4.3 Shared Memory Structure

The Minirel system consists of several processes, each trying to access and update a single database. Thus, some form of sharing is required between processes to access common data structures. One example of a shared data structure is the log tail. This section describes the data structure maintained by the log manager in shared memory.

The data structure maintained in shared memory is an object of class shm_log (header file: logshm.h). It contains the following fields:

1. The log tail. This is used to write log records into the log. When the log tail is full, it is flushed to disk.

2. The first LSN. This is the LSN of the earliest log record that the recovery subsystem is interested in. Log records with LSNs before this will never be used for a future restart.

3. The next LSN. This is the LSN that will be assigned to the next log record that is written.

4. The log file header. This contains the master log record, the maximum size of the log (in pages), and the log tail size (in pages). Currently, the log tail size is fixed at 1 page.

5. The number of transactions that are currently sharing this shared memory structure.

6. A semaphore to mutually exclude log opens from one another.