server Block

The server block configures the Nomad agent to operate in server mode to participate in scheduling decisions, register with service discovery, handle join failures, and more.

server {
  enabled          = true
  bootstrap_expect = 3
  server_join {
    retry_join = [ "1.1.1.1", "2.2.2.2" ]
    retry_max = 3
    retry_interval = "15s"
  }
}

server Parameters

• authoritative_region (string: "") - Specifies the authoritative region, which provides a single source of truth for global configurations such as ACL Policies and global ACL tokens. Non-authoritative regions will replicate from the authoritative to act as a mirror. By default, the local region is assumed to be authoritative.

• bootstrap_expect (int: required) - Specifies the number of server nodes to wait for before bootstrapping. It is most common to use the odd-numbered integers 3 or 5 for this value, depending on the cluster size. A value of 1 does not provide any fault tolerance and is not recommended for production use cases.

• data_dir (string: "[data_dir]/server") - Specifies the directory to use for server-specific data, including the replicated log. By default, this is the top-level data_dir suffixed with "server", like "/opt/nomad/server". The top-level option must be set, even when setting this value. This must be an absolute path.

• enabled (bool: false) - Specifies if this agent should run in server mode. All other server options depend on this value being set.

• enabled_schedulers (array<string>: [all]) - Specifies which sub-schedulers this server will handle. This can be used to restrict the evaluations that worker threads will dequeue for processing.

• enable_event_broker (bool: true) - Specifies if this server will generate events for its event stream.

• encrypt (string: "") - Specifies the secret key to use for encryption of Nomad server's gossip network traffic. This key must be 32 bytes that are RFC4648 "URL and filename safe" base64-encoded. You can generate an appropriately-formatted key with the nomad operator keygen command. The provided key is automatically persisted to the data directory and loaded automatically whenever the agent is restarted. This means that to encrypt Nomad server's gossip protocol, this option only needs to be provided once on each agent's initial startup sequence. If it is provided after Nomad has been initialized with an encryption key, then the provided key is ignored and a warning will be displayed. See the encryption documentation for more details on this option and its impact on the cluster.

• event_buffer_size (int: 100) - Specifies the number of events generated by the server to be held in memory. Increasing this value enables new subscribers to have a larger look back window when initially subscribing. Decreasing will lower the amount of memory used for the event buffer.

• node_gc_threshold (string: "24h") - Specifies how long a node must be in a terminal state before it is garbage collected and purged from the system. This is specified using a label suffix like "30s" or "1h".

• job_gc_interval (string: "5m") - Specifies the interval between the job garbage collections. Only jobs who have been terminal for at least job_gc_threshold will be collected. Lowering the interval will perform more frequent but smaller collections. Raising the interval will perform collections less frequently but collect more jobs at a time. Reducing this interval is useful if there is a large throughput of tasks, leading to a large set of dead jobs. This is specified using a label suffix like "30s" or "3m". job_gc_interval was introduced in Nomad 0.10.0.

• job_gc_threshold (string: "4h") - Specifies the minimum time a job must be in the terminal state before it is eligible for garbage collection. This is specified using a label suffix like "30s" or "1h".

• eval_gc_threshold (string: "1h") - Specifies the minimum time an evaluation must be in the terminal state before it is eligible for garbage collection. This is specified using a label suffix like "30s" or "1h". Note that batch job evaluations are controlled via batch_eval_gc_threshold.

• batch_eval_gc_threshold (string: "24h") - Specifies the minimum time an evaluation stemming from a batch job must be in the terminal state before it is eligible for garbage collection. This is specified using a label suffix like "30s" or "1h". Note that the threshold is a necessary but insufficient condition for collection, and the most recent evaluation won't be garbage collected even if it breaches the threshold.

• deployment_gc_threshold (string: "1h") - Specifies the minimum time a deployment must be in the terminal state before it is eligible for garbage collection. This is specified using a label suffix like "30s" or "1h".

