Load Balancing with Apache

Load Balancing with Apache Bradley Holt (http://bradley-holt.com/)@BradleyHolt (http://twitter.com/BradleyHolt)

About Me

Co-Founder andTechnical Director

Contributor

Author http://oreilly.com/catalog/9781449303129/ http://oreilly.com/catalog/9781449303433/

Apache HTTP Server

About ApacheOpen sourceServes over 100 million websitesCan run in many diﬀerent modes, depending on your needs,via MultiProcessing Modules (MPMs)The Apache Software Foundation (ASF) supports many otheropen source software projects

Alternatives

Software Load BalancersHAProxyVarnishPoundPerlbalSquidnginxLinux-HA (High-Availability Linux) on Linux Standard Base (LSB)

Hosted Load BalancersAmazon’s Elastic Load BalancingRackspace Cloud Load Balancers

Shared Nothing Architecture (SN)

Properties of a SN SystemEach node operates independentlyNo single point of contentionPHP is shared nothing by default…

Breaking SNSessions require sharing (or require the use of sticky sessions)Databases are the most common single point of contention(this is why eventual consistency is important)Storing variables in memory between requests breaks SN(possible in Java and .NET)

Practical SN TechniquesStore sessions in a memcached cluster—this makes web nodes SNFor database applications: • send writes to a write-only master • replicate to multiple read-only nodes • read-only nodes are now eﬀectively SNClustering/partitioning/sharding can help, too (but painful).Alternatively, use a “NoSQL” database (e.g. CouchDB is SN).

Load Balancing Examples

Required Modulesmod_proxymod_proxy_http (assuming you’re load balancing HTTP requests)mod_proxy_balancermod_headers (for sticky sessions)mod_rewrite (for advanced con gurations)

Basic Load Balancing

# Create a load balancer named "web-nodes"<Proxy balancer://web-nodes> # Add three load balancer members BalancerMember http://www1.example.com BalancerMember http://www2.example.com BalancerMember http://www3.example.com</Proxy># Send all requests to the "web-nodes" balancerProxyPass / balancer://web-nodes

Apache allows traﬃc to bebalanced by number of requests(lbmethod=byrequests), bytestransferred (lbmethod=bytraffic),or by the number of currently pendingrequests (lbmethod=bybusyness).

Sticky Sessions in PHP(your mileage may vary)

# Create a load balancer named "web-nodes"<Proxy balancer://web-nodes> # Add three load balancer members BalancerMember http://www1.example.com BalancerMember http://www2.example.com BalancerMember http://www3.example.com # Use the PHPSESSID for sticky sessions ProxySet stickysession=PHPSESSID</Proxy># Send all requests to the "web-nodes" balancerProxyPass / balancer://web-nodes

Create Your Own Sticky Sessions

# Set a cookieHeader add Set-Cookie "NODE=%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED# Create a load balancer named "web-nodes"<Proxy balancer://web-nodes> # Add three load balancer members BalancerMember http://www1.example.com route=1 BalancerMember http://www2.example.com route=2 BalancerMember http://www3.example.com route=3 # Use the NODE cookie for sticky sessions ProxySet stickysession=NODE</Proxy># Send all requests to the "web-nodes" balancerProxyPass / balancer://web-nodes

Use a private network to connect yourload balancer to your balancermembers.This allows for dedicated bandwidthand opens up the possibility ofoﬄoading SSL handling to the loadbalancer.

Route Based on HTTP Method

# Enable mod_rewriteRewriteEngine On# Send POST, PUT, and DELETEs to "write" balancerRewriteCond %{REQUEST_METHOD} ^(POST|PUT|DELETE)$RewriteRule ^/(.*)$ balancer://write$1 [P]# Send GET, HEAD, and OPTIONS to "read" balancerRewriteCond %{REQUEST_METHOD} ^(GET|HEAD|OPTIONS)$RewriteRule ^/(.*)$ balancer://read$1 [P]# Modify HTTP response headers (e.g. Location)ProxyPassReverse / balancer://writeProxyPassReverse / balancer://read

Consider con guring multiple loadbalancers, removing the load balanceras a single point of failure.This typically involves having two ormore load balancers sharing the sameIP address, with one con gured as afailover.

Distributed Load Testing with Tsung

TsungDistributes load testing across multiple testing clientsCan generate huge numbers of concurrent usersMonitors CPU, memory, load, and network traﬃcSimulates dynamic sessions, as described in a con guration leRandomizes traﬃc patterns based on de ned probabilitiesRecording and playback of sessionsHTML reports and graphs

XML Con guration File

<?xml version="1.0"?><!DOCTYPE tsung SYSTEM "/usr/share/tsung/tsung-1.0.dtd"><tsung loglevel="notice" version="1.0"> </tsung>

Client Side Setup

<clients> <client host="test-a" weight="1" maxusers="10000" cpu="4" /> <client host="test-b" weight="1" maxusers="10000" cpu="4" /></clients>

Server Side Setup

<servers> <server host="www" port="80" type="tcp" /></servers>

Load Setup

<load> <arrivalphase phase="1" duration="5" unit="minute" > <users arrivalrate="200" unit="second" /> </arrivalphase></load>

Session Setup

<session name="default" probability="100" type="ts_http"> <thinktime value="1" random="true" /> <request> <http method="GET" url="/" /> </request></session>

Monitoring Setup

Tsung allows for monitoringusing Erlang, SNMP, or Munin

<monitoring> <monitor host="www" type="munin" /> <monitor host="www1" type="munin" /> <monitor host="www2" type="munin" /> <monitor host="www3" type="munin" /></monitoring>

Reports

Graphs

Questions?

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Load Balancing with Apache

by Bradley Holt

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