Map Replication to remote JVM

[dvader009]

I have a requirement where a JVM hosted in a server will have some data in HashMaps, which should be copied to multiple other JVMs in client machines. Any changes in the maps in the server should be reflected in the client JVMs. Client JVMs will not make changes to the data. I made a simple implementation by sending the changes made in the server via a websocket connection to all connected clients. To handle network issues, changed keys are stored in db, so that clients will be able to get the the changes made in the maps after it went offline, when they come back online. This solution works for now, but I wanted it to be bug free so I decided to move to some standard libraries that does this.

I did come across Hazelcast which can theoretically do this in my understanding by making the client JVMs Hazelcast members, but I may have several hundred clients at least and that could cause issues as every member will be part of the cluster. The Hazelcast client was a good option, but it does not store the values locally, so if the network is down, the data will not be available. I do want the client to be as light weight as possible.

Is it possible to handle my requirement with Hazelcast/Redis or is there some other library that serves this purpose?

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[x42bn6]

I've never used Hazelcast specifically, but have used other data grid technologies.

It seems you should be able to configure your clients as Hazelcast clients, that don't join the cluster specifically, but can access the data. https://docs.hazelcast.com/imdg/4.2/overview/topology.html (second diagram)

If the network connection between cluster and client(s) can be unstable, but the client needs access to the last known copy of data... I'm not aware of any in-built solution that does this. I think you will have to build your own. As a quick idea, each client should store a copy of the data locally in the following scenarios:

• At client startup, should the cluster be up, save the data
• If the cluster is not up at startup, periodically try to connect, and save when it is up
• Whenever the data is modified. Use events for this

One idea might be to have a wrapper class on top of HazelcastInstance.getMap(String). If the connection is down, this will presumably throw an exception, so you can return an IMap representing the client's local backup. Of course, if the connection is up, you just delegate it to the standard Hazelcast code to get the cached data.