External Interface

The public interface to the lock manager is contained in lock_manager.h All transactions wanting to use the lock manager must create an instance of the lock manger, class lock_manager_def(), and an instance of its memory manger, class sh_alloc(). Sh_alloc is defined in mem_manager.h When the sh_alloc class is created, an external pointer sh_alloc_ptr must be set to point to it.

//The enumerated type used to request the lock mode     
enum lock_type {Shared, Exclusive, IShared, IExclusive, None}; 

//The structure of a page id.
struct page_def 
{
        Page::Id page_id;           // The page to be locked in the file
        char db_name[MAXFILENAME];  // The name of the file in which the page resides
}

class LockMgr {
friend class deadlock_detector;
public:
    // returns Ok in status if the lock manager was created correctly
    lock_manager_def (Status &status);
    // This needs to connect the lock manager to the lock_table and the 
    // transaction info

    ~lock_manager_def() {};

    // The following public functions return OK on success, 
    // Restart if this transaction must be restarted.
    
    // Request a Shared/IShared or Exclusive/IExclusive lock on a page.  
    // If it is not possible, the OS will be asked to suspend this
    // process until the page becomes available. If the lock request
    // introduces a dependency cycle into the graph, the process
    // requesting the lock will be restarted.  A return code of Restart
    // indicates the transaction must be restarted. Lock page also handles
    // implicit upgrades. 

    Status lock_page(page_def &page,lock_type lock);

    // Upgrade a shared lock to an Exclusive lock.
    // An error code of Restart indicates the transaction must be restarted
    // "page" is the page to upgrade the lock on.  Lock is the new
    // lock you wish to hold on the page.
    Status upgrade_lock(page_def &page,lock_type lock); 

    // Unlock a page
    // Exclusive locks can only be released by calling unlockall()
    Status unlock_page(page_def &page);

    // Unlock all pages locked by this process.  This will usually
    // be called by the OS at commit or abort time.
    Status unlockall();

}