Difference between revisions of "Trove/Specs/Trove-v1-MySQL-Replication"

Revision as of 18:27, 28 April 2014

Description

Providing support for the various replication use cases is critical for use of Trove in production. For the first phase implementation of Replication in Trove we will implement the functionality laid out in the Trove V1 Replication Blueprint

Use Case Summary

The following use cases will be addressed by this V1 implementation:

A. Read Replicas (Slaves)

1. The master can exist before the slave such that the master already contains data
2. N Slaves for one master
3. Slaves can be marked read-only (read-only will be default)
4. A slave can be detached from "replication set" to act as independent site
5. A pre-existing non-replication site can become the master of a new "replication set"
6. The health of a slave will be monitor-able

Design

Trove API

Create Slaves

POST /instances/{id}/action

{
  "replicate": {
    "count": 2,
    "instance": {
      "availability_zone": "us-west-2",
      "flavorRef": "7",
      "volume": { "size": 1 }
    }
    "topology": {
                "slave_of": [{"id": "{id}"}],
                "read_only": true
    }
}

Notes:

• id in the resource URI is the id of the master instance to be replicated
• count allows multiple slaves to be created from a single snapshot of the master
• instance defines the template used for each slave. Certain instance properties (datastore_version, databases, users) will not be supported here
• instance may reference a configuration group, which would then be used for all the slave instances. If no configuration group is specified, the configuration group of the master will be used.
  • If the configuration group includes a server_id property, that property will be ignored as new ids will be generated.

Stop Replication

POST /instances/{id}/topology/action

{
    "detach": {}
}

Notes:

• id in the resource URI is the id of the slave to be detached from its current topology.

Python-Troveclient

trove replicate <master instance> <slave count> --read-only=<boolean>

trove detach_replication <slave instance>

Taskmanager

The taskmanager will implement 2 API calls:

• create_replicated_instances(master_id, slave_count, flavor, topology, volume_size, availability_zone, nics )
• detach_replication(slave_instance)

taskmanager.create_replication

The Create Replication task will be performed with the following steps:

1. Execute getReplicationSnapshot() on the master site, receiving "master snapshot results metadata"
2. N times:
   1. Create trove instance of given flavor, volume size, and any optional instance parameters
   2. generate a unique server_id for the slave.
   3. execute guestagent.create_replication_slave() on new instance
   4. Update instance metadata to add "topology" section for slave
3. Update topology of master to include slaves
4. delete replication snapshot from Swift

After the Create Replication task has completed, showing the topology of the master will list the newly created slave instances:

{
  "topology": {
    "members": [
      {
        "id": "{master-id}",
        "name": "master"
      },
      {
        "id": "{slave1-id}",
        "name": "slave1",
        "mysql": {
          "slave_of": [{"id": "{master-id}"}],
          "read_only": true
        }
      }
      {
        "id": "{slave2-id}",
        "name": "slave2",
        "mysql": {
          "slave_of": [{"id": "{master-id}"}],
          "read_only": true
        }
      }
    ]
  }
}

The topology for a slave will include its own properties:

  "topology": {
      {
        "id": "{slave1-id}",
        "name": "slave1",
        "mysql": {
          "slave_of": [{"id": "{master-id}"}],
          "read_only": true
        }
}

Notes:

• The currently proposed change set for the topology api supports a topology record for each slave but does not explicitly support a combined topology view of the master. To provide the kind of show output suggested here we will update both the master and slave topology records during replication. This is subject to further revision.

taskmanager.detach_replication

Executes guestagent.detach_replication_slave() for the selected instance.

Trove GuestAgent

There will be 3 new methods added to the guestagent API:

• get_replication_snapshot()
• attach_replication_slave()
• detach_replication_slave()

replication will be focused around a replication snapshot. This snapshot will contain the data necessary to set up a slave to replicate from the site which created the snapshot, typically a URI to the user's data set stored in Swift plus the metadata required to coordinate replication.

Each datastore implementation will need to implement these methods. The content of the image uploaded to swift is opaque to the taskmanager and higher components, so the guest agent is free to store whatever data it chooses, in whichever format is most appropriate. The content of the metadata is specific to the datastore, but will be represented as a JSON object.

Trove Guestagent - MySQL Datastore Implementation

get_replication_snapshot()

The MySQL guestagent will use xtrabackup to create a backup of the user's data and upload it to Swift. The metadata will include a URI of the uploaded backup data, along with the site's binlog position and network information required to set up replication.

{
    "master": {
        "host": "192.168.0.1",
        "port": 3306
    },
    "dataset": {
        "datastore": "mysql",
        "datastore_version": "mysql-5.5",
        "dataset_size": 2,
        "snapshot_href": "http://..."
    },
    "binlog_position": <binlog position>
}

attach_replication_slave()

Injects the copy of the master's data into the selected site, then configures the site to receive replicated updates from the master site.

detach_replication_slave()

Stops the slave from replicating from the master. After the instance has been detached from the master, it is an indepent copy of the master's data, and is a fully functional site on its own.

After a slave is detached the topology for the master will no longer contain the detached slave:

{
  "topology": {
    "members": [
      {
        "id": "{master-id}",
        "name": "master"
      },
      {
        "id": "{slave2-id}",
        "name": "slave2",
        "mysql": {
          "slave_of": [{"id": "{master-id}"}],
          "read_only": true
        }
      }
    ]
  }
}

The detached slave (slave1 in this example) will have no topology, as it is now a stand-alone instance.