RMI vs. CORBA vs. DCOM
Back in the 1990s, when networks and the internet were providing a way for computers to talk to each other, Service-Oriented Architecture was this new, amazing way to bind software systems together using Objects. This was a really big deal, because developers had these cool OO languages, but had to resort to all kinds of crap (EDI, anyone?) to communicate with other machines. The major computer companies had very expensive ways to solve this problem. They created a new class of technology called the Object Request Broker, or ORB. Microsoft had DCOM; Sun had Java RMI; Sun (I know), IBM, DEC, and 700 other companies had CORBA.
These companies knew that if they could get their distributed OO tech into the architectures of the new SOA systems being created, they could lock in their technology for decades to come. Naturally, DCOM and RMI couldn't talk to each other, and they were not about to cross-license. A war developed, and it promised to be long and nasty.
Most customers realized what was coming, and they yearned for freedom from lockin. Their desire for SOA gave them a vision of the Service interface backed by any vendor's software. Fortunately, some employees of these same companies wanted the same kind of freedom. Big companies are really a bunch of little companies that have to work together, and one of the little companies inside Microsoft, the WebData team, was working with Dave Winer on SOAP. The Simple Object Access Protocol was an insurgent technology meant to make SOA a reality.
The pressure of the growing internet forced the parties towards interoperability. Everyone realized pretty quickly that SOAP was the answer to their SOA needs, and as a result the competing technologies died a fairly quick death. SOAP was the Versailles Treaty of the ORB Wars.
SOAP Became "COAP"
Another reason SOAP was so successful was that it was much easier to understand than the other ORB protocols. The problem with SOAP however was that it became really complex as everyone started pouring their Object-Oriented concepts into it. The complexity rose, the performance dropped, and the price of distributed SOA architecture climbed. SOAP over the internet stagnated.
Many people found that the Object part of SOAP was actually a big problem because it didn't really model what the Internet was all about. People wanted to share Resources, not Objects. Roy Fielding crystallized this in his famous dissertation and termed it "REST." The good news is, many internet developers already had practice with representations and state transfers. They avoided SOAP and used CGI-BIN instead.
CGI-BIN, the original REST
Before JSON, if you were a "hacker" you used the cgi-bin/ dir of your web server to write apps that could consume http forms for State Transfers. HTML would show a form, the user would enter data, submit, and presto, your perl script on the server would update the database. Simple, easy to understand, and easy to consume.
Big vendors hated cgi-bin and actively fought hard against it. Many, many articles and presentations warned against using cgi-bin (It's nonstandard and must be confusing; It's interprocess, not scalable; It's a separate process, running rampant! Hackers use cgi-bin as their back door!! cgi-bin is Open Source, and who knows what could be hidden in there!!!) CIOs and vendors accepted this line of reasoning and cgi-bin died by strangulation.
REST Beat SOAP, Yay!
The need for some kind of REST only grew however and lots of alternatives emerged (thanks to XMLHttpRequest.responseText()—another Microsoft Gift to the World), driving the RESTful API age. Even SOAP vendors added a "REST" button to make your old SOAP services be RESTful. "REST" became synonymous with anything Not SOAP and adoption took off.
Clearly, SOAP is history and REST is the way to go for interconnected systems. The problem is, REST isn't really anything. You can't define something in the negative. Leonard Richardson has thoughtfully considered this problem and has developed the Richardson Maturity Model to help us understand the state of our world, but all this does for me is place us somewhere in the Middle Ages. Rome has fallen, but the Renaissance is still in progress.
The Internet Is Hand-Cranked
When you look at the growth and size of the internet, you quickly realize that the Internet must be completely powered by human effort. People are sitting at their desks, or on the bus, or in traffic jams, pushing and typing and swiping away at it. The internet itself has maybe 20,000 APIs in play; perhaps more, but really, nothing like the numbers of domains in existence:
Go read Robert Zakon's report for proof that APIs are a niche on the internet compared to all the human-powered interactions. What we call REST isn't making much of a dent in how we use the Internet at all.
The Internet of Things
The coming tidal wave of the Internet of Things is starting to hit. Given where REST is today, we are in for a messy ride because what these Things need is not what we have on offer.