Disaster recovery

Estimated reading time: 15 minutes

Disaster recovery procedures should be performed in the following order:

  1. Docker Swarm.
  2. Universal Control Plane (UCP).
  3. Docker Trusted Registry (DTR).

Swarm disaster recovery

Recover from losing the quorum

Swarm is resilient to failures and the swarm can recover from any number of temporary node failures (machine reboots or crash with restart) or other transient errors. However, a swarm cannot automatically recover if it loses a quorum. Tasks on existing worker nodes continue to run, but administrative tasks are not possible, including scaling or updating services and joining or removing nodes from the swarm. The best way to recover is to bring the missing manager nodes back online. If that is not possible, continue reading for some options for recovering your swarm.

In a swarm of N managers, a quorum (a majority) of manager nodes must always be available. For example, in a swarm with 5 managers, a minimum of 3 must be operational and in communication with each other. In other words, the swarm can tolerate up to (N-1)/2 permanent failures beyond which requests involving swarm management cannot be processed. These types of failures include data corruption or hardware failures.

If you lose the quorum of managers, you cannot administer the swarm. If you have lost the quorum and you attempt to perform any management operation on the swarm, an error occurs:

Error response from daemon: rpc error: code = 4 desc = context deadline exceeded

The best way to recover from losing the quorum is to bring the failed nodes back online. If you can’t do that, the only way to recover from this state is to use the --force-new-cluster action from a manager node. This removes all managers except the manager the command was run from. The quorum is achieved because there is now only one manager. Promote nodes to be managers until you have the desired number of managers.

# From the node to recover
$ docker swarm init --force-new-cluster --advertise-addr node01:2377

When you run the docker swarm init command with the --force-new-cluster flag, the Docker Engine where you run the command becomes the manager node of a single-node swarm which is capable of managing and running services. The manager has all the previous information about services and tasks, worker nodes are still part of the swarm, and services are still running. You need to add or re-add manager nodes to achieve your previous task distribution and ensure that you have enough managers to maintain high availability and prevent losing the quorum.

Force the swarm to rebalance

Generally, you do not need to force the swarm to rebalance its tasks. When you add a new node to a swarm, or a node reconnects to the swarm after a period of unavailability, the swarm does not automatically give a workload to the idle node. This is a design decision. If the swarm periodically shifted tasks to different nodes for the sake of balance, the clients using those tasks would be disrupted. The goal is to avoid disrupting running services for the sake of balance across the swarm. When new tasks start, or when a node with running tasks becomes unavailable, those tasks are given to less busy nodes. The goal is eventual balance, with minimal disruption to the end user.

In Docker 1.13 and higher, you can use the --force or -f flag with the docker service update command to force the service to redistribute its tasks across the available worker nodes. This causes the service tasks to restart. Client applications may be disrupted. If you have configured it, your service uses a rolling update.

If you use an earlier version and you want to achieve an even balance of load across workers and don’t mind disrupting running tasks, you can force your swarm to re-balance by temporarily scaling the service upward. Use docker service inspect --pretty <servicename> to see the configured scale of a service. When you use docker service scale, the nodes with the lowest number of tasks are targeted to receive the new workloads. There may be multiple under-loaded nodes in your swarm. You may need to scale the service up by modest increments a few times to achieve the balance you want across all the nodes.

When the load is balanced to your satisfaction, you can scale the service back down to the original scale. You can use docker service ps to assess the current balance of your service across nodes.

See also docker service scale and docker service ps.

UCP disaster recovery

In the event half or more manager nodes are lost and cannot be recovered to a healthy state, the system is considered to have lost quorum and can only be restored through the following disaster recovery procedure.

Recover a UCP cluster from an existing backup

  1. If UCP is still installed on the swarm, uninstall UCP using the uninstall-ucp command.

    Note: If the restore is happening on new machines, skip this step.

  2. Perform a restore from an existing backup on any node. If there is an existing swarm, the restore operation must be performed on a manager node. If no swarm exists, the restore operation will create one.

If your cluster has lost quorum, you can still perform a backup of one of the remaining nodes.

Important: Performing a backup after losing quorum is not guaranteed to succeed with no loss of running services or configuration data. To properly protect against manager failures, the system should be configured for high availability, and backups should be performed regularly in order to have complete backup data.

