Plain – Transactions

With a working draft of the VM I need to move on Plain Assembly syntax. By addressing syntax I challenge the VM as well and it will drive a few changes. I expect the number of instructions to increase as I cover topics. One such topic is how to maintain data integrity during multi-threaded transactions?

My Servo example assume that I only have one module accessing the Channel array. It also uses manual functions to synchronize array content. I want the assembler to handle aspects of multi-threading automatically, but I also need to cover integrity of data transactions and I wonder if I can do this with a single mechanism.

Using the 32xServo/IO Hat as an example we have an array of 32 objects. We have at least one C module and maybe several Plain modules reading/writing to that array simultaneously and we need a mechanism that ensure that they only access consistent data – not data that is in the middle of a change.

extern C Channel chan[32]

I already declare this data with the keyword “Extern” and a module name which in this case is “C”. This tells the assembler that this is an interface variable that need some special care. It might be that “Interface” is better than “Extern” in the syntax.

Transaction( chan[0])
            chan[0].mode = ChannelMode.Servo
            ...
            Commit( chan[0])
            ...
            if (error)
                        Rollback( chan[0])
            end
            ...
Update

In this draft I introduce 4 new keywords. We set a new rule that data declared extern only can be written to inside a transaction-Update block. We add Rollback and Commit with a rule that if we hit Update this is an automated Commit if no commit/rollback parameter is added.

Don’t worry about the keywords or syntax details for now. We can change this later if we find better alternatives.

This will add 4 instructions to our instruction set.

Transaction	Lock a list of variables from being changed by other modules. This can only be used on variables declared as extern.
Update	Complete a transaction. Will free the locked variables and terminate the transaction block. The only optional parameter is Commit or Rollback. No parameters means we Commit. Update Commit or Update will also raise events associated with the changes.
Commit	Commit allow us to commit selected variables and raise events associated with these changes. A commit statement will commit everything or only the parameters attached.
Rollback	Will rollback any changes to variables listed or all variables if we have no parameters. This is to ensure data integrity in case of errors.