horus/docs/coordinator/overview.md

Coordinator Overview

The Coordinator is the workflow orchestration layer in Horus. It executes DAG-based flows by managing job dependencies and dispatching ready steps to supervisors.

Architecture

Client → Coordinator → Supervisor(s) → Runner(s)

Responsibilities

1. Workflow Management

• Parse and validate DAG workflow definitions
• Track workflow execution state
• Manage step dependencies

2. Job Orchestration

• Determine which steps are ready to execute
• Dispatch jobs to appropriate supervisors
• Handle step failures and retries

3. Dependency Resolution

• Track step completion
• Resolve data dependencies between steps
• Pass outputs from completed steps to dependent steps

4. Multi-Supervisor Coordination

• Route jobs to specific supervisors
• Handle supervisor failures
• Load balance across supervisors

Workflow Definition

Workflows are defined as Directed Acyclic Graphs (DAGs):

workflow:
  name: "data-pipeline"
  steps:
    - id: "fetch"
      runner: "hero"
      payload: "!!http.get url:'https://api.example.com/data'"
      
    - id: "process"
      runner: "sal"
      depends_on: ["fetch"]
      payload: |
        let data = input.fetch;
        let processed = process_data(data);
        processed
        
    - id: "store"
      runner: "osiris"
      depends_on: ["process"]
      payload: |
        let model = osiris.model("results");
        model.create(input.process);

Features

DAG Execution

• Parallel execution of independent steps
• Sequential execution of dependent steps
• Automatic dependency resolution

Error Handling

• Step-level retry policies
• Workflow-level error handlers
• Partial workflow recovery

Data Flow

• Pass outputs between steps
• Transform data between steps
• Aggregate results from parallel steps

Monitoring

• Real-time workflow status
• Step-level progress tracking
• Execution metrics and logs

Workflow Lifecycle

1. Submission: Client submits workflow definition
2. Validation: Coordinator validates DAG structure
3. Scheduling: Determine ready steps (no pending dependencies)
4. Dispatch: Send jobs to supervisors
5. Tracking: Monitor step completion
6. Progression: Execute next ready steps
7. Completion: Workflow finishes when all steps complete

Use Cases

Data Pipelines

Extract → Transform → Load

CI/CD Workflows

Build → Test → Deploy

Multi-Stage Processing

Fetch Data → Process → Validate → Store → Notify

Parallel Execution

        ┌─ Task A ─┐
Start ──┼─ Task B ─┼── Aggregate → Finish
        └─ Task C ─┘

Configuration

# Start coordinator
coordinator --port 9090 --redis-url redis://localhost:6379

# With multiple supervisors
coordinator --port 9090 \
  --supervisor http://supervisor1:8080 \
  --supervisor http://supervisor2:8080

API

The Coordinator exposes an OpenRPC API:

• submit_workflow: Submit a new workflow
• get_workflow_status: Check workflow progress
• list_workflows: List all workflows
• cancel_workflow: Stop a running workflow
• get_workflow_logs: Retrieve execution logs

Advantages

• Declarative: Define what to do, not how
• Scalable: Parallel execution across multiple supervisors
• Resilient: Automatic retry and error handling
• Observable: Real-time status and logging
• Composable: Reuse workflows as steps in larger workflows