Oracle have released the core of their Coherence in-memory data grid (IMDG) product as free and open source software.
Named Coherence Community Edition, it has been released under the Open Source Initiative’s Universal Permissive License, Version 1.0, and is hosted on GitHub.
In its simplest form, an IMDG can be thought of as a Java Map that extends across multiple virtual machines and may optionally be persisted to disk. This simple mental model allows developers to quickly on-ramp to the technology, although the complexities of general distributed computing cannot be completely ignored (especially for advanced use cases).