Designing a Flexible Workflow Engine: From Simple Chains to Complex Nested Nodes

Initially, the boss requested a simple workflow engine, leading to a basic linked‑list design where approvers are added sequentially, the current approver moves forward after approval, and the process ends when the final node is reached.

Level 2 – Adding Multi‑Sign Nodes

To support multi‑sign nodes, the design shifted from a linear list to a tree structure, distinguishing simple nodes (single approver) and complex nodes (containers for sub‑nodes). Multi‑sign nodes activate all child nodes, completing only when every child is approved.

Level 3 – Introducing Parallel Nodes

Parallel nodes were added as complex nodes where any child’s approval completes the node; a new "Skip" status was defined to disable sibling nodes once a parallel path succeeds.

Level 4 – Unlimited Nesting

The tree‑based architecture naturally allowed unlimited nesting of any node type, enabling arbitrarily complex approval flows.

Level 5 – Conditional Nodes

Conditional nodes were introduced, functioning like parallel nodes but activating only those child branches whose conditions, based on form data, are satisfied.

Level 6 – Dynamic Approver Types

Simple nodes were expanded into three categories: fixed approvers, approvers derived from form fields, and approvers computed via a mapping function (e.g., get_manager("User")).

Level 7 – Rejection to Initiator

A rejection feature was added, allowing Ready‑state nodes to reject back to the initiator, effectively restarting the workflow.

Level 8 – Rejection to Previous Approver

Logic was implemented to reject to the immediate previous approver, handling the complexity of nested structures to locate the correct prior node.

Level 9 – Rejection to Arbitrary Node

The system now supports rejecting to any node by iteratively moving up the tree until a Ready node containing the target is found.

Level 10 – Time‑Limited Nodes

Nodes can now have a time limit; if not completed within the allotted period, they are marked as timed‑out.

Level 11 – Delegation

Delegation is modeled by creating a parallel parent node with a sibling representing the delegate, allowing both the original approver and delegate to complete the task; delegation can be nested indefinitely.

Level 12 – Cancel Delegation

Cancellation of delegation is implemented as the inverse operation, prohibited if the delegate has already approved.

Level 13 – Pre‑ and Post‑Conditions

Each node can now define pre‑conditions (required to enter the node) and post‑conditions (required to mark the node as complete), adding further control to the workflow.

Level 14 – Progress Metric

A progress percentage is calculated by measuring the distance from the leftmost node to the rightmost Ready node relative to the total width of the tree.

Level 15 – Execution Scripts

Two scripts can be attached to each node: one executed when the node starts approval and another when the node finishes approval.

The narrative concludes with a personal reflection on the demanding nature of these iterative requirements and the eventual commercial success of the workflow system.