Load balancing techniques

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  1. Load Balancing Techniques

This article details various load balancing techniques applicable to a MediaWiki installation. Effective load balancing is crucial for high availability, scalability, and performance, especially for sites with significant traffic. This guide is aimed at system administrators and server engineers familiar with basic server administration concepts.

What is Load Balancing?

Load balancing distributes network or application traffic across multiple servers. This prevents any single server from being overwhelmed, ensuring consistent responsiveness and preventing service disruptions. For a MediaWiki instance, this means distributing user requests (reading pages, editing, searching) across multiple web servers and database servers.

Why Use Load Balancing with MediaWiki?

  • High Availability: If one server fails, the load balancer automatically redirects traffic to the remaining healthy servers.
  • Scalability: Easily add more servers to handle increased traffic without downtime.
  • Performance: Distributing the load reduces response times and improves the user experience.
  • Resource Optimization: Makes efficient use of server resources.
  • Maintenance: Allows for server maintenance (updates, reboots) without impacting users.

Types of Load Balancing

There are several different approaches to load balancing. The choice depends on your specific needs and infrastructure.

Hardware Load Balancers

These are dedicated physical appliances designed specifically for load balancing. They offer very high performance and reliability but are generally expensive. Examples include F5 Networks BIG-IP and Citrix ADC.

Software Load Balancers

These are software applications running on standard servers. They are more flexible and cost-effective than hardware load balancers. Common examples include:

  • HAProxy: A popular open-source load balancer known for its speed and reliability.
  • Nginx: Often used as a reverse proxy and load balancer.
  • Apache: Can be configured for load balancing, though less common than HAProxy or Nginx.
  • Keepalived: Provides VRRP (Virtual Router Redundancy Protocol) for high availability and load balancing.

Cloud Load Balancers

Cloud providers offer load balancing as a managed service. This simplifies setup and maintenance. Examples include:

  • Amazon Elastic Load Balancing (ELB): AWS's load balancing service.
  • Google Cloud Load Balancing: Google Cloud's load balancing offering.
  • Azure Load Balancer: Microsoft Azure's load balancing service.

Load Balancing Algorithms

The load balancing algorithm determines how traffic is distributed across the servers.

Algorithm Description Use Cases
Round Robin Distributes requests sequentially to each server. Simple, good for evenly distributing load when servers are identical.
Least Connections Sends requests to the server with the fewest active connections. Ideal when requests vary in processing time.
Least Response Time Sends requests to the server with the fastest response time. Best for optimizing overall performance. Requires monitoring response times.
IP Hash Uses the client's IP address to determine which server to use. Ensures that requests from the same client are consistently routed to the same server (session affinity).
Weighted Round Robin Assigns weights to each server, determining the proportion of requests it receives. Useful when servers have different capacities.

MediaWiki Specifics and Configuration

When setting up load balancing for MediaWiki, consider the following:

  • Session Management: MediaWiki stores session data. Ensure that session data is either:
   * Shared: Use a shared session store (e.g., Redis, Memcached) accessible by all web servers.  This is the recommended approach. See Configuration settings for more details on session handling.
   * Sticky Sessions: Configure the load balancer to use IP Hash to route requests from the same client to the same server.  This is less scalable than a shared session store.
  • Database Load Balancing: Consider using database replication and connection pooling to distribute the load on the database server. See Database setup
  • Cache Configuration: Ensure that all web servers have access to the same cache (e.g., Memcached, Redis). See Caching
  • $wgSessionCacheType: Configure this in LocalSettings.php to use a shared cache.
  • $wgMainCacheType: Configure this in LocalSettings.php to use a shared cache.

Example Configuration (HAProxy)

This is a simplified example of an HAProxy configuration for load balancing two MediaWiki web servers.

Section Configuration
Frontend

frontend http_frontend 

   bind *:80
   mode http
   default_backend http_backend

Backend

backend http_backend 

   mode http
   balance roundrobin
   server webserver1 192.168.1.100:80 check
   server webserver2 192.168.1.101:80 check

This configuration listens on port 80 and distributes traffic between `webserver1` and `webserver2` using the round robin algorithm. The `check` option enables health checks to ensure that only healthy servers receive traffic. Consult the HAProxy documentation for more advanced configuration options. See Help:HAProxy for more information.

Monitoring and Troubleshooting

Regular monitoring is essential to ensure that the load balancing setup is working correctly. Monitor:

  • Server load (CPU, memory, disk I/O)
  • Response times
  • Error rates
  • Load balancer status

Tools like Nagios, Zabbix, and Prometheus can be used for monitoring. Log analysis is crucial for troubleshooting issues. See Logging for details on MediaWiki logging. Also, consider using tools like `tcpdump` and `wireshark` for network analysis. Review the MediaWiki performance checklist often.

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