• csi_volume_claim_gc_threshold (string: "1h") - Specifies the minimum age of a CSI volume before it is eligible to have its claims garbage collected. This is specified using a label suffix like "30s" or "1h".

• csi_plugin_gc_threshold (string: "1h") - Specifies the minimum age of a CSI plugin before it is eligible for garbage collection if not in use. This is specified using a label suffix like "30s" or "1h".

• acl_token_gc_threshold (string: "1h") - Specifies the minimum age of an expired ACL token before it is eligible for garbage collection. This is specified using a label suffix like "30s" or "1h".

• default_scheduler_config (scheduler_configuration: nil) - Specifies the initial default scheduler config when bootstrapping cluster. The parameter is ignored once the cluster is bootstrapped or value is updated through the API endpoint. See the example section for more details default_scheduler_config was introduced in Nomad 0.10.4.

• heartbeat_grace (string: "10s") - Specifies the additional time given beyond the heartbeat TTL of Clients to account for network and processing delays and clock skew. This is specified using a label suffix like "30s" or "1h". See Client Heartbeats below for details.

• min_heartbeat_ttl (string: "10s") - Specifies the minimum time between Client heartbeats. This is used as a floor to prevent excessive updates. This is specified using a label suffix like "30s" or "1h". See Client Heartbeats below for details.

• failover_heartbeat_ttl (string: "5m") - The time by which all Clients must heartbeat after a Server leader election. This is specified using a label suffix like "30s" or "1h". See Client Heartbeats below for details.

• max_heartbeats_per_second (float: 50.0) - Specifies the maximum target rate of heartbeats being processed per second. This allows the TTL to be increased to meet the target rate. See Client Heartbeats below for details.

• non_voting_server (bool: false) - (Enterprise-only) Specifies whether this server will act as a non-voting member of the cluster to help provide read scalability.

• num_schedulers (int: [num-cores]) - Specifies the number of parallel scheduler threads to run. This can be as many as one per core, or 0 to disallow this server from making any scheduling decisions. This defaults to the number of CPU cores.

• license_path (string: "") - Specifies the path to load a Nomad Enterprise license from. This must be an absolute path (ex. /etc/nomad.d/license.hclic). The license can also be set by setting NOMAD_LICENSE_PATH or by setting NOMAD_LICENSE as the entire license value. license_path has the highest precedence, followed by NOMAD_LICENSE and then NOMAD_LICENSE_PATH.

• plan_rejection_tracker (PlanRejectionTracker) - Configuration for the plan rejection tracker that the Nomad leader uses to track the history of plan rejections.

• raft_boltdb - This is a nested object that allows configuring options for Raft's BoltDB based log store.

  • no_freelist_sync - Setting this to true will disable syncing the BoltDB freelist to disk within the raft.db file. Not syncing the freelist to disk will reduce disk IO required for write operations at the expense of longer server startup times.

• raft_protocol (int: 3) - Specifies the Raft protocol version to use when communicating with other Nomad servers. This affects available Autopilot features and is typically not required as the agent internally knows the latest version, but may be useful in some upgrade scenarios. Must be 3 in Nomad v1.4 or later.

• raft_multiplier (int: 1) - An integer multiplier used by Nomad servers to scale key Raft timing parameters. Omitting this value or setting it to 0 uses default timing described below. Lower values are used to tighten timing and increase sensitivity while higher values relax timings and reduce sensitivity. Tuning this affects the time it takes Nomad to detect leader failures and to perform leader elections, at the expense of requiring more network and CPU resources for better performance. The maximum allowed value is 10.

  By default, Nomad will use the highest-performance timing, currently equivalent to setting this to a value of 1. Increasing the timings makes leader election less likely during periods of networking issues or resource starvation. Since leader elections pause Nomad's normal work, it may be beneficial for slow or unreliable networks to wait longer before electing a new leader. The tradeoff when raising this value is that during network partitions or other events (server crash) where a leader is lost, Nomad will not elect a new leader for a longer period of time than the default. The nomad.nomad.leader.barrier and nomad.raft.leader.lastContact metrics are a good indicator of how often leader elections occur and raft latency.

