BDB

Berkeley DB is an embedded database system that supports keyed access to data.

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With bdb >= 0.5.5 nil is stored as an empty string (when marshal is not used).

Open the database with

"store_nil_as_null" => true

if you want the old behavior (nil stored as `000')

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Developers may choose to store data in any of several different storage structures to satisfy the requirements of a particular application. In database terminology, these storage structures and the code that operates on them are called access methods.

* B+tree: Stores keys in sorted order, using a default function that does
  lexicographical ordering of keys.

* Hashing: Stores records in a hash table for fast searches based
  on strict equality, using a default that hashes on the key as a bit
  string. Extended Linear Hashing modifies the hash function used by the
  table as new records are inserted, in order to keep buckets underfull in
  the steady state.

* Fixed and Variable-Length Records: Stores fixed- or
  variable-length records in sequential order. Record numbers may be
  immutable, requiring that new records be added only at the end
  of the database, or mutable, permitting new records to be inserted
  between existing records.

Berkeley DB environment is an encapsulation of one or more databases, log files and shared information about the database environment such as shared memory buffer cache pages.

The transaction subsystem makes operations atomic, consistent, isolated, and durable in the face of system and application failures. The subsystem requires that the data be properly logged and locked in order to attain these properties. Berkeley DB contains all the components necessary to transaction-protect the Berkeley DB access methods and other forms of data may be protected if they are logged and locked appropriately.

A database cursor is a sequential pointer to the database entries. It allows traversal of the database and access to duplicate keyed entries. Cursors are used for operating on collections of records, for iterating over a database, and for saving handles to individual records, so that they can be modified after they have been read.

The lock subsystem provides interprocess and intraprocess concurrency control mechanisms. While the locking system is used extensively by the Berkeley DB access methods and transaction system, it may also be used as a stand-alone subsystem to provide concurrency control to any set of designated resources.

The logging subsystem is the logging facility used by Berkeley DB. It is largely Berkeley DB specific, although it is potentially useful outside of the Berkeley DB package for applications wanting write-ahead logging support. Applications wanting to use the log for purposes other than logging file modifications based on a set of open file descriptors will almost certainly need to make source code modifications to the Berkeley DB code base.