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A Quick Introduction to Consul

For reasons that (hopefully) will become clear in the relatively near future, I decided I needed to take a look at Consul, a distributed service discovery tool and key value store. I know Consul’s description sounds like a mouthful of buzzwords, but it’s pretty accurate. This post provides a quick introduction to Consul, in which I’ll break down what Consul does and how it works (at a high level). I’ll then build on this introduction in later posts.

There’s a lot to Consul, so let’s start by breaking down the description of Consul, which I provided as “a distributed service discovery tool and key value store”. What does this mean, exactly?

  • Consul is distributed. This means it runs as a cluster of systems so that there is no single point of failure. Consul uses a gossip protocol (known as Serf) to manage cluster membership, failure detection, and general orchestration. Managing cluster state via Serf is only part of the picture, though; the cluster must also manage consistency via a consensus protocol known as Raft. (Raft is the same consensus protocol used by etcd, for example.)
  • Consul is also a service discovery tool. Applications can register with Consul to provide a service, such as HTTP or MySQL. Other systems can then use Consul, either via HTTP or via DNS, to discover the providers of a given service. Part of this service discovery functionality includes some health checking functionality, to ensure that the provider of a service is actually working as expected.
  • Finally, Consul is a key/value store. In this respect, you could make a comparison between Consul and etcd, the distributed key/value store bundled with CoreOS Linux. Both Consul and etcd provide HTTP APIs to both store and retrieve key/value data in their distributed key/value store. (If you’re unfamiliar with etcd, have a look at this article I wrote.)

Installing Consul

Installing Consul is pretty easy, actually:

  1. Download the Consul software (binaries are provided for 32-bit Linux, 64-bit Linux, Mac OS X, and Windows).

  2. Unzip the downloaded file and save to a location of your choice.

That’s it.

Running Consul

Running Consul, on the other hand, is a bit more challenging. This is not a reflection on Consul, but rather a reflection of the complexity that is inherent in distributed systems. The single Consul binary you downloaded above can run either as a server (which actively participates in the consensus protocol) or as a client (which forwards requests to a server). If you are running Consul as a server, you’ll need to follow a set of instructions to bring up the Consul servers as a functioning cluster (see the Consul docs for an explanation of how this is done).

If, on the other hand, you’re running Consul as a client to connect to an existing Consul cluster, not too much additional is required:

  • If you’re running Consul (as a client) on a system where Consul is already running (as a server/daemon), you can just run consul directly. For example, if you wanted to see a list of the members of the Consul cluster, you could just run consul members.
  • If you running Consul (as a client) on a system that is not part of the Consul cluster, then you’ll need to tell Consul where to find the cluster by adding the -rpc-addr parameter. For example, if a host with IP address was part of the Consul cluster, you’d run consul members -rpc-addr= (8400 is the default port for Consul to listen for RPCs) to see a list of the members in the cluster.

Interacting with Consul

The consul command is really only for interacting with the cluster itself, such as to execute a command on the Consul cluster nodes, or to reload a Consul node’s configuration. Applications would interact with Consul either via service definition files, via the HTTP API, or via DNS.

  • Service definition files are JSON files that tell Consul what services are available and where. (Not familiar with JSON? See here.)
  • The HTTP API allows applications to register themselves, or to query Consul for information about a service (or for a list of services). For example, if you want to get a list of all the services about which Consul was aware, you’d just run curl http://<IP address of Consul cluster node>:8500/v1/catalog/services (obviously substituting a valid IP address in that command). Optionally, you might want to pipe it through python -m json.tool to “prettify” the output. From that list of services, you could then get more information by running curl http://<IP address of Consul cluster node>:8500/v1/catalog/service/<service name>. The HTTP API is pretty well-documented here.
  • Users and applications can also use DNS to discover services. To see this in action for yourself, just run dig @<IP address of Consul cluster node> <service name>.service.consul and Consul will happily return DNS information about the hosts/nodes that provide that service. This includes support for DNS SRV records as well, which means Consul can report on the port being used by a particular instance of a service (which might be handy in environments where ports are dynamically assigned…like say environments running Docker containers).

<aside>In my opinion, the DNS support is huge. To make it even more powerful, you can configure your DNS servers to forward requests to Consul to make service discovery even more seamless.</aside>

Now, if you’re wondering how you might put this to use in your environment, I’ll simply provide one word: Docker. In a distributed micro-service architecture, services may pop up on any node on a dynamically assigned port. Using Consul, you could provide a mechanism for discovering these distributed service instances, and you could provide it over DNS, a service that most applications are using today anyway. That’s pretty powerful, and that’s even before we get into stuff like Consul’s health checks and such.

So that’s a quick introduction to Consul. Have I whetted your appetite for more? I hope so, because I’ll be building on this introduction soon. Stay tuned.

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