> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# cursor.tailable() (mongosh method)

## Definition

**Important: mongosh Method**

This page documents a [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.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.](https://www.mongodb.com/docs/drivers/)

Marks the cursor as [tailable](https://www.mongodb.com/docs/manual/core/tailable-cursors.md#std-label-tailable-cursors-landing-page) keeping it open even when the client exhausts all results.

For use against a [capped collection](https://www.mongodb.com/docs/manual/core/capped-collections.md#std-label-manual-capped-collection) only. Using `tailable()` against a non-capped collection returns an error.

[`cursor.tailable()`](https://www.mongodb.com/docs/manual/reference/method/cursor.tailable.md#mongodb-method-cursor.tailable) uses the following syntax:

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

`~cursor.tailable()` has the following parameter:

| Parameter | Type | Description |
| --- | --- | --- |
| `awaitData` | boolean | Optional. For use with [`DBQuery.Option.tailable`](https://www.mongodb.com/docs/manual/reference/method/cursor.addoption.md#mongodb-data-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`](https://www.mongodb.com/docs/manual/reference/command/getmore.md#mongodb-dbcommand-dbcmd.getMore) on a cursor with `awaitData` enabled. Defaults to `false`. |

The [cursor](https://www.mongodb.com/docs/manual/core/cursors.md#std-label-cursors) that `~cursor.tailable()` is attached to.

## Compatibility

This method is available in deployments hosted in the following environments:

- [MongoDB Atlas](https://www.mongodb.com/docs/atlas): The fully managed service for MongoDB deployments in the cloud

**Note:**

This command is supported in all MongoDB Atlas clusters. For information on Atlas support for all commands, see [Unsupported Commands.](https://www.mongodb.com/docs/atlas/unsupported-commands/)

- [MongoDB Enterprise](https://www.mongodb.com/docs/manual/administration/install-enterprise.md#std-label-install-mdb-enterprise): The subscription-based, self-managed version of MongoDB

- [MongoDB Community](https://www.mongodb.com/docs/manual/administration/install-community.md#std-label-install-mdb-community-edition): The source-available, free-to-use, and self-managed version of MongoDB

## Behavior

A tailable cursor performs a collection scan over a [capped collection](https://www.mongodb.com/docs/manual/core/capped-collections.md#std-label-manual-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`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#std-program-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.](https://www.mongodb.com/docs/manual/core/tailable-cursors.md#std-label-tailable-cursors-landing-page)