key_prefix-curr");
  for(my $i=1; $i<=$last_key; $i++) {
    my $key = sprintf("$key_prefix-key-%d", $i);
    push @messages, $memd->get($key);
  }
  $memd->set("$key_prefix-curr", 0);
  $memd->set("$key_prefix-lock", 0);

  process_messages(@messages);

So that’s it. It should require only Cache::Memcached and Time::HiRes.

Update: if you run multiple memcached daemons and connect to them all (allowing key-hash load balancing), you will probably want to set the “no_rehash” flag and add a bit more error checking. If you allow Cache::Memcached to re-hash your keys if a server connection fails, you could end up losing the various keys temporarily.

I’m mostly a Perl guy (with secret love of Javascript), so I try to stay out of the Python stuff at dayjob where possible. But recently I’ve been taking the lead on a bunch of Memcached optimizations, which are starting to trickle over into the Python side.

A nice feature of the Perl Cache::Memcached module is the ability to define a “namespace” when you create the Memcached object:

my $memd = new Cache::Memcached (namespace => "foo_");

Then, any keys passed to the $memd object via get/set/etc. are automatically prefixed with “foo_”: $memd->get("123") actually requests the memcached key “foo_123”.

Python’s memcache module supports namespaces for the *_multi methods, but not on the individual get/set/etc calls. Also, the namespace must be passed on each call — you can’t specify it in the constructor. Well, subclassing saves the day again:

class Client(memcache.Client):
    def __init__(self, servers=None, debug=0, namespace=None):
        super(Client, self).__init__(servers, debug=debug)

        if namespace:
            self._namespace = namespace
        else:
            self._namespace=""

    # GET
    def get(self, key):
        try:
            val=self.get_multi([ key ])[key]
        except KeyError:
            val=None
        return val

    def get_multi(self, keys, key_prefix=''):
        if self._namespace: key_prefix=self._namespace + key_prefix
        return super(Client, self).get_multi(keys, key_prefix=key_prefix)

    # SET
    def set(self, key, val, time=0, min_compress_len=0):
        return self.set_multi({ key : val }, time=time, min_compress_len=min_compress_len)

    def set_multi(self, mapping, time=0, key_prefix='', min_compress_len=0):
        if self._namespace: key_prefix=self._namespace + key_prefix
        return super(Client, self).set_multi(mapping, time=time, key_prefix=key_prefix, min_compress_len=min_compress_len)

    # DELETE
    def delete(self, key, time=0):
        return self.delete_multi([key], time=time)

    def delete_multi(self, keys, seconds=0, key_prefix=''):
        if self._namespace: key_prefix=self._namespace + key_prefix
        return super(Client, self).delete_multi(keys, seconds=seconds, key_prefix=key_prefix)

    # EVERYTHING ELSE
    def add(self, key, val, time=0, min_compress_len=0):
        if self._namespace: key=self._namespace + str(key)
        super(Client, self).add(key, val, time=time, min_compress_len=min_compress_len)

    def incr(self, key, delta=1):
        if self._namespace: key=self._namespace + str(key)
        super(Client, self).incr(key, delta=delta)

    def replace(self, key, val, time=0, min_compress_len=0):
        if self._namespace: key=self._namespace + str(key)
        super(Client, self).replace(key, val, time=time, min_compress_len=min_compress_len)

    def decr(self, key, delta=1):
        if self._namespace: key=self._namespace + str(key)
        super(Client, self).decr(key, delta=delta)

The __init__ method is overridden to take an additional “namespace” parameter, which is stored in self._namespace. The get/set/delete methods all have namespace-capable *_multi versions, so for those I just pass the calls off to the appropriate one. The *_multi methods themselves are subclassed to check the self._namespace value as well as the namespace parameter, like normal. Finally, the add/incr/replace/decr methods are all modified to check the self._namespace value and prefix it to the key. Obviously, get/set/delete could have been done the same way.

I’ve been using Memcached for a few weeks, trying to offload some VERY heavy database load. It’s nice and blazing fast, but the implementation is sort of clunky. If I have this simple bit of code:

$key = "foobar";
$val = calculate_val($key);

It turns into this:

$key = "foobar";
$val = $memd->get($key);
if(! defined($val)) {
  $val = calculate_val($key);
  $memd->set($key, $val);
}

Repeatedly I came back to the idea of a get_or_set method that would handle this stuff, but until the obvious solution hit me, I couldn’t get it:

$key = "foobar";
$val = $memd->get_or_set($key, sub { calculate_val($key) });

A simple closure around the actual calculation block which is then passed to the get_or_set method as a callback. If the lookup finds a value the method returns it, otherwise it returns the result of calling the callback function.

The only change to Cache::Memcached is adding the get_or_set function. The easiest way is to just subclass Cache::Memcached:

package My::Memcached
use base qw(Cache::Memcached);
sub get_or_set {
  my $self = shift;
  my($key, $callback) = @_;
  my $val = $self->get($key);
  unless(defined $val) {
    $val = &$callback;
    $self->set($key, $val);
  }
  return $val;
}

1;

Now you have this simple interface:

use My::Memcached;

my $memd = new My::Memcached { servers => [...] };
my $foo = $memd->get_or_set("bar", sub { get_val("bar") });