cursor.tailable()

Definition

cursor.tailable()

Important

mongosh Method

This page documents a mongosh method. This is not the documentation for a language-specific driver, such as Node.js.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

Marks the cursor as tailable keeping it open even when the client exhausts all results.

For use against a capped collection only. Using ~cursor.tailable() against a non-capped collection returns an error.

cursor.tailable() uses the following syntax:

cursor.tailable( { awaitData : <boolean> } )

~cursor.tailable() has the following parameter:

Parameter	Type	Description
awaitData	boolean	Optional. For use with DBQuery.Option.tailable. Sets the cursor to block the query thread when no data is available and await data for a set time instead of immediately returning no data. The cursor returns no data only if the timeout expires. By default, if maxTimeMS is set on the command that created the cursor, then the timeout for awaitData is the remaining time. Otherwise, the default timeout is 1000 milliseconds. You can set a timeout when running getMore on a cursor with awaitData enabled. Defaults to false.

Returns:	The cursor that ~cursor.tailable() is attached to.

Behavior

A tailable cursor performs a collection scan over a capped collection. It remains open even after reaching the end of the collection. Applications can continue to iterate the tailable cursor as new data is inserted into the collection.

If awaitData is true, when the cursor reaches the end of the capped collection, mongod blocks the query thread for the timeout interval and waits for new data to arrive. When new data is inserted into the capped collection, mongod signals the blocked thread to wake and return the next batch to the client.

See Tailable Cursors.