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Load Balancer

What is a Load Balancer?

A load balancer is a device or software that distributes network or application traffic across multiple servers. Load balancers are used to increase the capacity and reliability of applications by sending requests only to servers that are online and have available capacity.

How Load Balancers Work

  1. Traffic Distribution: As requests come in to a website or application, the load balancer uses algorithms to determine which server should handle each request. Common methods include:

    • Round Robin: Requests are distributed sequentially across the server group.
    • Least Connections: New requests are sent to the server with the fewest current connections.
    • Hashing: A hash of the source IP is used to determine which server receives the request.

  2. Health Checking: The load balancer regularly checks the health of each server by sending pings, connecting to key ports, or requesting a specific application page. Non-responsive servers are taken out of rotation.

  3. Session Persistence: For applications that require a user’s requests to go to the same server for the duration of a session, the load balancer uses cookies or other methods to ensure requests are routed properly.

Benefits of Load Balancing

  • Scalability: As demand increases, new servers can easily be added to the resource pool.
  • Redundancy: If one server goes down, the load balancer will send requests to the remaining online servers.
  • Performance: By optimizing the load on each server, load balancers can reduce response times.
  • Flexibility: Load balancers make it easy to perform server maintenance by allowing you to take servers offline without impacting overall availability.

Types of Load Balancers

  • Hardware Load Balancers: Dedicated appliances designed for high performance and reliability. Vendors include F5, Citrix, and Cisco.
  • Software Load Balancers: Load balancing capabilities deployed as software on standard servers. NGINX and HAProxy are popular open source options.
  • Cloud Load Balancers: Managed load balancing services offered by cloud providers like AWS Elastic Load Balancing, Azure Load Balancer, and Google Cloud Load Balancing.

Key Takeaways

  • Load balancers distribute network traffic across multiple servers to improve performance, scalability, and reliability
  • They use algorithms like round robin, least connections, and hashing to determine which server should handle each request
  • Load balancers perform regular health checks to ensure traffic is only sent to online, available servers
  • Session persistence ensures a user’s requests are routed to the same server for the duration of their session
  • Load balancers are available as hardware appliances, software solutions, and managed cloud services

FAQs

What is the difference between Layer 4 and Layer 7 load balancing? Layer 4 load balancing operates at the transport layer and distributes traffic based on IP address and port. Layer 7 load balancing operates at the application layer and can make routing decisions based on the content of the HTTP header, cookies, and application data.
How does a load balancer handle SSL/TLS traffic? Load balancers can be configured to decrypt incoming SSL/TLS traffic, distribute the decrypted requests to the appropriate servers, and then re-encrypt the server responses before sending them back to the client. This is known as SSL termination and offloads the decryption/encryption work from the servers.
Can a load balancer also function as a reverse proxy? Yes, many load balancers can also function as reverse proxies. In addition to load balancing, they can perform tasks such as SSL termination, compression, caching, and request routing based on URL or other application-specific criteria.
Last updated on August 10, 2024
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Content Delivery Network (CDN)