http://www.cumulogic.com/multi-az-highly-available-database-on-cumulogic-platform/
We have been working on several features to enhance our cloud services platform and are happy to announce (beta) that users can launch Multi-AZ MySQL instances on CumuLogic Database-as-a-Service (DBaaS). The Multi-AZ database allows users to launch a standby replica of MySQL (Percona Server) on a second availability zone on any cloud. This option provides enhanced database availability and data durability and hence is suitable for deploying production workloads on most clouds. Each database instance runs on its own hardware / storage in a separate datacenter (or availability zone of the IaaS cloud) providing the highest availability of the database for production applications. CumuLogic health monitor performs an automatic switch of the DNS end point to the standby database instance, providing almost uninterrupted access to the data in case of complete failure of an Availability Zone or a datacenter.
How does it work?
Multi-AZ deployments use synchronous replication between the database instances so the data is always up to date on the standby instances. There may be a lag however (which can be viewed on the monitoring charts on the dashboard) depending on the cloud and the availability zones. CumuLogic health monitor continuously checks the database instances for most of failure conditions including storage, networking and compute failures and performs automatic DNS switchover to the standby database node. There is no manual administrative intervention required, and database operations can continue as soon as the DNS change gets propagated (usually between 3 – 5 minutes)
Multi-AZ Database
Enhanced Availability, Durability and Scalability
Multi-AZ deployments provide protection against entire datacenter or Availability Zone failures. Users can also increase the scalability of Multi-AZ database instances by creating read-only replicas of the primary database instances. Additional read-only replicas allow developers to configure applications to read from the replicas and use the primary database instance for write operations only. There can be up to 5 read-only replicas per database instance.
Availability of database is extended during automated backup operations as well. The standby mode is used to take scheduled backups limiting any impacts on the primary database instance. In Single AZ databse deployments, the database operations are blocked for the duration of the backup.
Launching Multi-AZ Databases
To launch, Multi-AZ databases, simple select the Multi-AZ checkbox in the Launch Instancescreen as shown below. You must have at least two target clouds (IaaS clouds) configured in your Profile to be able to use the Multi-AZ option. You can select the Availability Zone of the primary and standby nodes, their instance sizes and persistence storage volumes and backup options.
Multi-AZ Database
Please note that standby node is not visible in the database instance listing.
Note: Currently, it is not possible to convert Single AZ database instance to Multi-AZ and vice versa (feature planned for November 2013 release)
Other Replication Options
It is possible to use read-only replicas to provide higher availability and scalability of database instances. The difference between Multi-AZ instance and Single AZ instance with replicas is that read-replicas use asynchronous MySQL built-in replication which may have higher degree of lag between master and the replica depending on the IaaS cloud, availability zones and storage. In case of failure of a Single AZ database instance, all read replicas are promoted as individual masters. It is also possible to promote one or more replicas as masters manually. To achieve replica scalability and availability of Multi-AZ deployments, we recommend using an architecture with replicas of Multi-AZ instances deployed across primary and standby availability zones.
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