Add a new volume type for ephemeral use between tasks

[hajali-amine]

Proposal

I feel like there should be type of volume that is ephemeral, the moment the tasks are stopped, it gets removed and wiped.
Something similar to K8S' emptyDir.
For example;

job "job_a" {
  datacenters = ["dc1"]
  type        = "service"

  group "group_a" {
    
   volume "eph" {
      type      = "ephemeral"
    }

    task "task_a_prestart" {
      driver = "docker"

      config {
        image        = "busybox:1.28"
        command      = "sh"
        args         = ["-c", "echo 'hello world' >  path/to/somewhere/text.txt"]
        network_mode = "host"
      }

      resources {
        cpu    = 200
        memory = 128
      }

      volume_mount {
        volume      = "eph"
        destination = "path/to/somewhere"
      }

      lifecycle {
        hook    = "prestart"
        sidecar = false
      }
    }

    task "task_a" {
      driver = "docker"

      config {
        image   = "busybox"
        command = "sh"
        args    = ["-c", "cat path/in/task/text.txt"]
      }

      volume_mount {
        volume      = "eph"
        destination = "path/in/task"
      }

      resources {
        cpu    = 200
        memory = 128
      }
    }
  }
}

This allows two tasks to share a volume and fast without having to create a host volume or having something persistent.

Use-cases

Inter-task communication, especially when it's between prestart, poststart and a main task. It facilitates the choice on where to mount the ephemeral volume instead of having alloc and managing the movement of directories.

Attempted Solutions

None

[tgross]

Hi @hajali-amine! The allocation directory is intended for this use, and can be referenced inside each task at the path in the $NOMAD_ALLOC_DIR env var. See https://developer.hashicorp.com/nomad/docs/concepts/filesystem for more details on this.

We've also got an open feature request for Dynamic Host Volumes #15489 which will likely be a slightly more powerful version of the same concept.