f2ba54b62b
A common scenario in database interaction is to check some values in database and update them if some condition is met (or is not met). To prevent race conditions in such scenarios it is recomended to isolate appropriate transactions at read level, so two "select" operations are not executed in parallel. This may be achieved either by modifying transaction_isolation_level or by doing "selects" with a "for update" keyword, which puts a lock on the entries being selected thus blocking any other selects which may run in parallel transaction. The problem is that SELECT FOR UPDATE is not fully supported in Galera clusters which are a common tool for building HA openstack environments. Such statements may have non-determenistic results - see [1] for more details. This patch intoruduces a simple exclusive lock mechanics: it executes a simple insert or update operation into a special table. Any other transactions attempting to do the same will block until the first one completes in one way or another. This results in a simple but reliable locking solution, which may be not as effecient as dedicated distributed lock systems but still do its job nicely. Partial-bug: #1440094 Change-Id: Ib606f33b832b04a3ecac27e1e10018ea8fd503a5
48 lines
1.2 KiB
Python
48 lines
1.2 KiB
Python
# Copyright 2015 OpenStack Foundation.
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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
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# 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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from alembic import op
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import sqlalchemy as sa
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"""add_locks
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Revision ID: 007
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Revises: 006
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Create Date: 2015-04-08 14:01:06.458512
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"""
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# revision identifiers, used by Alembic.
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revision = '007'
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down_revision = '006'
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MYSQL_ENGINE = 'InnoDB'
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MYSQL_CHARSET = 'utf8'
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def upgrade():
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op.create_table(
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'locks',
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sa.Column('id', sa.String(length=50), nullable=False),
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sa.Column('ts', sa.DateTime(), nullable=False),
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sa.PrimaryKeyConstraint('id'),
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mysql_engine=MYSQL_ENGINE,
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mysql_charset=MYSQL_CHARSET)
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def downgrade():
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op.drop_table('locks')
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