Centralized vs. Distributed Messaging Systems


If your company is composed of offices all connected by high-bandwidth, reliable network connections, regardless of the distance between offices, you can implement a centralized messaging system. A centralized messaging system means that all of your Exchange servers are located and managed in a central data center and that you have a single routing group. When planning your messaging system, it is best to start with this model in mind because it is the most cost-effective and easily managed.

If your company contains remote offices with low-bandwidth, high-latency, unreliable network connections, you can introduce routing groups to control how messaging traffic is routed from one location to another. However, remote locations and multiple routing groups do not prevent you from centralizing your administrative model. In addition, with the features in Microsoft Windows Server™ 2003, Exchange 2003, and Microsoft Office Outlook® 2003, you also have the opportunity to consolidate your server hardware by removing Exchange servers from remote sites. With these changes, users can log on remotely to Microsoft Windows® services and Exchange 2003 and experience fewer problems related to performance degradation or connectivity.

This topic discusses the characteristics of centralized and distributed messaging systems and gives you some guidelines for planning each model.

Characteristics of a Centralized Messaging System

A centralized messaging system consists of a large data center that hosts all server resources, including the Active Directory® directory service global catalog servers, domain controllers, and Exchange servers. The data center supports all messaging system users, whether they connect locally or remotely. The following are characteristics of a centralized messaging system:

  • Data is hosted and managed in a centralized location regardless of whether the users are connected remotely. This contrasts with the distributed model, where users have local access to mailboxes but server administration is more complex.

  • Software upgrades can be rolled out from a centralized location.

  • The data center incorporates power-insulating devices such as an uninterruptible power supply (UPS) and "hot site" or "cold site" contingencies. A hot site is a full-service commercial site that provides all of the equipment needed for a company to continue operations in the event of a disaster. A cold site is a service that provides space but that the company must furnish and set up. A hot site gets the company up and running faster, but a cold site is a less expensive option.

Business requirements associated with reducing cost and security requirements are usually the driving forces behind centralizing systems. The requirements revolve around location centralization (reducing the number of sites that provide server resources), physical consolidation (replacing smaller servers with high-end servers), administrative consolidation, and data consolidation (centralizing storage solutions that provide backup and disaster recovery capabilities).

Important Considerations

Consider a centralized design only if prerequisites in the following areas are already met or are included in the project plan:

  • Client upgrades   If you plan to deploy Exchange 2003 but not Outlook 2003, offline Cached Exchange Mode is not available to users, and the experience is not improved. In fact, if network connections between client computers and the proposed data center site are slow and unreliable, you need to consider a distributed design.

  • Data center hardware costs   Weigh the cost of installing high-end servers and clusters in the data center against the administrative cost savings of centralizing the servers. It is recommended that you cluster the back-end servers to build high availability and redundancy into the system, but this involves greater costs up front. However, these costs may be more than offset by reductions in operational costs, infrastructure costs, reduced downtime, and greater scalability.

  • Contingency planning   When you centralize your server and data resources across the organization, you increase the possible single points of failure. You must formulate contingency plans in the event your data center is subjected to a catastrophic event.

  • Network outages   Consider the impact that a network outage will have on users in remote locations. If the users have Outlook Cached Exchange Mode enabled, this consideration is less of an issue.

  • Operational and administrative cost reductions   Centralizing server resources reduces operational costs because service capacity and growth are achieved with better use of resources. It also reduces infrastructure costs associated with storage and backup requirements.

  • Data storage   With larger centralized data volumes, you must use more reliable storage systems to improve the integrity of your data. In addition, by reducing the complexity of your server infrastructure, you can more readily restore services and data when a failure occurs.

  • LAN and WAN connectivity   If your current network does not provide the type of bandwidth and speed required for centralizing servers, you need to build a network upgrade into the project plan.

  • Security   A centralized model gives you easier security management, thus, a greater degree of control. This control makes it easier for security staff to maintain up-to-date virus signatures and take timely action in response to security incidents. Another advantage of a centralized design is that it locates your servers in a data center that you can physically secure.

Characteristics of a Distributed Messaging System

A branch office or distributed messaging deployment is one where numerous branch offices or smaller distributed sites have slow connections to a corporate hub or data center. The branches contain their own Exchange servers, domain controllers, and global catalog servers. A distributed messaging system is usually adopted when the network cannot handle traffic to a central hub for services, so the operating system and messaging servers are placed locally. User requirements may be another factor. If the requirements for user experience and availability cannot be met by connecting to a data center, you may have no choice but to place servers in the remote sites.

An Exchange branch office deployment has the following characteristics:

  • The messaging system consists of a large number of locations (branches), each containing an Exchange server, domain controllers, and at least one global catalog server.

  • The branch office locations usually contain a small or varying number of users.

  • The network is usually structured as a hub-and-spoke topology.

  • The network connections between the branch office locations and the central hub or data center are typically low-bandwidth, high-latency, or unreliable.

The main reasons behind deploying a distributed messaging system include the following:

  • The company's users are dispersed across sites.

  • The company's network infrastructure cannot handle traffic to a central hub for services.

  • The user requirements dictate that a server be placed locally to provide optimal user experience and availability.

Important Considerations

Consider the following issues when making your decision about a distributed design:

  • Software upgrades   Rolling out important updates and patches can be much more challenging in a distributed messaging system.

  • Using RPC over HTTP   If you want to use RPC over HTTP, all computers in your messaging environment that your users will need to use with RPC over HTTP communication must be running Windows Server 2003. This includes all global catalog servers and all Exchange servers that your Outlook 2003 users will access.

  • Operational and administrative costs   Distributed messaging systems require more servers and result in higher operational and administrative costs.

  • Data storage   With distributed servers, the service infrastructure is more complex, which makes it more difficult to restore services and data when a failure occurs.

  • Network connections   For remote offices, it is recommended that the network connection to the hub site or data center be no less than 56 Kbps between servers. Between a hub and an office, however, a higher connection speed is recommended.

  • Security   The physical security of servers in branch offices is a major consideration. In a branch office design, you must take precautions to ensure that servers are not located in open areas and that they are physically secured.