7.3: EDB database structure

7.3 EDB database structure

From Exchange 2000 onward, the Store uses two types of database: the EDB and the STM (a database set). The EDB is the traditional database and is organized into a set of 4-KB pages, numbered in sequence. Microsoft also refers to the EDB as a "property database," because its original function was to hold MAPI properties for messages and other items. Exchange holds message content and attachments as MAPI properties, although there is a limit to the amount of content that a particular property can hold, so the Store often uses extensions to hold large amounts of data. Exchange only needed a MAPI database as long as it only supported MAPI clients, but once Internet clients appeared some change was necessary to support the way they retrieve data. Exchange 5.0 and 5.5 handle Internet clients as if they were modified MAPI clients, but this is an inefficient mechanism, because it forces the Store to convert streamed data to MAPI always.

Microsoft then introduced the STM, or streaming database, in Exchange 2000 to handle the native MIME content generated by POP3 and IMAP4 clients. Every EDB database has a matching STM, and the Store manages the interaction between the EDB and STM databases invisible to clients. Whenever possible, the Store maintains content in its original format and only converts it when necessary, avoiding permanent conversion if not needed. For example, if an older MAPI client that cannot support MIME reads a message held in the STM, the...