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Lock Command, continued

Since all Lock does is check, add, and delete entries in the Lock Table, when using Lock, you don't even have to specify the name of an existing global. For example, you could decide to lock +^Person(id) (instead of lock +^PersonD(id)) in the code for editing ^PersonD, even though the ^Person global doesn't exist. It would work just as well, because ^Person would be added to the Lock Table, and all the code that edits the ^PersonD global will try to lock ^Person.

You might think that you also should lock ^PersonI when editing persons. But this is not necessary even though this global also contains person data. This is because you're not actually "locking" the arrays; you're simply adding an entry to the Lock Table, which has the effect of locking the code that appears between the lock and unlock. In an application, all the code that updates ^PersonD and ^PersonI would use this same convention. lock +^PersonD(id) is enough.

When ^PersonD(1) is in the Lock Table, another process can't lock ^PersonD(1), yet it could lock ^PersonD(2). In other words, another process can lock siblings of entries in the Lock Table. However, another process can't lock ancestors of entries in the lock Table (^PersonD in this case). This is because you wouldn't want to allow access to a higher level of the array if one of its lower nodes is locked. Similarly, another process can't lock descendants of entries in the Lock Table; ^PersonD(1, 2) for example. You wouldn't want to allow access to the descendants of a node if the node itself is locked.

The default form of Lock is an exclusive lock. As you've seen, exclusive locks prevent all other locks. InterSystems IRIS also supports shared locks, using the syntax lock +^PersonD(id)#"s" . Multiple processes may take out the same shared lock. But a shared lock prevents an exclusive lock, and an exclusive lock prevents a shared lock. Shared locks are useful for cases where one or more processes are viewing the same record, while at the same time preventing other processes from editing that record.

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