Recommend a fast & scalable persistent Map - Java

Question

I need a disk backed Map structure to use in a Java app. It must have the following criteria:

1. Capable of storing millions of records (even billions)
2. Fast lookup - the majority of operations on the Map will simply to see if a key already exists. This, and 1 above are the most important criteria. There should be an effective in memory caching mechanism for frequently used keys.
3. Persistent, but does not need to be transactional, can live with some failure. i.e. happy to synch with disk periodically, and does not need to be transactional.
4. Capable of storing simple primitive types - but I don't need to store serialised objects.
5. It does not need to be distributed, i.e. will run all on one machine.
6. Simple to set up & free to use.
7. No sql type queries required

Records keys will be strings or longs. As described above reads will be much more frequent than writes, and the majority of reads will simply be to check if a key exists (i.e. will not need to read the keys associated data). Each record will be updated once only and records are not deleted.

I'm currently using Bdb JE but am looking for other options and am interested in hearing what others either recommend or advise against using.

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Update

I have since improved the query performance on the existing BDB setup by reducing the dependency on secondary keys. Some of our queries required a join on two secondary keys. By reducing the number of secondary keys joined on (e.g by combining them into a composite key) we have removed a level of indirection in the lookup which speeds things up nicely.

Answer 1

I'd likely use a local database. Like say Bdb JE or HSQLDB. May I ask what is wrong with this approach? You must have some reason to be looking for alternatives.

In response to comments: As the problem performance and I guess you are already using JDBC to handle this it might be worth trying HSQLB and reading the chapter on Memory and Disk Use.

Answer 2

I think Hibernate Shards may easily fulfill all your requirements.

Answer 3

memcached provides an excellent scalable map-based distributable cache. If you use this and back it with one of the databases mentioned to provide persistance, you may well solve your performance problems, at least for frequently hit keys (as long as you provide enough RAM to cache all the values that are frequently accessed).

Answer 4

SQLite does this. I wrote a wrapper for using it from Java: http://zentus.com/sqlitejdbc

As I mention in a comment, I have successfully used SQLite with gigabytes of data and tables of hundreds of millions of rows. If you think out the indexing properly, it's very fast.

The only pain is the JDBC interface. Compared to a simple HashMap, it is clunky. I often end up writing a JDBC-wrapper for the specific project, which can add up to a lot of boilerplate code.

Answer 5

JBoss (tree) Cache is a great option. You can use it standalone from JBoss. Very robust, performant, and flexible.

Answer 6

You may want to look into OrientDB.

Answer 7

I've so far found Tokyo Cabinet to be the simplest persistent Hash/Map to integrate into my code.

This abbreviated example, taken from the docs, shows how simple it is to save and retrieve data from a persistent Hash:

    // create the object
    HDB hdb = new HDB();
    // open the database
    hdb.open("casket.tch", HDB.OWRITER | HDB.OCREAT);
    // add item 
    hdb.put("foo", "hop");
    hdb.close();