Extensible Storage Engine (ESE), also known as JET Blue, is an ISAM (indexed sequential access method) data storage technology from Microsoft. ESE is the core of Microsoft Exchange Server, Active Directory, and Windows Search. It's also used by a number of Windows components including Windows Update client and Help and Support Center. Its purpose is to allow applications to store and retrieve data via indexed and sequential access. ESE provides transacted data update and retrieval. A crash recovery mechanism is provided so that data consistency is maintained even in the event of a system crash. Transactions in ESE are highly concurrent making ESE suitable for server applications. ESE caches data intelligently to ensure high performance access to data. In addition, ESE is lightweight making it suitable for auxiliary applications. The ESE Runtime (ESENT.DLL) has shipped in every Windows release since Windows 2000, with native x64 version of the ESE runtime shipping with x64 versions of Windows XP and Windows Server 2003. Microsoft Exchange, up to Exchange 2003 shipped with only the 32-bit edition, as it was the only supported platform. With Exchange 2007, it ships with the 64-bit edition. Databases A database is both a physical and logical grouping of data. An ESE database looks like a single file to Windows. Internally the database is a collection of 2, 4, 8, 16, or 32 KB pages (16 and 32 KB page options are only available in Windows 7 and Exchange 2010), arranged in a balanced B-tree structure. These pages contain meta-data to describe the data contained within the database, data itself, indexes to persist interesting orders of the data, and other information. This information is intermixed within the database file but efforts are made to keep data used together clustered together within the database. An ESE database may contain up to 232 pages, or 16 terabytes of data, for 8 kilobyte sized pages. ESE databases are organized into groups called instances. Most applications use a single instance, but all applications can also use multiple instances. The importance of the instance is that it associates a single recovery log series with one or more databases. Currently, up to 6 user databases may be attached to an ESE instance at any time. Each separate process using ESE may have up to 1024 ESE instances. A database is portable in that it can be detached from one running ESE instance and later attached