Add support for Redis replication
Redis does not currently support the APIs for replication. This spec addresses that deficiency. Change-Id: I6b22d0c5af92854fb4534a6361d95a7b1cbadf28 Related: blueprint redis-replication
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Peter Stachowski
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specs/liberty/redis-replication.rst
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specs/liberty/redis-replication.rst
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..
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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Sections of this template were taken directly from the Nova spec
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template at:
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https://github.com/openstack/nova-specs/blob/master/specs/template.rst
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..
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This template should be in ReSTructured text. The filename in the git
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repository should match the launchpad URL, for example a URL of
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https://blueprints.launchpad.net/trove/+spec/awesome-thing should be named
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awesome-thing.rst.
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Please do not delete any of the sections in this template. If you
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have nothing to say for a whole section, just write: None
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Note: This comment may be removed if desired, however the license notice
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above should remain.
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=================
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Redis Replication
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=================
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.. If section numbers are desired, unindent this
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.. sectnum::
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.. If a TOC is desired, unindent this
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.. contents::
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Replication functionality needs to be added to the Redis datastore.
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Launchpad Blueprint:
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https://blueprints.launchpad.net/trove/+spec/redis-replication
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Problem Description
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===================
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At present, only single instances of Redis can be created. While useful,
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having multiple slaves that replicate off of a designated master is also
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desirable. This functionality will be addressed in this spec.
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Proposed Change
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===============
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Redis replication is a very simple to use (and configure) master-slave
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replication. It allows Redis slaves to be exact copies of master servers. [1]_
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Replication
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-----------
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Redis replication has the following features:
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- Asynchronous
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- Multiple slaves
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- Slave-of-slave
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- Non-blocking initialization of slaves
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To improve performance, persistence can be turned off on the master node. This
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however can lead to a loss of data if the system reboots and Redis starts
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automatically. For this reason, the master node will be required to have
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persistence enabled.
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Creating a Redis replication network is handled by the Redis SLAVEOF command.
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A new instance (or set of instances) will be created and the SLAVEOF command
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executed on each one. Having Trove create a backup and restore it is not
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necessary, as Redis has this capability built into the SLAVEOF command. This
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means that the Redis replication strategy will need to bypass the creation of a
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backup to add to the snapshot info, and the taskmanager will need to be
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modified to handle this case.
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::
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Note: Redis replication could be enabled using the current backup/restore
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implementation, however once the slave restarts (or starts for the first
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time), it will automatically do a full resync, thus rendering the backup
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obsolete. [1]_
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Enough disk space must be available on the master node to allow Redis to
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persist its data to storage.
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::
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Note: Starting in version 2.8.18, Redis has the (experimental) ability to
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stream the backup directly to the slave. Since this behaviour is still
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considered experimental (in version 3.0), a specific version of Redis will
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not be required - beyond being >= 2.8 - as the feature could be removed in
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a future release. If it exists, however, it can be used by Redis to
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increase performance on systems with slow disks. A configuration parameter
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will be provided to allow operators to turn this feature on.
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The Redis configuration file on each slave will have the corresponding values
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set so that subsequent starting of the database will maintain the slave status.
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As part of the slave configuration, all slaves will also be set to read-only.
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As with the MySQL implementation, slave-of-slave will not be allowed. The
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feature could be augmented to include this in the future.
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The steps to create a replication network is as follows:
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* Create the necessary configuration file. This will have the following
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settings:
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- slaveof <master_ip> <master_port>
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- slave-read-only
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- repl-diskless-sync-delay (if more than one slave is specified)
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* Create 'n' new slave instances with the correct configuration file
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Detach Replica
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--------------
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The current API for detach-replica will need to be implemented. No additions
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to the API are anticipated.
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Failover
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--------
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The current APIs for failover (both eject-replica-source and
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promote-to-replica-source) will need to be implemented. When ejecting the
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current replica source, a slave needs to be chosen as the new one. This will
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be done by overriding the _most_current_replica() method and having it query
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each slave and choose the one with the smallest value for
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'master_last_io_seconds_ago.' This, presumably, will be the one with the most
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current data.
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Configuration
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-------------
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The default values for the following config options will need to be updated:
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* replication_strategy
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Database
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--------
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None
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Public API
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----------
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None
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Public API Security
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-------------------
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None
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Python API
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----------
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Existing Python bindings are sufficient, and no changes are anticipated.
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CLI (python-troveclient)
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------------------------
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Once these changes are implemented, the following Trove CLI
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commands will now be fully functional with Redis:
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- create --replica_of <id> --replica_count <n>
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- eject-replica-source
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- promote-to-replica-source
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- detach-replica
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Internal API
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------------
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None
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Guest Agent
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-----------
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The following files will need to be added to the guest agent, where the
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corresponding implementation will reside:
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.. code-block:: bash
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guestagent/strategies/replication/experimental/redis_sync.py
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The following existing files will be updated:
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.. code-block:: bash
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guestagent/datastore/experimental/redis/manager.py
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guestagent/datastore/experimental/redis/service.py
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guestagent/datastore/experimental/redis/system.py
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No backwards compatibility issues are anticipated.
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Alternatives
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------------
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No alternative solutions are proposed at this time.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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peterstac
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Milestones
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----------
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Target Milestone for completion:
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Liberty-2
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Work Items
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----------
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- Create replication strategy for Redis.
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- Implement API calls for detach_replica, promote_to_replica_source and
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eject_replica_source.
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Upgrade Implications
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====================
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None
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Dependencies
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============
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None
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Testing
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=======
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No new tests are deemed to be required (beyond the requisite unit tests). The
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int-tests group for Redis will be modified to run replication-related commands
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during integration test runs.
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Documentation Impact
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====================
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Datastore specific documentation should be modified to indicate that
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replication is now supported by Redis, along with the corresponding
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detach/failover commands.
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References
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==========
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.. [1] Redis Replication: http://redis.io/topics/replication
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