coleslaw/docs/hacking.md

Coleslaw: A Hacker's Guide

Here we'll provide an overview of key concepts and technical decisions in coleslaw and a few suggestions about future directions. Please keep in mind that coleslaw was written on a lark when 3 friends had the idea to each complete their half-dreamed wordpress replacement in a week. Though it has evolved considerably since it's inception, like any software some mess remains.

Overall Structure

Conceptually, coleslaw processes a blog as follows:

1. Coleslaw loads the user's config, then reads the blog repo loading any .post files or other content. CONTENT and INDEX objects are created from those files.

2. The CONTENT and INDEX objects are then fed to the templating engine to produce HTML files in a config-specified staging directory, usually under /tmp.

3. A deploy method (possibly customized via plugins) is called with the staging directory. It does whatever work is needed to make the generated HTML files (and any static content) visible to the web.

Core Concepts

Data and Deployment

Coleslaw is pretty fundamentally tied to the idea of git as both a backing data store and a deployment method (via git push). The consequence is that you need a bare repo somewhere with a post-recieve hook. That post-recieve hook (example) will checkout the repo to a $TMPDIR and call (coleslaw:main $TMPDIR).

It is then coleslaw's job to load all of your content, your config and templates, and render the content to disk. Deployment is done by moving the files to a location specified in the config and updating a symlink. It is assumed a web server is set up to serve from that symlink. However, there are plugins for deploying to Heroku, S3, and Github Pages.

Plugins

Coleslaw strongly encourages extending functionality via plugins. The Plugin API is well-documented and flexible enough for many use cases. Do check the API docs when contemplating a new feature and see if a plugin would be appropriate.

Templates and Theming

User configs are allowed to specify a theme. A theme consists of a directory under "themes/" containing css, images, and at least 3 templates: Base, Index, and Post.

Coleslaw uses cl-closure-template exclusively for templating. cl-closure-template is a well documented CL implementation of Google's Closure Templates. Each template file should contain a namespace like coleslaw.theme.theme-name.

Each template creates a lisp function in the theme's package when loaded. These functions take a property list (or plist) as an argument and return rendered HTML. Coleslaw defines a helper called theme-fn for easy access to the template functions. Additionally, there are RSS, ATOM, and sitemap templates coleslaw uses automatically. No need for individual themes to reimplement a standard, after all!

The Lifecycle of a Page

• (load-content)

A page starts, obviously, with a file. When coleslaw loads your content, it iterates over a list of content types (i.e. subclasses of CONTENT). For each content type, it iterates over all files in the repo with a matching extension, e.g. ".post" for POSTs. Objects of the appropriate class are created from each matching file and inserted into the an in-memory data store. Then the INDEXes are created from the loaded content and added to the data store.

• (compile-blog dir)

Compilation starts by ensuring the staging directory (/tmp/coleslaw/ by default) exists, cd'ing there, and copying over any necessary theme assets. Then coleslaw iterates over all the content types and index classes, rendering all of their instances and writing the HTML to disk. After this, an 'index.html' symlink is created pointing to the first reverse-chronological index.

• (deploy dir)

Finally, we move the staging directory to a timestamped path under the the config's :deploy-dir, delete the directory pointed to by the old '.prev' symlink, point '.curr' at '.prev', and point '.curr' at our freshly built site.

Blogs vs Sites

Coleslaw is blogware. When I designed it, I only cared that it could replace my server's wordpress install. As a result, the code until very recently was structured in terms of POSTs and INDEXes. Roughly speaking, a POST is a blog entry and an INDEX is a collection of POSTs or other content. An INDEX really only serves to group a set of content objects on a page, it isn't content itself.

This isn't ideal if you're looking for a full-on static site generator. Thankfully, Content Types were added in 0.8 as a step towards making coleslaw suitable for more use cases. Any subclass of CONTENT that implements the document protocol counts as a content type. However, only POSTs are currently included in the basic INDEXes since there isn't yet a formal relationship to determine which content types should be included on which indexes. Users may easily implement their own dedicated INDEX for new Content Types.

The Document Protocol

The document protocol was born during a giant refactoring in 0.9.3. Any object that will be rendered to HTML should adhere to the protocol. Subclasses of CONTENT (content types) that implement the protocol will be seamlessly picked up by coleslaw and included on the rendered site.

All current Content Types and Indexes implement the protocol faithfully. It consists of 2 "class" methods, 2 instance methods, and an invariant.

There are also 4 helper functions provided that should prove useful in implementing new content types.

