Usage¶

Locking a function (local to a process)¶

To ensure that a function (which is not thread safe) is only used in a thread safe manner (typically such type of function should be refactored to avoid this problem but if not then the following can help):

@lockutils.synchronized('not_thread_safe')
def not_thread_safe():
    pass

Once decorated later callers of this function will be able to call into this method and the contract that two threads will not enter this function at the same time will be upheld. Make sure that the names of the locks used are carefully chosen (typically by namespacing them to your app so that other apps will not chose the same names).

Locking a function (local to a process as well as across process)¶

To ensure that a function (which is not thread safe or multi-process safe) is only used in a safe manner (typically such type of function should be refactored to avoid this problem but if not then the following can help):

@lockutils.synchronized('not_thread_process_safe', external=True)
def not_thread_process_safe():
    pass

Once decorated later callers of this function will be able to call into this method and the contract that two threads (or any two processes) will not enter this function at the same time will be upheld. Make sure that the names of the locks used are carefully chosen (typically by namespacing them to your app so that other apps will not chose the same names).

Enabling fair locking¶

By default there is no requirement that the lock is fair. That is, it’s possible for a thread to block waiting for the lock, then have another thread block waiting for the lock, and when the lock is released by the current owner the second waiter could acquire the lock before the first. In an extreme case you could have a whole string of other threads acquire the lock before the first waiter acquires it, resulting in unpredictable amounts of latency.

For cases where this is a problem, it’s possible to specify the use of fair locks:

@lockutils.synchronized('not_thread_process_safe', fair=True)
def not_thread_process_safe():
    pass

When using fair locks the lock itself is slightly more expensive (which shouldn’t matter in most cases), but it will ensure that all threads that block waiting for the lock will acquire it in the order that they blocked.

The exception to this is when specifying both external and fair locks. In this case, the ordering within a given process will be fair, but the ordering between processes will be determined by the behaviour of the underlying OS.

Common ways to prefix/namespace the synchronized decorator¶

Since it is highly recommended to prefix (or namespace) the usage of the synchronized there are a few helpers that can make this much easier to achieve.

An example is:

myapp_synchronized = lockutils.synchronized_with_prefix("myapp")

Then further usage of the lockutils.synchronized would instead now use this decorator created above instead of using lockutils.synchronized directly.

Command Line Wrapper¶

oslo.concurrency includes a command line tool for use in test jobs that need the environment variable OSLO_LOCK_PATH set. To use it, prefix the command to be run with lockutils-wrapper. For example:

$ lockutils-wrapper env | grep OSLO_LOCK_PATH
OSLO_LOCK_PATH=/tmp/tmpbFHK45