---
layout: docs
page_title: server Block - Agent Configuration
description: |-
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` 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.
```hcl
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](/nomad/docs/configuration#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: [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][encryption] 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][update-scheduler-config]:
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][update-scheduler-config]. See [the
example section](#configuring-scheduler-config) 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](#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](#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](#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](#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 (`/opt/nomad/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](#plan_rejection_tracker-parameters)) -
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 trade-off
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](/nomad/docs/operations/metrics-reference) are a good
indicator of how often leader elections occur and Raft latency.
- `raft_snapshot_threshold` `(int: "8192")` - Specifies the minimum number of
Raft logs to be written to disk before the node is allowed to take a snapshot.
This reduces the frequency and impact of creating snapshots. During node
startup, Raft restores the latest snapshot and then applies the individual
logs to catch the node up to the last known state. This can be tuned during
operation by a hot configuration reload.
- `raft_snapshot_interval` `(string: "120s")` - Specifies the minimum time between
checks if Raft should perform a snapshot. The Raft library randomly staggers
between this value and twice this value to avoid the entire cluster performing
a snapshot at once. Nodes are eligible to snapshot once they have exceeded the
`raft_snapshot_threshold`. This value can be tuned during operation by a hot
configuration reload.
- `raft_trailing_logs` `(int: "10240")` - Specifies how many logs are retained
after a snapshot. These logs are used so that Raft can quickly replay logs on
a follower instead of being forced to send an entire snapshot. This value can
be tuned during operation by a hot configuration reload.
- `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](/nomad/tutorials/manage-clusters/autopilot).
- `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][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][server-join] 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](/nomad/tutorials/manage-clusters/autopilot).
- `search` ([search][search]: nil) - Specifies configuration parameters
for the Nomad search API.
### Deprecated Parameters
- `retry_join` `(array: [])` - Specifies a list of server addresses to
retry joining if the first attempt fails. This is similar to
[`start_join`](#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][server-join]
section for more information on the format of the string. This field is
deprecated in favor of the [server_join block][server-join].
- `retry_interval` `(string: "30s")` - Specifies the time to wait between retry
join attempts. This field is deprecated in favor of the [server_join
block][server-join].
- `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][server-join].
- `start_join` `(array: [])` - 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](/nomad/docs/configuration/server_join#server-address-format)
section for more information on the format of the string. This field is
deprecated in favor of the [server_join block][server-join].
### `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][monitoring_nomad_progress] 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
```hcl
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.
```hcl
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](/nomad/tutorials/manage-clusters/clustering).
### 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:
```hcl
server {
enabled = true
enabled_schedulers = ["batch", "service"]
num_schedulers = 7
}
```
### Bootstrapping with a Custom Scheduler Config ((#configuring-scheduler-config))
While [bootstrapping a cluster], you can use the `default_scheduler_config` block
to prime the cluster with a [`SchedulerConfig`][update-scheduler-config]. The
scheduler configuration determines which scheduling algorithm is configured—
spread scheduling or binpacking—and which job types are eligible for preemption.
~> **Warning:** Once the cluster is bootstrapped, you must configure this using
the [update scheduler configuration][update-scheduler-config] API. This
option is only consulted during bootstrap.
The structure matches the [Update Scheduler Config][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.
```hcl
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 ((#client-heartbeats))
~> This is an advanced topic. It is most beneficial to clusters over 1,000
nodes or with unreliable networks or nodes (eg some edge deployments).
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`][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:
```
/
```
Other factors modify this base TTL:
- A random factor up to `2x` is added to the base TTL to prevent the
[thundering herd][herd] problem where a large number of clients attempt to
heartbeat at exactly the same time.
- [`min_heartbeat_ttl`](#min_heartbeat_ttl) is used as the lower bound to
prevent small clusters from sending excessive heartbeats.
- [`heartbeat_grace`](#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.
| Clients | Client TTL | Server TTL | Safe after elections |
| ------- | ----------- | ----------- | -------------------- |
| 10 | 10s - 20s | 20s - 30s | yes |
| 100 | 10s - 20s | 20s - 30s | yes |
| 1000 | 20s - 40s | 30s - 50s | yes |
| 5000 | 100s - 200s | 110s - 210s | yes |
| 10000 | 200s - 400s | 210s - 410s | NO (see below) |
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 * ( / )) + (10 * )
# 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.
[encryption]: /nomad/tutorials/transport-security/security-gossip-encryption 'Nomad Encryption Overview'
[server-join]: /nomad/docs/configuration/server_join 'Server Join'
[update-scheduler-config]: /nomad/api-docs/operator/scheduler#update-scheduler-configuration 'Scheduler Config'
[bootstrapping a cluster]: /nomad/docs/faq#bootstrapping
[rfc4648]: https://tools.ietf.org/html/rfc4648#section-5
[monitoring_nomad_progress]: /nomad/docs/operations/monitoring-nomad#progress
[`nomad operator keygen`]: /nomad/docs/commands/operator/keygen
[search]: /nomad/docs/configuration/search
[encryption key]: /nomad/docs/operations/key-management
[max_client_disconnect]: /nomad/docs/job-specification/group#max-client-disconnect
[herd]: https://en.wikipedia.org/wiki/Thundering_herd_problem