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---# TWiki Plugins

_Plug in enhanced feature add-ons, with a Plugin API for developers_

---++ Overview

You can add Plugins to extend TWiki functionality, without altering the core code. A plug-in approach lets you:

	* add virtually unlimited features while keeping the main TWiki code compact and efficient;
	* heavily customize an installation and still do clean updates to new versions of TWiki;
	* rapidly develop new TWiki functions in Perl using the Plugin API.

Everything to do with TWiki Plugins - demos, new releases, downloads, development, general discussion - is available at TWiki.org, in the <b> TWiki:Plugins </b> web.

TWiki plugins are developed and contributed by interested members of the community. Plugins are provided on an 'as is' basis; they are not a part of TWiki, but are independently developed and maintained.

#InstallPlugin
---++ Installing Plugins

Each TWiki Plugin comes with its own documentation: step-by-step installation instructions, a detailed description of any special requirements, version details, and a working example for testing. Many plugins have an install script that automates these steps for you.

*Special Requirements:* Some Plugins need certain Perl modules to be preinstalled on the host system. Plugins may also use other resources, like graphics, other modules, applications, and templates. You should be able to find detailed instructions in the Plugin's documentation.

Each Plugin has a standard release topic, located in the TWiki:Plugins web at TWiki.org. There's usually a number of other related topics, such as a developers page, and an appraisal page.

#PreTesting
---+++ On-Site Pretesting

The recommended approach to testing new Plugins before making them public is to create a second local TWiki installation, and test the plugin there. You can allow selected users access to the test area. Once you are satisifed that it won't compromise your main installation, you can install it there as well.

InstalledPlugins shows which Plugins are: 1) installed, 2) loading properly, and 3) what TWiki:Codev.PluginHandlers they invoke. Any failures are shown in the Errors section. The =[[TWikiVariables#VarFAILEDPLUGINS][%<nop>FAILEDPLUGINS%]]= variable can be used to debug failures. You may also want to check your webserver error log and the various TWiki log files.

---+++ Some Notes on Plugin Performance

The performance of the system depends to some extent on the number of Plugins installed and on the Plugin implementation. Some Plugins impose no measurable performance decrease, some do. For example, a Plugin might use many Perl libraries that need to be initialized with each page view (unless you run mod_perl). You can only really tell the performance impact by installing the Plugin and by measuring the performance with and without the new Plugin. Use the TWiki:Plugins.PluginBenchmarkAddOn, or test manually with the Apache =ab= utility. Example on Unix:%BR% =time wget -qO /dev/null <nop>%SCRIPTURLPATH{"view"}%/%TWIKIWEB%/AbcPlugin=

%T% If you need to install an "expensive" Plugin, and you need its functionality only in one web, you can place the Plugin topic into that web. TWiki will initialize the Plugin only if the Plugin topic is found (which won't be the case for other webs.)

#ManagingPlugins
---++ Managing Installed Plugins

Some Plugins require additional settings or offer extra options that you have to select. Also, you may want to make a Plugin available only in certain webs, or temporarily disable it. And may want to list all available Plugins in certain topics. You can handle all of these management tasks with simple procedures:

---+++ Enabling Plugins

Plugins can be enabled and disabled with the [[%SCRIPTURLPATH{configure}%][configure]] script. An installed Plugin needs to be enabled before it can be used.

---+++ Plugin Evaluation Order

By default, TWiki executes Plugins in alphabetical order on Plugin name. It is possible to change the order, for example to evaluate database variables before the speadsheet CALCs. This can be done with ={PluginsOrder}= in the Plugins section of [[%SCRIPTURLPATH{configure}%][configure]].

---+++ Plugin-Specific Settings

Plugin-specific settings are done in individual Plugin topics. Two settings are standard for each Plugin:
	1 One line description, used to form the bullets describing the Plugins in the %TWIKIWEB%.TextFormattingRules topic:
		* =Set SHORTDESCRIPTION = Create dynamic foo bar reports=
	1 Debug Plugin, output can be seen in =data/debug.txt=. Set to 0=off or 1=on:
		* =Set DEBUG = 0=
	* The settings can be retrieved as Preferences variables like =%&lt;pluginname&gt;_&lt;var&gt;%=. For example, =%<nop>DEFAULTPLUGIN_SHORTDESCRIPTION%= shows the description of the <nop>DefaultPlugin.

---+++ Listing Active Plugins

Plugin status variables let you list all active Plugins wherever needed.

---++++ =%<nop>ACTIVATEDPLUGINS%=
On this TWiki site, the enabled Plugins are: %ACTIVATEDPLUGINS%.

