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group Block

Placementjob -> group

The group block defines a series of tasks that should be co-located on the same Nomad client. Any task within a group will be placed on the same client.

job "docs" {
  group "example" {
    # ...
  }
}

group Parameters

  • constraint (Constraint: nil) - This can be provided multiple times to define additional constraints.

  • affinity (Affinity: nil) - This can be provided multiple times to define preferred placement criteria.

  • spread (Spread: nil) - This can be provided multiple times to define criteria for spreading allocations across a node attribute or metadata. See the Nomad spread reference for more details.

  • count (int) - Specifies the number of instances that should be running under for this group. This value must be non-negative. This defaults to the min value specified in the scaling block, if present; otherwise, this defaults to 1.

  • consul (Consul: nil) - Specifies Consul configuration options specific to the group. These options will be applied to all tasks and services in the group unless a task has its own consul block.

  • ephemeral_disk (EphemeralDisk: nil) - Specifies the ephemeral disk requirements of the group. Ephemeral disks can be marked as sticky and support live data migrations.

  • disconnect (disconnect: nil) - Specifies the disconnect strategy for the server and client for all tasks in this group in case of a network partition. The tasks can be left unconnected, stopped or replaced when the client disconnects. The policy for reconciliation in case the client regains connectivity is also specified here.

  • meta (Meta: nil) - Specifies a key-value map that annotates with user-defined metadata.

  • migrate (Migrate: nil) - Specifies the group strategy for migrating off of draining nodes. Only service jobs with a count greater than 1 support migrate blocks.

  • network (Network: <optional>) - Specifies the network requirements and configuration, including static and dynamic port allocations, for the group.

  • prevent_reschedule_on_lost (bool: false) - Defines the reschedule behaviour of an allocation when the node it is running on misses heartbeats. When enabled, if the node it is running on becomes disconnected or goes down, this allocations wont be rescheduled and will show up as unknown until the node comes back up or it is manually restarted.

    This behaviour will only modify the reschedule process on the server. To modify the allocation behaviour on the client, see stop_after_client_disconnect.

    The unknown allocation has to be manually stopped to run it again.

    `nomad alloc stop  <alloc ID>`

    Setting max_client_disconnect and prevent_reschedule_on_lost = true at the same time requires that rescheduling is disabled entirely.

    This field was deprecated in favour of replace on the [disconnect] block, see example below for more details about migrating.

  • reschedule (Reschedule: nil) - Allows to specify a rescheduling strategy. Nomad will then attempt to schedule the task on another node if any of the group allocation statuses become "failed".

  • restart (Restart: nil) - Specifies the restart policy for all tasks in this group. If omitted, a default policy exists for each job type, which can be found in the restart block documentation.

  • service (Service: nil) - Specifies integrations with Nomad or Consul for service discovery. Nomad automatically registers each service when an allocation is started and de-registers them when the allocation is destroyed.

  • shutdown_delay (string: "0s") - Specifies the duration to wait when stopping a group's tasks. The delay occurs between Consul or Nomad service deregistration and sending each task a shutdown signal. Ideally, services would fail health checks once they receive a shutdown signal. Alternatively, shutdown_delay may be set to give in-flight requests time to complete before shutting down. A group level shutdown_delay will run regardless if there are any defined group services and only applies to these services. In addition, tasks may have their own shutdown_delay which waits between de-registering task services and stopping the task.

  • stop_after_client_disconnect (string: "") - Specifies a duration after which a Nomad client will stop allocations, if it cannot communicate with the servers. By default, a client will not stop an allocation until explicitly told to by a server. A client that fails to heartbeat to a server within the heartbeat_grace window and any allocations running on it will be marked "lost" and Nomad will schedule replacement allocations. The replaced allocations will normally continue to run on the non-responsive client. But you may want them to stop instead — for example, allocations requiring exclusive access to an external resource. When specified, the Nomad client will stop them after this duration. The Nomad client process must be running for this to occur. This setting cannot be used with max_client_disconnect.

    This field was deprecated in favour of stop_after on the [disconnect] block.

  • max_client_disconnect (string: "") - Specifies a duration during which a Nomad client will attempt to reconnect allocations after it fails to heartbeat in the heartbeat_grace window. See the example code below for more details. This setting cannot be used with stop_after_client_disconnect.

    This field was deprecated in favour of lost_after on the [disconnect] block.

  • task (Task: <required>) - Specifies one or more tasks to run within this group. This can be specified multiple times, to add a task as part of the group.

  • update (Update: nil) - Specifies the task's update strategy. When omitted, a default update strategy is applied.

  • vault (Vault: nil) - Specifies the set of Vault policies required by all tasks in this group. Overrides a vault block set at the job level.

  • volume (Volume: nil) - Specifies the volumes that are required by tasks within the group.

group Examples

The following examples only show the group blocks. Remember that the group block is only valid in the placements listed above.

