These different types of lock and locking strategies are designed to reflect the normal pattern of modeling tool usage, to give the most efficient and invisible support.

Starting and ending sessions and transactions are actions that are explicitly carried out by the user. Locking, on the other hand, is performed automatically by MetaEdit+ on behalf of the user, based on the user’s actions. Why this distinction? Virtually every action in MetaEdit+ requires some kind of locking operation or check, and the burden on the user of manually setting and releasing the locks would be huge. In addition, the safety of the work done in a transaction depends on the correct locks being obtained at the correct times: any mistake, and the transaction would probably be unable to commit because of conflicting changes with another user.

For our purposes, we can divide data in MetaEdit+ into three kinds:

A representational graph is a diagram, matrix or table. A conceptual graph is the ‘real’ data that underlies that representational graph: a conceptual graph may have several different representations. Information stored by a representational graph and its elements includes the positions of individual symbols, the order of items on an axis in a matrix, and the widths of the columns in a table. Correspondingly a conceptual graph stores information about which objects belong to the graph, how they are connected together via relationships, and what other graphs they explode to.

These first two types of data are session locked in connection with opening an editor on a representational graph. MetaEdit+ will automatically try to obtain locks both for the representational graph, and for the underlying conceptual graph. The success of these locks determines which actions the user may perform in that editor: if one or both locks fail, the editor will still open, but some of the menu items will be hidden, and other e.g. mouse operations may have no effect. You can view lock information in editors through Info Tool (Graph | Graph Info).

For instance, if a diagram is opened and locked successfully, but the conceptual graph cannot be locked, the user will be able to move symbols around in the diagram, but not be able to add new objects or relationships to the graph. He will however be able to add a symbol for an object that already exists in the conceptual graph (Shift-click). If neither lock was obtained, the user’s actions will be restricted to scrolling, zooming, viewing selected types, and editing the properties of the objects etc. in the graph.

For as long as the editor remains open, the user will keep the locks he initially obtained on opening it. If he wants to relinquish the locks, or to try to gain more locks, he should close and reopen the editor on that representation graph.

Whilst locks in editors are generally held for a while, maybe over several transactions (session locks), other locks in MetaEdit+ are often held only for a single transaction (although their effects will be felt by other users throughout any transaction overlapping the one where these were locked). These are the transaction locks on individual objects, relationships, and roles, and on their individual properties.

The most common use of such locks is in property dialogs. When a user opens a dialog on an object’s properties, MetaEdit+ attempts to lock all that object’s properties, so they can be changed. If one or more of the locks fail, then no locks are taken, and the user can only view the properties in the property dialog: the OK button will be grayed. If all locks were obtained, the user will be able to press the OK button to accept his changes.

Similar short-lived locks are taken in various other circumstances, often so that their success is not noted in any particular way, but a failure to obtain a lock will be shown to the user as a warning dialog. In general these warnings are no real cause for concern, and the user will be able to continue working as normal.

When a user creates a new graph, it is automatically added to the collection of graphs owned by the current default project. This collection is of course also used by other users, and so changes to it by two users in overlapping transactions would normally not be possible. MetaEdit+ uses a special locking strategy for these collections, though, which normally allows many users to add to them simultaneously. Under certain circumstances however it may happen that the collection is locked so that a user cannot create a new graph in that project.

As with other locking problems, the solution is to have the other user whose lock conflicts with yours commit their transaction, and then for you to commit your transaction. In that way you will see the new graph the other user created, and also be able to lock the collection to add your new graph.