Application Programming Interface (API) #

An API defines the contract between client and server.

The API specifies three (3) things:

1. The types of possible requests
2. The data that is needed to accompany the requests
3. The results that will be obtained.

On the web, HTTP is the de facto communication protocol that is used for data exchange.

Horizontal Scaling #

Horizontal scaling is the act of increasing processing capacity by adding new processing nodes in the network.

There are two (2) prerequisites that allow horizontal scaling to work:

1. Statelessness - Processing services must never have state so that requests can be sent to any replicas within the system.

2. Load Balancers - A service that distributes requests to processing services.

Load Balancing #

Load balancing is the process of distributing network requests over to a collection of services such that the collective capacity is ideally utilized.

Load Distribution Policies #

Load distribution policies dictate how the load balancer chooses a target service to process a request.

There are a number of policies available depending on the vendor.

The following are four (4) of the most commonly supported policies across all load balancers:

1. Round Robin - The load balancer distributes requests to available servers in a round-robin algorithm.
2. Least connections - The load balancer distributes new requests to the server with the least open connections.
3. HTTP Header Field - The load balancer directs requests based on the contents of a specific HTTP header field.
4. HTTP Operation - The load balancer directs requests based on the HTTP verb in the request.

Health Monitoring #

A load balancer monitors the health of its collective services by periodically pinging each service in the load balancing pool.

If a service becomes unresponsive or fails connection attempts, it will be removed from the load balancing pool.

Elasticity #

Is the capability of a system to dynamically provision new services to handle increased traffic loads.

Session Affinity #

Session affinity, or sticky sessions, are a load balancer feature for stateful services.

With sticky sessions, the load balancer sends all requests from the same client to the same service instance.

It is generally not a good idea to use sticky sessions as this will cause a load imbalance and complicates scalability.