Specification vs. Workflow Instance¶
One critical concept to know about SpiffWorkflow is the difference between a
the difference between a
In order to understand how to handle a running workflow consider the following process:
Choose product -> Choose amount -> Produce product A `--> Produce product B
As you can see, in this case the process resembles a simple tree. Choose product, Choose amount, Produce product A, and Produce product B are all specific kinds of task specifications, and the whole process is a workflow specification.
But when you execute the workflow, the path taken does not necessarily have the same shape. For example, if the user chooses to produce 3 items of product A, the path taken looks like the following:
Choose product -> Choose amount -> Produce product A |--> Produce product A `--> Produce product A
This is the reason why you will find two different categories of objects in Spiff Workflow:
- Specification objects (WorkflowSpec and TaskSpec) represent the workflow definition, and
- derivation tree objects (Workflow and Task) model the task tree that represents the state of a running workflow.
Understanding task states¶
The following task states exist:
The states are reached in a strict order and the lines in the diagram show the possible state transitions.
The order of these state transitions is violated only in one case: A Trigger task may add additional work to a task that was already COMPLETED, causing it to change the state back to FUTURE.
- MAYBE means that the task will possibly, but not necessarily run at a future time. This means that it can not yet be fully determined as to whether or not it may run, for example, because the execution still depends on the outcome of an ExclusiveChoice task in the path that leads towards it.
- LIKELY is like MAYBE, except it is considered to have a higher probability of being reached because the path leading towards it is the default choice in an ExclusiveChoice task.
- FUTURE means that the processor has predicted that this this path will be taken and this task will, at some point, definitely run. (Unless the task is explicitly set to CANCELLED, which can not be predicted.) If a task is waiting on predecessors to run then it is in FUTURE state (not WAITING).
- WAITING means I am in the process of doing my work and have not finished. When the work is finished, then I will be READY for completion and will go to READY state. WAITING is an optional state.
- READY means “the preconditions for marking this task as complete are met”.
- COMPLETED means that the task is done.
- CANCELLED means that the task was explicitly cancelled, for example by a CancelTask operation.
Associating data with a workflow¶
The difference between specification objects and derivation tree objects is also important when choosing how to store data in a workflow. Spiff Workflow supports storing data in two ways:
- Task spec data is stored in the TaskSpec object. In other words, if a task causes task spec data to change, that change is reflected to all other instances in the derivation tree that use the TaskSpec object.
- Task data is local to the Task object, but is carried along to the children of each Task object in the derivation tree as the workflow progresses.
A derivation tree is created based off of the spec using a hierarchy of
SpiffWorkflow.Task objects (not
Each Task contains a reference to the TaskSpec that generated it.
Think of a derivation tree as tree of execution paths (some, but not all, of which will end up executing). Each Task object is basically a node in the derivation tree. Each task in the tree links back to its parent (there are no connection objects). The processing is done by walking down the derivation tree one Task at a time and moving the task (and its children) through the sequence of states towards completion.
You can serialize/deserialize specs. You can also serialize/deserialize a running workflow (it will pull in its spec as well).
There’s a decent eventing model that allows you to tie in to and receive
events (for each task, you can get event notifications from its TaskSpec).
The events correspond with how the processing is going in the derivation
tree, not necessarily how the workflow as a whole is moving.
SpiffWorkflow.specs.TaskSpec for docs on events.
You can nest workflows (using the
The serialization code is done well which makes it easy to add new formats if we need to support them.