Class Methods:

Class Methods don't really exist in Common Lisp, as methods are defined on generic functions and not on the class itself. But since it's useful to think about a Class as being responsible for its instances in the case of a blog, we implement class methods by eql-specializing on the class, e.g.

(defmethod foo ((doc-type (eql (find-class 'bar))))
  ... )

• discover: Create instances for documents of the class and put them in the in-memory database with add-document.

For CONTENT, this means checking the blog repo for any files with a matching extension and loading them from disk. If your class is a subclass of CONTENT, it inherits a pleasant default method for this.

For INDEXes, this means iterating over any relevant CONTENT in the database, and creating INDEXes in the database that include that content.

• publish: Iterate over all instances of the class, rendering each one to HTML and writing it out to the staging directory on disk.

Instance Methods:

• page-url: Generate a relative path for the object on the site, usually sans file extension. If there is no extension, an :around method adds "html" later. The slug slot on the instance is conventionally used to hold a portion of the path that corresponds to a unique Primary Key or Object ID.

• render: A method that calls the appropriate template with theme-fn, passing it any needed arguments and returning rendered HTML.

Invariants:

• Any Content Types (subclasses of CONTENT) are expected to be stored in the site's git repo with the lowercased class-name as a file extension, i.e. (".post" for POST files).

Protocol Helpers:

• add-document: Add the document to coleslaw's in-memory database. It will error if the page-url of the document is not unique. Such a hash collision represents content on the site being shadowed/overwritten. This should be used in your discover method.

• write-document: Write the document out to disk as HTML. It takes an optional template name and render-args to pass to the template. This should be used in your publish method.

• find-all: Return a list of all documents of the requested class. This is often used in the publish method to iterate over documents of a given type.

• purge-all: Remove all instances of the requested class from the DB. This is primarily used at the REPL or for debugging but it is also used in a :before method on discover to keep it idempotent.

Current Content Types & Indexes

There are 5 INDEX subclasses at present: TAG-INDEX, MONTH-INDEX, NUMERIC-INDEX, FEED, and TAG-FEED. Respectively, they support grouping content by tags, publishing date, and reverse chronological order. Feeds exist to special case RSS and ATOM generation. Currently, there is only 1 content type: POST, for blog entries. PAGE, a content type for static page support, is available as a plugin.

Areas for Improvement

New Content Type: Shouts!

I've also toyed with the idea of a content type called a SHOUT, which would be used primarily to reference or embed other content, sort of a mix between a retweet and a del.icio.us bookmark. We encounter plenty of great things on the web. Most of mine winds up forgotten in browser tabs or stored on twitter's servers. It would be cool to see SHOUTs as a plugin, probably with a dedicated SHOUT-INDEX, and some sort of oEmbed/embed.ly/noembed support.

Better Content Types

Creating a new content type is both straightforward and doable as a plugin. All that is really required is a subclass of CONTENT with any needed slots, a template, a render method to call the template with any needed options, a page-url method for layout, and a publish method.

Unfortunately, this does not solve:

1. The issue of compiling the template at load-time and making sure it was installed in the theme package. The plugin would need to do this itself or the template would need to be included in 'core'. Thankfully, this should be easy with cl-closure-template.
2. More seriously, there is no formal relationship between content types and indexes. Consequentially, INDEXes include only POST objects at the moment. Whether the INDEX should specify what Content Types it includes or the CONTENT which indexes it appears on is not yet clear.

Incremental Compilation

Incremental compilation is doable, even straightforward if you ignore indexes. It is also preferable to building the site in parallel as avoiding work is better than using more workers. Moreover, being able to determine (and expose) what files just changed enables new functionality such as plugins that cross-post to tumblr.

Git's post-receive hook is supposed to get a list of refs on $STDIN. A brave soul could update our post-receive script to iterate over those lines as shown in step 3 of this blog post. There would be two primary benefits:

1. We could pass the oldrev (previous revision) of the blog repo to coleslaw:main. That could be used in conjunction with get-updated-files to get a list of the files modified in the last push. This would enable cross-posting plugins to be developed as well as opening the door for incremental compilation.
2. More seriously, Coleslaw currently deploys for any branch pushed to the bare repo. This change would also ensure that we only invoke coleslaw:main when a push is made to the master branch.

This is a cool project and the effects are far reaching. Among other things the existing deployment model would not work as it involves rebuilding the entire site. In all likelihood we would want to update the site 'in-place'. How to replace the compilation and deployment model via a plugin has not yet been explored. Atomicity of filesystem operations would be a reasonable concern. Also, every numbered INDEX would have to be regenerated along with any tag or month indexes matching the modified files. If incremental compilation is a goal, simply disabling the indexes may be appropriate for certain users.