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Apache HTTP ServerLoad-Balancing with Apache HTTPD 2.2 and later
Erik Abelewww.eatc.de
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About Me
• Working internationally as IT Consultant
• Areas: Administration & Operations
• Working on and with Open Source
• Apache Software Foundation
• Other projects (Ruby, Zenoss, ...)
• Living in South Germany
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Content
• Basics
• Strategies
• Examples
• Questions
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Basics
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Why Load-Balancing?
• Optimizing Resource Utilization
• Maximizing Quality of Service
• Increasing Reliability & Availability
• Improving Security
• Lowering Administration Risks
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Approaches
• DNS-Based Load-Balancing
• Round Robin
• Geographical Scheduling
• Layer 4 Load-Balancing
• Layer 7 Load-Balancing
• Hardware-/Software-Based Load-Balancers
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DNS-Based Balancing
• Clients resolveFQDN
• Response withseveral IPs orchoosenrandomly ore.g. accordingto proximity
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Layer 4 Balancing
• Transport Layer
• Deals with TCP connections
• Routes pure packets
• NAT with port and transaction awareness
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Layer 7 Balancing
• Application Layer
• Deals with HTTP requests
• Forwards whole requests
• Gateway / Reverse Proxy
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Layer 4-7 Balancing
• Combination: routingpackets but aware ofHTTP content
• Load-Balancer actsas a multilayer switch
• Basic schema alwaysthe same
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Hardware-Based LBs
• Mostly real routers/switches with ML capabilites
• Pros
• Very fast
• Cons
• Expensive
• Proprietary
• Examples: Cisco LoadDirector, F5 Big/ip, Barracuda
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Software-Based LBs
• Software-based appliances or just packages
• Pros
• Cheaper and also available as Open Source
• Simpler to configure and often more flexible
• Cons
• Not as fast as hardware based solutions
• Examples: Pen, Pound, Squid, Linux VS, Zeus ZXTM
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Balancer Features 1
• TCP Offloading
• TCP Buffering
• SSL Offloading & Acceleration
• HTTP Caching
• HTTP Compression
• Client Authentication
• Attack Protection & Firewalling
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Balancer Features 2
• Content-Aware Load-Balancing
• Request Rate Shaping
• Bandwidth Shaping
• Traffic Valuation & Prioritization
• Programmatic Traffic Manipulation
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Balancing Algorithms
• Random Choice
• Round Robin
• Weighted Round Robin
• Request counting algorithm
• Weighted traffic counting algorithm
• Based on other factors (load, response time, ...)
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Strategies
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Gateway
• “Reverse Proxy” - the frontend:
• Apache HTTPD with mod_proxy and mod_proxy_balancer as well as one of:
• mod_proxy_http
• mod_proxy_ajp
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Application Servers
• A pool of servers - the backend:
• Apache HTTP with PHP, Perl, ...
• Apache Tomcat, JBoss, Glassfish, ...
• Zope, CherryPy, Mongrel, WebRick
• ...
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Focus: Scalability• A single load-balancer in front of a pool of
application servers
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Persistence
• “Session Stickiness”
• A problem when not able to use a shared data store for the application (e.g. when using multiple physical locations where replication becomes problematic)
• Alternative: using cookies as data store
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Focus: Resilience• Two or more load-balancers in front of a pool of
application servers
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Failover
• Active/Active
• Both load-balancers are actively serving requests
• Active/Passive
• Only one load-balancer is actively serving requests, the other one takes over in case of failure
• There needs to be a way to detect failures
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Focus: Globalization• Several groups of load-balancers in more
than one physical location
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Location Selection
• DNS-Based
• Round Robin
• Based on geographical proximity
• Based on utilization of location
• FQDN-Based (e.g. wwwN.example.com)
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Examples
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HTTPD as Gateway
• Required Modules:
• mod_proxy
• mod_proxy_balancer
• mod_proxy_http / mod_proxy_ajp
• mod_proxy_ftp
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HTTP Backends
• Requires extension module:mod_proxy_http
• Usage:BalancerMember http://1.1.1.1:8080 ...
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AJP Backends
• Requires extension module:mod_proxy_ajp
• Usage:BalancerMember ajp://1.1.1.1: 8009 ...
