PalDB | An embeddable write-once key-value store written in Java | Key Value Database library

I'm currently implementing a solution for one pretty old system, which needs a lightweight key-value file-based (a value for a key is stored as a file) storage with simple ConcurrentHashMap-like operations: put(), delete(), get() and listAll() operations. It has to privide an ability to have concurrent read and write operations with a fully-featured transactional behavior (ACID + repeatable read isolation).

Seems like this component should look pretty similar to the TableFS...

An initial idea is to take a lightweight embeddable persistent key-value storage and use it to store a metadata for the value files located in a file system (file key, file path, size, other attributes...). Also, it should provide a quick file searching engine, based on indexes. So that, such storage plays a role of a register...

I have been searching for quite a while for such the key-value storage that can meet the following requirements:

• Open source;
• Embeddable;
• Provide a support for Java maps and collections;
• Machine crash/power-off tolerance (write-ahead logs and other approaches);
• Providing ACID transaction;
• Being a read-write storage (comparing to the read-only PalDB).

and came up with the MapDb and LevelDb options.

As far as I need to ensure the concurrency resolution for those read/write user transactions, I need to establish global XA transactions, involving the key-value storage and a file system. For the file system there are the XADisk and Jakarta Commons-Transactions transaction systems, also for the global archestration there is the Atomikos Essintials transaction manager.

However, I cannot find any article that explains how to configure the MapDB or LevelDB with an XA transaction manager...

Could you please suggest on the following?

1. whether my approach makes sense or there are some well-known solutions/implementations for the described problem?
2. what pitfall can you see in this task/approach?
3. are there any resources that describe how to configure such key-value storages for participating in xa transactions?

There is another database - SQLite - which can be integrated with Atomikos. It does not mimic Java collections, but it is pretty easy to emulate all the opeations manually. The only pitfall that I can see for now, is the way of resolving concurrent key modifications by concurrent transactions (lost update problem). Seems like there would be a requirement of writing a kind of stored procedure with the test-and-set feature based on a key's timestamp...