---++++ =%<nop>PLUGINDESCRIPTIONS%=
%PLUGINDESCRIPTIONS%

---++++ =%<nop>FAILEDPLUGINS%=
%FAILEDPLUGINS%

#PluginAPI
---++ The TWiki Plugin API

The Application Programming Interface (API) for TWiki Plugins provides the specifications for hooking into the core TWiki code from your external Perl Plugin module.

---+++ Available Core Functions

The TWikiFuncDotPm module (=lib/TWiki/Func.pm=) describes *all* the interfaces available to Plugins. Plugins should *only* use the interfaces described in this module.

__%X% Note:__ If you use other core functions not described in =Func.pm=, you run the risk of creating security holes. Also, your Plugin will likely break and require updating when you upgrade to a new version of TWiki.

---+++ Predefined Hooks

In addition to TWiki core functions, Plugins can use *predefined hooks*, or *callbacks*, as described in the =lib/TWiki/Plugins/EmptyPlugin.pm= module.

	* All but the initPlugin are disabled. To enable a callback, remove =DISABLE_= from the function name.

TWiki:Codev/StepByStepRenderingOrder helps you decide which rendering handler to use.

#FastPluginHints
---+++ Hints on Writing Fast Plugins

	* Delay initialization as late as possible. For example, if your plugin is a simple syntax processor, you might delay loading extra Perl modules until you actually see the syntax in the text.
	* For example, use an =eval= block like this:%BR%
	  =eval { require IPC::Run }= %BR%
	  =return "&lt;font color=\"red\"&gt;SamplePlugin: Can't load required modules ($@)&lt;/font&gt;" if $@;=
	* You can use a flag to avoid running the initialization twice
	* The TWiki:Plugins.PluginBenchmarkAddOn comes in handy to measure and fine-tune the performance of your Plugin

---+++ Plugin Version Detection

To eliminate the incompatibility problems bound to arise from active open Plugin development, a Plugin versioning system is provided for automatic compatibility checking.

	* All modules require a =$VERSION='0.000'= variable, beginning at =1.000=.

	* The =initPlugin= handler should check all dependencies and return TRUE if the initialization is OK or FALSE if something went wrong.
		* The Plugin initialization code does not register a Plugin that returns FALSE (or that has no =initPlugin= handler).

	* =$TWiki::Plugins::VERSION= in the =TWiki::Plugins= module contains the TWiki Plugin API version, currently *%PLUGINVERSION{}%*.
		* You can also use the =[[TWikiVariables#VarPLUGINVERSION][%<nop>PLUGINVERSION{}%]]= variable to query the Plugin API version or the version of installed Plugins.

---+++ Security

	* Badly written plugins can open huge security holes in TWiki. This is especially true if care isn't taken to prevent execution of arbitrary commands on the server.
	* Don't allow sensitive configuration data to be edited by users. it is better to add sensitive configuration options to the =%TWiki::cfg= hash than adding it as preferences in the plugin topic
		* See TWiki:Plugins.MailInContrib for an example of this
		* TWiki:Plugins.BuildContrib can help you with this
	* Always use the TWiki::Sandbox to execute commands.
	* Always audit the plugins you install, and make sure you are happy with the level of security provided. While every effort is made to monitor plugin authors activities, at the end of the day they are uncontrolled user contributions.

#CreatePlugins
---++ Creating Plugins

With a reasonable knowledge of the Perl scripting language, you can create new Plugins or modify and extend existing ones. Basic plug-in architecture uses an Application Programming Interface (API), a set of software instructions that allow external code to interact with the main program. The [[#PluginAPI][TWiki Plugin API]] Plugins by providing a programming interface for TWiki.

---+++ Anatomy of a Plugin

A basic TWiki Plugin consists of two elements:

	* a Perl module, ex: =MyFirstPlugin.pm=
	* a documentation topic, ex: =MyFirstPlugin.txt=

The Perl module can be a block of code that connects with TWiki alone, or it can include other elements, like other Perl modules (including other Plugins), graphics, TWiki templates, external applications (ex: a Java applet), or just about anything else it can call.
In particular, files that should be web-accessible (graphics, Java applets ...) are best placed as attachments of the =MyFirstPlugin= topic. Other needed Perl code is best placed in a =lib/TWiki/Plugins/MyFirstPlugin/= directory.

The Plugin API handles the details of connecting your Perl module with main TWiki code. When you're familiar with the [[#PluginAPI][Plugin API]], you're ready to develop Plugins.

