- Berkeley DB Reference Guide:
- Locking Subsystem
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All the 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 32-bit 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 locker ID of a different length than the values used by Berkeley DB.
All of the access methods other than Queue use a simple multiple-reader/single writer page locking scheme. The standard read/write locks (DB_LOCK_READ and DB_LOCK_WRITE) and conflict matrix, as described in Standard lock modes are used. An operation that returns data (e.g., DB->get, DBcursor->c_get) obtains a read lock on all the pages accessed while locating the requested record. When an update operation is requested (e.g., DB->put, 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 writelock, even though they are only reading the data. While this may reduce concurrency somewhat, it reduces the probability of deadlock.
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 and for DBC operations, record locks are held until subsequent records are returned or the cursor is closed.
Under non-transaction operation, 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. As cursors maintain a position in a file, they must hold locks across calls and will, in fact, hold locks until the cursor is closed. Furthermore, each cursor is assigned its own unique locker ID when it is created, so cursor operations can conflict with one another. (Each cursor is assigned its own locker ID because 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 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 or uses a combination of cursor and non-cursor operations, these 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 if we block waiting for the write lock, the read lock can never be released. 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.
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 locker ID. Alternatively, 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.
As mentioned earlier, most of the Berkeley DB access methods use page level locking. During Btree traversal, lock-coupling is used to traverse the tree. Note that the tree traversal that occurs during an update operation can also use lock-coupling; it is not necessary to retain locks on internal Btree pages even if the item finally referenced will be updated. Even in the presence of transactions, locks obtained on internal pages of the Btree may be safely released as the traversal proceeds. This greatly improves concurrency. The only time internal locks become crucial is when internal pages are split or merged. When traversing duplicate data items for a key, the lock on the key value also acts as a lock on all duplicates of that key. Therefore, two conflicting threads of control cannot access the same duplicate set simultaneously.
The Recno access method uses a Btree as its underlying data representation and follows similar locking conventions. However, as the Recno access method must keep track of the number of children for all internal pages, it must obtain write locks on all internal pages during read and write operations. In the presence of transactions, these locks are not released until transaction commit.
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