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Wiki Farms

Also see: Cookbook:Farm Setup By Example, Cookbook:Wiki Farm Alternative

A WikiFarm is a collection of two or more wikis running on the same web server and sharing a set of common components. The term is based on the computing phrase “server farm”.

This page provides some background information about WikiFarms and describes how to turn a “normal” configuration into a farm by adding a wiki. (Click here to go directly to instructions on configuring a farm.) There are many ways to configure wiki farms; this page describes only one, in an effort to make it as simple as possible for the administrator who is creating a farm for the first time.

This page will discuss 3 ways to organize content:

1. Use WikiGroups
2. Use independent wiki sites with a shared code base (a “farm”)
3. Use independent wiki sites with a complete PmWiki installation per site

Choosing between separate wiki-sites and WikiGroups

Why use WikiGroups?

When you divide content between independently installed wikis (i.e., with their own separate URL), it is difficult (but not impossible) to provide services that require access to more than one wiki. For example, the PmWiki search function can only search within one wiki. Using a farm as a way of subdividing related content is generally a bad idea. A much better way to subdivide content is to use WikiGroups.

Why use separate wiki-sites?

When content is largely unrelated and there will be little or no need for sharing the data between the sites, it makes sense to divide the wikis into independently installed sites.

Choosing between separate, independent installations of PmWiki and a WikiFarm

Once you have decided that you need a separate wiki (with its own URL), you have two basic choices:

1. Do a complete installation of PmWiki in a new directory. This gives you two totally independent wikis that are completely self-contained. This is not a wiki farm.
2. Create a wiki farm using an existing wiki as the “home wiki” where most of the shared PmWiki components will live.

The primary motivation for using a wiki farm is to reduce the amount of administrative work involved in managing several wikis. In a farm, most of the PmWiki code is stored in one place and is shared by all the wikis. An administrator can (for example) upgrade to a new version of PmWiki on every wiki in the farm by simply updating the shared components in a single location.

From a reader’s point of view, there is no difference between separate, self-contained installations of PmWiki and separate wikis within a WikiFarm: each wiki in a farm is completely independent, and appears as a separate web site. Each wiki in a farm:

• has its own URL, and the URLs can be in different domains
• can have its own look and feel by using different skins
• can have its own add-ons or “recipes” from the Cookbook
• can have its own administrator responsible for local configuration

Why to use independent, self-contained installations of PmWiki

• it is not a wiki farm, and requires no additional administrative knowledge - it’s just two installations
• if you decide to move one of the wikis to another server, you can simply copy the wiki directory structure to the second server, and it will work (assuming there is a web server and PHP in place).
• you can run different versions of PmWiki on each wiki (good for testing new versions)
• no matter how badly you mess up one installation, it doesn’t affect the other

Why to use a WikiFarm

• you can upgrade all wikis in your farm by simply upgrading your home wiki
• recipes can be shared across all wikis
• portions of your configuration can be shared across wikis
• most code is stored in one location and shared by all wikis in the farm

I still can’t decide if I need a farm …

The good news is that you don’t have to decide in advance. In fact, the recommended procedure is to first do a “normal” or single installation of PmWiki. Use it for a while. Create pages and edit them. Get to know how to add recipes. Be sure to try out WikiGroups (they may be all you need).

If you choose to create a wiki farm, then read on …

Creating/Configuring a WikiFarm

Prerequisites

Before you create a farm, make sure that:

• you have a working installation of PmWiki ready to become the home wiki for your farm
• all of the wikis in your farm will be on the same web server
• each wiki will have a unique URL, such as http://www.example.com/wiki1/, http://www.example.com/wiki2/, http://another.example.com/wiki1/ and so on.

Creating the home wiki

You do have a working installation of PmWiki at this point, don’t you? That’s good, because your existing wiki is about to become the home wiki of your farm.

In the directory that contains your existing wiki, create the file local/farmconfig.php. This file is used to hold any local customizations that apply across the whole farm. For example, you could assign an admin password in farmconfig.php that will be used by all of the wikis in your farm.

If the URL used to access your existing wiki is http://www.example.com/pmwiki/ then a minimal farmconfig.php file would look like this:

<?php if (!defined('PmWiki')) exit();
$FarmPubDirUrl = 'http://www.example.com/pmwiki/pub';

This loads the variable $FarmPubDirUrl with the URL location of your home wiki’s pub/ directory. All of the wikis in your farm share this pub/ directory. The pub/ directory holds skin definitions and GUI-edit buttons to be shared by all the wikis in the farm.

Amazing as it may sound, this completes all of the changes you need to make in order to turn your existing wiki into the home wiki of your farm.

Creating an additional wiki in your farm

Your new wiki is now set up, and your farm now contains 2 wikis. To add more wikis, just repeat these 3 steps.

Customization

Each wiki in a farm inherits the settings stored in farmconfig.php. Do any customization that you want to apply farm-wide (to all the wikis) in farmconfig.php.

Create a local/ directory within each wiki’s directory to hold local customizations that apply only to that wiki. You should at least create the local/config.php file with a new title, like so :

<?php if (!defined('PmWiki')) exit();
  ## Title of your farmed wiki
  $WikiTitle = 'New Wiki';

Farm-wide customizations are processed before the individual wiki local customizations.

The PmWiki variable $FarmD points to the directory in which pmwiki.php is installed, and your home wiki, and it is used as a prefix to allow the other wikis to share PmWiki components. For example:

• $FarmD/scripts/ points to the shared scripts/ directory
• $FarmD/pub/ points to the shared pub/ directory
• $FarmD/cookbook/ points to the shared cookbook/ directory

Any Cookbook scripts you include in farmconfig.php must be included with a line such as:

Password use/authorization on farm wikis:

How come when I switch to another wiki within a farm, I keep my same authorization?

PmWiki uses PHP sessions to keep track of authentication/authorization information, and by default PHP sets things up such that all interactions with the same server are considered part of the same session.

An easy way to fix this is to make sure each wiki is using a different cookie name for its session identifier. Near the top of one of the wiki’s local/config.php files, before calling authuser or any other recipes, add a line like:

You can pick any alphanumeric name for XYZSESSID; for example, for the cs559–1 wiki you might choose

This will keep the two wikis’ sessions independent of each other.

Notes

• The terminology used to describe wiki farms is not used consistently. See WikiFarmTerminology for more info.
• It is important to remember that not all of the recipes in the Cookbook have been written for or tested with farms. Be sure to look for instructions on how to use a recipe on a farm.
• There are many, many more things you can do with farms. Some are described on PmWiki:WikiFarmsAdvanced which also contains links to step-by-step examples of setting up a farm.