» Adding & Removing Servers

Consul is designed to require minimal operator involvement, however any changes to the set of Consul servers must be handled carefully. To better understand why, reading about the consensus protocol will be useful. In short, the Consul servers perform leader election and replication. For changes to be processed, a minimum quorum of servers (N/2)+1 must be available. That means if there are 3 server nodes, at least 2 must be available.

In general, if you are ever adding and removing nodes simultaneously, it is better to first add the new nodes and then remove the old nodes.

In this guide, we will cover the different methods for adding and removing servers.

» Manually Add a New Server

Manually adding new servers is generally straightforward, start the new agent with the -server flag. At this point the server will not be a member of any cluster, and should emit something like:

consul agent -server
[WARN] raft: EnableSingleNode disabled, and no known peers. Aborting election.

This means that it does not know about any peers and is not configured to elect itself. This is expected, and we can now add this node to the existing cluster using join. From the new server, we can join any member of the existing cluster:

$ consul join <Existing Node Address>
Successfully joined cluster by contacting 1 nodes.

It is important to note that any node, including a non-server may be specified for join. Generally, this method is good for testing purposes but not recommended for production deployments. For production clusters, you will likely want to use the agent configuration option to add additional servers.

» Add a Server with Agent Configuration

In production environments, you should use the agent configuration option, retry_join. retry_join can be used as a command line flag or in the agent configuration file.

With the Consul CLI:

$ consul agent -retry-join=["52.10.110.11", "52.10.110.12", "52.10.100.13"]

In the agent configuration file:

{
  "bootstrap": false,
  "bootstrap_expect": 3,
  "server": true,
  "retry_join": ["52.10.110.11", "52.10.110.12", "52.10.100.13"]
}

retry_join will ensure that if any server loses connection with the cluster for any reason, including the node restarting, it can rejoin when it comes back. In additon to working with static IPs, it can also be useful for other discovery mechanisms, such as auto joining based on cloud metadata and discovery. Both servers and clients can use this method.

» Server Coordination

To ensure Consul servers are joining the cluster properly, you should monitor the server coordination. The gossip protocol is used to properly discover all the nodes in the cluster. Once the node has joined, the existing cluster leader should log something like:

[INFO] raft: Added peer 127.0.0.2:8300, starting replication

This means that raft, the underlying consensus protocol, has added the peer and begun replicating state. Since the existing cluster may be very far ahead, it can take some time for the new node to catch up. To check on this, run info on the leader:

$ consul info
...
raft:
    applied_index = 47244
    commit_index = 47244
    fsm_pending = 0
    last_log_index = 47244
    last_log_term = 21
    last_snapshot_index = 40966
    last_snapshot_term = 20
    num_peers = 4
    state = Leader
    term = 21
...

This will provide various information about the state of Raft. In particular the last_log_index shows the last log that is on disk. The same info command can be run on the new server to see how far behind it is. Eventually the server will be caught up, and the values should match.

It is best to add servers one at a time, allowing them to catch up. This avoids the possibility of data loss in case the existing servers fail while bringing the new servers up-to-date.

» Manually Remove a Server

Removing servers must be done carefully to avoid causing an availability outage. For a cluster of N servers, at least (N/2)+1 must be available for the cluster to function. See this deployment table. If you have 3 servers and 1 of them is currently failing, removing any other servers will cause the cluster to become unavailable.

To avoid this, it may be necessary to first add new servers to the cluster, increasing the failure tolerance of the cluster, and then to remove old servers. Even if all 3 nodes are functioning, removing one leaves the cluster in a state that cannot tolerate the failure of any node.

Once you have verified the existing servers are healthy, and that the cluster can handle a node leaving, the actual process is simple. You simply issue a leave command to the server.

consul leave

The server leaving should contain logs like:

...
[INFO] consul: server starting leave
...
[INFO] raft: Removed ourself, transitioning to follower
...

The leader should also emit various logs including:

...
[INFO] consul: member 'node-10-0-1-8' left, deregistering
[INFO] raft: Removed peer 10.0.1.8:8300, stopping replication
...

At this point the node has been gracefully removed from the cluster, and will shut down.

Running consul leave on a server explicitly will reduce the quorum size. Even if the cluster used bootstrap_expect to set a quorum size initially, issuing consul leave on a server will reconfigure the cluster to have fewer servers. This means you could end up with just one server that is still able to commit writes because quorum is only 1, but those writes might be lost if that server fails before more are added.

To remove all agents that accidentally joined the wrong set of servers, clear out the contents of the data directory (-data-dir) on both client and server nodes.

These graceful methods to remove servers assumes you have a healthly cluster. If the cluster has no leader due to loss of quorum or data corruption, you should plan for outage recovery.

WARNING Removing data on server nodes will destroy all state in the cluster

» Manual Forced Removal

In some cases, it may not be possible to gracefully remove a server. For example, if the server simply fails, then there is no ability to issue a leave. Instead, the cluster will detect the failure and replication will continuously retry.

If the server can be recovered, it is best to bring it back online and then gracefully leave the cluster. However, if this is not a possibility, then the force-leave command can be used to force removal of a server.

consul force-leave <node>

This is done by invoking that command with the name of the failed node. At this point, the cluster leader will mark the node as having left the cluster and it will stop attempting to replicate.

» Summary

In this guide we learned the straightforward process of adding and removing servers including; manually adding servers, adding servers through the agent configuration, gracefully removing servers, and forcing removal of servers. Finally, we should restate that manually adding servers is good for testing purposes, however, for production it is recommended to add servers with the agent configuration.