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  1. System Design

Load Balancer

PreviousTCP vs UDPNextCache

Last updated 4 years ago

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Load balancers can route traffic based on various metrics, including:

  • Random

  • Least loaded

  • Session/cookies

Layer 4 load balancing

Layer 4 load balancers look at info at the to decide how to distribute requests. Generally, this involves the source, destination IP addresses, and ports in the header, but not the contents of the packet. Layer 4 load balancers forward network packets to and from the upstream server, performing .

Layer 7 load balancing

Layer 7 load balancers look at the to decide how to distribute requests. This can involve contents of the header, message, and cookies. Layer 7 load balancers terminate network traffic, reads the message, makes a load-balancing decision, then opens a connection to the selected server. For example, a layer 7 load balancer can direct video traffic to servers that host videos while directing more sensitive user billing traffic to security-hardened servers.

At the cost of flexibility, layer 4 load balancing requires less time and computing resources than Layer 7, although the performance impact can be minimal on modern commodity hardware.

Round robin or weighted round robin
Layer 4
Layer 7
transport layer
Network Address Translation (NAT)
application layer