In a typical production deployment, one would expect the majority of external client browsers to not be able to communicate using WebSockets. There are several reasons for this:

These issues in combination make it more likely than not that a corporate web user will be unable to use WebSockets.

That said, if a browser doesn't support WebSocket, it will fail over to the next driver immediately. On the other hand, if the browser does support WebSockets, but an intermediate proxy or reverse proxy server doesn't, then the WebSocket HTTP handshake would, in some circumstances, result in the proxy immediately returning an error response to the browser. Under these conditions, the browser will retry the WebSocket driver a maximum of three times (incurring the "3 second" delay as per the "back-off" rules described above) before failing over to the next driver. The worse case scenario is a client using a proxy that simply ignores the WebSocket HTTP Protocol Upgrade handshake; here you are at the mercy of the client timing out, as the UM server would never receive the WebSocket upgrade request. Although you can set these timeouts to whatever values you wish, the default values are set to support clients on slow connections - lowering them will cause clients on slow connections to frequently fail to initialize sessions.

For client environments that support it, WebSocket is an excellent choice of driver. Whether you wish to risk a 3-second delay (or, in the rare cases described above, much higher) for those client environments that don't support it is down to your distribution of customers. If you are building applications targeting customers whose infrastructure you understand, then it is worth using the WebSocket driver. If the audience is more generally distributed, then a good proportion of the clients that are unable to use WebSockets will incur a session initialization delay. You may therefore wish to exclude the WEBSOCKET driver from your application's session configuration.

Unlike WebSocket, this driver does not rely on anything other than standard HTTP requests and a streaming HTTP response. If the client supports the driver, the only issues here are intermediate infrastructure components interfering with the real-time stream of data sent from the UM server to the client:

Although Internet Explorer 8 and Internet Explorer 9 support the XHR object, they cannot use the XHR object to make cross-domain requests. This driver therefore instead uses a Microsoft-specific object called the XDomainRequest object which can support streaming responses to fully cross-domain requests.

As mentioned in its description, this driver can not send client cookies to the server. This is because Microsoft chose to prevent cookies being sent via the XDR object. As a result, any intermediate DMZ infrastructure (such as a load balancer) that relies on client-side HTTP cookies to maintain, for example, load balancer server "stickiness", will be unable to maintain stickiness (since no cookies that the load balancer might set will appear in subsequent client requests when using an XDR based driver). If the load-balancer is fronting more than one UM server, then this setup can result in load balancers sending a client's post-session-initialization requests, such as a subscription request, to a random back-end UM server rather than to the server with which a client had initialized a (load-balancer-proxied) session. This will cause a client to reinitialize their session, and repeat ad infinitum. In these cases, if load balancers are indeed using cookie-based stickiness, then you have two options: either explicitly configure JavaScript sessions with a set of drivers that exclude all XDR variants, or change the load balancer configuration to use client IP-based stickiness instead of cookie-based stickiness instead

In all likelihood, a client browser will settle on one of the three drivers discussed above before it would fail over to this driver. This driver is really only of use in environments where, for some reason, a browser is unable to create an instance of an XHR object. This used to be the the case in some older versions of Internet Explorer if, for example, a Windows Administrator's policy prevented Internet Explorer from being able to invoke ActiveX objects (such as the XMLHTTPRequest object). In modern versions of Internet Explorer, however, XMLHTTPRequest is a native non-ActiveX object so this is less of an issue.

The downsides of this driver are the same as those for the XHR_STREAMING_CORS driver: proxy and antivirus product interference.

This driver is really only useful to a small subset of modern browsers such as Opera. It does not rely on any unusual HTTP behaviour, and is therefore only subject to the same negatives as the other streaming drivers: proxy and antivirus product interference.

This is the most efficient long-polling driver for Internet Explorer 8 and Internet Explorer 9. Its downside is the lack of cookie support. This is only an issue if dealing with certain load balancer configurations (see discussion of XDR_STREAMING).

This is the most efficient long-polling driver for non-Internet Explorer browsers and for Internet Explorer 10+.

As with all long polling drivers (including XHR_LONGPOLL_POSTMESSAGE and JSONP_LONGPOLL discussed below), the browser will maintain an open connection to the server until data is available for transport to the browser. As soon as data arrives for delivery to the browser, the server will wait for a short period of time (known as the server-side "Long Poll Active Delay" timeout, which defaults to 100ms) before sending the data to the browser, and closing the connection. The browser will then immediately make a new long-poll connection to the server, in preparation for more data. Since the server closes the connection immediately after sending any data, any intermediary proxy servers or antivirus products will not be buffering the response in the same was as they might buffer a streaming response, but will instead immediately relay the response to the browser.

Note that if no data is available to be sent to the browser (if, say, the client is subscribed to a channel which rarely contains events) then the long-poll connection will remain open for a limited period before being closed by the server (at which point the browser will automatically create a new long-poll request and wait for data once again). The length of time for which such "quiet" connections stay open is defined by the "Long Poll Idle Delay" value, which can be set using Enterprise Manager (see the JavaScript tab for the relevant Interface). It is important that this value is lower than the value of any intermediary proxy servers' own timeouts (which are often 60 seconds, but sometimes as low as 30 seconds). A suitable production value for "Long Poll Idle Delay" might therefore be 25000ms (25 seconds).

This driver, like all long polling drivers, is good for browsers that want low-latency but infrequent updates. Long polling drivers are not a good choice for browsers that receive a continual stream of many messages per second, however (as the browser may end up making several requests per second to the server, depending upon the "Long Poll Active Delay" value). In such cases it would be prudent to increase the "Long Poll Active Delay" value significantly from its default value of 100ms, to perhaps 1000ms or more (while acknowledging that the browsers using a long polling driver in such a scenario would no longer be receiving "near-real-time" data). For browsers that are instead subscribed to channels (or members of datagroups) that have relatively infrequent updates, the "Long Poll Active Delay" can potentially be lowered to 0, resulting in real-time delivery despite the use of a long-polling driver.

This is the most efficient long-polling driver for non-Internet Explorer browsers that do not support CORS, and is a cookie-supporting alternative to XDR_LONGPOLL for Internet Explorer 8 and Internet Explorer 9. See the XHR_LONGPOLL_CORS discussion for details on how long polling functions.

This is a streaming driver that is subject to the same issues as the other streaming drivers: proxy and antivirus product interference.

In addition, unlike all the other drivers, which are fully cross-domain (allowing communication to a UM server on a different domain to the server that serves the applications HTML/JS), this driver only permits communication back to the same server that served the HTML/JS. This implies that the application's HTML/JS must be served from a file plugin on the UM server in question.

This is the least efficient long-poll driver, but the most likely one to work in all circumstances. See the XHR_LONGPOLL_CORS discussion for details on how long polling functions.