# mongoc_collection_watch()

## Synopsis

    mongoc_change_stream_t*
    mongoc_collection_watch (const mongoc_collection_t *coll,
                             const bson_t *pipeline,
                             const bson_t *opts) BSON_GNUC_WARN_UNUSED_RESULT;

A helper function to create a change stream. It is preferred to call this function over using a raw aggregation to create a change stream.

This function uses the read preference and read concern of the collection. If the change stream needs to re-establish connection, the same read preference will be used. This may happen if the change stream encounters a resumable error.

Warning

A change stream is only supported with majority read concern.

This function is considered a retryable read operation. Upon a transient error (a network error, errors due to replica set failover, etc.) the operation is safely retried once. If `retryreads` is false in the URI (see mongoc_uri_t) the retry behavior does not apply.

## Parameters

- `coll`: A mongoc_collection_t specifying the collection which the change stream listens to.

- `pipeline`: A `bson_t` representing an aggregation pipeline appended to the change stream. This may be an empty document.

- `opts`: A `bson_t` containing change stream options.

`opts` may be NULL or a BSON document with additional command options:

- `batchSize`: An `int32` representing number of documents requested to be returned on each call to mongoc_change_stream_next()

- `resumeAfter`: A `Document` representing the logical starting point of the change stream. The result of mongoc_change_stream_get_resume_token() or the `_id` field of any change received from a change stream can be used here. This option is mutually exclusive with `startAfter` and `startAtOperationTime`.

- `startAfter`: A `Document` representing the logical starting point of the change stream. Unlike `resumeAfter`, this can resume notifications after an “invalidate” event. The result of mongoc_change_stream_get_resume_token() or the `_id` field of any change received from a change stream can be used here. This option is mutually exclusive with `resumeAfter` and `startAtOperationTime`.

- `startAtOperationTime`: A `Timestamp`. The change stream only provides changes that occurred at or after the specified timestamp. Any command run against the server will return an operation time that can be used here. This option is mutually exclusive with `resumeAfter` and `startAfter`.

- `maxAwaitTimeMS`: An `int64` representing the maximum amount of time a call to mongoc_change_stream_next() will block waiting for data

- `fullDocument`: An optional UTF-8 string. Set this option to “default”, “updateLookup”, “whenAvailable”, or “required”, If unset, The string “default” is assumed. Set this option to “updateLookup” to direct the change stream cursor to lookup the most current majority-committed version of the document associated to an update change stream event.

- `fullDocumentBeforeChange`: An optional UTF-8 string. Set this option to “whenAvailable”, “required”, or “off”. When unset, the default value is “off”. Similar to “fullDocument”, but returns the value of the document before the associated change.

- `comment`: A `bson_value_t` specifying the comment to attach to this command. The comment will appear in log messages, profiler output, and currentOp output. Only string values are supported prior to MongoDB 4.4.

## Returns

A newly allocated mongoc_change_stream_t which must be freed with mongoc_change_stream_destroy() when no longer in use. The returned mongoc_change_stream_t is never `NULL`. If there is an error, it can be retrieved with mongoc_change_stream_error_document(), and subsequent calls to mongoc_change_stream_next() will return `false`.

See also

mongoc_client_watch()
mongoc_database_watch()