In this last years a lot of high traffic web sites have been built in PHP. One of the main problem to design a distributed PHP architecture is how to share session data between multiple servers. In this presentation we showed the most used solutions to scale a PHP application along multiple servers. We presented different solutions to share session data using open source solutions (nfs, databases, memcached, redis, etc). Moreover we talk about Zend Server Cluster Manager, an enterprise­ ready Web Application Server for running and managing an HA Cluster of PHP servers.

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How to scale PHP applications
by Jan Burkl, Enrico Zimuel
Zend Technologies
http://www.zend.com
jan.burkl@zend.com, enrico@zend.com
30th
October 2010 – PHP Barcelona Conference

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About us
● Jan Burkl (jan.burkl@zend.com)
● Senior System Engineer at Zend
Technologies in Stuttgart (Germany)
since 2006
● Enrico Zimuel (enrico@zend.com)
● Senior Consultant & Architect at Zend
Technologies in Milan (Italy) since 2008
● Blog on web dev't: http://www.zimuel.it/blog

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Summary
● Scalability of a web application
● How to scale a PHP application
● PHP session management
● Sharing session data using:
▶ Network file system
▶ Database
▶ Memcached
▶ Redis
▶ Zend Server Cluster Manager

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Scalability of a web
application

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Scalability: general definition
“Scalability is a desirable property of a
system, a network, or a process, which
indicates its ability to either handle
growing amounts of work in a graceful
manner or to be enlarged”
Source: Wikipedia

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Scalability of a web application
A web application is scalable when is
able to manage a growing traffic with
additional resources (CPU, RAM)
without software changes

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Scale vertically vs. Scale horizontally
● Scale vertically (scale up)
▶ Add resources to a single node in a system
▶ Enhance the server (more CPU, more RAM, etc)
▶ High availability difficult to implement
● Scale horizontally (scale out)
▶ Add mores nodes to a system
▶ More servers, distributing the load
▶ High availability easy to implement

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Scale up vs. Scale out
Scale up Scale out
vs.

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The web scale out
● As Google taught, the best way to scale an high traffic
web application is horizontally
● No expensive servers to scale horizontally
● We need load balancers to split the traffic
between all the servers
A Google server

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A typical load balancer architecture
Load Balancer
Web Servers
Internet
Firewall

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How to scale a PHP application?
● The PHP application uses session/local data?
▶ Yes = we have to manage the session/local
data across multiple servers
● Using a persistent load balancer
● Share the session data
▶ No = we can scale very easy (stateless)
● Most of the PHP applications are not stateless,
they use session data

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Persistent load balancer
● A client request is assigned always to the same
server (means same session/local data)
● Pros:
▶ No software architecture changes
● Cons:
▶ No fault tolerant, if a server goes down the
session data are lost!
▶ The load balancer is the bottleneck
▶ The persistent load balancer are expensive

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Share the session data
● Manage the session data across the servers
● Pros:
▶ Fault tolerant, if a server goes down the load
balancer can user another server
▶ No bottleneck in the load balancer
▶ The stateless load balancer are cheaper
● Cons:
▶ Software architecture changes

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PHP session management

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Sessions in PHP
● Session support in PHP consists of a way to preserve certain
data across subsequent accesses
● To identify the subsequent accesses, from the same client,
PHP uses a cookie variable (PHPSESSID)
▶ Example: PHPSESSID= tclq3an1ri8dsfiuo43845loo1
● By default, session data are stored in the file system of the
server
● In PHP we manage the session data using the $_SESSION
global variable

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PHP session management
● Configure the PHP session management (php.ini directives):
▶ session.name
● name of the session cookie identifier
(PHPSESSID by default)
▶ session.save_handler
● defines the name of the handler which is used
for storing and retrieving data associated with
a session (files by default).
▶ session.save_path
● defines the argument which is passed to the
save handler (with files handler is the path to
store the session data)

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Example (PHP sessions using files)
● <?php
session_start();
$_SESSION['user']= 'enrico';
● In session folder (for instance, /tmp) the PHP creates a file
named sess_fvi9r84f14sjel8r28o6aqspr2 (where
fvi9r84f14sjel8r28o6aqspr2 is PHPSESSID) that contains:
user|s:6:"enrico";

