Understanding Databases
A database in MarkLogic Server serves as a layer of abstraction between forests and HTTP, WebDAV, or XDBC servers. A database is made up of data forests that are configured on hosts within the same cluster but not necessarily in the same group. It enables a set of one or more forests to appear as a single contiguous set of content for query purposes. See Understanding Forests for more detail on forests.
Multiple HTTP, XDBC, and WebDAV servers can be connected to the same database, allowing different applications to be deployed over a common content base. A database can also span forests that are configured on multiple hosts enabling data scalability through hardware expansion. To ensure database consistency, all forests that are attached to a database must be available in order for the database to be available.
Warning
All system master databases — Security, Schemas, Triggers, Modules, Extensions, Last-Login and App-Services — MUST be single forest databases. For high availability, you do need to configure one or two replica forests for each system database. But there is no benefit to having multiple master forests in the database.