Microsoft Exchange Server 2007 has several different Mailbox Server high availability features included with the product.  Each of the features is similar to the others in some ways but also very different.

In this post I will explain each of the high availability features and which types of scenarios they are suitable for.

What is High Availability?

High availability is a term used to describe the avoidance of unplanned downtime for a computer system through the implementation of hardware and/or software solutions.  Generally speaking a high availability solution will involve the elimination of and single points of failure in the system, often by duplicating or replicating components of the system so that if one fails the other is able to continue performing the role.

An example of downtime would be an email server that has suffered a hard disk crash and is unavailable to users who are then unable to send or receive email.  An example of a high availability solution in this case would be the use of a RAID volume to protect from single disk failures.Exchange Server 2007 contains several high availability features in the Mailbox Server role that can protect a system from multiple types of failure.  These features are a combination of database replication and server clustering technology.

What is Asynchronous Log Shipping?

Some of the features I am going to describe will include the term “asynchronous log shipping“.  “Asynchronous” means “not synchronised“, and “log shipping” refers to the copying of a transaction log file from one location to another where it is then replayed into a replica of the original database to keep it updated with the changes made to the source database.

Exchange Server 2007 writes transaction log files of 1Mb in size, meaning as each log file reaches 1Mb it is closed off and the next transaction log file is created.  Asynchronous log shipping occurs after a transaction log file is closed off and no longer in use as the active log file.

In essence, asynchronous log shipping is how Exchange Server 2007 database replication occurs.

What is Clustering?

A server cluster is two or more servers working together to perform a particular role so that it appears to be performed by a single system.  There are several different types of clustering commonly used.

Compute Clusters – this refers to the combination of processing power to perform tasks at a high speed than a single system is capable of.  A compute cluster usually involves a master node and several slave nodes.  The master node hands off computational tasks to the slaves and then receives the completed tasks back from them.  For example, many animated movies are created using computer graphics that are rendered by compute cluster farms, with individual frames of animation being processed by different slave nodes.

Load Balanced Clusters – this refers to the combination of several systems to act as a single system by distributing workload across all of the cluster nodes.  For example, a cluster of two web servers will load balance web page requests, so that approximately half of the requests are served by one web server and half by the other.  Very highly trafficked web sites that need to handle millions of visitors each month will operate on load balanced clusters.

High Availability Clusters – these clusters, also commonly known as Failover Clusters, provide high availability for servers by having redundant nodes that are able to take over serving requests if the active node should fail.  Exchange Server 2007 clustering makes use of failover clustering.

Local Continuous Replication

In the next part of this series I will demonstrate the Exchange Server 2007 Local Continuous Replication feature.