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Share session data
● How to share PHP sessions between multiple
servers?
▶ Using a Network File System
▶ Using a Database
▶ Using Memcached
▶ Using Redis
▶ Using Zend Server Cluster Manager
▶ etc

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session_set_save_handler()
● You can write your PHP session handler using
the session_set_save_handler():
bool session_set_save_handler (
callback $open,
callback $close,
callback $read,
callback $write,
callback $destroy,
callback $gc)
● More info: http://php.net/manual/en/function.session-set-save-handler.php

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Session sharing
using NFS

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Session sharing using NFS
● Use the default PHP session handler
(session.session_handler= files)
● NFS to store the session data files
(session.save_path= NFS folder)
● Pros:
▶ No changes on the PHP side
● Cons:
▶ Highly inefficient
▶ Not designed for high read/write ratio
▶ Performance problems and data corruptions

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Session sharing
using Database

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Session sharing using DB
● Use the user PHP session handler
(session.session_handler= user)
● Use the session_set_save_handler() to implement
your db session handler
● Which db?
▶ MySQL
http://www.php.net/manual/en/function.session-set-save-handler.php#81761
▶ Ms SQL Server
● http://www.zimuel.it/blog/?p=402
▶ PostgreSQL, Oracle, etc

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Session sharing using DB (2)
● Pros:
▶ Solves the scalability limitation
▶ A lot of best practices available
▶ Wide installation base
▶ DB is (normally) available
● Cons:
▶ Sessions have almost 1:1 read/write ratio
▶ Connection overhead
▶ Single point of failure
▶ Performance bottleneck

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Session sharing
using Memcached

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Session sharing using Memcached
● Use the memcached PHP session handler
(session.session_handler= memcache)
● Pros:
▶ Native session handler
▶ Very fast (works in RAM)
▶ Can be clustered
● Cons:
▶ No persistent data (it's a caching system)
▶ Cyclic memory (data can be overwritten if the
memory is full)

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Session sharing
using Redis

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What is Redis?
● Redis is an open-source,
networked, in-memory, persistent,
journaled, key-value data store
(NoSQL).
● It's similar to memcached but the
dataset is not volatile
● Developed by: Salvatore Sanfilippo
● Sponsored by vmware
● http://code.google.com/p/redis/

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Session sharing using Redis
● Different PHP extensions for Redis:
▶ Predis, http://github.com/nrk/predis/
▶ Rediska, http://rediska.geometria-lab.net/
▶ redis.php, http://github.com/antirez/redisdotphp
▶ PHPRedis!, http://github.com/owlient/phpredis
● Custom session handler for Redis:
▶ http://github.com/ivanstojic/redis-session-php

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Session sharing using Redis (2)
● Pros:
▶ Fast (works in RAM)
▶ Reliable (master-slave replication)
● Cons:
▶ PHP Redis extensions are 0.x version
▶ Custom PHP session handler
▶ Not so scalable* (data are stored in RAM)
▶ No High Availability* (master-slave limitation)
* but Redis developers are working on a Cluster version!

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Session sharing
using Zend Server
Cluster Manager

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What is Zend Server and Cluster Manager?
● Zend Server is a complete, enterprise-ready
Web Application Server for running and
managing PHP applications that require a
high level of reliability, performance and
security on Linux, Windows or IBM i.
● Zend Server Cluster Manager (ZSCM)
extends the benefits of Zend Server across
large-scale PHP deployments.
● With ZSCM you can build a real PHP cluster
stack.

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Session sharing with ZSCM
● Session Clustering Extension
● Session Clustering Daemon (SCD)
● Storage backends: Disk / Memory
● Peer-to-peer protocol to communicate
between nodes
● Session management
with master-backup
architecture

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Zend Server Cluster Manager architecture
Load Balancer
MySQL
Firewall

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High availability: session cluster

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High availability: session cluster (2)

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Session cluster with ZSCM
● Pros:
▶ Very Fast
▶ Reliable (data are stored in master-backup
servers)
▶ High Availability
▶ Scalable (session data are stored in all the
nodes of the cluster)
▶ No changes to the PHP code
● Cons:
▶ You need Zend Server on each node of the
cluster

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Questions?

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Thank you for attending!
More info:
http://www.zend.com
http://phpconference.es