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Introduction to Consul
Welcome to the intro guide to Consul! This guide is a the best place to start with Consul. We cover what Consul is, what problems it can solve, how it compares to existing software, and a quick start for using Consul. If you are already familiar with the basics of Consul, the documentation provides more of a reference for all available features.
What is Consul?
Consul has multiple components, but as a whole, it is tool for discovering and configuring services in your infrastructure. It provides several key features:
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Service Discovery: Clients of Consul can provide a service, such as
apiormysql, and other clients can use Consul to discover providers of a given service. Using either DNS or HTTP, applications can easily find the services they depend upon. -
Health Checking: Consul clients can provide any number of health checks, either associated with a given service ("is the webserver returning 200 OK"), or with the local node ("is memory utilization below 90%"). This information can be used by an operator to monitor cluster health, and it is used by the service discovery components to route traffic away from unhealthy hosts.
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Key/Value Store: Applications can make use of Consul's hierarchical key/value store for any number of purposes including dynamic configuration, feature flagging, coordination, leader election, etc. The simple HTTP API makes it easy to use.
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Multi Datacenter: Consul supports multiple datacenters out of the box. This means users of Consul do not have to worry about building additional layers of abstraction to grow to multiple regions.
Basic Architecture of Consul
Consul is a distributed, highly available system. There is an in-depth architecture overview available, but this section will cover the basics so you can get an understanding of how Consul works. This section will purposely omit details to quickly provide an overview the architecture.
Every node that provides services to Consul runs a Consul agent. Running an agent is not required for discovering other services or getting/setting key/value data. The agent is responsible for health checking the services on the node as well as the node itself.
The agents talk to one or more Consul servers. The Consul servers are where data is stored and replicated. The servers themselves elect a leader. While Consul can function with one server, 3 to 5 is recommended to avoid data loss scenarios. A cluster of Consul servers is recommended for each datacenter.
Components of your infrastructure that need to discover other services or nodes can query any of the Consul servers or any of the Consul agents. The agents forward queries to the servers automatically.
Each datacenter runs a cluster of Consul servers. When a cross-datacenter service discovery or configuration request is made, the local Consul servers forward the request to the remote datacenter and return the result.
Next Steps
See the page on how Consul compares to other software to see just how it fits into your existing infrastructure. Or continue onwards with the getting started guide to get Consul up and running and see how it works.