- Berkeley DB Reference Guide:
- Architecture
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The Berkeley DB subsystems can be accessed through interfaces from multiple languages. The standard library interface is ANSI C. Applications can also use Berkeley DB via C++ or Java, as well as from scripting languages. Environments can be shared among applications written using any of theses APIs. For example, you might have a local server written in C or C++, a script for an administrator written in Perl or Tcl, and a web based user interface written in Java, all sharing a single database environment.
The Berkeley DB library is written entirely in ANSI C. C applications use a single include file:
#include <db.h>
The C++ classes provide a thin wrapper around the C API, with the major advantages being improved encapsulation and an optional exception mechanism for errors. C++ applications use a single include file:
#include <db_cxx.h>
The classes and methods are named in a fashion that directly corresponds to structures and functions in the C interface. Likewise, arguments to methods appear in the same order as the C interface, except to remove the explicit this pointer. The #defines used for flags are identical between the C and C++ interfaces.
As a rule, each C++ object has exactly one structure from the underlying C API associated with it. The C structure is allocated with each constructor call and deallocated with each destructor call. Thus, the rules the user needs to follow in allocating and deallocating structures are the same between the C and C++ interfaces.
To ensure portability to many platforms, both new and old, Berkeley DB makes as few assumptions as possible about the C++ compiler and library. For example, it does not expect STL, templates or namespaces to be available. The newest C++ feature used is exceptions, which are used liberally to transmit error information. Even the use of exceptions can be disabled at runtime.
The Java classes provide a layer around the C API that is almost identical to the C++ layer. The classes and methods are, for the most part identical to the C++ layer. Db constants and #defines are represented as "static final int" values. Error conditions are communicated as Java exceptions.
As in C++, each Java object has exactly one structure from the underlying C API associated with it. The Java structure is allocated with each constructor or open call, but is deallocated only by the Java garbage collector. Because the timing of garbage collection is not predictable, applications should take care to do a close when finished with any object that has a close method.
Berkeley DB supports the standard UNIX interfaces dbm (or its ndbm variant) and hsearch. After including a new header file and recompiling, dbm programs will run orders of magnitude faster and their underlying databases can grow as large as necessary. Historic dbm applications fail when some number of entries were inserted into the database, where the number depends on the effectiveness of the hashing function on the particular data set.
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