Apache mod_proxy_balancer Load Balancing

Apache HTTPD provides powerful load balancing through the mod_proxy_balancer module. Define upstream groups with the balancer:// scheme and distribute requests using various algorithms (lbmethod).

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Enabling Required Modules

The following modules must be enabled for load balancing in Apache.

# Ubuntu/Debian
sudo a2enmod proxy
sudo a2enmod proxy_http
sudo a2enmod proxy_balancer
sudo a2enmod lbmethod_byrequests   # Round Robin
sudo a2enmod lbmethod_bytraffic    # Traffic-based
sudo a2enmod lbmethod_bybusyness   # Least connections
sudo a2enmod slotmem_shm           # Shared memory (required for balancer)
sudo systemctl restart apache2

# CentOS/RHEL (uncomment in httpd.conf)
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
LoadModule lbmethod_byrequests_module modules/mod_lbmethod_byrequests.so
LoadModule lbmethod_bytraffic_module modules/mod_lbmethod_bytraffic.so
LoadModule lbmethod_bybusyness_module modules/mod_lbmethod_bybusyness.so
LoadModule slotmem_shm_module modules/mod_slotmem_shm.so

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Basic Load Balancer Configuration

Simplest Setup (byrequests)

# /etc/apache2/sites-available/load-balancer.conf

<VirtualHost *:80>
    ServerName example.com

    ProxyRequests Off
    ProxyPreserveHost On

    # Define upstream group
    <Proxy "balancer://mycluster">
        BalancerMember "http://10.0.0.1:8080"
        BalancerMember "http://10.0.0.2:8080"
        BalancerMember "http://10.0.0.3:8080"

        ProxySet lbmethod=byrequests   # Round Robin
    </Proxy>

    # Forward all requests to the load balancer
    ProxyPass        "/" "balancer://mycluster/"
    ProxyPassReverse "/" "balancer://mycluster/"
</VirtualHost>

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lbmethod Algorithms

byrequests (Request Count Based, Round Robin)

Distributes based on the number of requests processed by each server, weighted by loadfactor. Behaves like Round Robin by default.

<Proxy "balancer://mycluster">
    BalancerMember "http://10.0.0.1:8080" loadfactor=3
    BalancerMember "http://10.0.0.2:8080" loadfactor=1
    ProxySet lbmethod=byrequests
</Proxy>

loadfactor works like Nginx's weight. App1 handles 75%, App2 handles 25%.

bytraffic (Traffic Byte Based)

Distributes based on processed traffic (bytes). Useful when response sizes vary widely, like file downloads.

<Proxy "balancer://mycluster">
    BalancerMember "http://10.0.0.1:8080"
    BalancerMember "http://10.0.0.2:8080"
    ProxySet lbmethod=bytraffic
</Proxy>

When 100MB file downloads and 1KB API responses are mixed, byrequests simply counts requests, while bytraffic accounts for actual data processed. Best for image/video serving servers.

bybusyness (Minimum Active Requests)

Routes to the server with the fewest currently active requests. Similar to Nginx's least_conn.

<Proxy "balancer://mycluster">
    BalancerMember "http://10.0.0.1:8080"
    BalancerMember "http://10.0.0.2:8080"
    BalancerMember "http://10.0.0.3:8080"
    ProxySet lbmethod=bybusyness
</Proxy>

Best for API servers or backends with heavy DB queries.

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Complete BalancerMember Parameter Reference

<Proxy "balancer://mycluster">
    BalancerMember "http://10.0.0.1:8080"
        loadfactor=3        # Weight (default: 1)
        retry=60            # Retry interval after failure (seconds), 0 = permanent exclusion
        timeout=30          # Connection timeout (seconds)
        acquire=3000        # Max connection wait time (ms)
        max=100             # Max concurrent connections
        ttl=60              # Idle connection lifetime (seconds)
        route=jvm1          # JVM route ID for mod_jk integration
        status=+H           # Force status (+H=Hot Standby, +D=Disabled, +S=Stop, +I=Ignore errors)

    BalancerMember "http://10.0.0.2:8080" loadfactor=1 status=+H
    # +H: Hot Standby — only used when all other servers are down

    ProxySet lbmethod=bybusyness
    ProxySet stickysession=JSESSIONID   # Session affinity
</Proxy>

Status Flag Reference

Flag	Meaning	When to Use
+D	Disabled	Block traffic during maintenance
+H	Hot Standby	Standby server (unused in normal operation)
+S	Stopped	Block new connections, keep existing ones
+I	Ignore errors	Ignore errors (exclude from health checks)
-D	Remove flag	Re-enable

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Sticky Session

Configure so the same user always connects to the same server for session-based applications.

