While some developers are exhausted, and brains are about full with all they can get with all the announcements and exiting news of a new Windows Cloud OS Azure, Office Web Applications/Office 14, and Windows 7 a few developers have had some spare cycles to respond to my "Just Say NO to Custom Site Definitions" (Updated 10/29).  Wherein I discouraged the use of custom site definitions, not intended as a scare tactic, but to discourage the use for two main reasons... 1) The difficulty of managing custom site definitions and my 2) concern of a difficult and challenging future upgrade for custom site definitions (How can the product team understand what people are doing in their own site definitions?). (More below on this one.)

Since that time and sifting through many very solid arguments, not by lazy devs, but from the best in our industry, I have to concede a few points soften my message and even have a few "a ha" moments.

First, I updated my post and took out some of the shrewd comments, but not in the spirit of the message more in ensuring it can be appreciated over time.  The new title "Do you really need custom site definitions?" where in I added the common ground, and provide some quotes from these extraordinary devs most of which have been awarded community SharePoint MVPs for good reason.

Common Ground - Summarized by Robert Bogue (See his post on SharePoint Guidance - Patterns and Practices).

1) Do site definitions when you need to.  (Mostly I think we agree this is when you need a unique ID/name -- but there are other cases as well)

2) If you build a site definition is should be very minimal -- everything should be built around features.  The features can be hidden.

Lots of attitude and perspective here.  Lots of great ideas and considerations.  Thanks for keeping the vulgarity to a minimum. :)

Say No to Heavy Site Definitions

"I think calling out bad practices (heavy site definitions) is absolutely appropriate.  However, I think that saying they're all bad is taking it too far." - Robert Bogue

You don't HAVE to create custom site defs

"For me, you post is too strong. My post that you reference [You don't need to create site definitions] is more how I feel about it… simply tell people you don’t HAVE to create site def. There are reasons when you have to create each one. In fact, while I hate site def’s, EVERY WCM project I do starts with a custom site def. Mind you it’s slimmed WAY down as I only need something with a name (for the user to pick) and ID (to attach Features to), but it is a site def none the less. " -- Andrew Connell

If SharePoint had the proper mechanism I wouldn't use them

"In my opinion it all boils down to the last sentence AC said.  “as I only need something with a name (for the user to pick) and ID (to attach Features to), but it is a site def none the less."

If SharePoint had another mechanism for us to provide this functionality then we could probably forego custom site defs altogether.  I follow the same approach AC outlines on his blog, come to think of it, the article on my blog you linked to just provides the instructions to implement AC’s approach." -- Todd Baginski

Site Definitions are a tool, you must consider the tools in your toolbox (don't take away this tool)

Custom Site Definitions are a tool.  Like any tool, they have benefits and drawbacks.  Used properly, they provide much value.  Used improperly, they cause pain.
Even the body of the article contradicts the sensationalist-headline by saying that there are some things you can't do without a custom site def.  The article of AC's that this links to, and it's comments, talk about a solution that is a totally blank CUSTOM SITE DEFINITION, that is then built up properly with Features/Solutions.  They also mention publishing scenarios that the recommended approach is a custom site def.
So, the best approach is to do your homework.  If a custom site def REALLY is the best approach, then feel free to use them.  Just make it a conscious decision, knowing the trade-offs, not your default reaction because it's easier.  -- David Mann

Everything else is a work around

Joel, my friend, you are way out in left field on this one.  Custom Site Definitions, when built properly, are EXACTLY THE SAME as what Microsoft provides out of the box.  They are nothing more than a "skeleton" that gives developers the ability to attach programmatic elements to in the form of Features.  Nothing more, nothing less.  There are many scenarios in which they are required:
1. Automated Provisioning - Self-contained solutions that have all necessary functionality baked in (think hosting and SAS models).
2. Repeatability - They migrate better from dev to staging to production better than any other method.
3. Maintainability - New Features can be added or removed as required and the solution upgraded.  Try doing that with an STP file.
4. One Click Deployment - The user simply selects the proper definition on the site creation page, to which you can add descriptive text and sample images (what do you think all those other options are?  They're OOTB site defs, that's what).
5. Control - Nothing beats a site def for restricting what features site owners can and cannot use.  Very important in many enterprise environments.
6. Ease of use - There are lots of workarounds for the power and flexibility that site defs provide but all require a great deal more code than a simple site def with stapled features.
Sorry to burst your bubble but you're wrong on this one.  Next time, ask a developer with experience doing Site Definitions the proper way before you go off on an opinionated rant.  I'd be happy to help! -- Eric Shupps

Speaking of Passion, Eric Shupps has written two blog posts recently that are very relevant in the context of this conversation... "Site Definitions – the Good, the Bad, and the Ugly" and "Site Definitions – the Good, the Bad, and the Ugly."

