Using With and Without Threads

You can use a single Connection object in a single thread, and attain reasonable performance by having one Connection object per thread. However, you cannot use the same object from multiple threads concurrently (but see below)

As couchbase is a C extension, it is helpful to know how Python deals with threads in general, and how it handles C extensions in this context.

Python utilizes something called a Global Interpreter Lock (GIL) to allow Python to simulate concurrency. The basic idea is that the interpreter can only ever utilize a single CPU core. Threading is acheived by having the interpreter switch to a different thread every n instructions (where n) is typically 100 - but is set in sys.getcheckinterval() and sys.setcheckinterval(). When C extension code is being executed however, Python has no way of knowing how many ‘instructions’ have passed and thus by default keeps the interpreter lock on for the duration of the extension function.

See http://docs.python.org/2/c-api/init.html#thread-state-and-the-global-interpreter-lock for more details on how Python handles the GIL.

Since couchbase does I/O operations, it is inefficient to keep the entire interpreter locked during the wait for I/O responses; thus it has the ability to unlock the GIL right before it begins an I/O operation, and lock it back again when the operation is complete.

When operating in a threaded environment (i.e. with other Python threads) running, it is helpful to have the GIL handling enabled, as it may potentially block other threads’ execution. When using in non-threaded mode (i.e. where no other Python threads operate within the entire application), it would be good to disable the GIL handling. This reduces locking/unlocking overhead and potentially makes your program faster.

The default is to have GIL handling enabled, so as to not unexpectedly hang other threads in the case where an I/O operation takes a prolonged amount of time.

This behavior itself can be controlled by the unlock_gil attribute

Using a Connection from multiple threads

Sometimes it may be necessary to use a Connection object from multiple threads. This is normally not a good option as there is no concurrency gained from multiple Python threads (as they do not run in parallel, as above) it might be necessary to have a single object which is being used from an already-existing framework using threads, where there is typically little contention between them.

You may utilize the lockmode constructor option to enforce a specific behavior when the Connection object is accessed from multiple threads

couchbase.connection.LOCKMODE_EXC

This is the default lockmode. If it is detected that the object is being used from multiple threads, an exception will be raised indicating such.

Internally this uses the C equivalent of the threading.Lock object (i.e. PyThread_allocate_lock()). Upon each entry to a function it will try to acquire the lock (without waiting). If the acquisition fails, the exception is raised.

couchbase.connection.LOCKMODE_WAIT

In this mode, a lock is also used, only in this case an exception is not raised. Rather, the current thread patiently waits until the other thread has completed its operation and the lock is then acquired. It is released once the current thread has finished performing the operation.

This is the default lockmode

Without this option, odd behavior may be exhibited (including some crashes). If you are sure that the Connection object will never be used from multiple threads, or if you have some other locking mechanism in place, then you may use LOCKMODE_NONE

couchbase.connection.LOCKMODE_NONE

No thread safety checks

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