import time
import uuid
from hazelcast.errors import (
LockOwnershipLostError,
LockAcquireLimitReachedError,
SessionExpiredError,
WaitKeyCancelledError,
IllegalMonitorStateError,
)
from hazelcast.future import ImmediateExceptionFuture, Future
from hazelcast.protocol import RaftGroupId
from hazelcast.protocol.codec import (
fenced_lock_lock_codec,
fenced_lock_try_lock_codec,
fenced_lock_unlock_codec,
fenced_lock_get_lock_ownership_codec,
)
from hazelcast.proxy.cp import SessionAwareCPProxy
from hazelcast.util import thread_id, to_millis
_NO_SESSION_ID = -1
[docs]class FencedLock(SessionAwareCPProxy["BlockingFencedLock"]):
"""A linearizable, distributed lock.
FencedLock is CP with respect to the CAP principle. It works on top
of the Raft consensus algorithm. It offers linearizability during crash-stop
failures and network partitions. If a network partition occurs, it remains
available on at most one side of the partition.
FencedLock works on top of CP sessions. Please refer to CP Session
documentation section for more information.
By default, FencedLock is reentrant. Once a caller acquires
the lock, it can acquire the lock reentrantly as many times as it wants
in a linearizable manner. You can configure the reentrancy behaviour
on the member side. For instance, reentrancy can be disabled and
FencedLock can work as a non-reentrant mutex. One can also set
a custom reentrancy limit. When the reentrancy limit is reached,
FencedLock does not block a lock call. Instead, it fails with
``LockAcquireLimitReachedError`` or a specified return value.
Please check the locking methods to see details about the behaviour.
It is advised to use this proxy in a blocking mode. Although it is
possible, non-blocking usage requires an extra care. FencedLock
uses the id of the thread that makes the request to distinguish lock
owners. When used in a non-blocking mode, added callbacks or
continuations are not generally executed in the thread that makes the
request. That causes the code below to fail most of the time since the
lock is acquired on the main thread but, unlock request is done in another
thread. ::
lock = client.cp_subsystem.get_lock("lock")
def cb(_):
lock.unlock()
lock.lock().add_done_callback(cb)
"""
INVALID_FENCE = 0
def __init__(self, context, group_id, service_name, proxy_name, object_name):
super(FencedLock, self).__init__(context, group_id, service_name, proxy_name, object_name)
self._lock_session_ids = dict() # thread-id to session id that has acquired the lock
[docs] def lock(self) -> Future[int]:
"""Acquires the lock and returns the fencing token assigned to the
current thread.
If the lock is acquired reentrantly, the same fencing token is returned,
or the ``lock()`` call can fail with ``LockAcquireLimitReachedError``
if the lock acquire limit is already reached.
If the lock is not available then the current thread becomes disabled
for thread scheduling purposes and lies dormant until the lock has been
acquired.
Fencing tokens are monotonic numbers that are incremented each time
the lock switches from the free state to the acquired state. They are
simply used for ordering lock holders. A lock holder can pass
its fencing to the shared resource to fence off previous lock holders.
When this resource receives an operation, it can validate the fencing
token in the operation.
Consider the following scenario where the lock is free initially ::
lock = client.cp_subsystem.get_lock("lock").blocking()
fence1 = lock.lock() # (1)
fence2 = lock.lock() # (2)
assert fence1 == fence2
lock.unlock()
lock.unlock()
fence3 = lock.lock() # (3)
assert fence3 > fence1
In this scenario, the lock is acquired by a thread in the cluster. Then,
the same thread reentrantly acquires the lock again. The fencing token
returned from the second acquire is equal to the one returned from the
first acquire, because of reentrancy. After the second acquire, the lock
is released 2 times, hence becomes free. There is a third lock acquire
here, which returns a new fencing token. Because this last lock acquire
is not reentrant, its fencing token is guaranteed to be larger than the
previous tokens, independent of the thread that has acquired the lock.
