Rolling Member Upgrades

Hazelcast members operating on binaries of the same major and minor version numbers are compatible regardless of patch version. For example, in a cluster with members running on version 4.2.1, it is possible to perform a rolling upgrade to 4.2.2 by shutting down, upgrading to hazelcast-4.2.2.jar binary and starting each member one by one. Rolling upgrade for patch versions is supported in the open source, Pro, or Enterprise editions of Hazelcast IMDG.

Also, each minor version is compatible with the previous one (back until Hazelcast IMDG 3.8). For example, it is possible to perform a rolling upgrade on a cluster running Hazelcast IMDG Enterprise 4.1 to Hazelcast IMDG Enterprise 4.2. Rolling upgrade for minor versions is supported in the Enterprise edition of Hazelcast IMDG.

The compatibility guarantees described above are given in the context of rolling member upgrades and only apply to GA (general availability) releases. It is never advisable to run a cluster with members running on different patch or minor versions for prolonged periods of time.

Let’s assume a cluster with four members running on codebase version 4.1.0 with cluster version 4.1, that should be upgraded to codebase version 4.2.0 and cluster version 4.2. The rolling upgrade process for this cluster, i.e., replacing existing 4.1.0 members one by one with an upgraded one at version 4.2.0, includes the following steps which should be repeated for each member:

The version in brackets ([4.2]) still denotes the member’s codebase version (running on the hypothetical hazelcast-enterprise-4.2.jar binary). Once the member locates the existing cluster members, it sends its join request to the master. The master validates that the new member is allowed to join the cluster and lets the new member know that the cluster is currently operating at 4.1 cluster version. The new member sets 4.1 as its cluster version and starts operating normally.

At this point all members of the cluster have been upgraded to codebase version 4.2.0 but the cluster still operates at cluster version 4.1. In order to use 4.2 features the cluster version must be changed to 4.2.

You have the following options to upgrade the cluster version:

Note that you need to enable the REST API to use either of the above methods to upgrade your cluster version. For this, enable the CLUSTER_WRITE REST endpoint group (its default is disabled). See the Using the REST Endpoint Groups section section on how to enable them.

Also note that you need to upgrade your Management Center version before upgrading the member version if you want to change the cluster version using Management Center. Management Center is compatible with the previous minor version of Hazelcast. For example, Management Center 4.2 works with both Hazelcast IMDG 4.1 and 4.2. To change your cluster version to 4.2, you need Management Center 4.2.

The cluster can automatically upgrade its version. As soon as it detects that all its members have a version higher than the current cluster version, it upgrades the cluster version to match it. This feature is disabled by default. To enable it, set the system property hazelcast.cluster.version.auto.upgrade.enabled to true.

There is one tricky detail here: as you are shutting down and upgrading the members one by one, when you shut down the last one, all the members in the remaining cluster have the newer version, but you don’t want the auto-upgrade to kick in before you have successfully upgraded the last member as well. To avoid this, you can use the hazelcast.cluster.version.auto.upgrade.min.cluster.size system property. You should set it to the size of your cluster, and then Hazelcast will wait for the last member to join before it can proceed with the auto-upgrade.

In the event of network partitions which split your cluster into two subclusters, split-brain handling works as explained in the Network Partitioning chapter, with the additional constraint that two subclusters only merge as long as they operate on the same cluster version. This is a requirement to ensure that all members participating in each one of the subclusters are able to operate as members of the merged cluster at the same cluster version.

With regards to rolling upgrades, the above constraint implies that if a network partition occurs while a change of cluster version is in progress, then with some unlucky timing, one subcluster may be upgraded to the new cluster version and another subcluster may have upgraded members but still operate at the old cluster version.

In order for the two subclusters to merge, it is necessary to change the cluster version of the subcluster that still operates on the old cluster version, so that both subclusters will be operating at the same, upgraded cluster version and able to merge as soon as the network partition is fixed.

The following provide answers to the frequently asked questions related to rolling member upgrades.

How is the cluster version set?

When a new member starts, it is not yet joined to a cluster; therefore its cluster version is still undetermined. In order for the cluster version to be set, one of the following must happen:

What if a new Hazelcast minor version changes fundamental cluster protocol communication, like join messages?

On startup, as answered in the above question (How is the cluster version set?), the cluster version is not yet known to a member that has not joined any cluster. By default the newly started member uses the cluster protocol that corresponds to its codebase version until this member joins a cluster (so for codebase 4.2.0 this means implicitly assuming cluster version 4.2). If, hypothetically, major changes in discovery & join operations have been introduced which do not allow the member to join a 4.1 cluster, then the member should be explicitly configured to start assuming a 4.1 cluster version.

Do I have to upgrade clients to work with rolling upgrades?

Clients which implement the Open Binary Client Protocol are compatible with Hazelcast version 3.6 and newer minor versions. Thus older client versions are compatible with next minor versions. Newer clients connected to a cluster operate at the lower version of capabilities until all members are upgraded and the cluster version upgrade occurs.

Can I stop and start multiple members at once during a rolling member upgrade?

It is not recommended due to potential network partitions. It is advised to always stop and start one member in each upgrade step.

Can I upgrade my business app together with Hazelcast while doing a rolling member upgrade?

Yes, but make sure to make the new version of your app compatible with the old one since there will be a timespan when both versions interoperate. Checking if two versions of your app are compatible includes verifying binary and algorithmic compatibility and some other steps.

It is worth mentioning that a business app upgrade is orthogonal to a rolling member upgrade. A rolling business app upgrade may be done without upgrading the members.