Site Definitions are best practice

Wow, way to create and document a development practice and then slam the use of it!
Site defs are a way to package up things into user-createable units, much as MS did with the "new team site" template. Ideally they're implemented by preconfiguring a set of lists and features, and are very lightweight. But this post is way off base. --Daniel Larson

If you must use a Site Definition, understand the ramifications

None of this is to say that Site Definitions are obsolete, or should never be used. You just need to be aware that this is a very fundamental construct. Once you build a site from a site definition, you cannot change it to a different definition later. You can still tweak it with additional features, but you can't change the "kind" of site it is. It will not be a "SharePoint Team Site", easily and transparently upgradable. It will be an "Acme Group Site", or whatever you have called it, and you need to plan for its entire life-cycle (Much like adopting a puppy). There may be extra steps in the upgrade process, for example (though, at this time, we don't know what those steps might be). You may have trouble getting support if the person who created the Site Definition moves on to greener pastures. This may be worth it for your environment, but it is something you need to take into account. -- Woody Windishmann

One area that I would be careful about is using custom site definitions.

These were available in WSS 2.0 and virtual discovered by the development community by reverse engineering soon after RTM. This approach to SharePoint customization lead to some documentation and even some sample site templates being released but then along came WSS 3.0 and only OOTB site definitions were upgraded automatically. WSS 3.0 also offers new feature and solution deployment options which can help you to add functionality to the out-of-the-box site definitions in a supported way. I suspect the OOTB site definition + Features may provide a cleaner upgrade path for you in the future (assuming your feature meets the evaluation criteria in the Code Acceptance Checklist ;-)

If your SharePoint farm is used for multiple applications, both OOTB and custom, such as Intranet Publishing, collaboration and a 3^rd part project management solution, I would use the Code Isolation design principal by installing the 3^rd party solution in its own SharePoint Web Application with a dedicated IIS Web Application Pool. -- Ian Shandling

Administrator Site Definitions vs. Features Upgrade Trade offs

Custom Site Definitions are not one time expenses, and just like other customizations requires continual support after implementation, especially at times like upgrade. However custom features also require maintenance and upgrade support, so maybe the cost is not that different.  Sure much of a site definition can be upgraded out of (convert schemas to features), but the core site definition IDs and template names last forever. That means being stuck trying to implement new UI changes for site definitions at each major upgrade when all you really initially wanted was a set of custom templates people could choose that had maybe a bit of UI changes, and sets of features. If you can live with just using features initially, then I would suggest skipping the additional burden of the site collection, because at some point someone will want agility that the custom site collection doesn’t easily or cheaply provide.

A couple of other potential options:

· SharePoint does have a mechanism to staple features to sites and it is global, but one could also use OM, via an external process or even activated through some custom master feature, to programmatically determine which features to staple at site creation. Or this could be a feature that has a first visit action to provision the customizations (not super friendly, but I know of some who do this).

· Another consideration is to create new custom site templates based on existing ones (like STS Blank) and then adding references to custom features or directly adding custom changes. Technically, I would say the burden of re-fiddling with the definition at each major upgrade vs. creating new site templates at each major version and ensuring they have the right customizations and features is fairly close in cost, but one can never become un-customized with the custom site definition, and the custom template can become un-customized. Either way though, the custom features still need to be confirmed for upgrade-ability and may need some adjustment in areas such as UI to work correctly or at least not stand out negatively in the new version.

For the area they say about creating a new definition just to be a UI placeholder and having all other aspect handled by features, sure it works. Based on historic experience, when they plan to upgrade they will have to revisit their definition or feature pages to update the UI. The operations person will have to keep the original and/or updated definitions perpetually, or at least until they deprecate the sites using them (whereas a feature only solution though would only require deprecating the feature, and then cleaning up whatever it creates)

I think the biggest caveat to new/custom site definitions is that there is no good way to migrate out of them. If you adopt them, you are stuck with them since [they] encode the sites onet.xml and setuppath values to the existing definitions copy of the files.  -- Anonymous voice that matters.

So I've softened the language, but you've gotten the gist, and I think my post achieved the goal of helping us narrow down why they are some times needed. I do enjoy the points that custom site definitions are NOT one time expenses, and this should definitely be considered in any solutions that include them.

