Wednesday, April 04, 2012

Implementing Namespaces with Memcached and PHP

    Memcached is a tool (with PHP extensions) for storing key-value pairs of data in RAM and it is used for boost applications where persistent storage is needed and fetching data from datasource (file, database, internet...) is more expensive than fetching from RAM (from performance point of view).

    Despite its advantages and the fact that by using it your application loads data much faster when it is in cache, it lacks any kind of keyword data removal nor any other capability to do so.

    It may not sound as a real problem, that can be a real problem making your application return invalid data in some scenarios.

    Here I will provide some scenarios where memcached can lead to problems and will post some approaches to resolve this limitation.

Scenario 1

We have pagination in our system, and keep that pagination in cache for better performance. When deleting, inserting or altering new data, that pagination becomes obsolete, and may be already cached without the posibility of deleting it because, for example, we don't have pagination cache's key id from the function that adds, deletes, or alters its data.

First approach

    The idea behind that (in which my work is based) is to emulate namespaces by adding some keys with version, are the official pseudo-namespace proposal, which allows to workarround invalid data returning by emulating namespaces and appending a version to the key. This way, if you have foo stored in cache, you update namespace to automatically try to get foo+latest_version, which may force cache regeneration.

    This approach have some troubles though: You can't bind data to several namespaces at the same time! And that was the original reason I extended this idea to a new approach which can be explained below:

Scenario 2

    Imagine we have a database with two tables, one for accounts, and the other for storing friend relationship related between them.

Table Account Table Friendship
username TEXT,
owner INTEGER,
friend INTEGER,
PRIMARY KEY (owner,friend),
FOREIGN KEY (owner) REFERENCES Account(id),
FOREIGN KEY (friend) REFERENCES Account(id)

    In this scenario, suppose account id 3 have these friend ids: 1,2,5,6,8. When asking database for friend, it will return array (1,2,5,6,8) we are going to put into cache, but what about if account id 2 is deleted, or have its id changed?

    Then, our cache is immediatelly invalid but the real data isn't (supposing we provided mechanisms for automatically updating foreign keys in DB!).

    I started to work on a class for memcache to address this problem (and possibly other issues) at the cost of having a bit of namespace data inside cache.

    My approach is to store all the keys a namespace contains as an array for allowing the system to delete all those keys when a namespace has expired.

    For example, in Scenario 2, when we add a new friend, we can bind it to owner account namespace as well as friend account namespace.

    This way, when something changes, we can expire one of those two namespaces and will delete related data from cache.


    You can grab the source code (PHP) from github


    In order to use this code you will need:
  • Memcached (of course!)
  • pecl-memcache PHP extension
  • A config file where $cache_enabled is set to TRUE
Usage example

    Check github for an usage example


    I tested it in my personal server, with postgreSQL, session handling and storing in DB and PHP 5.4.1_RC1 with the following results:
    Sessions are cached when user logs in via my DBCache class.
Without cache hit:
~35 SQL queries
~0.37 seconds page generation
With cache hits:
~0 SQL queries
~0.04 seconds page generation

Decreasing production server downtime with kexec

    When managing a production server, one of the most important thing is the tradeoff between server downtime and keeping server's software updated.

    While most of the updates can be applied from little to no downtime, a kernel update is always problematic since it requires typically a full reboot, and a significant downtime. To prevent that, many servers do not issue kernel updates as often as they should, specially those cheap rented servers.

    On the other hand, there are servers which like to presume of having a high uptime. While that might look good, it is in fact, quite the opposite: a high uptime in a server means they might not have updated their server's software!

    So I will introduce kexec and a benchmark to show how it can reduce downtime by reducing reboot time. But first, let's look how a unix-like system boots and shutdowns.

    In a typical boot/shutdown action, this are (aproximatelly) the steps that will be made by the machine:

  • Boot
    1. BIOS stage
    2. Bootloader load
    3. Kernel load
    4. INIT
      1. Kernel init
      2. Hardware initialisation
      3. Checking and mounting partitions
    5. Start services
  • Shutdown
    1. Stop services
    2. Sync discs
    3. Unmount partitions
    4. Hardware stop
    5. Hardware power off
    By using kexec, some of those steps are skipped, since it will change kernel from a running system. These are (aproximatelly) the steps for kexec reboot:

  1. INIT
    1. Kernel init
    2. Checking and (re)mounting partitions
  2. Start services
    To prove that reboot time decreased I created a little bash script to measure downtime ( and tested in my personal server running a Gentoo system: 

    To use provided script, you must run it after apache have been stopped with:
time ./ SERVER_WWW_URI 2&>1 > /dev/null
    The commands I used for this benchmark are (via SSH):
Normal Reboot: /etc/init.d/apache2 stop && echo "Now you can exec time measurement script" && reboot
kexec reboot: kexec -l KERNELIMAGE --reuse-cmdline && /etc/init.d/apache2 stop && echo "Now you can exec time measurement script" && kexec -e

    These are the results I got:
Full reboot:
real    1m21.996s
user    0m3.241s
sys     0m2.833s
kexec reboot:
real    0m31.415s
user    0m1.872s
sys     0m1.684s
    So to sum up, despite it still takes time to perform kernel update, it is reduced significantly, so for most servers out there, now that is not an excuse to have system not updated anymore!