Tailable Cursors
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By default, MongoDB automatically closes a cursor when the client
exhausts all results in the cursor. However, for capped
collections you can use a tailable cursor that remains open after the client exhausts the results in the
initial cursor. Tailable cursors are conceptually equivalent to the
tail Unix command with the -f option ("follow" mode). After
clients insert additional documents into a capped collection, the
tailable cursor continues to retrieve documents.
Use Cases
Use tailable cursors on capped collections that have high write volumes where indexes aren't practical. For instance, MongoDB replication uses tailable cursors to tail the primary's oplog.
Note
If your query is on an indexed field, use a regular cursor instead of a tailable cursor. Keep track of the last value of the indexed field returned by the query. To retrieve the newly added documents, query the collection again using the last value of the indexed field in the query criteria. For example:
db.<collection>.find( { indexedField: { $gt: <lastvalue> } } )
Get Started
To create a tailable cursor in mongo, see
cursor.tailable().
To see tailable cursor methods for your driver, see your driver documentation.
Behavior
Consider the following behaviors related to tailable cursors:
Tailable cursors do not use indexes. They return documents in natural order.
Because tailable cursors do not use indexes, the initial scan for the query may be expensive. After initially exhausting the cursor, subsequent retrievals of the newly added documents are inexpensive.
A tailable cursor can become invalid if the data at its current position is overwritten by new data. For example, this can happen if the speed of data insertion is faster than the speed of cursor iteration.