---+++ Creating the Perl Module 

Copy file =lib/TWiki/Plugins/EmptyPlugin.pm= to =&lt;name&gt;Plugin.pm=. The =EmptyPlugin.pm= module contains mostly empty functions, so it does nothing, but it's ready to be used. Customize it. Refer to the [[#PluginAPI][Plugin API]] specs for more information.

If your Plugin uses its own modules and objects, you must include the name of the Plugin in the package name. For example, write =Package <nop>MyFirstPlugin::Attrs;= instead of just =Package Attrs;=. Then call it using:
<pre>
use TWiki::Plugins::MyFirstPlugin::Attrs;
$var = MyFirstPlugin::Attrs->new();
</pre>

#MeasurePerformance
---+++ Measuring and Improving the Plugin Performance

A high quality Plugin performs well. You can use the TWiki:Plugins.PluginBenchmarkAddOn to measure your TWiki:Plugins.PluginBenchmarks. The data is needed as part of the Documentation Topic.

See also [[#FastPluginHints][Hints on Writing Fast Plugins]].

#CreatePluginTopic
---+++ Writing the Documentation Topic

The Plugin documentation topic contains usage instructions and version details. It serves the Plugin files as %TWIKIWEB%.FileAttachments for downloading. (The doc topic is also included _in_ the [[#CreatePluginPackage][distribution package]].) To create a documentation topic:

	1. *Copy* the Plugin topic template from TWiki.org. To copy the text, go to TWiki:Plugins/PluginPackage and:
		* enter the Plugin name in the "How to Create a Plugin" section
		* click Create
		* select all in the Edit box &amp; copy 
		* Cancel the edit
		* go back to your site to the %TWIKIWEB% web
		* In the GoBox enter your Plugin name, for example =MyFirstPlugin=, press enter and create the new topic
		* paste &amp; save new Plugin topic on your site
	1. *Customize* your Plugin topic.
		* Important: In case you plan to publish your Plugin on TWiki.org, use Interwiki names for author names and links to TWiki.org topics, such as TWiki:Main/%WIKINAME%. This is important because links should work properly in a Plugin topic installed on any TWiki, not just on TWiki.org.
	1. *Document* the performance data you gathered while [[#MeasurePerformance][measuring the performance]]
	1. *Save* your topic, for use in [[#CreatePluginPackage][packaging]] and [[#PublishPlugin][publishing]] your Plugin.

<blockquote style="background-color:#f5f5f5">
*OUTLINE: Doc Topic Contents* <br />
Check the Plugins web on TWiki.org for the latest Plugin doc topic template. Here's a quick overview of what's covered:

*Syntax Rules:* &lt;<i>Describe any special text formatting that will be rendered.</i>&gt;" 

*Example:* &lt;<i>Include an example of the Plugin in action. Possibly include a static HTML version of the example to compare if the installation was a success!</i>&gt;"

*Plugin Settings:* &lt;<i>Description and settings for custom Plugin %<nop>VARIABLES%, and those required by TWiki.</i>&gt;"

	* *Plugins Preferences* &lt;<i>If user settings are needed, explain... Entering values works exactly like %TWIKIWEB%.TWikiPreferences and WebPreferences: six (6) spaces and then:</i>&gt;"
		* *Set &lt;<i>EXAMPLE = value added</i>&gt;*

*Plugin Installation Instructions:* &lt;<i>Step-by-step set-up guide, user help, whatever it takes to install and run, goes here.</i>&gt;"

*Plugin Info:* &lt;<i>Version, credits, history, requirements - entered in a form, displayed as a table. Both are automatically generated when you create or edit a page in the TWiki:Plugins web.</i>&gt;"
</blockquote>

#CreatePluginPackage
---+++ Packaging for Distribution

The TWiki:Plugins.BuildContrib is a powerful build environment that is used by the TWiki project to build TWiki itself, as well as many of the plugins. You don't *have* to use it, but it is highly recommended!

If you don't want (or can't) use the !BuildContrib, then a minimum Plugin release consists of a Perl module with a WikiName that ends in =Plugin=, ex: =MyFirstPlugin.pm=, and a documentation page with the same name(=MyFirstPlugin.txt=).

	1. Distribute the Plugin files in a directory structure that mirrors TWiki. If your Plugin uses additional files, include them all:
		* =lib/TWiki/Plugins/MyFirstPlugin.pm=
		* =data/TWiki/MyFirstPlugin.txt=
		* =pub/TWiki/MyFirstPlugin/uparrow.gif= [a required graphic]
	2. Create a zip archive with the Plugin name (=MyFirstPlugin.zip=) and add the entire directory structure from Step 1. The archive should look like this:
		* =lib/TWiki/Plugins/MyFirstPlugin.pm=
		* =data/TWiki/MyFirstPlugin.txt=
		* =pub/TWiki/MyFirstPlugin/uparrow.gif=

#PublishPlugin
---+++ Publishing for Public Use

You can release your tested, packaged Plugin to the TWiki community through the TWiki:Plugins web. All Plugins submitted to TWiki.org are available for download and further development in TWiki:Plugins/PluginPackage.

