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Wednesday, October 25. 2006
Imagine a webserver...
Imagine a webserver that is completely dis-entangled from the filesystem. Where requests are handled as they should be: as requests for representations of resources. Imagine a webserver based on very simple rules that can be composed into more complex rules (url is foo, host is bar, accept-language is baz, method is blarg). Imagine a webserver where typical HTTP headers and methods are understood, but extensions can be easily dealt with. Thats right, a server that could respond to the new FROBNICATE method is possible.
This server, is called zen-garden. zen-garden is my pet project. It is my tilting at a windmill, and it is rapidly approaching a 0.1 release. The fundamental difference between zen-garden and most other webservers is that zen-garden is built, and based around simple rules. You want to log every request to a database table? Write a rule that acts on every request, and writes the request info to the table. You want to run all of your HTML files through Tidy and or gzip to compress them? Write a rule to act on every response that doesn't already have gzip encoding, and has a content-type of text/html. As you can see, the rule-based webserver is a very powerful concept indeed.
Now you might be asking "How do you handle a request for a single page, a single CSS file?" The answer is simple: a rule to output the page or CSS file when the request method is GET, and the URI is /path/to/your/file. But imagine this, instead of making the webserver find the file, load it, and send it across the network, what if you told the webserver it should compile the serving of that file into itself on deploy time? What if part of the deploy is taking a source file in XML and doing some transformations on it, and then compiling the transformations into itself before deploying? But what if all this was developer friendly so that when working in a development environment, you don't have to deal with the daily grind of compile-test-debug, you could make a change to your file, and test it on the spot? zen-gardens host language is Scheme on Gambit, so it is interpreted for a really fast development cycle, but Gambit can also be compiled to C (which of course can be compiled to native code) so when it is time to deploy, your server will be fast.
The crux to zen-garden is the request-chains and request-handlers. A request-handler is a data structure, with 2 important parts: a predicate stating whether or not the handler should execute, and the handler itself. Both of these are functions that return booleans. A request-chain is really just a series of request handlers that are executed in a particular order, split out into 4 convenient phases: initialization, handling, fallback and finalization. A request-handler can be bound to any one of these 4 phases. If a request is not handled during the 'handling' phase, the fallback phase is executed, otherwise it is skipped.
The real interesting thing about this structure is that a request-handler can spawn a new request-chain. So you can have a request-handler that executes when the 'Host' header is set to 'my.foo.domain.com' that spawns a new request-chain. On top of that, there is nothing stopping you from dealing with requests in other ways, in fact, any way you see fit. Don't like my method of predicates and request chains and what have you? Write your own method of doing things. zen-garden is modular in design. Pull out the routine that I use to handle requests, and install your own.
What is in the future of zen-garden? Well, ZSP's, or Zen-Server-Pages, which is a JSP/PHP method of page generation. Unlike these other technologies, I plan on getting the separation of business logic and display logic right. No top-heavy tag libraries with presentational logic getting confused with document structure, yes JSP and the entire J2EE stack, I am looking at you. No business logic tied up with your presentation, dirty, nasty PHP! What else is in the future? My(and other)SQL connection pooling, GD integration, regular expressions, optional continuation based web programming, RESTful resources, specialized preprocessing of CSS and Javascript files (only send the browser the CSS it needs, and you want it to have).
Oh, and it is all under a BSD License.
For now, everything is available in a subversion repository, svn://panda-ba.sanriowasteland.net/zengarden/trunk . You will need the Gambit-C compiler and interpreter as well.
This server, is called zen-garden. zen-garden is my pet project. It is my tilting at a windmill, and it is rapidly approaching a 0.1 release. The fundamental difference between zen-garden and most other webservers is that zen-garden is built, and based around simple rules. You want to log every request to a database table? Write a rule that acts on every request, and writes the request info to the table. You want to run all of your HTML files through Tidy and or gzip to compress them? Write a rule to act on every response that doesn't already have gzip encoding, and has a content-type of text/html. As you can see, the rule-based webserver is a very powerful concept indeed.
Now you might be asking "How do you handle a request for a single page, a single CSS file?" The answer is simple: a rule to output the page or CSS file when the request method is GET, and the URI is /path/to/your/file. But imagine this, instead of making the webserver find the file, load it, and send it across the network, what if you told the webserver it should compile the serving of that file into itself on deploy time? What if part of the deploy is taking a source file in XML and doing some transformations on it, and then compiling the transformations into itself before deploying? But what if all this was developer friendly so that when working in a development environment, you don't have to deal with the daily grind of compile-test-debug, you could make a change to your file, and test it on the spot? zen-gardens host language is Scheme on Gambit, so it is interpreted for a really fast development cycle, but Gambit can also be compiled to C (which of course can be compiled to native code) so when it is time to deploy, your server will be fast.
The crux to zen-garden is the request-chains and request-handlers. A request-handler is a data structure, with 2 important parts: a predicate stating whether or not the handler should execute, and the handler itself. Both of these are functions that return booleans. A request-chain is really just a series of request handlers that are executed in a particular order, split out into 4 convenient phases: initialization, handling, fallback and finalization. A request-handler can be bound to any one of these 4 phases. If a request is not handled during the 'handling' phase, the fallback phase is executed, otherwise it is skipped.
The real interesting thing about this structure is that a request-handler can spawn a new request-chain. So you can have a request-handler that executes when the 'Host' header is set to 'my.foo.domain.com' that spawns a new request-chain. On top of that, there is nothing stopping you from dealing with requests in other ways, in fact, any way you see fit. Don't like my method of predicates and request chains and what have you? Write your own method of doing things. zen-garden is modular in design. Pull out the routine that I use to handle requests, and install your own.
What is in the future of zen-garden? Well, ZSP's, or Zen-Server-Pages, which is a JSP/PHP method of page generation. Unlike these other technologies, I plan on getting the separation of business logic and display logic right. No top-heavy tag libraries with presentational logic getting confused with document structure, yes JSP and the entire J2EE stack, I am looking at you. No business logic tied up with your presentation, dirty, nasty PHP! What else is in the future? My(and other)SQL connection pooling, GD integration, regular expressions, optional continuation based web programming, RESTful resources, specialized preprocessing of CSS and Javascript files (only send the browser the CSS it needs, and you want it to have).
Oh, and it is all under a BSD License.
For now, everything is available in a subversion repository, svn://panda-ba.sanriowasteland.net/zengarden/trunk . You will need the Gambit-C compiler and interpreter as well.