From the Blogosphere
SPDY versus HTML5 WebSockets
SPDY So much alike, yet so vastly a different impact on the data center …
By: Lori MacVittie
Jun. 16, 2012 09:00 AM
A recent post on the HTTP 2.0 War beginning garnered a very relevant question regarding WebSockets and where it fits in (what might shape up to be) an epic battle.
The answer to the question, “Why not consider WebSockets here?” could be easily answered with two words: HTTP headers. It could also be answered with two other words: infrastructure impact.
But I’m guessing Nagesh (and others) would like a bit more detail on that, so here comes the (computer) science.
Different Solutions Have Different Impacts
Due to a simple (and yet profound) difference between the two implementations, WebSockets is less likely to make an impact on the web (and yet more likely to make an impact inside data centers, but more on that another time). Nagesh is correct in that in almost all the important aspects, WebSockets and SPDY are identical (if not in implementation, in effect). Both are asynchronous, which eliminates the overhead of “polling” generally used to simulate “real time” updates a la Web 2.0 applications. Both use only a single TCP connection. This also reduces overhead on servers (and infrastructure) which can translate into better performance for the end-user. Both can make use of compression (although only via extensions in the case of WebSockets) to reduce size of data transferred resulting, one hopes, in better performance, particularly over more constrained mobile networks.
Both protocols operate “outside” HTTP and use an upgrade mechanism to initiate. While WebSockets uses the HTTP connection header to request an upgrade, SPDY uses the Next Protocol Negotiation (proposed enhancement to the TLS specification). This mechanism engenders better backwards-compatibility across the web, allowing sites to support both next-generation web applications as well as traditional HTTP.
Both specifications are designed, as pointed out, to solve the same problems. And both do, in theory and in practice. The difference lies in the HTTP headers – or lack thereof in the case of WebSockets.
WebSockets does not use HTTP headers, SPDY does. This seemingly simple difference has an inversely proportional impact on supporting infrastructure.
The Impact on Infrastructure
The impact on infrastructure is why WebSockets may be more trouble than its worth – at least when it comes to public-facing web applications. While both specifications will require gateway translation services until (if) they are fully adopted, WebSockets has a much harsher impact on the intervening infrastructure than does SPDY.
WebSockets effectively blinds infrastructure. IDS, IPS, ADC, firewalls, anti-virus scanners – any service which relies upon HTTP headers to determine specific content type or location (URI) of the object being requested – is unable to inspect or validate requests due to its lack of HTTP headers. Now, SPDY doesn’t make it easy – HTTP request headers are compressed – but it doesn’t make it nearly as hard, because gzip is pretty well understood and even intermediate infrastructure can deflate and recompress with relative ease (and without needing special data, such as is the case with SSL/TLS and certificates).
Let me stop for a moment and shamelessly quote myself from a blog on this very subject, “Oops! HTML5 Does it Again”:
SPDY, however, does not eliminate these Very-Important-to-Infrastructure-Services HTTP headers, it merely compresses them. Which makes SPDY a much more compelling option than WebSockets. SPDY can be enabled for an entire data center via the use of a single component: a SPDY gateway. WebSockets ostensibly requires the upgrade or replacement of many more infrastructure services and introduces risks that may be unacceptable to many organizations.
And thus my answer to the question "Why not consider WebSockets here” is simply that the end-result (better performance) of implementing the two may be the same, WebSockets is unlikely to gain widespread acceptance as the protocol du jour for public facing web applications due to the operational burden it imposes on the rest of the infrastructure.
That doesn’t mean it won’t gain widespread acceptance inside the enterprise. But that’s a topic for another day…
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