job_cancellation_example.cpp

Showcases four scenarios: basic cancellation via worker stop, non-cooperative job anti-pattern, pool-level multi-worker cancellation, and immediate versus graceful shutdown comparison.

See also

thread_pool, job, cancellation_token

/*****************************************************************************
BSD 3-Clause License
 
Copyright (c) 2024, 🍀☀🌕🌥 🌊
All rights reserved.
*****************************************************************************/
 
#include <kcenon/thread/core/thread_pool.h>
#include <kcenon/thread/core/job.h>
#include <kcenon/thread/core/thread_worker.h>
#include <kcenon/thread/utils/formatter.h>
 
#include <iostream>
#include <chrono>
#include <thread>
#include <atomic>
 
using namespace kcenon::thread;
using kcenon::thread::utils::formatter;
 
class cancellable_long_job : public job
{
public:
    cancellable_long_job(const std::string& name, int iterations = 100)
        : job(name), iterations_(iterations)
    {
    }
 
    kcenon::common::VoidResult do_work() override
    {
        std::cout << formatter::format("[{}] Starting job with {} iterations\n",
                                       get_name(), iterations_);
 
        for (int i = 0; i < iterations_; ++i)
        {
            // ✅ BEST PRACTICE: Check cancellation periodically
            if (cancellation_token_.is_cancelled())
            {
                std::cout << formatter::format("[{}] Job cancelled at iteration {}/{}\n",
                                               get_name(), i, iterations_);
 
                return kcenon::common::error_info{
                    error_code::operation_canceled,
                    formatter::format("Cancelled at iteration {}", i),
                    "job_cancellation_example"};
            }
 
            // Simulate work (100ms per iteration)
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
 
            // Log progress every 10 iterations
            if (i % 10 == 0)
            {
                std::cout << formatter::format("[{}] Progress: {}/{}\n",
                                               get_name(), i, iterations_);
            }
        }
 
        std::cout << formatter::format("[{}] Job completed successfully\n", get_name());
        return kcenon::common::ok();
    }
 
private:
    int iterations_;
};
 
class non_cancellable_job : public job
{
public:
    non_cancellable_job(const std::string& name, int iterations = 50)
        : job(name), iterations_(iterations)
    {
    }
 
    kcenon::common::VoidResult do_work() override
    {
        std::cout << formatter::format("[{}] Starting non-cancellable job\n", get_name());
 
        for (int i = 0; i < iterations_; ++i)
        {
            // ❌ BAD PRACTICE: Never checking cancellation
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
 
            if (i % 10 == 0)
            {
                std::cout << formatter::format("[{}] Progress: {}/{} (ignoring cancellation)\n",
                                               get_name(), i, iterations_);
            }
        }
 
        std::cout << formatter::format("[{}] Job completed (never checked cancellation)\n",
                                       get_name());
        return kcenon::common::ok();
    }
 
private:
    int iterations_;
};
 
void demo_basic_cancellation()
{
    std::cout << "\n========================================\n";
    std::cout << "Demo 1: Basic Job Cancellation\n";
    std::cout << "========================================\n\n";
 
    // Create pool with single worker
    auto pool = std::make_shared<thread_pool>("cancellation_demo_pool");
 
    auto worker = std::make_unique<thread_worker>();
    pool->enqueue(std::move(worker));
    pool->start();
 
    // Submit a long-running cancellable job (10 seconds total)
    auto long_job = std::make_unique<cancellable_long_job>("long_task", 100);
    pool->enqueue(std::move(long_job));
 
    // Let job run for 2 seconds
    std::cout << "Letting job run for 2 seconds...\n";
    std::this_thread::sleep_for(std::chrono::seconds(2));
 
