The DBDirectClient class is an implementation of DBClientBase that simply calls the server's message handling code. It provides a unified API (the DBClientBase API) for performing "sub-operations", and is often used as a simple way for a command implementation to perform some subtask.

Use of DBDirectClient is generally considered harmful for a number of reasons, including that it:

• Subverts the API of the subsystem being called into, and introduces a large number of dependencies for the calling subsystem
• Introduces performance overhead through unnecessary message creation and message parsing
• Prevents yielding of locks during execution of the sub-operation, when the calling operation is already holding a lock
• Is hard for new developers to understand
• Has (historically) been a reliable source of bugs

As of version ~3.1.0 of the server, the following lists all current uses of DBDirectClient in the server. They are roughly ordered in decreasing order of difficulty to remove.

• V8Scope: provides connection objects for the eval command (e.g. the connection object associated with the "db" variable)
• dbtests/: various uses for testing
• Cloner: provides implementation of "copydb" within same host
• MapReduceCommand: miscellaneous collection admin tasks (drop/count/renameCollection/splitVector), reading documents for reduce post-processing
• DocumentSourceOut: agg $out miscellaneous collection admin tasks (create/listIndexes/createIndexes/renameCollection), inserting documents into $out collection
• AuthzManagerExternalState: CRUD operations (with write concern) on "system.users" collection
• DocumentSourceGeoNear: generates documents for $geoNear agg operator
• Trivial uses in ApplyOpsCmd, CmdFileMD5, ReplSource, initOpTimeFromOplog(), checkIfReplMissingFromCommandLine()