nomad-dtree

Nomad doesn't have its own dependency management plugin between the jobs. Especially with rise in the microservices pattern, it becomes difficult to maintain order of deployment using Nomad. For eg. Database service must be up before the api service is started. The order of deployment of the jobs has to be handled separately, using some external tool like Rundeck.

Nomad also lacks feature of post/pre hook. (Check issue: https://github.com/hashicorp/nomad/issues/419 and https://github.com/hashicorp/nomad/issues/2851). Many of services a pre-setup step or a post setup. For example, when we run a postgres or any db container, we need to create a user, db and grant permissions. Another example, on bringing up a rabbitmq cluster we might need to create vhost, user etc.

nomad-dtree is an attempt to address these problems.

nomad-dtree expects everything to be a nomad job. Even a post/pre hook should be a nomad jobs. It also expects a JSON where the dependency is mentioned.

How it works

Consider a small service which has three components:

• nginx
• api
• postgres

The order of dependency is same as above. The postgres service has a post hook, a small step of creating a user and db which is used by the api service. So we have nomad jobs:

• nginx.nomad
• api.nomad
• postgres.nomad
• postgres_setup.nomad

The dependency file for the above servcies is as follows:

{
  "dependencies": {
    "nginx": {
      "wait_cond": 5,
      "pre": {
        "job": "api"
      }
    },
    "api": {
      "wait_cond": 7,
      "pre": {
        "job": "postgres"
      }
    },
    "postgres": {
      "post": {
        "job": "postgres_setup",
        "wait_cond": 3
      },
      "wait_cond": 5
    }
  }
}

Now, each block in the dependencies json denotes a nomad job. It has three parts, consider api:

    "api": {
      "wait_cond": 7,
      "pre": {
        "job": "postgres"
      }
    },

• wait_cond (required) - It the wait_time, time taken for the api service to be healthy.
• pre (optional) - Pre-requisite job that has to be done before running the api, in this case postgres should be running before running postgres.
• post(optional) - Post hook, job that needs to be run after api is running (and after waiting for wait_cond seconds).

Now, when we run the following command:

nomad-dtree \
  run \
  --job nginx \  (note we are submitting nginx job here)
  --server-addr <nomad-address> \
  --store filesystem \
  --fs-depfile-path <path to dependency json file> \
  --fs-jobs-path <path to directory containing all nomad jobs>

The order of execution will be as follows:

postgres.nomad -> postgres_setup.nomad -> api.nomad -> nginx.nomad

Similar to run you can also stop the jobs. Stop command has two flags:

• purge (optional): When used, Job will be purged from the system.
• deep (optional): When used, the mentioned job and all the dependent child jobs will be stopped.

nomad-dtree \
  stop --purge --deep \
  --server-addr <nomad-address> \
  --store filesystem \
  --fs-depfile-path <path to dependency json file> \
  --fs-jobs-path <path to directory containing all nomad jobs>

For more examples check Examples