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Updating the Session object

We maintain a set `Front' that contains all the latest database commands that do not have any other database commands executed after them. When a new database command executes, it is connected to the commands in `Front', which thereafter is reset to contain the one command that was just executed, see figure 13.

 


:  The solution to the problem with redundant arcs.

The only control structure needed is the set of front commands, i.e. the database commands that have no database commands after them. To that end we use a control object, called ch object (Command History object), which is also the handle to the command history as a whole. Each version of the ch object has one link `Front' to exactly one front command node. The set of latest ch object versions, through their `Front' links, forms the set of front commands in the command history, see figure 14.

 


:  The Ch object for the command history in



Martin Sjolin
Thu Jun 15 20:41:59 MET DST 1995