StampedLock is an alternative to using a ReadWriteLock (implemented by ReentrantReadWriteLock). The main differences between StampedLock and ReentrantReadWriteLock are that: StampedLocks allow optimistic locking for read operations ReentrantLocks are reentrant (StampedLocks are not) So if you have a scenario where you have contention (otherwise you may as well use synchronized or a simple Lock) and … Read more
You’ve got it correct. In a “SELECT FOR UPDATE” with a JOIN, any rows that contribute to the returned rows will be locked. You can change this behavior by adding an “OF table_a” to the “FOR UPDATE” so that only the rows from table_a will be locked. You can read more about this in the … Read more
(I’m assuming modern SQL databases in this answer.) tl;dr Transactions are not locks, but hold locks that are acquired automatically during operations. And django does not add any locking by default, so the answer is No, it does not lock the database. E.g. if you were do: @transaction.atomic def update_db(): cursor.execute(‘UPDATE app_model SET model_name TO … Read more
try running (if gradle in installed on your machine and in the PATH) gradle –stop or ./gradlew –stop in your project directory from the command line. worked for me.
It’s about locking the memory bus for that address. The Intel 64 and IA-32 Architectures Software Developer’s Manual – Volume 3A: System Programming Guide, Part 1 tells us: 7.1.4 Effects of a LOCK Operation on Internal Processor Caches. For the Intel486 and Pentium processors, the LOCK# signal is always asserted on the bus during a … Read more
While the answer above is true in that a SELECT … FOR UPDATE will prevent concurrent sessions / transactions from inserting the same record, that is not the full truth. I am currently fighting with the same problem and have come to the conclusion that the SELECT … FOR UPDATE is nearly useless in that … Read more
[WRONG: Please see answers below] No, it is not thread-safe by default. You shoud use locking-related SQLiteHelper methods to provide thread safety. [EDIT]: SQLiteDatabase class provides a locking mechanism by default (see comments) and if you are running on multithread, you don’t have to consider changing anything to have thread-safety. Search for ‘thread’ in this … Read more
Just use a threading.RLock which is re-entrant meaning it can be acquired multiple times by the same thread. http://docs.python.org/library/threading.html#rlock-objects For clarity, the RLock is used in the with statements, just like in your sample code: lock = threading.RLock() def func1(): with lock: func2() def func2(): with lock: # this does not block even though the … Read more
It has a large impact. The Update lock takes an Update lock on the row, Intent update on the page and a shared lock on the table / database. This does not stop other queries from accessing the data within the table, since the locks on the page / database are purely share locks. They … Read more
The practical difference between flock() and lockf() is in the semantics (behaviour with respect to closing and passing), applicability over NFS and other shared filesystems, and whether the advisory locks are visible to other processes using fcntl() locks or not. The library you’re using simply has logic to pick the desired semantics based on the … Read more