In general, the term replication means copy, duplicate or an imitation of the original. Dual things make life easier in day to day life. The second copy proves highly advantageous when there is some issue with the original, similarly replication has its own importance in Exchange environment. The concept of replication was first introduced in Exchange 2007 and later on its improved format was included in later version i.e. 2010 and 2013. It is implemented to overcome loss of data in case of corruption in EDB files that leaves data info inaccessible. However, Exchange 2013 database replication ranks top in comparison to replication disaster recovery feature in ES 2007 and 2010.
Importance of Database Replication in Exchange
Exchange Server, which is usually setup at organizational level for admin control, stores almost all information in user’s mailbox that makes up database (DB). The DB stores almost all critical and crucial data of the concerned establishment, the loss of which results in disastrous situation as data means a lot. To avoid data loss due to damaged Exchange EDB file the concept of replication has been introduced. In this process replica of database which is located in one mailbox server is created and stored in several other server mailboxes as copy. This is done so that if the original or the active database file gets corrupt the passive or the replica can be brought in use to access data.
Replication Technologies for High Data Availability in ES
The page includes information about three basic types of Continuous Replication of database used in Exchange and they are as follows:
- Cluster Continuous Replication (CCR)
- Local Continuous Replication (LCR)
- Standby Continuous Replication (SCR)
Though they utilize the same asynchronous log shipping and log replay
technology for replication purpose they differ from each other in many aspects. The different fields in which they differ are mentioned below:
- Methods and steps to enable and disable replication
- Environments where they should be deployed
- Configuration of disk storage
- Management of disk volumes
- Management of replication databases
- Status Information for enabled storage groups
- Ways to view configuration information
- Traditions to verify a passive copy
- Management of replication and replay
- Way-out for recovery from corruption
To understand each kind of replication technologies all the above mentioned ten fields are necessary to know. However the most important field is enabling because clustering can be used only when they are enabled.
Transaction log replication and replay
– It is used to copy the changed data and update the passive copy's database. The replication functionality copies the log files to the passive node as each log file is generated.
How Is Continuous Replication Asynchronous?
- CR is asynchronous in the sense that logs are not copied until they are closed so that usage by mailbox server is discontinued. It symbolizes that usually passive nodes do not have copy of every log file that exists on the active node.
Local Continuous Replication
It utilizes only one server in count to produce and maintain a copy of a storage group on a second set of disks that are connected to the same server as the production storage group (PSG). PSR is referred to as the active copy whereas the copy of storage group maintained on different sets of disk is known as the passive copy. In this way same data is stored in both active and passive LCR copy. In addition, it provides a quick manual switch known as activation to the secondary copy which helps in reducing recovery time for data-level disasters, if enabled. Also the total cost of ownership is minimized by reducing the number of regular full backups created for data protection. Its quick recovery of current data also lays in the fact that it uses only a single server.
Cluster Continuous Replication
It has been developed by the combination of asynchronous log shipping and replay technology with failover as well as management features furnished by the Cluster service
. In the CCR environment, cluster replication ability is integrated with the Cluster service to deliver high availability result. In addition to providing data and service availability, CCR also provides for scheduled outages. When updates need to be installed or when maintenance needs to be performed, administrator is required to move a clustered mailbox server (called an Exchange Virtual Server in earlier versions of server) manually to passive node.
Standby Continuous Replication
SCR which was introduced in Exchange 2007 SP1, extends the existing continuous replication features. As name implies, it is developed to enable use of standby recovery servers. It enables to use continuous replication to replicate mailbox server data from a stand-alone mailbox server (either with or without LCR) or from a clustered mailbox server in SCC or in CCR.
SCR In–Comparison to LCR and CCR
Though SCR is similar to LCR and CCR in many aspects, it has some unique features that are differentiated in the table below:
LCR and CCR
- Supports numerous replication targets per storage group.
- Includes in-built delay for replay that prevents logical corruption.
- Completely managed by EMS only.
High Availability and Site Resilience in Exchange 2013
- Supports single replication target per storage group.
- No delays to avoid logical corruption.
- Managed by EMC as well as EMS.
Exchange Server 2013 mailbox databases can be protected by configuring mailbox servers and databases for high availability as well as site resilience while minimizing the cost by use of Database availability groups (DAG). It enables to deploy message continuity service in establishments of all sizes by enhancing the basic replication capability that was introduced in Exchange 2010 i.e. DAG, which is again a combination of SCR and CCR that were introduced in ES 2007. DAG is the basic component built in Exchange 2013 that can use a group of about 16 mailbox servers for disaster recovery. Not just DAG but also Active Manager, Datacenter Activation Coordination mode and multiple mailbox database copies also adds to increased server availability and enhanced Exchange 2013 database replication technology.