Using Berkeley DB with Apache

In general, it is dangerous to run Berkeley DB in a multiprocess system without some facility to coordinate database recovery between processes sharing the database environment after application or system failure. Failure to run recovery after failure can include process hangs and an inability to access the database environment. The mod_db4 Apache module oversees the proper management of Berkeley DB database environment resources. Developers building applications using Berkeley DB as the storage manager within an Apache module should employ this technique for proper resource management.

Specifically, mod_db4 provides the following facilities:

1. Transparent caching of open DB_ENV and handles.
2. Automatic detection of unexpected failures (segfaults, or a module actually calling exit() and avoiding shut down phases), and automatic termination of all child processes with open database resources to attempt consistency.

mod_db4 is designed to be used as an alternative interface to Berkeley DB. To have another Apache module (for example, mod_foo) use mod_db4, do not link mod_foo against the Berkeley DB library. In your mod_foo makefile, you should:

In mod_foo, to create a mod_db4 managed DB_ENV handle, use the following:

which takes identical arguments to .

To create a mod_db4 managed DB handle, use the following:

Otherwise the API is completely consistent with the standard Berkeley DB API.

The mod_db4 module requires the Berkeley DB library be compiled with C++ extensions and the MM library. (The MM library provides an abstraction layer which allows related processes to share data easily. On systems where shared memory or other inter-process communication mechanisms are not available, the MM library emulates them using temporary files. MM is used in several operating systems to provide shared memory pools to Apache modules.)

To build this apache module, perform the following steps: