One of the improvements made in Exchange 2010 is the redundancy built-in to the routing of messages through the Exchange environment. Microsoft built-in a technology they call “Shadow Redundancy” which the concept behind shadow redundancy is that a message is not deleted from the queue until the next hop has confirmed delivery to the subsequent hop. If confirmation is not received, the message is resubmitted. If the next hop server is down, the message is resubmitted to another server. Bottomline, messages are no longer lost when a routing server fails.
NOTE: Assured delivery requires that there be redundant Hub Transport and Edge Transport servers to resubmit to if a failure of any given transport server occurs.
The components of the shadow redundancy follow:
As an excerpt from my book “Exchange 2010 Unleashed”, one of my co-authors, Chris Amaris, describes a mail flow using shadow redundancy in the following example. In the example, Chris with mailbox on Exchange Server 2010 Mailbox server MB1 is sending a message to Michelle with a mailbox on Exchange Server 2010 Mailbox server MB2. There are two Exchange Server 2010 Hub Transport servers, HT1 and HT2. The process is:
Shadow redundancy gives the Exchange Server 2010 self-healing capabilities for mail flow. It enables the infrastructure to intelligently fail over between redundant paths if messages have not been delivered in a timely manner.
An administrator needs to do nothing to get Shadow Redundancy to work in Exchange 2010. This feature is built-in, and as long as you have redundant servers in your environment, message routing will be made highly available and routing redundancy will be performed automatically in event of a routing (Hub Transport) server failure.
This design change in Exchange 2010 does beg the question “so what” in terms of how we might architect Exchange 2010 servers. Because routing is made redundant and assurances are made to the delivery of messages between servers, an organization is better off with 2 Hub Transport servers with non-fault tolerant hardware (ie: basic server with no RAID / no reduant power supplies) in their environment than a single Hub Transport server with a lot of hardware fault tolerance. If a basic Hub Transport server fails, messages are routed through the secondary or subsequent server system(s). For organizations virtualizing their servers, having 2 (or more) basic hub transport servers will provide redundancy for the messaging routing better than clustering the guest sessions. This is the strategy that Exchange has taken for redundancy is to have multiple lowend systems (ie: scaling out) than to have a single highly available (ie: scaling up) configuration.
The technology and model work very well, and our architecture of Exchange 2010 environments now take in account this new design structure and high availability and redundancy technology built-in to Exchange 2010.
Rand Morimoto has been in the computer industry for more than 30 years and has authored, co-authored, or been a contributing writer for a couple dozen books on Microsoft Windows, Security, Exchange email, BizTalk Server, and remote and mobile computing. Rand is the president of Convergent Computing, an IT consulting firm that has been one of the key early adopter program partners with Microsoft, implementing beta versions of Microsoft technologies 2-3 years before the product releases to the public. This provides Rand and the consultants in his company extensive knowledge on the technologies long before the products are generally available.
Besides speaking at more than 50 conferences and conventions around the world in the past year on tips, tricks, and best practices on planning, migrating, and implementing technologies, Rand is also head judge for the worldwide Imagine Cup competition, is a Board member for the Chabot Space and Science Center and planetarium, and a Regent for the Board of Saint Mary's College of California.
Rand's book Exchange Server 2010 Unleashed was selected as the November, 2009, book of the month book giveaway. Read a free sample chapter of this book,, hosted exclusively by Microsoft Subnet. Buy the book now from InformIT.
Rand's latest book, Microsoft System Center Enterprise Suite Unleashed has been selected as the April, 2010, Microsoft Subnet book giveaway. Read an excerpt of Microsoft System Center Enterprise Suite Unleashed.
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