Disaster recovery for Consul clusters

Depending on your deployment configuration, it may take only a single server failure for cluster unavailability. Recovery requires an operator to intervene, but the process is straightforward.

This datacenter outlines the processes for recovering from a Consul outage due to a majority of server nodes in a cluster being lost. There are several types of outages, depending on the number of server nodes and number of failed server nodes. This tutorial will outline how to recover from:

• Failure of a single server cluster. This is when you have a single Consul server and it fails.
• Failure of a server in a multi-server cluster. This is when one server fails, but the Consul cluster has three or more servers.
• Failure of multiple servers in a multi-server cluster. This when more than one server fails in a cluster of three or more servers. This scenario is potentially the most serious, because it can result in data loss.

Failure of a single server cluster

If you had only a single server and it has failed, simply restart it. A single server configuration requires the -bootstrap or -bootstrap-expect=1 flag.

$ consul agent -bootstrap-expect=1

If the server cannot be recovered, you need to bring up a new server using the deployment guide.

In the case of an unrecoverable server failure in a single server cluster and there is no backup procedure, data loss is inevitable since data was not replicated to any other servers. This is why a single server deploy is never recommended.

Any services registered with agents will be re-populated when the new server comes online as agents perform anti-entropy.

Failure of a single server cluster: after upgrading Raft protocol

After upgrading the server's -raft-protocol from version 2 to 3, server's running Raft protocol version 3 will no longer allow servers running an older version to be added.

In a single server cluster scenario, restarting the server may result in the server not being able to elect itself as a leader, rendering the cluster unusable.

To recover from this, go to the -data-dir of the failing Consul server. Inside the data directory there will be a raft/ sub-directory. Create a raft/peers.json file in the raft/ directory.

For Raft protocol version 3 and later, this should be formatted as a JSON array containing the node ID, address:port, and suffrage information of the Consul server, like this:

[
  {
    "id": "<node-id>",
    "address": "<node-ip>:8300",
    "non_voter": false
  }
]

Make sure you replace node-id and node-ip with the correct value for your Consul server.

Finally, restart your Consul server.

Failure of a server in a multi-server cluster

If the failed server is recoverable, the best option is to bring it back online and have it rejoin the cluster with the same IP address. This will return the cluster to a fully healthy state. Similarly, if you need to rebuild a new Consul server, to replace the failed node, you may wish to do that immediately. Note, the rebuilt server needs to have the same IP address as the failed server. Again, once this server is online and has rejoined, the cluster will return to a fully healthy state.

$ consul agent -bootstrap-expect=3 -bind=192.172.2.4 -retry-join=192.172.2.3

Both of these strategies involve a potentially lengthy time to reboot or rebuild a failed server. If this is impractical or if building a new server with the same IP isn't an option, you need to remove the failed server. Usually, you can issue a consul force-leave command to remove the failed server if it's still a member of the cluster.

$ consul force-leave <node.name.consul>

If consul force-leave isn't able to remove the server, you have two methods available to remove it, depending on your version of Consul:

• In Consul 0.7 and later, you can use the consul operator command to remove the stale peer server on the fly with no downtime if the cluster has a leader.

• In versions of Consul prior to 0.7, you can manually remove the stale peer server using the raft/peers.json recovery file on all remaining servers. See the section below for details on this procedure. This process requires a Consul downtime to complete.

In Consul 0.7 and later, you can use the consul operator command to inspect the Raft configuration:

$ consul operator raft list-peers

Node     ID              Address         State     Voter RaftProtocol
alice    10.0.1.8:8300   10.0.1.8:8300   follower  true  3
bob      10.0.1.6:8300   10.0.1.6:8300   leader    true  3
carol    10.0.1.7:8300   10.0.1.7:8300   follower  true  3

Failure of multiple servers in a multi-server cluster

In the event that multiple servers are lost, causing a loss of quorum and a complete outage, partial recovery is possible using data on the remaining servers in the cluster. There may be data loss in this situation because multiple servers were lost, so information about what's committed could be incomplete. The recovery process implicitly commits all outstanding Raft log entries, so it's also possible to commit data that was uncommitted before the failure.

See the section below on manual recovery using peers.json for details of the recovery procedure. You simply include just the remaining servers in the raft/peers.json recovery file. The cluster should be able to elect a leader once the remaining servers are all restarted with an identical raft/peers.json configuration.

Any new servers you introduce later can be fresh with totally clean data directories and joined using Consul's join command.

$ consul agent -retry-join=192.172.2.3

In extreme cases, it should be possible to recover with just a single remaining server by starting that single server with itself as the only peer in the raft/peers.json recovery file.

Prior to Consul 0.7 it wasn't always possible to recover from certain types of outages with raft/peers.json because this was ingested before any Raft log entries were played back. In Consul 0.7 and later, the raft/peers.json recovery file is final, and a snapshot is taken after it is ingested, so you are guaranteed to start with your recovered configuration. This does implicitly commit all Raft log entries, so should only be used to recover from an outage, but it should allow recovery from any situation where there's some cluster data available.