Publish your plugin by following these steps:
	1. *Post* the Plugin documentation topic in the TWiki:Plugins/PluginPackage:
		* enter the Plugin name in the "How to Create a Plugin" section, for example =MyFirstPlugin=
		* paste in the topic text from [[#CreatePluginTopic][Writing the Documentation Topic]] and save
	1. *Attach* the distribution zip file to the topic, ex: =MyFirstPlugin.zip=
	1. *Link* from the doc page to a new, blank page named after the Plugin, and ending in =Dev=, ex: =MyFirstPluginDev=. This is the discussion page for future development. (User support for Plugins is handled in TWiki:Support.)
	1. *Put* the Plugin into the SVN repository, see TWiki:Plugins/ReadmeFirst (optional)

%N% Once you have done the above steps once, you can use the !BuildContrib to upload updates to your plugin.

Thank you very much for sharing your Plugin with the TWiki community :-)

#RecommendedStorageOfPluginData
---++ Recommended Storage of Plugin Specific Data

Plugins sometimes need to store data. This can be Plugin internal data such as cache data, or data generated for browser consumption such as images. Plugins should store data using TWikiFuncDotPm functions that support saving and loading of topics and attachments.

---+++ Plugin Internal Data

You can create a Plugin "work area" using the =TWiki::Func::getWorkArea()= function, which gives you a persistant directory where you can store data files. By default they will not be web accessible. The directory is guaranteed to exist, and to be writable by the webserver user. For convenience, =TWiki::Func::storeFile()= and =TWiki::Func::readFile()= are provided to persistently store and retrieve simple data in this area.

---+++ Web Accessible Data

__Topic-specific data__ such as generated images can be stored in the topics attachment area, which is web accessible. Use the =TWiki::Func::saveAttachment()= function to store the data.

Recommendation for file name:
	* Prefix the filename with an underscore (the leading underscore avoids a nameclash with files attached to the same topic)
	* Identify where the attachment originated from, typically by including the Plugin name in the file name
	* Use only alphanumeric characters, underscores, dashes and periods to avoid platform dependency issues and URL issues
	* Example: =_FooBarPlugin_img123.gif=

__Web specific data__ can be stored in the Plugin's attachment area, which is web accessible. Use the =TWiki::Func::saveAttachment()= function to store the data.

Recommendation for file names in Plugin attachment area:
	* Prefix the filename with an underscore
	* Include the name of the web in the filename
	* Use only alphanumeric characters, underscores, dashes and periods to avoid platform dependency issues and URL issues
	* Example: =_Main_roundedge-ul.gif=

---++ Handling deprecated functions

From time-to-time, the TWiki developers will add new functions to the interface (either to TWikiFuncDotPm, or new handlers). Sometimes these improvements mean that old functions have to be deprecated to keep the code manageable.

FIXME: The following is not KISS

When this happens, the deprecated functions will be supported in the interface for at least one more TWiki release (i.e. one more after the one they are first deprecated in). When a plugin uses (or defines) the deprecated function, a warning will be generated in the TWiki warnings log. Admins who see these warnings in their logs should check TWiki.org and if necessary, contact the plugin author, for an updated version of the plugin.

Updated plugins may still need to define deprecated handlers for compatibility with old TWiki versions. In this case, the plugin package that defines old handlers can set a flag to prevent the warnings from being issued. This is done by defining a _global hash_ that maps from the handler name to the =TWiki::Plugins= version _in which the handler was first deprecated_. For example, if we need to define the =endRenderingHandler= for compatibility with =TWiki::Plugins= versions before 1.1, we would add this to the plugin:
<verbatim>
package TWiki::Plugins::SinkPlugin;
use vars qw( %TWikiCompatibility );
$TWikiCompatibility{endRenderingHandler} = 1.1;
</verbatim>
If the currently-running TWiki version is 1.1 _or later_, then the _handler will not be called_ and _the warning will not be issued_. TWiki with versions of =TWiki::Plugins= before 1.1 will still call the handler as required.

There is no way to prevent warnings from being issued when deprecated functions in the TWikiFuncDotPm interface are called. In this case, plugins authors should be encouraged recode their plugins as described in the TWikiFuncDotPm documentation of the deprecated function.

__Related Topics:__ DeveloperDocumentationCategory, AdminDocumentationCategory
