Re: Searching a disk-backed Map

Tom Anderson <>
Tue, 18 Aug 2009 23:02:31 +0100
On Tue, 18 Aug 2009, Tom Anderson wrote:

On Tue, 18 Aug 2009, Patricia Shanahan wrote:

Stefan Ram wrote:

  This should be a common need. Yet I am not aware of anything
  like it in Java SE. What is the most common (pure Java)
  solution to it?

  I would like to have an implementation of java.util.Map,
  which is constructed with an int ?m? and a ?f?.

  It will use no more than ?m? bytes of memory, but ?swap? out
  (the least often used) entries to the file ?f?, when they do
  not fit into the given memory size anymore.

Have you considered putting the data in a database instead, and using
java.sql to access it? The data structures and algorithms that Java uses
for in-memory maps are not very suitable for disk-based maps. Database
managers use structures and algorithms designed for the job.

'The job' in question being relational data access. Stefan doesn't want that,
he wants to do stores and lookups by key, and nothing else (well, that and
removals, and iteration - but i would imagine the priority is fast storage
and lookup). Yes, this is a subset of what you can do with a relational data
store, but it's quite possible that an implementation which does keyed
storage and nothing else will do it faster and more efficiently.

And if you don't believe me - how about Oracle?

  Relational databases are the most sophisticated tool available to the
  developer for data storage and analysis. Most persisted object data is
  never analyzed using ad-hoc SQL queries; it is usually simply retrieved
  and reconstituted as Java objects. The overhead of using a sophisticated
  analytical storage engine is wasted on this basic task of object
  retrieval. The full analytical power of the relational model is not
  required to efficiently persist Java objects. In many cases, it is
  unnecessary overhead. In contrast, Berkeley DB Java Edition does not have
  the overhead of an ad-hoc query language like SQL, and so does not incur
  this penalty.

  The result is faster storage, lower CPU and memory requirements, and a
  more efficient development process.

That software is freeware; if i was going to implement a disk-backed map,
it's where i'd start.


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