Returns:
The fencing token.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
LockAcquireLimitReachedError: If the lock call is reentrant
and the configured lock acquire limit is already reached.
"""
current_thread_id = thread_id()
invocation_uuid = uuid.uuid4()
return self._do_lock(current_thread_id, invocation_uuid)
[docs] def try_lock(self, timeout: float = 0) -> Future[int]:
"""Acquires the lock if it is free within the given waiting time,
or already held by the current thread at the time of invocation and,
the acquire limit is not exceeded, and returns the fencing token
assigned to the current thread.
If the lock is acquired reentrantly, the same fencing token is returned.
If the lock acquire limit is exceeded, then this method immediately
returns :const:`INVALID_FENCE` that represents a failed lock attempt.
If the lock is not available then the current thread becomes disabled
for thread scheduling purposes and lies dormant until the lock is
acquired by the current thread or the specified waiting time elapses.
If the specified waiting time elapses, then :const:`INVALID_FENCE`
is returned. If the time is less than or equal to zero, the method does
not wait at all. By default, timeout is set to zero.
A typical usage idiom for this method would be ::
lock = client.cp_subsystem.get_lock("lock").blocking()
fence = lock.try_lock()
if fence != lock.INVALID_FENCE:
try:
# manipulate the protected state
finally:
lock.unlock()
else:
# perform another action
This usage ensures that the lock is unlocked if it was acquired,
and doesn't try to unlock if the lock was not acquired.
See Also:
:func:`lock` function for more information about fences.
Args:
timeout: The maximum time to wait for the lock in seconds.
Returns:
The fencing token if the lock was acquired and
:const:`INVALID_FENCE` otherwise.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
"""
current_thread_id = thread_id()
invocation_uuid = uuid.uuid4()
timeout = max(0.0, timeout)
return self._do_try_lock(current_thread_id, invocation_uuid, timeout)
[docs] def unlock(self) -> Future[None]:
"""Releases the lock if the lock is currently held by the current
thread.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
IllegalMonitorStateError: If the lock is not held by
the current thread
"""
current_thread_id = thread_id()
session_id = self._get_session_id()
# the order of the following checks is important
try:
self._verify_locked_session_id_if_present(current_thread_id, session_id, False)
except LockOwnershipLostError as e:
return ImmediateExceptionFuture(e)
if session_id == _NO_SESSION_ID:
self._lock_session_ids.pop(current_thread_id, None)
return ImmediateExceptionFuture(self._new_illegal_monitor_state_error())
def check_response(f):
try:
still_locked_by_the_current_thread = f.result()
if still_locked_by_the_current_thread:
self._lock_session_ids[current_thread_id] = session_id
else:
self._lock_session_ids.pop(current_thread_id, None)
self._release_session(session_id)
except SessionExpiredError:
self._invalidate_session(session_id)
self._lock_session_ids.pop(current_thread_id, None)
raise self._new_lock_ownership_lost_error(session_id)
except IllegalMonitorStateError as e:
self._lock_session_ids.pop(current_thread_id, None)
raise e
return self._request_unlock(session_id, current_thread_id, uuid.uuid4()).continue_with(
check_response
)
[docs] def is_locked(self) -> Future[bool]:
"""Returns whether this lock is locked or not.
Returns:
``True`` if this lock is locked by any thread in the cluster,
``False`` otherwise.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
"""
current_thread_id = thread_id()
session_id = self._get_session_id()
try:
self._verify_locked_session_id_if_present(current_thread_id, session_id, False)
except LockOwnershipLostError as e:
return ImmediateExceptionFuture(e)
def check_response(f):
state = _LockOwnershipState(f.result())
if state.is_locked_by(session_id, current_thread_id):
self._lock_session_ids[current_thread_id] = session_id
return True
self._verify_no_locked_session_id_present(current_thread_id)
return state.is_locked()
return self._request_get_lock_ownership_state().continue_with(check_response)
[docs] def is_locked_by_current_thread(self) -> Future[bool]:
"""Returns whether the lock is held by the current thread or not.