  1. On one of the remaining manager nodes, perform docker swarm init --force-new-cluster. You might also need to specify an --advertise-addr parameter, which is equivalent to the --host-address parameter of the docker/ucp install operation. This instantiates a new single-manager swarm by recovering as much state as possible from the existing manager. This is a disruptive operation and existing tasks might be either terminated or suspended.
  2. Create a backup of the remaining manager node.
  3. If UCP is still installed on the swarm, uninstall UCP using the uninstall-ucp command.
  4. Perform a restore on the recovered swarm manager node.
  5. Log in to UCP and browse to the nodes page, or use the CLI docker node ls command.
  6. If any nodes are listed as down, you’ll have to manually remove these nodes from the swarm and then re-join them using a docker swarm join operation with the swarm’s new join-token.
  7. Create a backup of the restored cluster.

Recreate objects within Orchestrators that Docker Enterprise supports

Kubernetes currently backs up the declarative state of Kube objects in etcd. However, for Swarm, there is no way to take the state and export it to a declarative format, since the objects that are embedded within the Swarm raft logs are not easily transferable to other nodes or clusters.

For disaster recovery, to recreate swarm related workloads requires having the original scripts used for deployment. Alternatively, you can recreate workloads by manually recreating output from docker inspect commands.

DTR disaster recovery

Docker Trusted Registry is a clustered application. You can join multiple replicas for high availability. For a DTR cluster to be healthy, a majority of its replicas (n/2 + 1) need to be healthy and be able to communicate with the other replicas. This is also known as maintaining quorum.

This means that there are three failure scenarios possible.

Replica is unhealthy but cluster maintains quorum

One or more replicas are unhealthy, but the overall majority (n/2 + 1) is still healthy and able to communicate with one another.

Failure scenario 1

In this example the DTR cluster has five replicas but one of the nodes stopped working, and the other has problems with the DTR overlay network.

Even though these two replicas are unhealthy the DTR cluster has a majority of replicas still working, which means that the cluster is healthy.

In this case you should repair the unhealthy replicas, or remove them from the cluster and join new ones.

Repair a single replica

When one or more DTR replicas are unhealthy but the overall majority (n/2 + 1) is healthy and able to communicate with one another, your DTR cluster is still functional and healthy.

Cluster with two nodes unhealthy

Given that the DTR cluster is healthy, there’s no need to execute any disaster recovery procedures like restoring from a backup.

Instead, you should:

  1. Remove the unhealthy replicas from the DTR cluster.
  2. Join new replicas to make DTR highly available.

Since a DTR cluster requires a majority of replicas to be healthy at all times, the order of these operations is important. If you join more replicas before removing the ones that are unhealthy, your DTR cluster might become unhealthy.

Split-brain scenario

To understand why you should remove unhealthy replicas before joining new ones, imagine you have a five-replica DTR deployment, and something goes wrong with the overlay network connection the replicas, causing them to be separated in two groups.

Cluster with network problem

Because the cluster originally had five replicas, it can work as long as three replicas are still healthy and able to communicate (5 / 2 + 1 = 3). Even though the network separated the replicas in two groups, DTR is still healthy.

If at this point you join a new replica instead of fixing the network problem or removing the two replicas that got isolated from the rest, it’s possible that the new replica ends up in the side of the network partition that has less replicas.

cluster with split brain

When this happens, both groups now have the minimum amount of replicas needed to establish a cluster. This is also known as a split-brain scenario, because both groups can now accept writes and their histories start diverging, making the two groups effectively two different clusters.

Remove replicas

To remove unhealthy replicas, you’ll first have to find the replica ID of one of the replicas you want to keep, and the replica IDs of the unhealthy replicas you want to remove.

