open-consul/website/source/docs/upgrade-specific.html.markdown

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docs	Upgrading Specific Versions	docs-upgrading-specific	Specific versions of Consul may have additional information about the upgrade process beyond the standard flow.

Upgrading Specific Versions

The upgrading page covers the details of doing a standard upgrade. However, specific versions of Consul may have more details provided for their upgrades as a result of new features or changed behavior. This page is used to document those details seperately from the standard upgrade flow.

Consul 0.5

Consul version 0.5 adds two features that complicate the upgrade process:

• ACL system includes service discovery and registration
• Internal use of tombstones to fix behavior of blocking queries in certain edge cases.

Users of the ACL system need to be aware that deploying Consul 0.5 will cause service registration to be enforced. This means if an agent attempts to register a service without proper privileges it will be denied. If the acl_default_policy is "allow" then clients will continue to work without an updated policy. If the policy is "deny", then all clients will begin to have their registration rejected causing issues.

To avoid this situation, all the ACL policies should be updated to add something like this:

# Enable all services to be registered
service "" {
    policy = "write"
}

This will set the service policy to write level for all services. The blank service name is the catch-all value. A more specific service can also be specified:

# Enable only the API service to be registered
service "api" {
    policy = "write"
}

The ACL policy can be updated while running 0.4, and enforcement will being with the upgrade to 0.5. The policy updates will ensure the availability of the cluster.

The second major change is the new internal command used for tombstones. The details of the change are not important, however to function the leader node will replicate a new command to its followers. Consul is designed defensively, and when a command that is not recognized is received, the server will panic. This is a purposeful design decision to avoid the possibility of data loss, inconsistensies, or security issues caused by future incompatibility.

In practice, this means if a Consul 0.5 node is the leader, all of its followers must also be running 0.5. There are a number of ways to do this to ensure cluster availability:

• Add new 0.5 nodes, then remove the old servers. This will add the new nodes as followers, and once the old servers are removed, one of the 0.5 nodes will become leader.

• Upgrade the followers first, then the leader last. Using consul info, you can determine which nodes are followers. Do an in-place upgrade on them first, and finally upgrade the leader last.

• Upgrade them in any order, but ensure all are done within 15 minutes. Even if the leader is upgraded to 0.5 first, as long as all of the followers are running 0.5 within 15 minutes there will be no issues.

Finally, even if any of the methods above are not possible or the process fails for some reason, it is not fatal. The older version of the server will simply panic and stop. At that point, you can upgrade to the new version and restart the agent. There will be no data loss and the cluster will resume operations.