Returns:
``True`` if the lock is held by the current thread, ``False``
otherwise.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
"""
current_thread_id = thread_id()
session_id = self._get_session_id()
try:
self._verify_locked_session_id_if_present(current_thread_id, session_id, False)
except LockOwnershipLostError as e:
return ImmediateExceptionFuture(e)
def check_response(f):
state = _LockOwnershipState(f.result())
locked_by_the_current_thread = state.is_locked_by(session_id, current_thread_id)
if locked_by_the_current_thread:
self._lock_session_ids[current_thread_id] = session_id
else:
self._verify_no_locked_session_id_present(current_thread_id)
return locked_by_the_current_thread
return self._request_get_lock_ownership_state().continue_with(check_response)
[docs] def get_lock_count(self) -> Future[int]:
"""Returns the reentrant lock count if the lock is held by any thread
in the cluster.
Returns:
The reentrant lock count if the lock is held by any thread in the
cluster.
Raises:
LockOwnershipLostError: If the underlying CP session was
closed before the client releases the lock
"""
current_thread_id = thread_id()
session_id = self._get_session_id()
try:
self._verify_locked_session_id_if_present(current_thread_id, session_id, False)
except LockOwnershipLostError as e:
return ImmediateExceptionFuture(e)
def check_response(f):
state = _LockOwnershipState(f.result())
if state.is_locked_by(session_id, current_thread_id):
self._lock_session_ids[current_thread_id] = session_id
else:
self._verify_no_locked_session_id_present(current_thread_id)
return state.lock_count
return self._request_get_lock_ownership_state().continue_with(check_response)
[docs] def destroy(self) -> Future[None]:
self._lock_session_ids.clear()
return super(FencedLock, self).destroy()
[docs] def blocking(self) -> "BlockingFencedLock":
return BlockingFencedLock(self)
def _do_lock(self, current_thread_id, invocation_uuid):
def do_lock_once(session_id):
session_id = session_id.result()
self._verify_locked_session_id_if_present(current_thread_id, session_id, True)
def check_fence(fence):
try:
fence = fence.result()
except SessionExpiredError:
self._invalidate_session(session_id)
self._verify_no_locked_session_id_present(current_thread_id)
return self._do_lock(current_thread_id, invocation_uuid)
except WaitKeyCancelledError:
self._release_session(session_id)
error = IllegalMonitorStateError(
"Lock(%s) not acquired because the lock call on the CP group "
"is cancelled, possibly because of another indeterminate call "
"from the same thread." % self._object_name
)
raise error
except Exception as e:
self._release_session(session_id)
raise e
if fence != self.INVALID_FENCE:
self._lock_session_ids[current_thread_id] = session_id
return fence
self._release_session(session_id)
error = LockAcquireLimitReachedError(
"Lock(%s) reentrant lock limit is already reached!" % self._object_name
)
raise error
return self._request_lock(session_id, current_thread_id, invocation_uuid).continue_with(
check_fence
)
return self._acquire_session().continue_with(do_lock_once)
def _do_try_lock(self, current_thread_id, invocation_uuid, timeout):
start = time.time()
def do_try_lock_once(session_id):
session_id = session_id.result()
self._verify_locked_session_id_if_present(current_thread_id, session_id, True)
def check_fence(fence):
try:
fence = fence.result()
except SessionExpiredError:
self._invalidate_session(session_id)
self._verify_no_locked_session_id_present(current_thread_id)
remaining_timeout = timeout - (time.time() - start)
if remaining_timeout <= 0:
return self.INVALID_FENCE
return self._do_try_lock(current_thread_id, invocation_uuid, remaining_timeout)
except WaitKeyCancelledError:
self._release_session(session_id)
return self.INVALID_FENCE
except Exception as e:
self._release_session(session_id)
raise e
if fence != self.INVALID_FENCE:
