created: 201308251429
modified: 201310312218
tags: mechanism
title: BootMechanism

!Introduction

At its heart, TiddlyWiki5 is a relatively small boot kernel that runs either under Node.js or in the browser with all other functionality added via dynamically loaded [[modules|Modules]].

The kernel boots just enough of the TiddlyWiki environment to allow it to load and execute module tiddlers. The module system is compatible with CommonJS and [[Node.js]].

There are many [[different types of module|ModuleType]]: parsers, deserializers, widgets etc. It goes much further than you might expect. For example, individual tiddler fields are modules, too: there's a module that knows how to handle the `tags` field, and another that knows how to handle the special behaviour of the `modified` and `created` fields. Some plugin modules have further sub-plugins: the wikitext parser, for instance, accepts parsing rules as individual plugin modules.

!Plugins

In TiddlyWiki5, [[Plugins]] are bundles of tiddlers that are distributed and managed as one; [[Modules]] are JavaScript tiddlers with a module type identifying when and how they should be executed.

The tiddler [[$:/boot/boot.js]] is a barebones TiddlyWiki kernel that is just sufficient to load the core plugin modules and trigger a startup module to load up the rest of the application.

The boot kernel includes:

* Several short shared utility functions
* A handful of methods implementing the module mechanism
* The `$tw.Tiddler` class (and field definition plugins)
* The `$tw.Wiki` class (and tiddler deserialization methods)
* Code for the browser to load tiddlers from the HTML DOM
* Code for the server to load tiddlers from the file system

Each module is an ordinary `Node.js`-style module, using the `require()` function to access other modules and the `exports` global to return JavaScript values. The boot kernel smooths over the differences between `Node.js` and the browser, allowing the same plugin modules to execute in both environments.

In the browser, `core/boot.js` is packed into a template HTML file that contains the following elements in order:

* Ordinary and system tiddlers, packed as HTML `<DIV>` elements
* `core/bootprefix.js`, containing a few lines to set up the plugin environment
* JavaScript modules, packed as HTML `<SCRIPT>` blocks
* `core/boot.js`, containing the boot kernel

On the server, `core/boot.js` is executed directly. It uses the `Node.js` local file API to load plugins directly from the file system in the `core/modules` directory. The code loading is performed synchronously for brevity (and because the system is in any case inherently blocked until plugins are loaded).

The boot kernel sets up the `$tw` global variable that is used to store all the state data of the system.