Introduction

We implemented a page level lock manager for the Minirel DB system which provides the following locking modes: Intention Shared, Intention Exclusive, Shared and Exclusive locks on file pages. In addition, the lock manager also handles deadlock detection. Locking is done via the use of the following functions:

 Status lock_page(page_def &page,lock_type lock);
 Status upgrade_lock(page_def &page,lock_type lock); 
 Status unlock_page(page_def &page);
 Status unlockall();

The lock manager maintains a table which keeps track of the pages that each transaction has locked. Every time a new lock is requested, the subgraph dealing with that page will be check for cycles. If a cycle is detected, then the lock manager will select the transaction which just completed the cycle for restarting. It will then return this error to the transaction requesting the lock. Upon receiving this error, the transaction is then responsible for restarting itself. Whenever a transaction commits or aborts, it is the operating systems responsibility to call the lock manager function, unlockall(), to unlock all the pages that the transaction currently has locked. The lock manager is intended to be used by functions are responsible for accessing the database at the page level. Before requesting a page from the buffer manager, the function should first call the lock manager to get a lock on that page.

Components Used by the Lock Manager

The lock manager depends upon the database operating system to run. The operating system is responsible for setting up the lock manager's data structures in shared memory and providing access to them. It does this by either maintaining global pointers to these structures, or providing functions that return pointers to these structures. In the implementation, both of these methods are used. The operating system also maintains a shared region around the lock managers data structures so that only one instance of the lock manager can be manipulating the data structures at any given time. It is the lock managers responsibility to lock its data structures in memory before accessing it and to unlock it when it is done.