self._lock_session_ids[current_thread_id] = session_id
else:
self._release_session(session_id)
return fence
return self._request_try_lock(
session_id, current_thread_id, invocation_uuid, timeout
).continue_with(check_fence)
return self._acquire_session().continue_with(do_try_lock_once)
def _verify_locked_session_id_if_present(self, current_thread_id, session_id, release_session):
lock_session_id = self._lock_session_ids.get(current_thread_id, None)
if lock_session_id and lock_session_id != session_id:
self._lock_session_ids.pop(current_thread_id, None)
if release_session:
self._release_session(session_id)
raise self._new_lock_ownership_lost_error(lock_session_id)
def _verify_no_locked_session_id_present(self, current_thread_id):
lock_session_id = self._lock_session_ids.pop(current_thread_id, None)
if lock_session_id:
raise self._new_lock_ownership_lost_error(lock_session_id)
def _new_lock_ownership_lost_error(self, lock_session_id):
error = LockOwnershipLostError(
"Current thread is not the owner of the Lock(%s) because its "
"Session(%s) is closed by the server." % (self._proxy_name, lock_session_id)
)
return error
def _new_illegal_monitor_state_error(self):
error = IllegalMonitorStateError(
"Current thread is not the owner of the Lock(%s)" % self._proxy_name
)
return error
def _request_lock(self, session_id, current_thread_id, invocation_uuid):
codec = fenced_lock_lock_codec
request = codec.encode_request(
self._group_id, self._object_name, session_id, current_thread_id, invocation_uuid
)
return self._invoke(request, codec.decode_response)
def _request_try_lock(self, session_id, current_thread_id, invocation_uuid, timeout):
codec = fenced_lock_try_lock_codec
request = codec.encode_request(
self._group_id,
self._object_name,
session_id,
current_thread_id,
invocation_uuid,
to_millis(timeout),
)
return self._invoke(request, codec.decode_response)
def _request_unlock(self, session_id, current_thread_id, invocation_uuid):
codec = fenced_lock_unlock_codec
request = codec.encode_request(
self._group_id, self._object_name, session_id, current_thread_id, invocation_uuid
)
return self._invoke(request, codec.decode_response)
def _request_get_lock_ownership_state(self):
codec = fenced_lock_get_lock_ownership_codec
request = codec.encode_request(self._group_id, self._object_name)
return self._invoke(request, codec.decode_response)
[docs]class BlockingFencedLock(FencedLock):
__slots__ = ("_wrapped",)
def __init__(self, wrapped: FencedLock):
self._wrapped = wrapped
[docs] def lock( # type: ignore[override]
self,
) -> int:
return self._wrapped.lock().result()
[docs] def try_lock( # type: ignore[override]
self,
timeout: float = 0,
) -> int:
return self._wrapped.try_lock(timeout).result()
[docs] def unlock( # type: ignore[override]
self,
) -> None:
return self._wrapped.unlock().result()
[docs] def is_locked( # type: ignore[override]
self,
) -> bool:
return self._wrapped.is_locked().result()
[docs] def is_locked_by_current_thread( # type: ignore[override]
self,
) -> bool:
return self._wrapped.is_locked_by_current_thread().result()
[docs] def get_lock_count( # type: ignore[override]
self,
) -> int:
return self._wrapped.get_lock_count().result()
[docs] def destroy( # type: ignore[override]
self,
) -> None:
return self._wrapped.destroy().result()
[docs] def get_group_id(self) -> RaftGroupId:
return self._wrapped.get_group_id()
[docs] def blocking(self) -> "BlockingFencedLock":
return self
def __repr__(self) -> str:
return self._wrapped.__repr__()
class _LockOwnershipState:
def __init__(self, state):
self.fence = state["fence"]
self.lock_count = state["lock_count"]
self.session_id = state["session_id"]
self.thread_id = state["thread_id"]
def is_locked(self):
return self.fence != FencedLock.INVALID_FENCE
def is_locked_by(self, session_id, current_thread_id):
return (
self.is_locked()
and self.session_id == session_id
and self.thread_id == current_thread_id
)