You can find the list of replicas by navigating to Shared Resources > Stacks or Swarm > Volumes (when using swarm mode) on the UCP web interface, or by using the UCP client bundle to run:

$ docker ps --format "{{.Names}}" | grep dtr

# The list of DTR containers with <node>/<component>-<replicaID>, e.g.
# node-1/dtr-api-a1640e1c15b6

Another way to determine the replica ID is to SSH into a DTR node and run the following:

$ REPLICA_ID=$(docker inspect -f '{{.Name}}' $(docker ps -q -f name=dtr-rethink) | cut -f 3 -d '-')
&& echo $REPLICA_ID

Then use the UCP client bundle to remove the unhealthy replicas:

$ docker container run \
  --rm \
  --interactive \
  docker/dtr:2.7.1 remove \
  --existing-replica-id <healthy-replica-id> \
  --replica-ids <unhealthy-replica-id> \
  --ucp-insecure-tls \
  --ucp-url <ucp-url> \
  --ucp-username <user> \
  --ucp-password <password>

You can remove more than one replica at the same time, by specifying multiple IDs with a comma.

Healthy cluster

Join replicas

Once you’ve removed the unhealthy nodes from the cluster, you should join new ones to make sure your cluster is highly available.

Use your UCP client bundle to run the following command which prompts you for the necessary parameters:

$ docker container run \
  --rm \
  --interactive \
  docker/dtr:2.7.1 join \
  --ucp-node <ucp-node-name> \

Learn more about high availability.

The majority of replicas are unhealthy

If a majority of replicas are unhealthy, making the cluster lose quorum, but at least one replica is still healthy, or at least the data volumes for DTR are accessible from that replica, you can repair the cluster without having to restore from a backup. This minimizes the amount of data loss. The following image provides an example of this scenario.

Failure scenario 2

Repair a cluster

For a DTR cluster to be healthy, a majority of its replicas (n/2 + 1) need to be healthy and be able to communicate with the other replicas. This is known as maintaining quorum.

In a scenario where quorum is lost, but at least one replica is still accessible, you can use that replica to repair the cluster. That replica doesn’t need to be completely healthy. The cluster can still be repaired as the DTR data volumes are persisted and accessible.

Unhealthy cluster

Repairing the cluster from an existing replica minimizes the amount of data lost. If this procedure doesn’t work, you’ll have to restore from an existing backup.

Diagnose an unhealthy cluster

When a majority of replicas are unhealthy, causing the overall DTR cluster to become unhealthy, operations like docker login, docker pull, and docker push present internal server error.

Accessing the /_ping endpoint of any replica also returns the same error. It’s also possible that the DTR web UI is partially or fully unresponsive.

Perform an emergency repair

Use the docker/dtr emergency-repair command to try to repair an unhealthy DTR cluster, from an existing replica.

This command checks the data volumes for the DTR

This command checks the data volumes for the DTR replica are uncorrupted, redeploys all internal DTR components and reconfigured them to use the existing volumes.

It also reconfigures DTR removing all other nodes from the cluster, leaving DTR as a single-replica cluster with the replica you chose.

Start by finding the ID of the DTR replica that you want to repair from. You can find the list of replicas by navigating to Shared Resources > Stacks or Swarm > Volumes (when using swarm mode) on the UCP web interface, or by using a UCP client bundle to run:

$ docker ps --format "{{.Names}}" | grep dtr

# The list of DTR containers with <node>/<component>-<replicaID>, e.g.
# node-1/dtr-api-a1640e1c15b6

Another way to determine the replica ID is to SSH into a DTR node and run the following:

$ REPLICA_ID=$(docker inspect -f '{{.Name}}' $(docker ps -q -f name=dtr-rethink) | cut -f 3 -d '-')
&& echo $REPLICA_ID

Then, use your UCP client bundle to run the emergency repair command:

$ docker container run \
  --rm \
  --interactive \
  docker/dtr:2.7.1 emergency-repair \
  --ucp-insecure-tls \
  --existing-replica-id <replica-id>

If the emergency repair procedure is successful, your DTR cluster now has a single replica. You should now join more replicas for high availability.

Healthy cluster

If the emergency repair command fails, try running it again using a different replica ID. As a last resort, you can restore your cluster from an existing backup.

All replicas are unhealthy

This is a total disaster scenario where all DTR replicas are lost, causing the data volumes for all DTR replicas to get corrupted or be lost.

Failure scenario 3

In a disaster scenario like this, you’ll have to restore DTR from an existing backup. Restoring from a backup should be only used as a last resort, since doing an emergency repair might prevent some data loss.

Create a backup.

Where to go next

enterprise, recovery, disaster recovery, dtr, ucp, swarm