Graphics Courtesy: Multi-Headed Drupal by Larry Garfield at Drupalcon Munich
Why Domain Access?
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Good for syndicating content from the main site onto multiple regional (pseudo)sites at one click.
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Easier content administration from a single console (only when you have 5-6 sites). When you have more than 20 sites, the same attributes to confusion.
Why NOT Domain Access?
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Single Database - Single point of failure
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An ad campaign on one country affects performance of all other country sites
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Duplicate content / menu link titles (like Home, About Us from different regional sites) make the administration of content confusing. This also makes debugging a pain.
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PathAuto requires unique aliases. Can not use same alias like (about-us) on different sites
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Has known issues with Organic Groups (in case you wish to use it in the future)
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Even basic modules like Views are not compatible out of the box with DA and require additional modules to add compatibility (domain_views is not actively maintained too). And such DA-enablement modules are not available for most of the contrib Modules
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You will lose the freedom to pick one of those 8K modules that D7 has got and use them. For every module that you plan to use, you will have to think about how it plays with DA
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You can not enable/disable modules on specific regional sites (Because you just have 1 site basically)
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If you have 2 custom modules for every new country site that you launch, by the time you launch 100 sites, you will have 200 modules enabled bogging down your site which is already crippled by performance issues due to using a single database for your 100 sites.
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Permissions/Security Issues: Most modules do not offer per-domain configuration and the administrator of one regional site can be able to modify configuration of another regional site intentionally or accidentally.
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You can end up with dozens of views/panels/“name you structure here” that are needed by a single site but are present for all the others.
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The number of nodes / users / taxonomy terms is also bound to grow and may affect every site in cases there is a specific problem with one site
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It is very hard to maintain differences between sites and introducing any change will impact all sites
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Under certain configuration, Domain Access may add prefixes to table names for each site (eg taxonomy), creating new tables for every site. At scale this could would be unsustainable.