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A lot of times there is some ignorance with developers about the possible package installation options. This also included myself. Sort of like an old habit I always use the "overwrite" option for data templates and layouts and the merge-clear option for content-items.
Looking at the description that is shown in the popup dialog during package installation, got me confused about the difference between the options and sub-options.
The descriptions of the installation options in the package installation wizard says:
|Overwrite||Replace the entire subtree with the subtree in the package|
|Merge - Clear||Leave the subtree and replace any matching items and versions with those from the package but do not replace any subitems.|
|Merge - Append||Leave the subtree. Overwrite nothing. Append any matching items to the existing subtree by giving them incremental numbers.|
|Merge - Overwrite||Leave the subtree and replace any matching items and versions with those from the package.|
|Skip||Take no action and move on to the next item.|
Looking at these descriptions, the Overwrite and Skip seem obvious although the description could be a bit better. But what about the merge-clear, merge-append and merge-overwrite? Those got me really confused.
The correct description of what each option does should be something like this:
|Overwrite||Items with the same ID (along with it's descendants) will be removed and replaced by items from the package.|
|Skip||Items with the same ID from the target database will remain unchanged; the item from the package will be skipped.|
|Merge - Clear||all existing versions for a language of the item being installed are removed prior to adding new versions. This options 'clears out' the versions of the language and creates one new version.|
|Merge - Append||item versions from the package are added 'on-top' of the existing versions of the item. This preserves history, but numbers the package versions with numbers higher than the existing version numbers. A user can merge information between versions afterwards.|
|Merge - Merge||if there is a version with the same number in the item, the Installation Wizard will overwrite it, otherwise a new version with the specific number is added. This makes it possible to replace specific versions of items.|
As you can see, the Merge-options doesn't remove items, but instead, merges versions of an item.
The Merge-Append option could be useful when updating content from a testing or acceptance environment to a production environment (or vice versa) where the customer has already started creating content without losing any of the previous content.