Enable the experimental WAL LogStore backend

This topic describes how to safely configure and test the WAL backend in your Consul deployment.

The overall process for enabling the WAL LogStore backend for one server consists of the following steps. In production environments, we recommend starting by enabling the backend on a single server . If you eventually choose to expand the test to further servers, you must repeat these steps for each one.

1. Enable log verification.
2. Select target server to enable WAL.
3. Stop target server gracefully.
4. Remove data directory from target server.
5. Update target server's configuration.
6. Start the target server.
7. Monitor target server raft metrics and logs.

Requirements

• Consul v1.15 or later is required for all servers in the datacenter. Refer to the standard upgrade procedure and the 1.15 upgrade notes for additional information.
• A Consul cluster with at least three nodes are required to safely test the WAL backend without downtime.

We recommend taking the following additional measures:

• Take a snapshot prior to testing.
• Monitor Consul server metrics and logs, and set an alert on specific log events that occur when WAL is enabled. Refer to Monitor Raft metrics and logs for WAL for more information.
• Enable WAL in a pre-production environment and run it for a several days before enabling it in production.

Known issues

The following issues were discovered after release of Consul 1.15.1 and will be fixed in a future patch release.

• A follower that is disconnected may be unable to catch up if it is using the WAL backend.
• Restoring user snapshots can break replication to WAL-enabled followers.
• Restoring user snapshots can cause a WAL-enabled leader to panic.

Risks

While their likelihood remains low to very low, be aware of the following risks before implementing the WAL backend:

• If WAL corrupts data on a Consul server agent, the server data cannot be recovered. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
• WAL may corrupt data or contain a defect that causes the server to panic and crash. WAL may not restart if the defect recurs when WAL reads from the logs on startup. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
• If WAL corrupts data, clients may read corrupted data from the Consul server, such as invalid IP addresses or unmatched tokens. This outcome is unlikely even if a recurring defect causes WAL to corrupt data because replication uses objects cached in memory instead of reads from disk. Restart the server with an empty data directory and reload its state from the leader to resolve the issue.
• If you enable a Consul OSS server to use WAL or enable WAL on a voting server with Consul Enterprise, WAL may corrupt the server's state, become the leader, and replicate the corrupted state to all other servers. In this case, restoring from backup is required to recover a completely uncorrupted state. Test WAL on a non-voting server in Enterprise to prevent this outcome. You can add a new non-voting server to the cluster to test with if there are no existing ones.

Enable log verification

You must enable log verification on all voting servers in Enterprise and all servers in OSS because the leader writes verification checkpoints.

1. On each voting server, add the following to the server's configuration file:

   raft_logstore {
     verification {
       enabled = true
       interval = "60s"
     }
   }
   

2. Restart the server to apply the changes. The consul reload command is not sufficient to apply raft_logstore configuration changes.

3. Run the consul operator raft list-peers command to wait for each server to become a healthy voter before moving on to the next. This may take a few minutes for large snapshots.

When complete, the server's logs should contain verifier reports that appear like the following example:

2023-01-31T14:44:31.174Z [INFO]  agent.server.raft.logstore.verifier: verification checksum OK: elapsed=488.463268ms leaderChecksum=f15db83976f2328c rangeEnd=357802 rangeStart=298132 readChecksum=f15db83976f2328c

Select target server to enable WAL

If you are using Consul OSS or Consul Enterprise without non-voting servers, select a follower server to enable WAL. As noted in Risks, Consul Enterprise users with non-voting servers should first select a non-voting server, or consider adding another server as a non-voter to test on.

Retrieve the current state of the servers by running the following command:

$ consul operator raft list-peers

Stop target server

Stop the target server gracefully. For example, if you are using systemd, run the following command:

$ systemctl stop consul

If your environment uses configuration management automation that might interfere with this process, such as Chef or Puppet, you must disable them until you have completely enabled WAL as a storage backend.

Remove data directory from target server

Temporarily moving the data directory to a different location is less destructive than deleting it. We recommend moving it in cases where you unsuccessfully enable WAL. Do not use the old data directory (/data-dir/raft.bak) for recovery after restarting the server. We recommend eventually deleting the old directory.

The following example assumes the data_dir in the server's configuration is /data-dir and renames it to /data-dir.bak.

$ mv /data-dir/raft /data-dir/raft.bak

When switching backends, you must always remove the entire raft directory, not just the raft.db file or wal directory. The log must always be consistent with the snapshots to avoid undefined behavior or data loss.

Update target server configuration

Add the following to the target server's configuration file:

raft_logstore {
  backend = "wal"
  verification {
    enabled = true
    interval = "60s"
  }
}

Start target server

Start the target server. For example, if you are using systemd, run the following command:

$ systemctl start consul

Watch for the server to become a healthy voter again.

$ consul operator raft list-peers

Monitor target server Raft metrics and logs

Refer to Monitor Raft metrics and logs for WAL for details.

We recommend leaving the cluster in the test configuration for several days or weeks, as long as you observe no errors. An extended test provides more confidence that WAL operates correctly under varied workloads and during routine server restarts. If you observe any errors, end the test immediately and report them.

If you disabled configuration management automation, consider reenabling it during the testing phase to pick up other updates for the host. You must ensure that it does not revert the Consul configuration file and remove the altered backend configuration. One way to do this is add the raft_logstore block to a separate file that is not managed by your automation. This file can either be added to the directory if -config-dir is used or as an additional -config-file argument.

Next steps

• If you observe any verification errors, performance anomalies, or other suspicious behavior from the target server during the test, you should immediately follow the procedure to revert back to BoltDB. Report failures through GitHub.

• If you do not see errors and would like to expand the test further, you can repeat the above procedure on another target server. We suggest waiting after each test expansion and slowly rolling WAL out to other parts of your environment. Once the majority of your servers use WAL, any bugs not yet discovered may result in cluster unavailability.

• If you wish to permanently enable WAL on all servers, repeat the steps described in this topic for each server. Even if backend = "wal" is set in logs, servers continue to use BoltDB if they find an existing raft.db file in the data directory.