Specifying Count

This example specifies that 5 instances of the tasks within this group should be running:

group "example" {
  count = 5
}

Tasks with Constraint

This example shows two abbreviated tasks with a constraint on the group. This will restrict the tasks to 64-bit operating systems.

group "example" {
  constraint {
    attribute = "${attr.cpu.arch}"
    value     = "amd64"
  }

  task "cache" {
    # ...
  }

  task "server" {
    # ...
  }
}

Metadata

This example show arbitrary user-defined metadata on the group:

group "example" {
  meta {
    my-key = "my-value"
  }
}

Network

This example shows network constraints as specified in the network block which uses the bridge networking mode, dynamically allocates two ports, and statically allocates one port:

group "example" {
  network {
    mode = "bridge"
    port "http" {}
    port "https" {}
    port "lb" {
      static = "8889"
    }
  }
}

Service Discovery

This example creates a service in Consul. To read more about service discovery in Nomad, please see the Nomad service discovery documentation.

group "example" {
  network {
    port "api" {}
  }

  service {
    name = "example"
    port = "api"
    tags = ["default"]

    check {
      type     = "tcp"
      interval = "10s"
      timeout  = "2s"
    }
  }

  task "api" { ... }
}

Stop After Client Disconnect

This example shows how stop_after_client_disconnect interacts with other blocks. For the first group, after the default 10 second heartbeat_grace window expires and 90 more seconds passes, the server will reschedule the allocation. The client will wait 90 seconds before sending a stop signal (SIGTERM) to the first-task task. After 15 more seconds because of the task's kill_timeout, the client will send SIGKILL. The second group does not have stop_after_client_disconnect, so the server will reschedule the allocation after the 10 second heartbeat_grace expires. It will not be stopped on the client, regardless of how long the client is out of touch.

Note that if the server's clocks are not closely synchronized with each other, the server may reschedule the group before the client has stopped the allocation. Operators should ensure that clock drift between servers is as small as possible.

Note also that a group using this feature will be stopped on the client if the Nomad server cluster fails, since the client will be unable to contact any server in that case. Groups opting in to this feature are therefore exposed to an additional runtime dependency and potential point of failure.

group "first" {
  stop_after_client_disconnect = "90s"

  task "first-task" {
    kill_timeout = "15s"
  }
}

group "second" {

  task "second-task" {
    kill_timeout = "5s"
  }
}

Max Client Disconnect

max_client_disconnect specifies a duration during which a Nomad client will attempt to reconnect allocations after it fails to heartbeat in the heartbeat_grace window.

By default, allocations running on a client that fails to heartbeat will be marked "lost". When a client reconnects, its allocations, which may still be healthy, will restart because they have been marked "lost". This can cause issues with stateful tasks or tasks with long restart times.

Instead, an operator may desire that these allocations reconnect without a restart. When max_client_disconnect or disconnect.lost_after is specified, the Nomad server will mark clients that fail to heartbeat as "disconnected" rather than "down", and will mark allocations on a disconnected client as "unknown" rather than "lost". These allocations may continue to run on the disconnected client. Replacement allocations will be scheduled according to the allocations' disconnect.replace settings. until the disconnected client reconnects. Once a disconnected client reconnects, Nomad will compare the "unknown" allocations with their replacements and will decide which ones to keep according to the disconnect.replace setting. If the max_client_disconnect or disconnect.losta_after duration expires before the client reconnects, the allocations will be marked "lost". Clients that contain "unknown" allocations will transition to "disconnected" rather than "down" until the last max_client_disconnect or disconnect.lost_after duration has expired.

In the example code below, if both of these task groups were placed on the same client and that client experienced a network outage, both of the group's allocations would be marked as "disconnected" at two minutes because of the client's heartbeat_grace value of "2m". If the network outage continued for eight hours, and the client continued to fail to heartbeat, the client would remain in a "disconnected" state, as the first group's max_client_disconnect is twelve hours. Once all groups' max_client_disconnect durations are exceeded, in this case in twelve hours, the client node will be marked as "down" and the allocation will be marked as "lost". If the client had reconnected before twelve hours had passed, the allocations would gracefully reconnect without a restart.

Max Client Disconnect is useful for edge deployments, or scenarios when operators want zero on-client downtime due to node connectivity issues. This setting cannot be used with stop_after_client_disconnect.

# server_config.hcl

server {
  enabled         = true
  heartbeat_grace = "2m"
}

# jobspec.nomad

group "first" {
  max_client_disconnect = "12h"

  task "first-task" {
    ...
  }
}

group "second" {
  max_client_disconnect = "6h"

  task "second-task" {
    ...
  }
}

Max Client Disconnect and Prevent Reschedule On Lost

Setting max_client_disconnect and prevent_reschedule_on_lost = true at the same time requires that rescheduling is disabled entirely.

# jobspec.nomad

group "first" {
  max_client_disconnect      = "12h"
  prevent_reschedule_on_lost = true

  reschedule {
    attempts  = 0
    unlimited = false
  }

  task "first-task" {
    ...
  }
}

If max_client_disconnect is set and prevent_reschedule_on_lost = true, allocations on disconnected nodes will be unknown until the max_client_disconnect window expires, at which point the node will be transition from disconnected to down. The allocation will remain as unknown and won't be rescheduled.

Migration to disconnect block

The new configuration fileds in the disconnect block work exactly the same as the ones they are replacing:

To keep the same behaviour as the old configuration upon reconnection, the reconcile option should be set to best_score.

The following example shows how to migrate from the old configuration to the new one:

job "docs" {
  group "example" {
    max_client_disconnect        = "6h"
    stop_after_client_disconnect = "2h"
    prevent_reschedule_on_lost   = true
  }
}

Can be directly translated to:

job "docs" {
    group "example" {
      disconnect {
        lost_after = "6h"
        stop_after = "2h"
        replace = false
        reconcile = "best_score"
      }
    }
  }

All use constrains still apply with the disconnect block as they did before:

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