Which phrase best describes the function of the CometD library in the Streaming API?

The function of the CometD library in the Streaming API is best described as implementing the Bayeux protocol. The Bayeux protocol is a simple, long-polling protocol that allows clients to subscribe to a channel and receive events asynchronously. In the context of Salesforce's Streaming API, CometD facilitates real-time web communication by enabling clients to maintain a persistent connection to the server. This allows clients to receive notifications of changes in Salesforce data immediately after they occur, which is essential for applications that require up-to-date information without the overhead of constant polling.

By acting as a client-side library that listens to server-published events, CometD streamlines the process by managing the intricacies of the protocol, allowing developers to focus on building their applications’ core functionality rather than dealing with the underlying communication mechanics.