• redundancy_zone (string: "") - (Enterprise-only) Specifies the redundancy zone that this server will be a part of for Autopilot management. For more information, see the Autopilot Guide.

• rejoin_after_leave (bool: false) - Specifies if Nomad will ignore a previous leave and attempt to rejoin the cluster when starting. By default, Nomad treats leave as a permanent intent and does not attempt to join the cluster again when starting. This flag allows the previous state to be used to rejoin the cluster.

• root_key_gc_interval (string: "10m") - Specifies the interval between encryption key metadata garbage collections.

• root_key_gc_threshold (string: "1h") - Specifies the minimum time that an encryption key must exist before it can be eligible for garbage collection.

• root_key_rotation_threshold (string: "720h") - Specifies the minimum time that an encryption key must exist before it is automatically rotated on the next garbage collection interval.

• server_join (server_join: nil) - Specifies how the Nomad server will connect to other Nomad servers. The retry_join fields may directly specify the server address or use go-discover syntax for auto-discovery. See the server_join documentation for more detail.

• upgrade_version (string: "") - A custom version of the format X.Y.Z to use in place of the Nomad version when custom upgrades are enabled in Autopilot. For more information, see the Autopilot Guide.

• search (search: nil) - Specifies configuration parameters for the Nomad search API.

Deprecated Parameters

• retry_join (array<string>: []) - Specifies a list of server addresses to retry joining if the first attempt fails. This is similar to start_join, but only invokes if the initial join attempt fails. The list of addresses will be tried in the order specified, until one succeeds. After one succeeds, no further addresses will be contacted. This is useful for cases where we know the address will become available eventually. Use retry_join with an array as a replacement for start_join, do not use both options. See the server_join section for more information on the format of the string. This field is deprecated in favor of the server_join block.

• retry_interval (string: "30s") - Specifies the time to wait between retry join attempts. This field is deprecated in favor of the server_join block.

• retry_max (int: 0) - Specifies the maximum number of join attempts to be made before exiting with a return code of 1. By default, this is set to 0 which is interpreted as infinite retries. This field is deprecated in favor of the server_join block.

• start_join (array<string>: []) - Specifies a list of server addresses to join on startup. If Nomad is unable to join with any of the specified addresses, agent startup will fail. See the server address format section for more information on the format of the string. This field is deprecated in favor of the server_join block.

plan_rejection_tracker Parameters

The leader plan rejection tracker can be adjusted to prevent evaluations from getting stuck due to always being scheduled to a client that may have an unexpected issue. Refer to Monitoring Nomad for more details.

• enabled (bool: false) - Specifies if plan rejections should be tracked.

• node_threshold (int: 100) - The number of plan rejections for a node within the node_window to trigger a client to be set as ineligible.

• node_window (string: "5m") - The time window for when plan rejections for a node should be considered.

If you observe too many false positives (clients being marked as ineligible even if they don't present any problem) you may want to increase node_threshold.

Or if you are noticing jobs not being scheduled due to plan rejections for the same node_id and the client is not being set as ineligible you can try increasing the node_window so more historical rejections are taken into account.

server Examples

Common Setup

This example shows a common Nomad agent server configuration block. The two IP addresses could also be DNS, and should point to the other Nomad servers in the cluster

server {
  enabled          = true
  bootstrap_expect = 3

  server_join {
    retry_join     = [ "1.1.1.1", "2.2.2.2" ]
    retry_max      = 3
    retry_interval = "15s"
  }
}

Configuring Data Directory

This example shows configuring a custom data directory for the server data.

server {
  data_dir = "/opt/nomad/server"
}

Automatic Bootstrapping

The Nomad servers can automatically bootstrap if Consul is configured. For a more detailed explanation, please see the automatic Nomad bootstrapping documentation.

