How do I configure task in group to read a different volume for different instances?

Metadata

• Nomad v1.7.2
• OS: Windows
• Scheduler: service
• Driver: exec / raw_exec

Architecture

I have 10 windows servers running nomad client nodes.

I want to run 3 instances of a stateful service using Nomad. Each instance (01, 02, 03) writes checkpoints and data to filesystem (eg. /tmp/instance01, /tmp/instance02, /tmp/instance03). When the instance restarts, it will continue from the latest checkpoint. Each instance can be allocated to any host. However, each instance should be configured to use the same directory as the previously failed instance.

So basically:

• 01 <--> /tmp/instance01
• 02 <--> /tmp/instance02
• 03 <--> /tmp/instance03

For simplicity, assume these 3 directories are created in NAS, and the NAS is mounted on all servers running nomad client node. Also assume all groups / tasks has RW access to these 3 directories.

Issue

There are a few ways I can configure the directory that the service uses to read/write state data:

• Command Line Argument
• Configuration File, via environment variable
• Template block, via Nomad template interpolation

How can I pass a different value to the same task running in different instances of a group?

i.e. How do I give each instance a unique and consistent tag, so it can be reliably identified?

What I’ve considered

• Parameterized Block --> Does not work for Service jobs
• Template Block -> Every task instance will receive the same data
• Env Var --> Every task instance will receive the same data
• Meta Block --> Every task instance will receive the same data
• Variable Block --> Every task instance will receive the same data
• Dynamic Block --> Possible solution, but this is essentially repeating Group Block
• Repeating Group Block --> Trying to avoid this
• Multiple Job Spec --> Trying to avoid this

It seems like it's possible to loop within a nomad task, but not across tasks.

Any solution appreciated, even hacky ones. TIA!

Job Spec

job "app-write" {
  datacenters = ["dc1"]
  type = "service"
  node_pool = "default"

  # Write
  group "app-write" {
    count = 3
    
    # /tmp/instance01
    volume "app01" {
      type = "host"
      read_only = false
      source = "tmpapp01"
    }

    # /tmp/instance02
    volume "app02" {
      type = "host"
      read_only = false
      source = "tmpapp02"
    }

    # /tmp/instance03
    volume "app03" {
      type = "host"
      read_only = false
      source = "tmpapp03"
    }
    
    network {
      port "http" { }  // 3100
      port "grpc" { } // 9095
      port "gossip" { }  // 7946
      # port "lb" { static = 8080 }
    }

    service {
      name = "app-write"
      address_mode = "host"
      port = "http"
      tags = ["http"]
      provider = "nomad"
    }

    service {
      name = "app-write"
      address_mode = "host"
      port = "grpc"
      tags = ["grpc"]
      provider = "nomad"
    }

    service {
      name = "app-write"
      address_mode = "host"
      port = "gossip"
      tags = ["gossip"]
      provider = "nomad"
    }

    task "app-write" {
      driver = "exec" # or "raw_exec"
      
      volume_mount {
        volume = "app01"
        destination = "/tmp/app01"
        read_only = false
      }

      volume_mount {
        volume = "app02"
        destination = "/tmp/app02"
        read_only = false
      }

      volume_mount {
        volume = "app03"
        destination = "/tmp/app03"
        read_only = false
      }

      config {
        command = "/usr/bin/app"
        args = [
          "-config.file=local/app/config.yaml",
          "-working-directory=/tmp/app01" # <-- Need this to change for each instance
          ]
      }

      resources {
        cpu = 100
        memory = 128
      }

      # Can change this for each instance too
      template {
        source = "/etc/app/config.yaml.tpl"
        destination = "local/app/config.yaml"
        change_mode = "restart" // restart
      }
    }
  }
}

How do I configure task in group to read a different volume for different instances?

Consider just writing the 3 different groups. They are different.

job "app-write" {
  group "app-write1" {
    volume "app01" {
    }
  }
  group "app-write2" {
    volume "app02" {
    }
  }
  group "app-write3" {
    volume "app03" {
    }
  }
}

"-working-directory=/tmp/app01" # <-- Need this to change for each instance

That is rather simple. You would need a shell to calculate it.

   command = "sh"
   args = [ "-xc", <<EOF
      idx=$(printf "%02d" "$((NOMAD_ALLOC_INDEX + 1))")
      /usr/bin/app \
         -config.file=local/app/config.yaml \
         -working-directory=/tmp/app$idx
     EOF
  ]