在什么情况下 AJAX 长/短轮询优于 HTML5 WebSockets?

WebSockets 现在绝对是未来.

WebSockets is definitely the future now.

长轮询是一种肮脏的解决方法，可以防止像 AJAX 那样为每个请求创建连接 - 但是当 WebSockets 不存在时会创建长轮询.现在由于 WebSockets，长轮询正在消失.

Long polling is a dirty workaround to prevent creating connections for each request like AJAX does - but long polling was created when WebSockets didn't exist. Now due to WebSockets, long polling is going away no more.

WebRTC 允许点对点通信.

WebRTC allows for peer-to-peer communication.

我建议学习WebSockets.

网络上不同的通信技术

• AJAX - request → response.创建到服务器的连接，发送带有可选数据的请求标头，从服务器获取响应，然后关闭连接.所有主流浏览器都支持.

• AJAX - request → response. Creates a connection to the server, sends request headers with optional data, gets a response from the server, and closes the connection. Supported in all major browsers.

长轮询 - request → wait → response.像 AJAX 一样创建到服务器的连接，但保持一个保持活动连接打开一段时间(虽然不长).在连接过程中，开放的客户端可以从服务器接收数据.由于超时或数据 eof，客户端必须在连接关闭后定期重新连接.在服务器端，它仍然被视为 HTTP 请求，与 AJAX 相同，除了请求的答案将在现在或将来的某个时间发生，由应用程序逻辑定义.支持图表(完整) |维基百科

Long poll - request → wait → response. Creates a connection to the server like AJAX does, but maintains a keep-alive connection open for some time (not long though). During connection, the open client can receive data from the server. The client has to reconnect periodically after the connection is closed, due to timeouts or data eof. On server side it is still treated like an HTTP request, same as AJAX, except the answer on request will happen now or some time in the future, defined by the application logic. support chart (full) | wikipedia

WebSockets - client ↔ server.创建到服务器的 TCP 连接，并在需要时保持打开状态.服务器或客户端可以轻松关闭连接.客户端通过 HTTP 兼容的握手过程.如果成功，那么服务器和客户端可以随时双向交换数据.如果应用程序需要以两种方式进行频繁的数据交换，则它是有效的.WebSockets 确实有数据帧，包括对从客户端发送到服务器的每条消息进行掩码，因此数据被简单地加密.支持图表(非常好) |维基百科

WebSockets - client ↔ server. Create a TCP connection to the server, and keep it open as long as needed. The server or client can easily close the connection. The client goes through an HTTP compatible handshake process. If it succeeds, then the server and client can exchange data in both directions at any time. It is efficient if the application requires frequent data exchange in both ways. WebSockets do have data framing that includes masking for each message sent from client to server, so data is simply encrypted. support chart (very good) | wikipedia

WebRTC - peer ↔ peer.传输在客户端之间建立通信并且与传输无关，因此它可以使用 UDP、TCP 甚至更多抽象层.这通常用于高容量数据传输，例如视频/音频流，其中可靠性是次要的，并且可以牺牲一些帧或质量进展的降低以有利于响应时间，并且至少是一些数据传输.双方(对等点)可以独立地相互推送数据.虽然它可以完全独立于任何中央服务器使用，但它仍然需要某种方式来交换端点数据，在大多数情况下，开发人员仍然使用中央服务器来链接"终端数据.同行.这仅用于交换用于建立连接的基本数据，之后不需要中央服务器.支持图表(中) |维基百科

WebRTC - peer ↔ peer. Transport to establish communication between clients and is transport-agnostic, so it can use UDP, TCP or even more abstract layers. This is generally used for high volume data transfer, such as video/audio streaming, where reliability is secondary and a few frames or reduction in quality progression can be sacrificed in favour of response time and, at least, some data transfer. Both sides (peers) can push data to each other independently. While it can be used totally independent from any centralised servers, it still requires some way of exchanging endPoints data, where in most cases developers still use centralised servers to "link" peers. This is required only to exchange essential data for establishing a connection, after which a centralised server is not required. support chart (medium) | wikipedia

服务器发送的事件 - client ← server.客户端与服务器建立持久和长期的连接.只有服务器可以向客户端发送数据.如果客户端想要向服务器发送数据，则需要使用另一种技术/协议来完成.该协议与 HTTP 兼容且易于在大多数服务器端平台中实现.这是替代长轮询的首选协议.支持图表(良好，IE 除外) |维基百科

Server-Sent Events - client ← server. Client establishes persistent and long-term connection to server. Only the server can send data to a client. If the client wants to send data to the server, it would require the use of another technology/protocol to do so. This protocol is HTTP compatible and simple to implement in most server-side platforms. This is a preferable protocol to be used instead of Long Polling. support chart (good, except IE) | wikipedia

WebSockets 服务器端的主要优势在于，它不是 HTTP 请求(握手后)，而是基于适当消息的通信协议.这使您能够获得巨大的性能和架构优势.例如，在 node.js 中，您可以为不同的套接字连接共享相同的内存，因此它们可以访问共享变量.因此，您不需要使用数据库作为中间的交换点(就像使用 AJAX 或使用 PHP 等语言的长轮询).您可以将数据存储在 RAM 中，甚至可以直接在套接字之间重新发布.

The main advantage of WebSockets server-side, is that it is not an HTTP request (after handshake), but a proper message based communication protocol. This enables you to achieve huge performance and architecture advantages. For example, in node.js, you can share the same memory for different socket connections, so they can each access shared variables. Therefore, you don't need to use a database as an exchange point in the middle (like with AJAX or Long Polling with a language like PHP). You can store data in RAM, or even republish between sockets straight away.

人们经常关注WebSockets的安全性.现实情况是，它几乎没有什么区别，甚至将 WebSockets 作为更好的选择.首先，使用 AJAX，MITM 的几率更高，因为每个请求都是一个新的 TCP 连接，它正在穿越 Internet 基础设施.使用 WebSockets，一旦连接起来，在两者之间进行拦截就更具挑战性，当数据从客户端流式传输到服务器时额外强制执行帧掩码以及额外的压缩，这需要更多的努力来探测数据.所有现代协议都支持:HTTP 和 HTTPS(加密).

People are often concerned about the security of WebSockets. The reality is that it makes little difference or even puts WebSockets as better option. First of all, with AJAX, there is a higher chance of MITM, as each request is a new TCP connection that is traversing through internet infrastructure. With WebSockets, once it's connected it is far more challenging to intercept in between, with additionally enforced frame masking when data is streamed from client to server as well as additional compression, which requires more effort to probe data. All modern protocols support both: HTTP and HTTPS (encrypted).

请记住，WebSockets 通常具有非常不同的网络逻辑方法，更像是一直以来的实时游戏，而不是 http.

Remember that WebSockets generally have a very different approach of logic for networking, more like real-time games had all this time, and not like http.