Microservices based architecture and individual cache for each node

Question

Is it considered bad practice to use separated, local cache for each node in distributed microservice application? I've heard that in monolithic application it's OK to use local EHCache as 2nd level cache provider for Hibernate, but in distributed environment it's common practice to use distributed caches, such as Memcached, Redis or Hazelcast. What are the consequences of using separated cache for each node?

Answer 1

"There are only two hard problems in Computer Science:cache invalidation and naming things."-- Phil Karlton

The main problem with local cache in app-server is that it makes cache invalidation much more hard that it was before.

Each time a resource change, it has to be invalidated (and updated) on all the local caches. This would require a system that knows about all the cache servers running at any point of time. This system would have to be informed about all updates so that it can co-ordinate the data invalidation on all servers. It will also have to take care of retries, handling failed servers, etc.

If your application server has it's own local cache, you will have to solve these problems yourselves using a separate system or in the application code. A distributed caching system, would have solved those problems for you. You can make an update call and on success have a guarantee of data consistency (or eventual consistency).

It is separation of concerns. With a separate cache cluster, the caching logic and the associated problems are handled at one place. The same cluster can be reused for multiple applications easily, rather than redoing the same for each application you develop.

Another minor disadvantage is that you would have to warm up the cache each time you spawn a new server, if you don't want a performance degradation. This would lead to longer time to spawn servers.