~/Building a Workflow Engine in C

Apr 12, 2021

A workflow engine interprets and executes process definitions, coordinating task flow between components or systems. Implementing a workflow engine in C requires expertise in data structures, state machines, and process orchestration.

Core Components

A minimal workflow engine must handle:

• Definition model for task and control flow
• Execution engine for running the workflow
• Persistence for state

Definition Model

Workflows can be modeled as a Directed Acyclic Graph or a set of linked states and transitions.

Example structure in C:

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typedef struct WorkflowNode {
    int id;
    void (*task)(void *);
    struct WorkflowNode **next_nodes;
    int next_count;
} WorkflowNode;

typedef struct Workflow {
    WorkflowNode **nodes;
    int node_count;
} Workflow;

Task Scheduling

A simple task scheduling loop processes nodes with all dependencies satisfied.

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void execute_workflow(Workflow *wf, void *context) {
    int executed[wf->node_count];
    memset(executed, 0, sizeof(executed));
    int progress = 1;
    while (progress) {
        progress = 0;
        for (int i = 0; i < wf->node_count; i++) {
            if (!executed[i] && dependencies_satisfied(wf->nodes[i], executed)) {
                wf->nodes[i]->task(context);
                executed[i] = 1;
                progress = 1;
            }
        }
    }
}

Dependency checking is implemented by inspecting predecessor nodes.

State Machines

Workflow progress is managed via a finite state machine. Each node represents a state, with transitions defined in the workflow specification.

Persistence

State could be written to file storage or databases such as SQLite.

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FILE *f = fopen("state.dat", "wb");
fwrite(executed, sizeof(int), wf->node_count, f);
fclose(f);

Event Handling

Event-driven workflows use callbacks or signals.

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void on_task_complete(int task_id) {
    // handle downstream actions
}

Error Handling

Robust engines support retry policies and compensation logic.

Example:

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int max_retries = 3;
for (int i = 0; i < max_retries; i++) {
    if (wf->nodes[i]->task(context) == 0) break;
}

Extensibility

Use function pointers for custom task logic. Load dynamic tasks via shared libraries with dlopen. Integrate JSON or XML for workflow definitions.

Example

A workflow that prints messages in sequence:

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void task1(void *ctx) { printf("Task 1\n"); }
void task2(void *ctx) { printf("Task 2\n"); }
WorkflowNode node2 = { 2, task2, NULL, 0 };
WorkflowNode *node1_next[] = { &node2 };
WorkflowNode node1 = { 1, task1, node1_next, 1 };
WorkflowNode *nodes[] = { &node1, &node2 };
Workflow wf = { nodes, 2 };
execute_workflow(&wf, NULL);

Relevant Libraries

• libevent
• libuv
• sqlite for persistence

Design Considerations

For advanced needs, support concurrency with threads, parallelism, and robust error handling.

For distributed workflows, integrate with message queues such as RabbitMQ or Kafka.

Conclusion

A workflow engine in C is feasible using state machines, function pointers, and persistent state. External libraries enhance concurrency and persistence. For more on this topic, consult source code examples, academic papers, and existing open source engines.