Directory

The directory provides a single, central location where users and applications can look up and configure information about an object. It stores all the information about a Microsoft Exchange Server organization (such as addresses, mailboxes, distribution lists, public folders) and configuration information about sites and servers. The directory also generates the Address Book, which users can use to look up information about other users and to address messages. The Address Book contains all of the recipients in the organization and is organized into lists, such as the global address list.

The directory makes the following tasks possible:

• Finding information about Microsoft Exchange Server users, custom recipients, and other recipients of electronic mail, such as distribution lists and public folders.
• Centrally managing all objects in a site by using either the Administrator program or a custom application that calls the Microsoft Exchange Server directory functions.
• Writing custom applications and add-on services that can store configuration information and make it available through an extension to the Administrator program as a custom property page. (This task is important to developers of third-party applications.)
• Using the directory to look up the information required to provide a specific service. For example, the system attendant looks up connector configuration information to build the routing table. The MTA looks up a user's address and uses it with the routing table to route a message.

All information about an organization is stored as configurable elements or objects in a database. Each object in an organization is represented in a hierarchical structure and has a set of attributes or properties. You can customize your organization by changing the properties for directory objects in the Administrator program. For example, you can set message storage limits for mailboxes by setting properties for the information store object for a server. You can also specify who has permission to make changes to directory objects.

The directory is automatically replicated among servers in a site to ensure that all servers have the same information. Among sites, the directory is replicated by directory bridgehead servers according to a schedule you set in the Administrator program. Each site has one or more directory bridgehead servers that are responsible for keeping the site's directory synchronized with the directories in other sites.

Note   Keeping the directory up-to-date is important. If directory information is out of date, messages might not reach their destinations. For example, if a new mailbox is added to a server, servers in another site will not have that information until the directory is replicated to that site. Also, users cannot send messages to the new mailbox until the directory on the mailbox's home server has been replicated. For more information about directory replication, see Microsoft Exchange Server Operations.

When the directory receives an address request, it performs the following tasks:

• Searches to see whether the address is already in memory (the buffer).
• Searches the directory database for the address if the address is not already in memory.
• Returns the requested address or a "not found" message to the requesting client.

When the directory receives an address change, it performs the following tasks:

• Logs the change in the directory transaction log.
• Writes the change to the memory buffer.
• Returns a confirmation to the user who requested the change.
• Writes the change to the directory database.

Users, administrators, and applications all have access to the directory.

Users Users have access to objects in the directory such as mailboxes, distribution lists, custom recipients, and public folders, all of which are addressable objects.

Administrators Administrators have access to the directory through the Administrator program, and they have permissions to perform the following tasks:

• Create or delete most types of individual directory objects.
• Modify the attribute values of directory objects.
• Perform certain import and export operations

  In most cases, you can perform these tasks on one object at a time. After a directory object has been created, you can edit its using property pages, which display the most commonly accessed attributes.

Applications Other applications running on Windows NT Server and custom applications developed for Microsoft Exchange Server have access to the directory to perform the following tasks:

• Manipulating a large number of directory objects, for example, during administration of custom add-on services
• Synchronizing the Microsoft Exchange Server directory with directories of foreign systems
• Automatically updating Windows NT user accounts when their associated directory objects (such as mailboxes) are updated
• Looking up routing tables, which are stored in the directory

The directory uses a memory cache to minimize the time that is required for processing requests and a transaction log buffer to minimize wait time while read and write requests are logged. The directory's memory cache increases performance by maintaining the most recently requested information in memory rather than reading from the disk for every request.

When an address is created or modified, the directory logs the transaction in the transaction log file. The log is written sequentially to increase performance. If the server fails before changes are written to the database, the directory uses this log to restore the changes.