How to use AtomicLong on Hazelcast CP Subsystem with just 2 Hazelcast cluster nodes?

[Renato Rodrigues]

I am updating to latest Hazelcast version [3.12] and I am facing problem to obtain a instance of AtomicLong. The new version, HZ introduces the concept of CAP Theorem, to grant Consistency and Partition Tolerance, but he problem is the CP subsystem must have at least 3 members.

Config config = new Config();
config.getCPSubsystemConfig().setCPMemberCount(3);
config.getCPSubsystemConfig().setGroupSize(3);

HazelcastInstance instance1 = Hazelcast.newHazelcastInstance(config);

How can I configure the CP Subsystem to provides me an instance of atomicLong with just two hazelcast nodes?

If I start my application with just one node, the follow message is printed:

MetadataRaftGroupManager.log:65 [127.0.0.1]:6000 [dev] [3.12] CP Subsystem is waiting for 3 members to join the cluster. Current member count: 1

I will have just two nodes, so, the CP Subsystem doesn't allow me to use an atomicLong because it will waits for ever for at leats 3 nodes..

The version 3.11 I just called hazelcast.getAtomicLong("count"). How can I handle with this?

[renato...@gmail.com]

0

[ky...@hazelcast.com]

You set the CP member count to 3:

config.getCPSubsystemConfig().setCPMemberCount(3);

Do you still see the error if it is set to 2?

https://docs.hazelcast.org/docs/3.12/javadoc/com/hazelcast/config/cp/CPSubsystemConfig.html

[Renato Rodrigues]

Yes, if I set to 2 , there is a validation cheking if the count is lower than MIN_GROUP_SIZE.

Group size must be an odd value between 3 and 7.

So, i can't define with count equals 2. In fact, I guess I can't any more have an application with two nodes, if I needs an atomicLong 

provided by CP SubSystem.. There is no much sense..

Atenciosamente,

Renato Rodrigues Silva
Analista de Sistemas
Graduado em Tecnologia da Informação
Pós-graduado em Segurança em Redes

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[Ensar Basri Kahveci]

Hi Renato,

You need to keep the majority, i.e., more than half, of the CP members up and running to preserve availability of the CP Subsystem. If you run 3 CP members (majority=2), they will tolerate failure of 1 CP member. When a CP member crashes out of 3, you still need the majority running in order to remove that member and add a new CP member as a replacement. However, majority of 2 CP members is still 2, so running 2 CP members means that you don't have fault tolerance. That is why we require an odd number of CP members to run the CP Subsystem. 

In the next release, we will provide something like a development-mode to run the CP Subsystem with 1 or maybe 2 CP members. This mode will likely to have the full-functionality but it will not be fault-tolerant because of this majority requirement.

Thanks for trying out the new CP Subsystem! 

Regards,

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[yukti...@gmail.com]

Hello Renato,

You can configure more than one Hazelcast member on the same JVM.

You dont neccessarily need additional physical nodes.

Warm Regards,

Yukti.

[yukti...@gmail.com]

Hi Ensar,

Has this feature been provided.If so, with which release?

Warm Regards,

Yukti

[Deger]

Hi Yukti,

CP Subsystem was released with Hazelcast IMDG v3.12 on April 2019. See Hazelcast IMDG v3.12 Release Notes: https://docs.hazelcast.org/docs/rn/index.html#3-12

Regards,

Deger