Manual recovery using peers.json

To begin, stop all remaining servers. You can attempt a graceful leave, but it will not work in most cases. Do not worry if the leave exits with an error. The cluster is in an unhealthy state, so this is expected.

In Consul 0.7 and later, the peers.json file is no longer present by default and is only used when performing recovery. This file will be deleted after Consul starts and ingests this file. Consul 0.7 also uses a new, automatically- created raft/peers.info file to avoid ingesting the raft/peers.json file on the first start after upgrading. Be sure to leave raft/peers.info in place for proper operation.

Using raft/peers.json for recovery can cause uncommitted Raft log entries to be implicitly committed, so this should only be used after an outage where no other option is available to recover a lost server. Make sure you don't have any automated processes that will put the peers file in place on a periodic basis.

The next step is to go to the -data-dir of each Consul server. Inside that directory, there will be a raft/ sub-directory. We need to create a raft/peers.json file. The format of this file depends on what the server has configured for its Raft protocol version.

For Raft protocol version 2 and earlier, this should be formatted as a JSON array containing the address and port of each Consul server in the cluster, like this:

["10.1.0.1:8300", "10.1.0.2:8300", "10.1.0.3:8300"]

For Raft protocol version 3 and later, this should be formatted as a JSON array containing the node ID, address:port, and suffrage information of each Consul server in the cluster, like this:

[
  {
    "id": "adf4238a-882b-9ddc-4a9d-5b6758e4159e",
    "address": "10.1.0.1:8300",
    "non_voter": false
  },
  {
    "id": "8b6dda82-3103-11e7-93ae-92361f002671",
    "address": "10.1.0.2:8300",
    "non_voter": false
  },
  {
    "id": "97e17742-3103-11e7-93ae-92361f002671",
    "address": "10.1.0.3:8300",
    "non_voter": false
  }
]

• id (string: <required>) - Specifies the node ID of the server. This can be found in the logs when the server starts up if it was auto-generated, and it can also be found inside the node-id file in the server's data directory.

• address (string: <required>) - Specifies the IP and port of the server. The port is the server's RPC port used for cluster communications.

• non_voter (bool: <false>) - This controls whether the server is a non-voter, which is used in some advanced Autopilot configurations. If omitted, it will default to false, which is typical for most clusters.

Simply create entries for all servers. You must confirm that servers you do not include here have indeed failed and will not later rejoin the cluster. Ensure that this file is the same across all remaining server nodes.

At this point, you can restart all the remaining servers. In Consul 0.7 and later you will see them ingest recovery file:

...
[INFO] consul: found peers.json file, recovering Raft configuration
...
[INFO] consul.fsm: snapshot created in 12.484µs
[INFO] snapshot: Creating new snapshot at /tmp/peers/raft/snapshots/2-5-1471383560779.tmp
[INFO] consul: deleted peers.json file after successful recovery
[INFO] raft: Restored from snapshot 2-5-1471383560779
[INFO] raft: Initial configuration (index=1): [{Suffrage:Voter ID:10.212.15.121:8300 Address:10.212.15.121:8300}]
...

If any servers managed to perform a graceful leave, you may need to have them rejoin the cluster using the join command:

$ consul join <Node Address>

Successfully joined cluster by contacting 1 nodes.

It should be noted that any existing member can be used to rejoin the cluster as the gossip protocol will take care of discovering the server nodes.

At this point, the cluster should be in an operable state again. One of the nodes should claim leadership and emit a log like:

[INFO] consul: cluster leadership acquired

In Consul 0.7 and later, you can use the consul operator command to inspect the Raft configuration:

$ consul operator raft list-peers

Node     ID              Address         State     Voter  RaftProtocol
alice    10.0.1.8:8300   10.0.1.8:8300   follower  true   3
bob      10.0.1.6:8300   10.0.1.6:8300   leader    true   3
carol    10.0.1.7:8300   10.0.1.7:8300   follower  true   3

Kubernetes-specific recovery steps

The file peers.json recovery method works fine in Kubernetes with one exception. After remote executing into the server pods and creating the peers.json file, you will typically restart the server by deleting its respective pod. Kubernetes then reschedules the Consul server node and a new pod is created and started. The problem is – that new pod also has a new IP address, which causes the information in the peers.json file to be outdated.

To work around this – follow this basic process:

1. Remote execute into each server pod and create a peers.json file as described above using the current IP addresses of each server pod.
2. Perform a consul leave on each server pod while you are inside the cluster through a remote execution. This will cause the consul process to exit and to be restarted by Kubernetes. Kubernetes will not reschedule the pod, therefore it will retain the same IP address.

Following the above steps should allow you to recover your Kubernetes cluster.

Next steps

In this tutorial, we reviewed how to recover from a Consul server outage. Depending on the quorum size and number of failed servers, the recovery process will vary. In the event of complete failure it is beneficial to have a backup process.

If you operate a multi-datacenter federated environment you will want to prepare for an outage having a disaster recovery strategy in case your primary datacenter becomes unavailable. Follow Disaster Recovery for the Primary Datacenter to learn how to recover from that.