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Basic Configuration<VirtualHost *:80> ServerName www.example.com ... ProxyRequests Off ProxyVia Off ProxyErrorOverride On ProxyPreserveHost On ProxyTimeout 30
ProxyPass / balancer://app_farm lbmethod=byrequests maxattempts=3 \ nofailover=Off stickysession=SID
<Proxy balancer://app_farm> BalancerMember http://1.1.1.1:8080 smax=15 max=50 lbfactor=2 BalancerMember http://1.1.1.2:8080 smax=15 max=50 lbfactor=2 BalancerMember http://1.1.1.3:8080 smax=10 max=25 lbfactor=1 </Proxy></VirtualHost>
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Connection Pooling
• minMinimum number of connections that will always be open to the backend server
• maxHard Maximum number of connections that will be allowed to the backend server
• smax & ttlSoft Maximum number of connections that will be created on demand, TTL for connect’s above smax
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Parameters 1
• lbmethodThe load-balancing scheduler method to use, e.g.
• weighted request counting
• weighted traffic counting
• lbsetAssigns a specific cluster set to members
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Parameters 2
• loadfactorDefines the normalized weighted load applied to this balancer member, e.g.
• loadfactor=1
• loadfactor=2 <- twice as much req’s / IO
• statusDefines the initial state of this member
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Hot-Standby
<VirtualHost *:80> ServerName www.example.com ...
ProxyPass / balancer://app_farm lbmethod=byrequests maxattempts=3 \ nofailover=Off stickysession=SID
<Proxy balancer://app_farm> BalancerMember http://1.1.1.1:8080 ... BalancerMember http://1.1.1.2:8080 ... BalancerMember http://1.1.1.3:8080 ... status=+H </Proxy></VirtualHost>
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Sticky Sessions
• Supported by stickysession flag:stickysession=PHPSESSIDstickysession=JSESSIONID|jsessionid
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Environment Variables
• Generated by mod_proxy
• X-Forwarded-For
• X-Forwarded-Host
• X-Forwarded-Server
• Influence mod_proxy
• SetEnv force-proxy-request-1.0 1
• SetEnv proxy-nokeepalive 1
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ProxyPassReverse
ProxyPass / balancer://app_farm ...
ProxyPassReverse / http://1.1.1.1:8080/ProxyPassReverse / http://1.1.1.2:8080/
<Proxy balancer://app_farm> BalancerMember http://1.1.1.1:8080 ... BalancerMember http://1.1.1.2:8080 ...</Proxy>
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URL RewritingProxyPass ~ ^/+(.*)$ balancer://app_farm/$1 ...
[or]
ProxyPassMatch ^/+(.*)$ balancer://app_farm/$1 ...
[or]
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/.balancerRewriteRule ^(.*)$ - [L]
RewriteRule ^/+(.*)$ balancer://app_farm/$1 [P,L]
ProxySet balancer://app_farm lbmethod=bytraffic
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Using Headers<VirtualHost *:443> ServerName www.example.com ... RequestHeader set Front-End-Https "On" ...
ProxyPass / balancer://app_farm lbmethod=byrequests maxattempts=3 \ nofailover=Off stickysession=SID
<Proxy balancer://app_farm> BalancerMember http://1.1.1.1:8080 smax=15 max=50 lbfactor=2 ... </Proxy></VirtualHost>
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Filtering Content
• mod_mime, e.g.AddOutputFilter INCLUDES .shtml
• mod_filter
• mod_proxy_html
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Using Compression
• We can use mod_deflate as usually:AddOutputFilterByType DEFLATE text/html
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Offloading SSL
• Simply configure SSL on the frontend server as you would do without a balancer
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Caching<VirtualHost *:80> ServerName www.example.com ... CacheEnable mem / CacheEnable disk / CacheDefaultExpire 1800 CacheMaxExpire 3600 ...
ProxyPass / balancer://app_farm lbmethod=byrequests maxattempts=3 \ nofailover=Off stickysession=SID
<Proxy balancer://app_farm> BalancerMember http://1.1.1.1:8080 smax=15 max=50 lbfactor=2 ... </Proxy></VirtualHost>
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Failover
• Automatically done
• if a member is in-error the next one is tried
• Can be switched off if backend servers do not support session replication
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Balancer Management
• Requires mod_status
• Allows for real-time monitoring
• Allows for dynamic updates of parameters of balancer members
• enable/disable balancer members
• specify balance factors, methods, sets
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Screenshot
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Example Configuration<Location /.balancer> SetHandler balancer-manager
AuthType Digest AuthName "Balancer Manager" AuthDigestDomain /.balancer AuthDigestProvider file AuthUserFile /www/etc/httpd/balancer.passwd
Require user operations</Location>
...
ProxyPass /.balancer !ProxyPass / balancer://app_farm ...
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Questions?