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Checkouts comes in two flavours:
  • Reserved
  • Unreserved
In ClearCase you can not get write access to a file unless it's checked out (well in snapshot views there's an exception, here you also have the possibility to hijack the file). This is quite different from how version control is implemented in most other systems (such as Subversion, CVS, git, Bazaar and Mercurial) where you don't have to "warn" the version control system in advance, that you have intentions of altering files, you just do it, and then take the consequences as you commit.

The closest ClearCase comes to this approach is unreserved checkouts. An unreserved checkout actually doesn't have any real consequence to anything except which files that are view-private files, until you choose to check-in the file again.

However, unreserved checkouts are not the default flavour in ClearCase. Reserved checkouts are. A reserved checkout is creating what could be described as a proxy-version on the branch (not stream!) you are checking out from. It's a proxy for the view private file created in the view and it just sits there and prevents other users from checking out the same element on the same branch elsewhere.

Other users are still allowed to do an unreserved checkout even if a reserved checkout is blocking the branch, but they will not be able to make any checkin until the reserved chekout is either checked in againg, or it's cancelled or changed to become an unreserved one.