What is a load balancer and what are the common load balancing algorithms?
Quick answer
A load balancer distributes incoming requests across several servers to improve capacity and availability, using algorithms such as round robin, least connections or consistent hashing.
Round robin sends each request to the next server in turn and works when requests cost about the same. Least connections sends the request to the server with the fewest active connections, which is better when request times vary. IP hash or consistent hashing sends the same client or key to the same server, which helps with caches and sticky sessions.
A load balancer also runs health checks and stops sending traffic to failed servers. Layer 4 balancers route by IP and port and are very fast, while layer 7 balancers understand HTTP and can route by path, host or header and terminate TLS. The load balancer itself must not be a single point of failure, so it is deployed redundantly.