Let me reiterate the checklist from MSDN on evaluating third party solutions.

In a very constructive email thread with a number of Dev SharePoint MVPs and myself we discussed point 2 and my concerns around upgrade.  I had such an earful in V2 and I've been concerned about O12-O14 (V3 to V4) upgrades, but Todd's comments really got me thinking that it could really be different this time around.

I’m not sure why there is such concern about upgrading a site definition that is nothing but a copy of an out of the box site definition with a bunch of Features stapled to it.  I’ve heard from several folks at MS since V3 was released that when V4 comes around the existing out of the box site definitions will be upgraded as part of the installation process.  As we know, an XML file is used to upgrade site defs from one version to another.  So, in the future, if MS writes the XML file to upgrade the Team Site during the upgrade process I’m not sure why I wouldn’t be able to use the exact same file to upgrade a site definition that is exactly the same as a Team Site with just a different identifier and a bunch of features stapled to it.  It doesn’t seem to me like an upgrade using this approach would take long at all; in fact I’ve heard MS people say the same thing.  Is there anything that leads you guys to believe what I am discussing in this paragraph doesn’t make sense?  More importantly, how can we work with the product team to ensure that we have a framework in place for the next version that allows us to add a line item to the list box on the create a site page and ensure that we have an approach that’s flexible and upgradeable?  Is it too late for that by now in the development cycle? -- Todd Baginski

I think this is brilliant.  If the devs will simply let us know that they are simply copies of the out of the box site definitions with a unique ID then in theory we should be able to use the same config.xml that is provided for upgrade.  If there are no custom lists and customization that's actually unique and bulky sitting there that needs custom feature mappings and junk, then an Admin in theory could have a fairly similar experience to an out of the box upgrade.  The important distinction would be that all these various "dev" best practices are producing in theory a COPY of the out of the box Site Defs, with no real difference other than the ID.  Knowing this, and understanding this as common the upgrade scanners should include features to check for such this case and the intelligence of both Dev and Admin should be to explain to each other the common ground of not FAT customized Site Definitions, but simple copies with a unique ID to both make the definition available for new features stapled to that new name, title, description, etc... 

I believe Todd, AC, and many of the others are saying the same thing, don't throw away the tools, use them when necessary (sure us admins hate them, and would prefer for them to be isolated in their own app pools and own web apps and maybe even separate farms,) but upgrade need not be a blocker if we can minimize what is truly done with the custom site definition to avoid what was done in V2, and minimize the amount of work done to the on disk, and yes, please please package this as the SharePoint Cowboy, AC, Robert Bogue, and the rest of us demand.  Package it or deny.  No willy nilly installs of DLLs.

As best practices begin to be incorporated into MSDN you'll see content like this section on Site Definitions that goes right along with our discussion.

"In earlier versions of SharePoint Products and Technologies, site definitions were a common method of implementing server-wide user interface changes, but in Office SharePoint Server 2007 and Windows SharePoint Services 3.0, the site definitions rely on individual features that are referenced from the site definitions for much of the user interface and functionality.

After you create a site by using a custom site definition, that site always relies on that site definition. Therefore, custom site definitions must be maintained for subsequent releases of SharePoint Products and Technologies. If an upgrade occurs, then the site definition also must be upgraded, requiring changes to the site definition and the new upgrade definitions to map the old site definition to the new one.

Site definitions are a powerful method of creating many sites with the same layout; however Features and feature stapling now allow you to give sites additional functionality without the development/support burden that can come with using a site definition."

Or this existing content that is as controversial as the conversations began... on Site Templates or Site Definitions, a conversation that I don't think can have a winner.  This one is definitely a religious question... From the WSS SDK -  Deciding Between Custom Templates and Definition

• "Are the changes you need to make simple or complex? If, for example, you need to make only minor changes in the look of certain pages and add a few fields in particular lists, you should create a custom site template. However, if you need to create new content types, add new Web Part definitions, and significantly restructure sites, you should create a custom site definition.

• Can you deploy changes to the front-end Web server? If you do not have access to the file system of the computers running Windows SharePoint Services, you have no choice but to create a custom site template."

I hope this has been as constructive for you as it has been for me.  Sorry if I didn't include your comments or posts.  Many of these developers are working with the patterns and practices group on dev best practices, and you can see just how disparate the conversations and voices truly are.  There is common ground as is suggested.  Using the right tool for the job, and understanding the implications...

I definitely can't say these developers are lazy, they are obviously reading blogs, and keeping up with the latest... and are concerned about the future as are the admins :)

Joel Oleson