Skip to content

Prerequisite Checks

Validate all preconditions before executing expensive or irreversible operations.

Key Insight

Check everything, then do everything. Consolidate validation into a dedicated phase before any work begins.

Overview

Prerequisite checks are a structured approach to fail fast validation. Instead of scattering validation throughout code, you consolidate all precondition checks into a dedicated phase that runs before any work begins.

flowchart LR
    subgraph prereq[Prerequisite Phase]
        A[Check Tools]
        B[Check Access]
        C[Check State]
        D[Check Resources]
    end

    subgraph gate[Gate]
        G{All Pass?}
    end

    subgraph exec[Execution Phase]
        E[Execute Operation]
    end

    A --> B --> C --> D --> G
    G -->|Yes| E
    G -->|No| F[Abort with Report]

    %% Ghostty Hardcore Theme
    style A fill:#65d9ef,color:#1b1d1e
    style B fill:#65d9ef,color:#1b1d1e
    style C fill:#65d9ef,color:#1b1d1e
    style D fill:#65d9ef,color:#1b1d1e
    style G fill:#fd971e,color:#1b1d1e
    style E fill:#a7e22e,color:#1b1d1e
    style F fill:#f92572,color:#1b1d1e

The key insight: check everything, then do everything.

Categories of Prerequisites

Category What to Check Example Guide
Environment Required tools and variables kubectl, $DATABASE_URL Environment
Access Permissions are granted API tokens, RBAC roles Permissions
State System is in expected state Resource exists, not locked State
Input Inputs are valid Required fields, formats Input
Dependencies Dependencies are ready Upstream jobs, services Dependencies

Quick Example

# GitHub Actions prerequisite check
- name: Validate prerequisites
  run: |
    errors=()

    # Environment
    [[ -n "${{ secrets.DEPLOY_TOKEN }}" ]] || errors+=("DEPLOY_TOKEN not set")

    # Tools
    command -v kubectl >/dev/null || errors+=("kubectl not installed")

    # Permissions
    kubectl auth can-i create deployments -n production || errors+=("No deploy permission")

    # State
    kubectl get namespace production >/dev/null || errors+=("Namespace missing")

    # Report
    if [ ${#errors[@]} -gt 0 ]; then
      echo "::error::Prerequisite check failed"
      printf '%s\n' "${errors[@]}"
      exit 1
    fi

    echo "All prerequisites met"

Check Categories

Environment Validation

Check that the environment has everything needed before starting work:

  • Required environment variables
  • Required tools installed
  • Network connectivity

Permission Checks

Verify access rights before attempting operations:

  • API token scopes
  • GitHub App permissions
  • Kubernetes RBAC
  • Cloud IAM roles

State Preconditions

Validate system state before operations:

  • Resource existence
  • No naming conflicts
  • Service health
  • Branch exists

Input Validation

Validate all inputs before processing:

  • Required inputs provided
  • Format validation
  • Cross-field validation

Dependency Checks

Verify dependencies are ready:

  • Upstream jobs succeeded
  • Required artifacts available
  • External services reachable
  • API rate limits

Implementation Guide

See Implementation Patterns for:

  • Check ordering strategy (cost-based)
  • Implementation patterns (fail-first vs collect-all vs structured)
  • Common CI/CD prerequisites checklist
  • Anti-patterns to avoid

Implementation Checklist

Before implementing prerequisite checks:

  • [ ] List all prerequisites for the operation
  • [ ] Categorize by type (tools, access, state, resources, config)
  • [ ] Order by cost (cheapest first)
  • [ ] Make checks read-only (no side effects)
  • [ ] Provide actionable errors (what failed, how to fix)
  • [ ] Collect all errors (don't stop at first failure, unless fast feedback required)
  • [ ] Test prerequisite failures explicitly
  • [ ] Document prerequisites for operators

When to Apply

Scenario Apply Prerequisite Checks? Reasoning
Kubernetes deployment Yes Check cluster access, namespace, resources
GitHub Actions workflow Yes Check secrets, tools, permissions
Database migration Yes Check connectivity, schema version, backup
API request handling Depends Check inputs yes, runtime state no
File processing Depends Check file exists yes, content format no

Decision rule: Use prerequisite checks for validation you can do upfront, not validation that requires starting the operation.

Relationship to Other Patterns

Pattern How Prerequisite Checks Applies
Fail Fast Prerequisite checks are structured fail-fast validation
Graceful Degradation Prerequisites determine if graceful degradation is even possible
Idempotency Check-before-act is a prerequisite pattern
Work Avoidance Prerequisites can include "work already done" checks

Further Reading

Comments