Is there a simple way of having a file backed Map?
The contents of the map are updated regularly, with some being deleted, as well as some being added. To keep the data that is in the map safe, persistence is needed. I understand a database would be ideal, but sadly due to constraints a database can't be used.
I have tried:
Writing the whole contents of the map to file each time it gets updated. This worked, but obviously has the drawback that the whole file is rewritten each time, the contents of the map are expected to be anywhere from a couple of entries to ~2000. There are also some concurrency issues (i.e. writing out of order results in loss of data).
Using a RandomAccessFile and keeping a pointer to each file's start byte so that each entry can be looked up using seek(). Again this had a similar issue as before, changing an entry would involve updating all of the references after it.
Ideally, the solution would involve some sort of caching, so that only the most recently accessed entries are kept in memory.
Is there such a thing? Or is it available via a third party jar? Someone suggested Oracle Coherence, but I can't seem to find much on how to implement that, and it seems a bit like using a sledgehammer to crack a nut.
You could look into MapDB which was created with this purpose in mind.
MapDB provides concurrent Maps, Sets and Queues backed by disk storage or off-heap-memory. It is a fast and easy to use embedded Java database engine.