Let me jump to the cool part:
Galaxy is a distributed RAM. It is not a key-value store. Rather, it is meant to be used as a infrastructure for building distributed data-structures. In fact, there is no way to query objects stored on Galaxy at all. Instead, Galaxy generates an ID for each item, that you can store in other items just like you’d store a normal reference in a plain object graph.
The application runs on all Galaxy nodes alongside with the portion of the data that is kept (in RAM) at each of the nodes, and when it wishes to read or write a data item, it requests the Galaxy API to fetch it.
At any given time an item is owned by exactly one node, but can be shared by many. Sharers store the item locally, but they can only read it. However, they remember who the owner is, and the owner maintains a list of all sharers. If a sharer (or any node) wants to update the item (a “write”) it requests the current owner for a transfer of ownership, and then receives the item and the list of sharers. Before modifying the item, it invalidates all sharers to ensure consistency. Even when the sharers are invalidated, they remember who the new owner is, so if they’d like to share or own the item again, they can request it from the new owner. If the application requests an item the local node has never seen (or it’s been migrated again after it had been validated), the node multicasts the entire cluster in search of it.
The idea is that when data access is predictable, expensive operations like item migration and a clueless lookup are rare, and more than offset by the common zero-I/O case. In addition, Galaxy uses some nifty hacks to eschew many of the I/O delays even in worst-case scenarios.
In the coming weeks I will post here the exact details of Galaxy’s inner-workings. What messages are transferred, how Galaxy deals with failures, and what tricks it employs to reduce latencies. In the meantime, I encourage you to read Galaxy’s documentation and take it for a spin.
May not fit your use case but like the man says, “take it for a spin.”
Jack Park sent this to my attention.