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.
## Load Balancer benefits
Additional benefits include:
- SSL termination: Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
- Removes the need to install [X.509 certificates](https://en.wikipedia.org/wiki/X.509) 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](https://github.com/donnemartin/system-design-primer#active-passive) or [active-active](https://github.com/donnemartin/system-design-primer#active-active) mode.
## Load Balancer route traffic
Load balancers can route traffic based on various metrics, including:
- Random
- Least loaded
- Seesion/cookies
- [Round robin or weighted round robin](http://g33kinfo.com/info/archives/2657)
Layer 4 load balancers look at info at the [transport layer](https://github.com/donnemartin/system-design-primer#communication) 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 [Network Address Translation (NAT) ](https://www.nginx.com/resources/glossary/layer-4-load-balancing/) .
## layer 7 load balancing
Layer 7 load balancers look at the [application layer](https://github.com/donnemartin/system-design-primer#communication) 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.
## Horizontal scaling
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.
## Disadvantage(s) : horizontal scaling
- 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](https://github.com/donnemartin/system-design-primer#database) (SQL, NoSQL) or a persistent [cache](https://github.com/donnemartin/system-design-primer#cache) (Redis, Memcached)
