Consul - Introduction

Consul is a Hashicorp based tool for discovering and configuring a variety of different services in your infrastructure. It is based and built on Golang. One of the core reasons to build Consul was to maintain the services present in the distributed systems. Some of the significant features that Consul provides are as follows.

• Service Discovery − Using either DNS or HTTP, applications can easily find the services they depend upon.

• Health Check Status − It can provide any number of health checks. It is used by the service discovery components to route traffic away from unhealthy hosts.

• Key/Value Store − It can make use of Consul's hierarchical key/value store for any number of purposes, including dynamic configuration, feature flagging, coordination, leader election, etc.

• Multi Datacenter Deployment − Consul supports multiple datacenters. It is used for building additional layers of abstraction to grow to multiple regions.

• Web UI − Consul provides its users a beautiful web interface using which it can be easy to use and manage all of the features in consul.

Service Discovery

Service discovery is one of the most important feature of Consul. It is defined as the detection of different services and network protocols using which a service is found. The usage of service discovery comes in as a boon for distributed systems. This is one of the main problems, which are faced by today's large-scale industries with the advancement of distributed systems in their environment.

Comparison with Etcd and Zookeeper

When we look at other service discovery tools in this domain, we have two popular options. Some major players in the software industry have been using it in the past. These tools are Etcd and Zookeeper.

Let us consider the following table for comparing different aspects of each tool. We will also understand what each one of them uses internally.

Properties	Consul	Etcd	Zoo Keeper
User Interface	Available
RPC	Available	Available
Health Check	HTTP API	HTTP API	TCP
Key Value	3 Consistency modes	Good Consistency	Strong Consistency
Token System	Available
Language	Golang	Golang	Java

Consul - Members and Agents

Consul members can be defined as the list of different agents and server modes using which a consul cluster is deployed. Consul provides us with a command line feature using which we can easily list all the agents associated with consul.

Consul agent is the core process of Consul. The agent maintains membership information, registers services, runs checks, responds to queries, etc. Any agent can be run in one of two modes: Client or Server. These two modes can be used according to their role as decided when using consul. The consul agent helps by providing us information, which is listed below.

• Node name − This is the hostname of the machine.

• Datacenter − The datacenter in which the agent is configured to run. Each node must be configured to report to its datacenter.

• Server − It indicates whether the agent is running in server or client mode. Server nodes participates in the consensus quorum, storing cluster state and handling queries.

• Client Addr − It is the address used for client interfaces by the agent. It includes the ports for the HTTP, DNS, and RPC interfaces.

• Cluster Addr − It is the address and the set of ports used for communication between Consul Agents in a cluster. This address must be reachable by all other nodes.

In the next chapter, we will understand the architecture for Consul.