Hazelcast is an open source In-Memory Data Grid platform
Terracotta is an open source JVM-level clustering software for Java developed by Terracotta, Inc.
In terms of implementation terracotta ehcache may be based on top of jgroups services and provides a specific set of apis specific of a caching system and so less generic than hazelcast
JGroups, Terracotta & Hazelcast
We started with ehcache terracotta server array cause it s well-known backed by terracotta and has bigger community support than hazelcast
Hazelcast vs ehcache