Consul with the Autopilot Pattern

Consul in Docker, designed to be self-operating according to the autopilot pattern. This application demonstrates support for configuring the Consul raft so it can be used as a highly-available discovery catalog for other applications using the Autopilot pattern.

Using Consul with ContainerPilot

This design starts up all Consul instances with the -bootstrap-expect flag. This option tells Consul how many nodes we expect and automatically bootstraps when that many servers are available. We still need to tell Consul how to find the other nodes, and this is where Triton Container Name Service (CNS) and ContainerPilot come into play.

The ContainerPilot configuration has a management script that is run on each health check interval. This management script runs consul info and gets the number of peers for this node. If the number of peers is not equal to 2 (there are 3 nodes in total but a node isn't its own peer), then the script will attempt to find another node via consul join. This command will use the Triton CNS name for the Consul service. Because each node is automatically added to the A Record for the CNS name when it starts, the nodes will all eventually find at least one other node and bootstrap the Consul raft.

When run locally for testing, we don't have access to Triton CNS. The local-compose.yml file uses the v2 Compose API, which automatically creates a user-defined network and allows us to use Docker DNS for the service.

Run it!

1. Get a Joyent account and add your SSH key.
2. Install the Docker Toolbox (including docker and docker-compose) on your laptop or other environment, as well as the Joyent Triton CLI (triton replaces our old sdc-* CLI tools).

Check that everything is configured correctly by running ./setup.sh. This will check that your environment is setup correctly and will create an _env file that includes injecting an environment variable for a service name for Consul in Triton CNS. We'll use this CNS name to bootstrap the cluster.

$ docker-compose up -d
Creating consul_consul_1

$ docker-compose scale consul=3
Creating and starting consul_consul_2 ...
Creating and starting consul_consul_3 ...

$ docker-compose ps
Name                        Command                 State       Ports
--------------------------------------------------------------------------------
consul_consul_1   /usr/local/bin/containerpilot...   Up   53/tcp, 53/udp,
                                                          8300/tcp, 8301/tcp,
                                                          8301/udp, 8302/tcp,
                                                          8302/udp, 8400/tcp,
                                                          0.0.0.0:8500->8500/tcp
consul_consul_2   /usr/local/bin/containerpilot...   Up   53/tcp, 53/udp,
                                                          8300/tcp, 8301/tcp,
                                                          8301/udp, 8302/tcp,
                                                          8302/udp, 8400/tcp,
                                                          0.0.0.0:8500->8500/tcp
consul_consul_3   /usr/local/bin/containerpilot...   Up   53/tcp, 53/udp,
                                                          8300/tcp, 8301/tcp,
                                                          8301/udp, 8302/tcp,
                                                          8302/udp, 8400/tcp,
                                                          0.0.0.0:8500->8500/tcp

$ docker exec -it consul_consul_3 consul info | grep num_peers
    num_peers = 2

Using this in your own composition

There are two ways to run Consul and both come into play when deploying ContainerPilot, a cluster of Consul servers and individual Consul client agents.

Servers

The Consul container created by this project provides a scalable Consul cluster. Use this cluster with any project that requires Consul and not just ContainerPilot/Autopilot applications.

The following Consul service definition can be dropped into any Docker Compose file to run a Consul cluster alongside other application containers.

version: '2.1'

services:

  consul:
    image: autopilotpattern/consul:1.0.0r43
    restart: always
    mem_limit: 128m
    ports:
      - 8500
    environment:
      - CONSUL=consul
      - LOG_LEVEL=info
    command: >
      /usr/local/bin/containerpilot

In our experience, including a Consul cluster within a project's docker-compose.yml can help developers understand and test how a service should be discovered and registered within a wider infrastructure context.

Environment Variables

• CONSUL_DEV: Enable development mode, allowing a node to self-elect as a cluster leader. Consul flag: -dev.
  • The following errors will occur if CONSUL_DEV is omitted and not enough Consul instances are deployed:

  [ERR] agent: failed to sync remote state: No cluster leader
  [ERR] agent: failed to sync changes: No cluster leader
  [ERR] agent: Coordinate update error: No cluster leader
  

• CONSUL_DATACENTER_NAME: Explicitly set the name of the data center in which Consul is running. Consul flag: -datacenter.
  • If this variable is specified it will be used as-is.
  • If not specified, automatic detection of the datacenter will be attempted. See issue #23 for more details.
  • Consul's default of "dc1" will be used if none of the above apply.

Clients

ContainerPilot utilizes Consul's HTTP Agent API for a handful of endpoints, such as UpdateTTL, CheckRegister, ServiceRegister and ServiceDeregister. Connecting ContainerPilot to Consul can be achieved by running Consul as a client to a cluster (mentioned above). It's easy to run this Consul client agent from ContainerPilot itself.

The following snippet demonstrates how to achieve running Consul inside a container and connecting it to ContainerPilot by configuring a consul-agent job.

{
  consul: 'localhost:8500',
  jobs: [
    {
      name: "consul-agent",
      restarts: "unlimited",
      exec: [
        "/usr/bin/consul", "agent",
          "-data-dir=/data",
          "-log-level=err",
          "-rejoin",
          "-retry-join", '{{ .CONSUL | default "consul" }}',
          "-retry-max", "10",
          "-retry-interval", "10s",
      ],
      health: {
        exec: "curl -so /dev/null http://localhost:8500",
        interval: 10,
        ttl: 25,
      }
    }
  ]
}

Many application setups in the Autopilot Pattern library include a Consul agent process within each container to handle connecting ContainerPilot itself to a cluster of Consul servers. This helps performance of Consul features at the cost of more clients connected to the cluster.

A more detailed example of a ContainerPilot configuration that uses a Consul agent co-process can be found in autopilotpattern/nginx.

Triton-specific availability advantages

Some details about how Docker containers work on Triton have specific bearing on the durability and availability of this service:

1. Docker containers are first-order objects on Triton. They run on bare metal, and their overall availability is similar or better than what you expect of a virtual machine in other environments.
2. Docker containers on Triton preserve their IP and any data on disk when they reboot.
3. Linked containers in Docker Compose on Triton are distributed across multiple unique physical nodes for maximum availability in the case of node failures.

Consul encryption

Consul supports TLS encryption for RPC and symmetric pre-shared key encryption for its gossip protocol. Deploying these features requires managing these secrets, and a demonstration of how to do so can be found in the Vault example.

Testing

The tests/ directory includes integration tests for both the Triton and Compose example stacks described above. Build the test runner by making sure you've pulled down the submodule with git submodule update --init and then make build/tester.

Running make test/triton will run the tests in a container locally but targeting Triton Cloud. To run those tests you'll need a Triton Cloud account with your Triton command line profile set up. The test rig will use the value of the TRITON_PROFILE environment variable to determine what data center to target. The tests use your own credentials mounted from your Docker host (your laptop, for example), so if you have a passphrase on your ssh key you'll need to add -it to the arguments of the test/triton Make target.

Credit where it's due

This project builds on the fine examples set by Jeff Lindsay's (Glider Labs) Consul in Docker work. It also, obviously, wouldn't be possible without the outstanding work of the Hashicorp team that made consul.io.