- Berkeley DB Reference Guide:
- Locking Subsystem
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All Berkeley DB access methods follow the same conventions for locking database objects. Applications that do their own locking and also do locking via the access methods must be careful to adhere to these conventions.
Whenever a Berkeley DB database is opened, the DB handle is assigned a unique locker ID. Unless transactions are specified, that ID is used as the locker for all calls that the Berkeley DB methods make to the lock subsystem. In order to lock a file, pages in the file, or records in the file, we must create a unique ID that can be used as the object to be locked in calls to the lock manager. Under normal operation, that object is a 28-byte value created by the concatenation of a unique file identifier, a page or record number, and an object type (page or record).
In a transaction-protected environment, database create and delete operations are recoverable and single-threaded. This single-threading is achieved using a single lock for the entire environment that must be acquired before beginning a create or delete operation. In this case, the object on which Berkeley DB will lock is a 4-byte unsigned integer with a value of 0.
If applications are using the lock subsystem directly while they are also using locking via the access methods, they must take care not to inadvertently lock objects that happen to be equal to the unique file IDs used to lock files. This is most easily accomplished by using a lock object with a length different from the values used by Berkeley DB.
All the access methods other than Queue use standard read/write locks in a simple multiple-reader/single writer page-locking scheme. An operation that returns data (for example, DB->get or DBcursor->c_get) obtains a read lock on all the pages accessed while locating the requested record. When an update operation is requested (for example, DB->put or DBcursor->c_del), the page containing the updated (or new) data is write-locked. As read-modify-write cycles are quite common and are deadlock-prone under normal circumstances, the Berkeley DB interfaces allow the application to specify the DB_RMW flag, which causes operations to immediately obtain a write lock, even though they are only reading the data. Although this may reduce concurrency somewhat, it reduces the probability of deadlock. In the presence of transactions, page locks are held until transaction commit.
The Queue access method does not hold long-term page locks. Instead, page locks are held only long enough to locate records or to change metadata on a page, and record locks are held for the appropriate duration. In the presence of transactions, record locks are held until transaction commit. For Berkeley DB operations, record locks are held until operation completion; for DBC operations, record locks are held until subsequent records are returned or the cursor is closed.
Under non-transaction operations, the access methods do not normally hold locks across calls to the Berkeley DB interfaces. The one exception to this rule is when cursors are used. Because cursors maintain a position in a file, they must hold locks across calls; in fact, they will hold locks until the cursor is closed.
In this mode, the assignment of locker IDs to DB and cursor handles is complicated. If the DB_THREAD option was specified when the DB handle was opened, each use of a DB has its own unique locker ID, and each cursor is assigned its own unique locker ID when it is created, so DB handle and cursor operations can all conflict with one another. (This is because when Berkeley DB handles may be shared by multiple threads of control the Berkeley DB library cannot identify which operations are performed by which threads of control, and it must ensure that two different threads of control are not simultaneously modifying the same data structure. By assigning each DB handle and cursor its own locker, two threads of control sharing a handle cannot inadvertently interfere with each other.)
This has important implications. If a single thread of control opens two cursors, uses a combination of cursor and non-cursor operations, or begins two separate transactions, the operations are performed on behalf of different lockers. Conflicts that arise between these different lockers may not cause actual deadlocks, but can, in fact, permanently block the thread of control. For example, assume that an application creates a cursor and uses it to read record A. Now, assume a second cursor is opened, and the application attempts to write record A using the second cursor. Unfortunately, the first cursor has a read lock, so the second cursor cannot obtain its write lock. However, that read lock is held by the same thread of control, so the read lock can never be released if we block waiting for the write lock. This might appear to be a deadlock from the application's perspective, but Berkeley DB cannot identify it as such because it has no knowledge of which lockers belong to which threads of control. For this reason, application designers are encouraged to close cursors as soon as they are done with them.
If the DB_THREAD option was not specified when the DB handle was opened, all uses of the DB handle and all cursors created using that handle will use the same locker ID for all operations. In this case, if a single thread of control opens two cursors or uses a combination of cursor and non-cursor operations, these operations are performed on behalf of the same locker, and so cannot deadlock or block the thread of control.
Complicated operations that require multiple cursors (or combinations of cursor and non-cursor operations) can be performed in two ways. First, they may be performed within a transaction, in which case all operations lock on behalf of the designated transaction. Second, they may be performed using a local DB handle, although, as DB->open operations are relatively slow, this may not be a good idea. Finally, the DBcursor->c_dup function duplicates a cursor, using the same locker ID as the originating cursor. There is no way to achieve this duplication functionality through the DB handle calls, but any DB call can be implemented by one or more calls through a cursor.
When the access methods use transactions, many of these problems disappear. The transaction ID is used as the locker ID for all operations performed on behalf of the transaction. This means that the application may open multiple cursors on behalf of the same transaction and these cursors will all share a common locker ID. This is safe because transactions cannot span threads of control, so the library knows that two cursors in the same transaction cannot modify the database concurrently.
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