2 Checkpoint Library

The client application can use this library to help ensure the flow of the program is developed in such a way in which it can recover from any type of unexpected problem.

This is accomplished by dividing the program structure into two levels, the sequence level, and the checkpoint level.

2.1 Sequences

A sequence consists of an associative array style data structure containing a list of functions that the sequence will call. Each set of functions in the list, known as a node, contains a normal function, a rollback function, and a policy function.

A sequence is invoked similarily to the way one might invoke a single function, except that it is done through the API of the checkpoint library.

The important features of the sequence structure

• The progression through the nodes in the sequence is linear or branching. It is not a dependancy map.

• The nodes are anonymous and uniform. There is no need to differentiate between setup, running, or teardown functions.

• The nodes are self-directing. The execution sequence is indicated to the checkpoint library by the policy function, which is described below.

• Sequences are independant and nestable. Inter-sequence dependancies are left to the domain of the developer's code.

• When a node is invoked, it will be pushed onto the stack as it is called. This allows a node's policy function to roll back to a previous node should it determine that an error can't be corrected in the context of the current node.

• Errors encountered during the execution of the sequence are reported by the developer's code to the checkpoint library.

2.2 Checkpoints

Checkpoints are a facility for allowing the rollback of a partially completed function. Checkpoints are a strictly linear enumeration of the resources allocated by the programmer's code.

The value of the last checkpoint set by a particular normal or rollback function is stored, then presented to the rollback function so that the programmer's rollback code can know what resources it needs to free and/or deconfigure.

The important features of the checkpoint structure

• The progression through the checkpoints is strictly linear, and as such is 100% predictable. Branching is handled in the scope of the sequence structure, not the checkpoints.

• The checkpoints are maintained by both the normal function and the rollback function. This is necassary in case the rollback function fails and needs to be re-executed.

• The checkpoint system is designed to be inobtrusive and lightweight. It does not directly affect the flow of execution.