SharePoint newbies often get confused with this question: When to create Site collection and when to use Subsites? What are the advantages of having a Ste collection rather subsite? Well, the choice of Site collection or subsite can be determined based on these factors:
- Security/Permission Management - Every Site collection uses unique security model, which means: security is isolated between site collections. Whereas subsites can inherit security from its parent site. In an another aspect - permission management between site collections could be a nightmare. But managing permissions in subsites can be much easier and less frustrating.
- Re-usability: Several SharePoint components such as web parts, content types, site columns (only in SharePoint 2007), etc can be reused within all sites in a site collection. But if you have to port them to other site collections, you may require considerable efforts.
- Hierarchy/Navigation: Subsites can share a common navigation and structure. But site collections uses unique navigation (OOTB! Although custom site map providers can be implemented, it involves considerable efforts)
- Governance/Scalability- Storage Quotas: Site collection's growth can be governed and controlled with storage Quotas. Whereas subsites doesn't has any such feature.We can't control storage growth of a subsite without splitting them into site collections that involves several known issues and URLs to be changed after converting subsites into site collection. But we can increase site collection's storage quota without affecting its URL structure. Site collections are easy to lock!
- Content Database Scalability: Site collections are mapped with content databases as One - Many relationship. So one site collection can be placed only in a single DB. But One DB can hold multiple site collections), So that site collections can be easily moved between content databases or to different web applications.
- Backup/Restore : Site collections completely supports backup-restore. Although subsites support export-import, certain artifacts such as security, recycle bin, workflows, alerts, etc will be lost during export-import process. So, site collections are easy to replicate between different SharePoint environments.
- Feature Scopes: Certain features are scoped at site collection level in SharePoint. E.g. Search scopes, Site collection Policies, Auditing, IRM, Recycle Bin, Usage reports, Site collection Administrators, site/list templates, Content and structure feature to copy/paste files within a site collection, etc.
- Branding: Branding features such as Master pages, Themes, Display Template, CSS/JS files can be inherited by sub-sites to provide consistent look and feel seamlessly. But between site collections we'll have to deploy these individually.
- URL Hierarchy: Site collections can have different URLs (Host-named site collections) or Unique URL underneath the managed path (such as: /sites/siteA or /teams/marketing), But subsites share the common Top Level URL and lay beneath it.
- Web Part Boundaries: If you want to aggregate data throughout your site hierarchy using Web Parts like the Content Query Web Part, Data View Web Part and other data aggregation Web Parts, they work only within the site collection scope. They can't fetch data from other site collections. (up to SharePoint 2010! We've Content search web part in SharePoint Enterprise Edition!)
- Site Templates - If you create your own site template by combining content, you'll end up creating only subsites from the template.
- Functional Classification: Create a site collection to host sites that are going to share something in common. E.g Site collection for the business unit. Each business unit may contain multiple subsites. E.g. http://sharepoint.com/Sites/Sales/us/, /Sales/NA, etc.
- Software Boundaries: A site collection supports up to 250,000 subsites. Number Site Collections per Content Database - Recommendation 10000 - as of SharePoint 2013.
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