---
layout: docs
page_title: Federated cluster failure scenarios
description: Failure scenarios in multi-region federated cluster deployments.
---

# Failure scenarios

When running Nomad in federated mode, failure situations and impacts are different depending on
whether the authoritative region is the impacted region or not, and what the failure mode is. In
soft failures, the region's servers have lost quorum but the Nomad processes are still up, running,
and reachable. In hard failures, the regional servers are completely unreachable and are akin to
the underlying hardware having been terminated (cloud) or powered-off (on-prem).

The scenarios are based on a Nomad deployment running three federated regions:
 * `asia-south-1`
 * `europe-west-1` - authoritative region
 * `us-east-1`

## Federated region failure: soft
In this situation the region `asia-south-1` has lost leadership but the servers are reachable and
up.

All server logs in the impacted region have entries such as this example.
```console
[ERROR] nomad/worker.go:504: worker: failed to dequeue evaluation: worker_id=d19e6bb5-5ec9-8f75-9caf-47e2513fe28d error="No cluster leader"
```

✅ Request forwarding continues to work between all federated regions that are running with
  leadership.

🟨 API requests, either directly or attempting to use request forwarding to the impacted region,
  fail unless using the `stale=true` flag.

✅ Creation and deletion of replicated objects, such as namespaces, is written to the
  authoritative region.

✅ Any federated regions with leadership is able to continue to replicate all objects detailed
  previously.

✅ Creation of local ACL tokens continues to work for all regions with leadership.

✅ Jobs **without** the [`multiregion`][] block deploy to all regions with leadership.

❌ Jobs **with** the [`multiregion`][] block defined fail to deploy.

## Federated region failure: hard
In this situation the region `asia-south-1` has gone down. When this happens, the Nomad server logs
for the other regions have log entries similar to this example.
```console
[DEBUG] go-hclog@v1.6.3/stdlog.go:58: nomad: memberlist: Failed UDP ping: asia-south-1-server-1.asia-south-1 (timeout reached)
[INFO]  go-hclog@v1.6.3/stdlog.go:60: nomad: memberlist: Suspect asia-south-1-server-1.asia-south-1 has failed, no acks received
[DEBUG] go-hclog@v1.6.3/stdlog.go:58: nomad: memberlist: Initiating push/pull sync with: us-east-1-server-1.us-east-1 192.168.1.193:9002
[DEBUG] go-hclog@v1.6.3/stdlog.go:58: nomad: memberlist: Failed UDP ping: asia-south-1-server-1.asia-south-1 (timeout reached)
[INFO]  go-hclog@v1.6.3/stdlog.go:60: nomad: memberlist: Suspect asia-south-1-server-1.asia-south-1 has failed, no acks received
```

✅ Request forwarding continues to work between all federated regions that are running with
  leadership.

❌ API requests, either directly or attempting to use request forwarding to the impacted region,
 fail.

✅ Creation and deletion of replicated objects, such as namespaces, are written to the
  authoritative region.

✅ Any federated regions with leadership continue to replicate all objects detailed
  above.

✅ Creation of local ACL tokens continues to work for all regions which are running with
  leadership.

✅ Jobs **without** the [`multiregion`][] block deploy to all regions with leadership.

❌ Jobs **with** the [`multiregion`][] block defined fail to deploy.

## Authoritative region failure: soft
In this situation the region `europe-west-1` has lost leadership but the servers are reachable and
up.

The server logs in the authoritative region have entries such as this example.
```console
[ERROR] nomad/worker.go:504: worker: failed to dequeue evaluation: worker_id=68b3abe2-5e16-8f04-be5a-f76aebb0e59e error="No cluster leader"
```

✅ Request forwarding continues to work between all federated regions that are running with
  leadership.

🟨 API requests, either directly or attempting to use request forwarding to the impacted region,
 fail unless using the `stale=true` flag.

❌ Creation and deletion of replicated objects, such as namespaces, fails.

❌ Any federated regions are able to read data to replicate as they use the stale flag, but no
  writes can occur to the authoritative region as described previously.

✅ Creation of local ACL tokens continues to work for all federated regions which are running
  with leadership.

✅ Jobs **without** the [`multiregion`][] block deploy to all federated regions which
  are running with leadership.

❌ Jobs **with** the [`multiregion`][] block defined fails to deploy.

## Authoritative region failure: hard
In this situation the region `europe-west-1` has gone down. When this happens, the Nomad server
leader logs for the other regions have log entries similar to this example.
```console
[ERROR] nomad/leader.go:544: nomad: failed to fetch namespaces from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:1767: nomad: failed to fetch policies from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:2498: nomad: failed to fetch ACL binding rules from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader_ent.go:226: nomad: failed to fetch quota specifications from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:703: nomad: failed to fetch node pools from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:1909: nomad: failed to fetch tokens from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:2083: nomad: failed to fetch ACL Roles from authoritative region: error="rpc error: EOF"
[DEBUG] nomad/leader_ent.go:84: nomad: failed to fetch policies from authoritative region: error="rpc error: EOF"
[ERROR] nomad/leader.go:2292: nomad: failed to fetch ACL auth-methods from authoritative region: error="rpc error: EOF"
[DEBUG] go-hclog@v1.6.3/stdlog.go:58: nomad: memberlist: Failed UDP ping: europe-west-1-server-1.europe-west-1 (timeout reached)
[INFO]  go-hclog@v1.6.3/stdlog.go:60: nomad: memberlist: Suspect europe-west-1-server-1.europe-west-1 has failed, no acks received
[DEBUG] go-hclog@v1.6.3/stdlog.go:58: nomad: memberlist: Failed UDP ping: europe-west-1-server-1.europe-west-1 (timeout reached)
```

✅ Request forwarding continues to work between all federated regions that are running with
  leadership.

❌ API requests, either directly or attempting to use request forwarding to the impacted region,
 fail.

❌ Creation and deletion of replicated objects, such as namespaces, fails.

❌ Any federated regions with leadership is not able to replicate objects detailed in the logs.

✅ Creation of local ACL tokens continues to work for all regions with leadership.

✅ Jobs **without** the [`multiregion`][] block deploy to regions with leadership.

❌ Jobs **with** the [`multiregion`][] block defined fail to deploy.

[`multiregion`]: /nomad/docs/job-specification/multiregion