Cookie-based Sticky Session (Recommended)

<Proxy "balancer://mycluster">
    BalancerMember "http://10.0.0.1:8080" route=server1
    BalancerMember "http://10.0.0.2:8080" route=server2
    BalancerMember "http://10.0.0.3:8080" route=server3

    ProxySet lbmethod=byrequests
    ProxySet stickysession=JSESSIONID  # Tomcat's default session cookie name
</Proxy>

Set jvmRoute in Tomcat's server.xml:

<!-- server.xml -->
<Engine name="Catalina" defaultHost="localhost" jvmRoute="server1">

Tomcat then appends .server1 to session IDs:

Set-Cookie: JSESSIONID=ABC123.server1; Path=/

Apache reads this cookie and routes to the server with route=server1.

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Balancer Manager (Web Management Interface)

Apache provides a management page to monitor load balancer status and change settings in real time from a browser.

<Location "/balancer-manager">
    SetHandler balancer-manager
    Require ip 127.0.0.1 10.0.0.0/8    # Allow internal network only
</Location>

# Enable and reload
sudo a2enconf balancer-manager
sudo systemctl reload apache2

# Access
curl http://localhost/balancer-manager

What you can do from the management page:

• View status, request count, and error count for each server
• Disable/enable servers (during maintenance)
• Change weights (loadfactor) in real time
• Reset session data

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Production Configuration: Complete Load Balancer Setup

# /etc/apache2/sites-available/production-lb.conf

<VirtualHost *:80>
    ServerName example.com
    ServerAlias www.example.com

    ProxyRequests Off
    ProxyPreserveHost On

    # Main app cluster
    <Proxy "balancer://appcluster">
        BalancerMember "http://10.0.0.1:8080" loadfactor=3 retry=5 route=jvm1
        BalancerMember "http://10.0.0.2:8080" loadfactor=3 retry=5 route=jvm2
        BalancerMember "http://10.0.0.3:8080" loadfactor=1 retry=5 route=jvm3 status=+H

        ProxySet lbmethod=bybusyness
        ProxySet stickysession=JSESSIONID
        ProxySet nofailover=Off
    </Proxy>

    # Static file cluster
    <Proxy "balancer://staticcluster">
        BalancerMember "http://10.0.1.1:80" loadfactor=1
        BalancerMember "http://10.0.1.2:80" loadfactor=1
        ProxySet lbmethod=byrequests
    </Proxy>

    # Static files: dedicated cluster
    ProxyPass        "/static/" "balancer://staticcluster/static/"
    ProxyPassReverse "/static/" "balancer://staticcluster/static/"

    # Admin page: single server (exclude from load balancing)
    ProxyPass        "/admin/" "http://10.0.0.1:8080/admin/"
    ProxyPassReverse "/admin/" "http://10.0.0.1:8080/admin/"

    # All other requests: app cluster
    ProxyPass        "/" "balancer://appcluster/"
    ProxyPassReverse "/" "balancer://appcluster/"

    ProxyTimeout 60

    # Balancer Manager (internal access only)
    <Location "/balancer-manager">
        SetHandler balancer-manager
        Require ip 127.0.0.1 10.0.0.0/8
    </Location>

    CustomLog /var/log/apache2/lb-access.log combined
    ErrorLog  /var/log/apache2/lb-error.log
</VirtualHost>

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Algorithm Selection Guide

Situation	Recommended lbmethod
Simple even distribution, identical specs	byrequests
File downloads, media streaming	bytraffic
API servers, variable processing times	bybusyness
Existing app without session sharing	byrequests + stickysession

The next page covers AJP-based load balancing with mod_jk.