Restricting Schedulers

This example shows restricting the schedulers that are enabled as well as the maximum number of cores to utilize when participating in scheduling decisions:

server {
  enabled            = true
  enabled_schedulers = ["batch", "service"]
  num_schedulers     = 7
}

Bootstrapping with a Custom Scheduler Config

While bootstrapping a cluster, you can use the default_scheduler_config block to prime the cluster with a SchedulerConfig. The scheduler configuration determines which scheduling algorithm is configured— spread scheduling or binpacking—and which job types are eligible for preemption.

The structure matches the Update Scheduler Config API endpoint, which you should consult for canonical documentation. However, the attributes names must be adapted to HCL syntax by using snake case representations rather than camel case.

This example shows configuring spread scheduling and enabling preemption for all job-type schedulers.

server {
  default_scheduler_config {
    scheduler_algorithm             = "spread"
    memory_oversubscription_enabled = true
    reject_job_registration         = false
    pause_eval_broker               = false # New in Nomad 1.3.2

    preemption_config {
      batch_scheduler_enabled    = true
      system_scheduler_enabled   = true
      service_scheduler_enabled  = true
      sysbatch_scheduler_enabled = true # New in Nomad 1.2
    }
  }
}

Client Heartbeats

Nomad Clients periodically heartbeat to Nomad Servers to confirm they are operating as expected. Nomad Clients which do not heartbeat in the specified amount of time are considered down and their allocations are marked as lost or disconnected (if max_client_disconnect is set) and rescheduled.

The various heartbeat related parameters allow you to tune the following tradeoffs:

• The longer the heartbeat period, the longer a down Client's workload will take to be rescheduled.
• The shorter the heartbeat period, the more likely transient network issues, leader elections, and other temporary issues could cause a perfectly functional Client and its workloads to be marked as down and the work rescheduled.

While Nomad Clients can connect to any Server, all heartbeats are forwarded to the leader for processing. Since this heartbeat processing consumes resources, Nomad adjusts the rate at which Clients heartbeat based on cluster size. The goal is to try to keep the resource cost of processing heartbeats constant regardless of cluster size.

The base formula for determining how often a Client must heartbeat is:

<number of Clients> / <max_heartbeats_per_second>

Other factors modify this base TTL:

• A random factor up to 2x is added to the base TTL to prevent the thundering herd problem where a large number of clients attempt to heartbeat at exactly the same time.
• min_heartbeat_ttl is used as the lower bound to prevent small clusters from sending excessive heartbeats.
• heartbeat_grace is the amount of extra time the leader will wait for a heartbeat beyond the base heartbeat.
• After a leader election all Clients are given up to failover_heartbeat_ttl to successfully heartbeat. This gives Clients time to discover a functioning Server in case they were directly connected to a leader that crashed.

For example, given the default values for heartbeat parameters, different sized clusters will use the following TTLs for the heartbeats. Note that the Server TTL simply adds the heartbeat_grace parameter to the TTL Clients are given.

Regardless of size, all clients will have a Server TTL of failover_heartbeat_ttl after a leader election. It should always be larger than the maximum Client TTL for your cluster size in order to prevent marking live Clients as down.

For clusters over 5000 Clients you should increase failover_heartbeat_ttl using the following formula:

(2 * (<number of Clients> / <max_heartbeats_per_second>)) + (10 * <min_heartbeat_ttl>)

# For example with 6000 Clients:
(2 * (6000 / 50)) + (10 * 10) = 340s (5m40s)

This ensures Clients have some additional time to failover even if they were told to heartbeat after the maximum interval.

The actual value used should take into consideration how much tolerance your system has for a delay in noticing crashed Clients. For example a failover_heartbeat_ttl of 30 minutes may give even the slowest clients in the largest clusters ample time to heartbeat after an election. However if the election was due to a datacenter-wide failure affecting Clients, it will be 30 minutes before Nomad recognizes that they are down and reschedules their work.