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Berkeley DB Reference Guide:
  Introduction

### Sleepycat Software's Berkeley DB products

Sleepycat Software licenses four different products that use the Berkeley DB technology. Each product offers a distinct level of database support. It is not possible to mix-and-match products, that is, each application or group of applications must use the same Berkeley DB product.

All four products are included in the single Open Source distribution of Berkeley DB from Sleepycat Software, and building that distribution automatically builds all four products. Each product adds new interfaces and services to the product that precedes it in the list. As a result, developers can download Berkeley DB and build an application that does only single-user, read-only database access, and easily add support later for more users and more complex database access patterns.

Users who distribute Berkeley DB must ensure that they are licensed for the Berkeley DB interfaces they use. Information on licensing is available directly from Sleepycat Software.

### Berkeley DB Data Store

The Berkeley DB Data Store product is an embeddable, high-performance data store. It supports multiple concurrent threads of control (including multiple processes and multiple threads of control within a process) reading information managed by Berkeley DB. When updates are required, only a single thread of control may be using the database. The Berkeley DB Data Store does no locking, and so provides no guarantees of correct behavior if more than one thread of control is updating the database at a time. The Berkeley DB Data Store is intended for use in read-only applications or applications which can guarantee no more than one thread of control will ever update the database at a time.

### Berkeley DB Concurrent Data Store

The Berkeley DB Concurrent Data Store product adds multiple-reader, single writer capabilities to the Berkeley DB Data Store product, supporting applications that need concurrent updates and do not want to implement their own locking protocols. Berkeley DB Concurrent Data Store is intended for applications that require occasional write access to a database that is largely used for reading.

### Berkeley DB Transactional Data Store

The Berkeley DB Transactional Data Store product adds full transactional support and recoverability to the Berkeley DB Data Store product. Berkeley DB Transactional Data Store is intended for applications that require industrial-strength database services, including excellent performance under high-concurrency workloads with a mixture of readers and writers, the ability to commit or roll back multiple changes to the database at a single instant, and the guarantee that even in the event of a catastrophic system or hardware failure, any committed database changes will be preserved.

### Berkeley DB High Availability

The Berkeley DB High Availability product support for data replication. A single master system handles all updates, and distributes them to as many replicas as the application requires. All replicas can handle read requests during normal processing. If the master system fails for any reason, one of the replicas takes over as the new master system, and distributes updates to the remaining replicas.

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