In a distributed Java web application, how to share a value between all servlets on all machines?

Question

If I have a distributed java web application deployed in a cluster and I have say 10 servlets & 10 JSPs running the show, and if I want to share some data, say a variable or a simple POJO between all the threads of all the servlets on all the machines, what is the way to do it?

No framework like Spring/Struts is used and let's say I'm only using the basic Servlets and JSPs. Usually we think about ServletConfig, ServletContext, HttpSession and HttpServletRequest objects to store information which needs to be passed/shared from one component to another. ServletContext has the largest scope because it's accessible from all the servlets and JSPs in the web app. But in case of distributed application I guess ServeltContext object would be created one per JVM, so even for a single web app every machine in the cluster will have a different java object for ServletContext, correct? So in such a scenario what should be done to share a POJO between all the servlets on all the machines of a single web app?

If it's not possible using plain Servlets and JSP, do any frameworks make is possible? Would appreciate any inputs. Many thanks!

Answer 1

The simplest solution is ConcurrentHashMap https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html

If you want to scale your application - you will need something like hazelcast - http://hazelcast.com/

Answer 2

You can use hazelcast, a framework as memcache but with auto-discovery for clustering . I use to used for the session and cache sharing on my Amazon cluster and works like a charm http://hazelcast.com/use-cases/caching/

But if you want keep in simple you can always use as I said before memcached http://memcached.org/

Answer 3

Sharing things between servers is:

• error prone
• sometimes complicated

The most common thing to want is user session data across a load balanced cluster of servers. If someone is talking to one server, then gets load balanced to a different server, you want to keep their session going. Tomcat Clusters does this, and it's already built in.

https://tomcat.apache.org/tomcat-7.0-doc/cluster-howto.html

The last time I played with that, it was touchy; don't count on session replication always working in any servlet container, and you'll be better off. Also, session replication is crazy expensive; once you're past a few machines, the cost (in RAM) of having all session data everywhere... starts to add up quickly, and you can't add more users easily anymore.

Wanting to share things between multiple JVMs is a code smell; if you can architect around it, do so. But other than clustering, you have the two normal options:

• a database. Tried, true, tested; keep details that need to change there.
• an in-memory store. If it gets called on every request, and/or must be really fast for whatever reason, just consider keeping it in memory; memcached is a multi-machine in-memory key-value-store that does just this.

Answer 4

you are using spring already, so maybe spring session project is a right choice for you - http://projects.spring.io/spring-session/. For sure its the easiest one to run.

Answer 5

In a distributed architecture, it is useful to think beyond objects and think about "services". There are several possible solutions for this but all of them would include some form of service you could access from any of your 10 nodes.

So, you could for example create an 11th machine and host an API for putting and getting objects (values/maps/etc?). That would create a shareable region between the nodes.

However, this opens a whole world of possible issues if not done correctly, because you need to think about sinchronization, deadlocks, dirty reads and other concurrent processing stuff in a cross-JVM mindset.

Also, many systems sinchronize their nodes via the database, but this approach is somehow deprecated nowadays in favor of the more recent "microservices" approach where persistence is distributed, not monolithic.