Microkernel Architecture

17th July 2014 at 9:16pm

This section describes the architecture of the TiddlyWiki-kernel. TiddlyWiki is based on a micro-kernel which provides only a small stack of functions. This design decision was made to introduce a cleaner mechanism for customisation of TiddlyWiki. This section also describes the data-model of TiddlyWiki called tiddler. And it gives a overview to the module system which developers can use to extend the functionality of the TiddlyWiki application.

Microkernel Description

The TiddlyWiki application is based on a microkernel architecture, that means it separate minimal functional core from the extended functionality. The microkernel provides the functionality to load external extensions to extend its core features. The TiddlyWiki microkernel provides a few helper methods but the main task of the TiddlyWiki kernel is to provide a basic functionality for storing data and loading the extension plug-ins. Within the TiddlyWiki architecture everything is stored as a tiddler. How the architecture of TiddlyWiki stores his data during the runtime of the application is shown in Datamodel, but the kernel provides this datamodel. It also prepares the functionality to store and create these tiddlers. In favour it creates a store to manage the tiddlers during the runtime. Without any extensions the microkernel is not able to persist the tiddlers, this means the data is only holded in the RAM. Here are some example interfaces for working with tiddlers in memory which are provided by the kernel:

$tw.Tiddler = function(/* [fields,] fields */)
$tw.Wiki.addTiddler = function(tiddler)
$tw.Wiki.deleteTiddler = function(title)
$tw.Wiki.getTiddler = function(title)

An additional feature of the microkernel is the ability to encrypt and decrypt a block of text. To provide this functionality the microkernel has a built in password-vault to store the current used password. The library used to encrypt and decrypt data is the StandfordJavaScript Crypto Libary. This feature allows the micro-kernel to load encrypted tiddlers from the TiddlyWiki file, but it also allows extension plug-ins to use the encrypt functionality e.g. to persist an encrypted TiddlyWiki.

In order to load extension plug-ins the kernel prepares a interface to load and execute these plug-ins. Therefore the micro-kernel provides some deserializers to extract different type of tiddlers e.g. from the TiddlyWiki-File. Within the microkernel a bunch of different deserializer are installed. These deserializer are needed because every tiddler can include a different type of data for example tiddlers can contain javaScript-code, text, html-text or ~JSON-data. Even after packaging a TiddlyWiki application every plug-in is stored as a tiddler within the TiddlyWiki-Document. This feature is specified in the Module System section. Therefore the micro-kernel need the functionality to parse all the different type of tiddlers. To differ between the different type of tiddlers every tiddler has a file type which are generated by the microkernel

The image below shows the startup process of the TiddlyWiki-kernel. The bootprefix is responsible for preparing the kernel to boot on different engines e.g. browsers and node.js. Afterwards the main boot process which includes the microkernel, with the startup-method, is started. After successfully running these steps the main architecture is loaded. The last step is to run the startup modules. These modules are described later. But in brief this is the point where the TiddlyWiki microkernel can be extended by own functionality. Every module marked as "startup" is started after finishing the boot process of the kernel.

The microkernel builds up the essential functions and structures and initiates a startup sequence.

Datamodel

The micro-kernel creates a TiddlyWiki specific data-structure called tiddler. Here you have to separate the different definition of tiddlers. In the architectural view a tiddler is a JavaScript object holding some data. In the overall concept a tiddler is similar to a wiki page in a normal wiki application like wikipedia. In this section we describe the architectural view of a tiddler. The listing below shows the JSON representation of a tiddler object. During the runtime of the TiddlyWiki everything is saved an object like this. Without any plug-in the architecture is not able to persist any kind of data. All the data is stored in a store. This store is a JavaScript object. This store is constructed like a Map with a bunch of key value pairs. Every tiddler has its name as the key in the map and the JavaScript-Object as the value. The tiddler concept is the main data-model within TiddlyWiki, everything from data up to plug-ins is stored as a tiddler.

{"fields":{
   "text":"example Text",
   "title":"Infrastruktur",
   "tags":["vs"],
   "modified":"2014-07-01T16:25:01.230Z",
   "myField":"myFieldValue",
   "created":"2014-07-01T16:22:10.673Z"
   }
}

Module System

After the boot kernel provides the functions used to load tiddlers, the rest of the TiddlyWiki application is loaded as modules. A module is a tiddler which has the type application/javascript and contains CommonJS compatible JavaScript code. This means a single module provides its public structures and functions in a variable called export. Other modules can obtain these structures and functions by using a global require function.

var Widget = require("$:/core/modules/widgets/widget.js").widget;
// ...
ButtonWidget.prototype = new Widget();

In most cases these module tiddlers are packed into a plug-in. Following the "everything is a tiddler" concept, a plug-in is a tiddler, which contains a bunch of other tiddlers. These tiddlers are first converted into a JSON structure which then becomes the body of the plug-in tiddler. This is not restricted to module tiddlers. A plug-in can contain any tiddlers. This way a developer can put for example simple modules, widgets, UI parts written with WikiText, even new filter operators or extensions to the WikiText parser into a plug-in tiddler. In fact the whole TW core is provided as a single plug-in. Tiddlers provided in a plug-in are called shadow tiddlers and can not be edited. Instead, when trying to edit a shadow tiddler, a new tiddler with the same name is created which then "overrides" the shadow tiddler.

Instead of requiring a specific module directly, a module developer can specify the type of the module he is developing by setting the field "module-type" of the containing tiddler. For example, by providing a module-type of "saver", TiddlyWiki knows that this module implements a way of saving the whole wiki and when the user clicks on the save button, TiddlyWiki automaticly considers the provided module to save the current state.