To understand CometD. We need to understand few terms first which are as follows:
What is Bayeux?
- Bayeux is a protocol for transporting asynchronous messages (primarily over web protocols such as HTTP and WebSocket), with low latency between a web server and web clients.
- What is latency? Latency is the time it takes for data to pass from one point on a network to another. Suppose Server A in New York sends a data packet to Server B in London
- The primary purpose of Bayeux is to support responsive bidirectional interactions between web clients, for example using AJAX, and the web server.
- Delivery of asynchronous messages from the server to a web client is often described as server push. The combination of server push techniques with an AJAX web application has been called Comet.
- CometD is a project to provide multiple implementation of the Bayeux protocol in several programming languages.
What is CometD?
- CometD is a scalable web event routing bus that allows you to write low-latency, server-side, event-driven web applications. Typical examples of such applications are stock
trading applications, web chat applications, online games, and monitoring
consoles.
- The CometD project provides Java and JavaScript libraries that allow you to write low-latency, server-side, event-driven web applications in a simple and portable way.
- You can therefore concentrate on the business side of your application rather than worrying about low-level details such as the transport (HTTP or WebSocket), the scalability and the robustness.
What is Long Polling?
- Long polling, also called Comet programming, allows emulation of an information push from a server to a client. Similar to a normal poll, the client connects and requests information from the server. However, instead of sending an empty response if information isn’t available, the server holds the request and waits until information is available (an event occurs). The server then sends a complete response to the client.
- The client continually maintains a connection to the server, so it’s always waiting to receive a response. In the case of server timeouts, the client connects again and starts over.
What is empAPI?
- EMP Connector is a thin wrapper around the CometD library. It hides the complexity of creating a CometD client and subscribing to Streaming API in Java.
- The CometD-based subscription mechanism in EMP Connector can receive any type of Salesforce event. Just pass in the channel name of the desired event. For example, the events that EMP Connector can receive include:
- The CometD-based subscription mechanism in EMP Connector can receive any type of Salesforce event. Just pass in the channel name of the desired event. For example, the events that EMP Connector can receive include: