Load balancer
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Load balancers distribute incoming client requests to computing resources such as application servers and databases. In each case, the load balancer returns the response from the computing resource to the appropriate client. Load balancers are effective at:
Preventing requests from going to unhealthy servers
Preventing overloading resources
Helping eliminate single points of failure
Load balancers can be implemented with hardware (expensive) or with software such as HAProxy.
Additional benefits include:
SSL termination - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations.
eg. Removes the need to install X.509 certificates on each server
Session persistence - Issue cookies and route a specific client's requests to same instance if the web apps do not keep track of sessions
To protect against failures, it's common to set up multiple load balancers, either in active-passive or active-active mode.
Load balancers can route traffic based on various metrics, including:
Random
Least loaded
Session/cookies
Layer 4
Layer 7
When the Layer 4 load balancer receives a request and makes the load balancing decision, it also performs Network Address Translation (NAT) on the request packet, changing the recorded destination IP address from its own to that of the content server it has chosen on the internal network. Similarly, before forwarding server responses to clients, the load balancer changes the source address recorded in the packet header from the server’s IP address to its own. (The destination and source TCP port numbers recorded in the packets are sometimes also changed in a similar way.)
Layer 7 load balancers look at the application layer to decide how to distribute requests. This can involve contents of the header, message, and cookies. Layer 7 load balancers terminates 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.
Load balancers can also help with horizontal scaling, improving performance and availability. Scaling out using commodity machines is more cost efficient and results in higher availability than scaling up a single server on more expensive hardware, called Vertical Scaling. It is also easier to hire for talent working on commodity hardware than it is for specialized enterprise systems.
Scaling horizontally introduces complexity and involves cloning servers
Servers should be stateless: they should not contain any user-related data like sessions or profile pictures
Sessions can be stored in a centralized data store such as a database (SQL, NoSQL) or a persistent cache (Redis, Memcached)
Downstream servers such as caches and databases need to handle more simultaneous connections as upstream servers scale out
The load balancer can become a performance bottleneck if it does not have enough resources or if it is not configured properly.
Introducing a load balancer to help eliminate single points of failure results in increased complexity.
A single load balancer is a single point of failure, configuring multiple load balancers further increases complexity.
Layer 4 load balancers look at info at the transport layer 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 .
