Distributed Cache in Java using ehcache RMI

Question

Requirement : I have 4 servers : A,B,C,D. They all connect to data provider, get the data and persist it into mongodb for N mins. So that if, next time, same request arrives to another server, it takes data from mongodb only instead of making a call to data provider.

|A|
|B|    |data provider|
|C|
|D|

But if, |data provider| response slow, there is a possibility that 2 different requests for same resource arrive to A, B. I want one request waiting until the response of first request is received. I am using queue for this which is fine for single server. But now I need need distributed cache due to multiple servers.

Implementation : After reading few articles over the net, I got to know that Distributed Cache in Java can be implemented using ehcache RMI replication. But I have few doubts before going ahead with ehchache. (Although there are more solutions like JCS etc, I decided to pick ehcache on the basis other answers on StackOverflow)

Doubts

• What if one of the servers gets down? Does ehcache handles this automatically?

Answer 1

Interesting situation, but I fail too see how an extra cache (of any kind) would help solve the problem. Ultimately your problem boils down to one of coordination between servers and a cache has little to add there.

Instead I would either use a queue shared between the four servers where only one request for a resource is allowed on at a time. Another possibility is a shared Map where each server will lock a resource name while retrieving it. Other servers can then wait on this lock and once it is released try to retrieve the resource from MongoDB.

I haven't tried using it, but a combination of redis and redisson looks like a good fit for such a task.

Answer 2

Ehcache with RMI replication is NOT a distributed cache and will not help in your situation because there will not be any shared state on which to queue/isolate your accesses.

Distributed Ehcache - that is backed by Terracotta - can help as you could combine strong consistency with a CacheLoader to obtain that only one thread across servers does load a given resource. But unless you are ready to stick with Terracotta 3.7.x, this is no longer an open source option.

But as Martin said, this may not be the best answer to your use case, as I feel you already use MongoDB as your fast access storage, which makes the cache redundant.