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What is the point of a read lock?
A reader/writer lock pair allows any number of readers to “own” the read lock at the same time, OR it allows one writer to own the write lock, but it never allows a reader and a writer at the same time, and it never allows more than one writer at the same time.
Why do we need locking?
Locks are used to guard a shared data variable, like the account balance shown here. If all accesses to a data variable are guarded (surrounded by a synchronized block) by the same lock object, then those accesses will be guaranteed to be atomic — uninterrupted by other threads.
What does read lock mean?
Read lock has another name: Shared lock) When a thread has a read lock on a row/table, no thread can update/insert/delete data from that table. (Even if the thread trying to write data doesn’t require a write lock.)
Can you read with write lock?
An RW lock allows concurrent access for read-only operations, while write operations require exclusive access. When a writer is writing the data, all other writers or readers will be blocked until the writer is finished writing.
What is two phase locking in DBMS?
In databases and transaction processing, two-phase locking (2PL) is a concurrency control method that guarantees serializability. The protocol utilizes locks, applied by a transaction to data, which may block (interpreted as signals to stop) other transactions from accessing the same data during the transaction’s life.
What is the overhead of locking?
lock overhead: the extra resources for using locks, like the memory space allocated for locks, the CPU time to initialize and destroy locks, and the time for acquiring or releasing locks.
Why is mutex necessary?
It ensures that only one thread is executing a key piece of code at a time, which in turns limits access to a data structure. It ensures that the both threads have a full and proper view of that memory irrespective of any CPU reordering. The mutex is an absolute necessity when doing concurrent programming.
What is two phase locking explain with example?
Two Phase Locking Protocol also known as 2PL protocol is a method of concurrency control in DBMS that ensures serializability by applying a lock to the transaction data which blocks other transactions to access the same data simultaneously. Two Phase Locking protocol helps to eliminate the concurrency problem in DBMS.
Do you need a lock to read a book?
Reading does not require a lock; as long as you don’t care about the ‘correctness’ of the read. It is only dangerous if you attempt to write without a lock. The original question asked if a lock was required for reading; not locking.
When to use read lock or write lock in Java?
If you have the read lock before the write lock, the read lock will block the write transactions until the reading transaction finishes. When a row/table has a write lock, it cannot be read by another thread if they have a read lock implemented in them but can be read by other threads if no read lock is implemented (i.e simple Select query)
Can a thread have a read lock at the same time?
Multiple threads can have a read lock at the same time but as soon as a thread requests a write lock all new request are blocked until it is complete. This sort of locking would be usefull if you are doing alot of reads and not many writes.
Can a row / table have a write lock at the same time?
(Even if the thread trying to write data doesn’t require a write lock.) A row/table cannot have a read and a write lock at the same time. When a row/table has a write lock, it cannot be read by another thread if they have a read lock implemented in them but can be read by other threads if no read lock is implemented (i.e simple Select query)