I have to say before you read this that I only have a theoritical understanding of how a web service benefits for being RESTful. Most of the topics I have explored are actually explained in Roy Fieldings section on REST
In the future, I would like to explore and test the assumptions I have made and come up with evidence of thier truthiness.
RESTful web services have been talked about ad-nauseum for a decade. In fact, REST felt like such a hipster's architecture that I pawned it off as a fad and waited this long to only begin to understand it. Prior to understanding a technology, I have to ask myself, "What motivates people to use this technology over others?". I think I may have figured it out.
The key to benefit of REST is the fact that it lives within the confines of HTTP/1.1. Within this world a RESTful web service benefits from all the solutions already solved by the HTTP/1.1 spec. A brief list of standard HTTP solutions that benefit RESTful web services are a uniform interface, caching, asynchronous processing, and security.
A RESTful web service, in particular a resource oriented RESTful web service, starts with a list of URLs that resources live on. Resources are basically entities that live inside your application. A few examples could be a Blog entry, a Forum post, a page hit. These entities don't have to map directly to your applications database, they could merely be logical resources, such as a page hit.
When a client sends a resource to the server, the server will act on that resource. If it's a blog post, it would store the blog post. If it's a logical resource like a page hit, the service would simply increment a counter somewhere.
Each resource has a limited number of verbs that can be done to that resource. The primary verbs are GET, POST, PUT and DELETE. There are inherent restrictions to what these verbs can do. GET never writes on the server; whereas POST/PUT and DELETE always write. Due to these restrictions you can do some simple optimizations to your web service to help it scale.
One of the limitations of SQL is replication and it is a hard problem to solve. In the most basic SQL replication architecture you have a master server and a number of slave servers. The master is written to and the slaves are read from. If your web service abides by the HTTP/1.1 read/write restrictions based on the HTTP method, you can easily route your database connection to your read slaves on GET and your write master on POST, PUT, and DELETE. This would be a more complex solution in a GET based RPC service because every GET to a URL could potentially write to the server. A special router would have to be written that have to know which procedure writes and which reads. You can gain the same benefit with a custom RPC if you support POST and limit your writes to just POST methods.
Another problem already solved by HTTP/1.1 is caching. If you can assume that a GET will always read and if you know when resource was last changed, you can send the Last-Modified header with the response. The client is then responsible for caching the content and storing the Last-Modified date. The client will now send the If-Modified-Since header with each new request for that resource. If the resource has not been modified, the service will respond with a "304 Not Modified" with no information. The service is done. All it had to do is check if the resource has been modified since the client last requested the content. There are other cache headers such as ETag and Cache-Expires, but I think Last-Modified is the simplest to support.
The beauty of your web service supporting HTTP/1.1 caching is that browsers already support it. All AJAX calls to your service will only return content if it's changed. This makes every web browser out there a replicated node for your service! Instant horizontal scaling, how's that feel?
However, you should not trust clients to cache your content, so you should implement a server side caching layer in front of your web service. Varnish can sit in front of your service and serve all cached content from memory. If you want to get really distributed, you can load balance an expandable pool of nginx servers caching content into a expandable pool of memcached servers that sit in front of your web service. If you always send a Last-Modified header, these super fast asynchronous servers can serve up cached content until the content is changed.
Sometimes it takes a long time to process a write and one solution is to make that request asynchronous. HTTP/1.1 has a standard response code for supporting this, "202 Accepted". This status code tells the client, "I have your content, I will process it when I can". This allows you to respond in a timely matter. The client knows that it's write is being worked on. Using a "202 Accepted" response in conjunction with a request with a Last-Modified header allows the client to poll the service for new content and the service will only work as hard as it has to get the Last-Modified value.
An example of asynchronous processing might be implemented in a search engine. Let's say your search engine indexer is time consuming. It parses out keywords, does named entity extraction, and searches for addresses to geocode. This would be a lot of things to do in one HTTP request. Don't worry HTTP/1.1 lets you tell the client you've queue it up. When a new piece of content needs to be index, the service would queue the document and respond with a "202 Accepted" and the client can move on. Eventually the search engine will index the content and the new content will show up in the search engine.
For security, HTTP/1.1 defines support username/password based authentication and SSL encryption so any web service has the ability to use these technologies.
A RESTful web service also has the benefit of having a limited number of actions that can act on a resource. This simplifies authorization and access control tremendously. If you want to restrict a client from writing to a resource you can forbid a client from doing a PUT, POST or DELETE on the URL. You can even set up an OAuth based system external to the web service that allows users to grant authorization to external applications to do things on their behalf.
For instance, a blogging system's OAuth based system could define a permission of "Add/Update blog entries", and this permission would allow PUT /blog/entry/:slug resources and POST /blog/entry/. The web service does not have to worry about authorizing access to that resource because a layer between the client and the service handles that for it.
These are small number of solutions that HTTP/1.1 provides web services if they abide by a RESTful resource oriented architecture and make use of the functionality that HTTP/1.1 has already defined for you.
There are a number of off the shelf products that can benefit RESTful web services such as caching servers, load balancers, and authentication frameworks. These solutions would have to be implemented within your web service or within your service's overloaded HTTP protocol. One example of this reimplementation is SOAP's WS-* suite of specifications. Reinventing the wheel is rampant in the web development world and this is mainly due to ignorance of existing solutions. There is no reason to keep reimplementing functionality solved by HTTP on top of HTTP.