218 lines
7.4 KiB
Python
218 lines
7.4 KiB
Python
# Copyright 2017 AT&T Intellectual Property. All other rights reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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"""
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api_lock provides for a rudimentary lock mechanism using
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the database to sync across multiple shipyard instances.
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Also provided is the api_lock decorator to allow for resources
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to easily declare the lock they should be able to acquire before
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executing.
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"""
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from enum import Enum
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import logging
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from functools import wraps
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import falcon
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import ulid
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from shipyard_airflow.db.db import SHIPYARD_DB
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from shipyard_airflow.errors import ApiError
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LOG = logging.getLogger(__name__)
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def api_lock(api_lock_type):
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"""
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Decorator to handle allowing a resource method to institute a lock
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based on the specified lock type.
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These locks are intended for use around methods such as on_post
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and on_get, etc...
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"""
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def lock_decorator(func):
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@wraps(func)
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def func_wrapper(self, req, resp, *args, **kwargs):
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lock = ApiLock(api_lock_type,
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req.context.external_marker,
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req.context.user)
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try:
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lock.acquire()
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return func(self, req, resp, *args, **kwargs)
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except ApiLockAcquireError:
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raise ApiError(
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title='Blocked by another process',
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description=(
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'Another process is currently blocking this request '
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'with a lock for {}. Lock expires in not more '
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'than {} seconds'.format(
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lock.lock_type_name,
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lock.expires
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)
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),
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status=falcon.HTTP_409,
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retry=False,
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)
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finally:
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lock.release()
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return func_wrapper
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return lock_decorator
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class ApiLockType(Enum):
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"""
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ApiLockType defines the kinds of locks that can be set up using
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this locking mechanism.
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"""
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CONFIGDOCS_UPDATE = {'name': 'configdocs_update', 'expires': 60}
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class ApiLock(object):
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"""
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Api Lock provides for a simple locking mechanism for shipyard's
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API classes. The lock provided is intended to block conflicting
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activity across containers by using the backing database
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to calculate if a lock is currently held.
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The mechanism is as follows:
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1) Attempt to write a lock record to the database such that:
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there is no lock in the database of the same kind already that
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is not either released or expired
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1a) If the insert fails, the lock is not acquired.
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2) Query the database for the latest lock of the type provided.
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If the lock's id matches the ID of the process trying to
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acquire the lock, the process should succeed.
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If the lock ID doesn't match the record returned, the
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process attempting to acquire a lock is blocked/failed.
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(Note that the intended use is not to queue requests, but
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rather fail them if something else holds the lock)
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3) Upon completion of the activity, the lock holder will update
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the lock record to indicate that it is released.
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The database query used for insert will only insert if there
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is not already an active lock record for the given type.
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If the insert inserts zero rows, this indicates that the lock
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is not acquired, and does not require a subsequent query.
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The subsequent query is always used when the lock record insert
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has been succesful, to handle race conditions. The select query
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orders by both date and id, Whereby the randomness of the id
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provides for a tiebreaker.
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All locks expire based on their lock type, and default to
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60 seconds
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"""
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def __init__(self,
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api_lock_type,
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reference_id,
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user,
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lock_db=SHIPYARD_DB):
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"""
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Set up the Api Lock, using the input ApiLockType.
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Generates a ULID to represent this lock
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:param api_lock_type: the ApiLockType for this lock
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:param reference_id: the calling process' id provided for
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purposes of correlation
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:param user: the calling process' user for purposes of
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tracking
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"""
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if (not isinstance(api_lock_type, ApiLockType) or
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api_lock_type.value.get('name') is None):
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raise ApiLockSetupError(
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message='ApiLock requires a valid ApiLockType'
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)
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self.lock_id = ulid.ulid()
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self.lock_type_name = api_lock_type.value.get('name')
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self.expires = api_lock_type.value.get('expires', 60)
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self.reference_id = reference_id
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self.user = user
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self.lock_db = lock_db
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def acquire(self):
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"""
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Acquires a lock
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Responds with an ApiLockAcquireError if the lock is not
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acquired
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"""
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LOG.info('Acquiring lock type: %s. Lock id: %s.',
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self.lock_type_name,
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self.lock_id)
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holds_lock = False
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insert_worked = self.lock_db.insert_api_lock(
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lock_id=self.lock_id,
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lock_type=self.lock_type_name,
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expires=self.expires,
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user=self.user,
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reference_id=self.reference_id
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)
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LOG.info('Insert lock %s %s',
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self.lock_id,
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'succeeded' if insert_worked else 'failed')
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if insert_worked:
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lock_retrieved = self.lock_db.get_api_lock(
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lock_type=self.lock_type_name
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)
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holds_lock = lock_retrieved == self.lock_id
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LOG.info(
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'Lock %s is currently held. This lock is %s. Match=%s',
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lock_retrieved,
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self.lock_id,
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holds_lock
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)
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if not holds_lock:
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LOG.info('Api Lock not acquired')
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raise ApiLockAcquireError()
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def release(self):
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"""
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Release the lock
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"""
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try:
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self.lock_db.release_api_lock(self.lock_id)
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except Exception as error:
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# catching Exception because this is a non-fatal case
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# and has no expected action to be taken.
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LOG.error('Exception raised during release of api lock: %s. '
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'Unreleased lock for %s will expire in not more than '
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'%s seconds. Exception: %s',
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self.lock_id,
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self.lock_type_name,
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self.expires,
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str(error))
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class ApiLockError(Exception):
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"""
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Base exception for all api lock exceptions
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"""
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def __init__(self, message=None):
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self.message = message
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super().__init__()
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class ApiLockSetupError(ApiLockError):
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"""
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Specifies that there was a problem during setup of the lock
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"""
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pass
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class ApiLockAcquireError(ApiLockError):
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"""
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Signals to the calling process that this lock was not acquired
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"""
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pass
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