    // Stop the pool (triggers cancellation)
    std::cout << "\n>>> Calling pool->stop() <<<\n\n";
    auto stop_start = std::chrono::steady_clock::now();
    pool->stop();
    auto stop_duration = std::chrono::duration_cast<std::chrono::milliseconds>(
        std::chrono::steady_clock::now() - stop_start);
 
    std::cout << formatter::format("\nPool stopped in {}ms (job cooperated with cancellation)\n",
                                   stop_duration.count());
}
 
void demo_non_cooperative_job()
{
    std::cout << "\n========================================\n";
    std::cout << "Demo 2: Non-Cooperative Job (Anti-Pattern)\n";
    std::cout << "========================================\n\n";
 
    auto pool = std::make_shared<thread_pool>("non_coop_pool");
 
    auto worker = std::make_unique<thread_worker>();
    pool->enqueue(std::move(worker));
    pool->start();
 
    // Submit a job that doesn't check cancellation (5 seconds total)
    auto stubborn_job = std::make_unique<non_cancellable_job>("stubborn_task", 50);
    pool->enqueue(std::move(stubborn_job));
 
    // Let job run for 1 second
    std::cout << "Letting job run for 1 second...\n";
    std::this_thread::sleep_for(std::chrono::seconds(1));
 
    // Stop the pool
    std::cout << "\n>>> Calling pool->stop() <<<\n";
    std::cout << "⚠️  Job is NOT checking cancellation token!\n";
    std::cout << "Worker must wait for job to complete...\n\n";
 
    auto stop_start = std::chrono::steady_clock::now();
    pool->stop();
    auto stop_duration = std::chrono::duration_cast<std::chrono::milliseconds>(
        std::chrono::steady_clock::now() - stop_start);
 
    std::cout << formatter::format("\nPool stopped in {}ms (job did NOT cooperate)\n",
                                   stop_duration.count());
    std::cout << "Notice how much longer this took!\n";
}
 
void demo_pool_level_cancellation()
{
    std::cout << "\n========================================\n";
    std::cout << "Demo 3: Pool-Level Multi-Worker Cancellation\n";
    std::cout << "========================================\n\n";
 
    auto pool = std::make_shared<thread_pool>("multi_worker_pool");
 
    // Add 3 workers
    for (int i = 0; i < 3; ++i)
    {
        auto worker = std::make_unique<thread_worker>();
        pool->enqueue(std::move(worker));
    }
    pool->start();
 
    // Submit multiple jobs to different workers
    for (int i = 0; i < 3; ++i)
    {
        auto job = std::make_unique<cancellable_long_job>(
            formatter::format("worker_{}_task", i), 100);
        pool->enqueue(std::move(job));
    }
 
    // Let jobs run for 2 seconds
    std::cout << "All workers running jobs...\n";
    std::this_thread::sleep_for(std::chrono::seconds(2));
 
    // Stop the entire pool
    std::cout << "\n>>> Calling pool->stop() - cancelling ALL workers <<<\n\n";
    auto stop_start = std::chrono::steady_clock::now();
    pool->stop();
    auto stop_duration = std::chrono::duration_cast<std::chrono::milliseconds>(
        std::chrono::steady_clock::now() - stop_start);
 
    std::cout << formatter::format("\nAll workers stopped in {}ms\n", stop_duration.count());
    std::cout << "All jobs received cancellation signal simultaneously!\n";
}
 
void demo_immediate_vs_graceful()
{
    std::cout << "\n========================================\n";
    std::cout << "Demo 4: Immediate vs. Graceful Shutdown\n";
    std::cout << "========================================\n\n";
 
    // Test 1: Graceful shutdown
    {
        std::cout << "--- Graceful Shutdown (immediately_stop = false) ---\n";
        auto pool = std::make_shared<thread_pool>("graceful_pool");
        auto worker = std::make_unique<thread_worker>();
        pool->enqueue(std::move(worker));
        pool->start();
 
        // Enqueue multiple jobs
        for (int i = 0; i < 5; ++i)
        {
            auto job = std::make_unique<cancellable_long_job>(
                formatter::format("graceful_job_{}", i), 20);
            pool->enqueue(std::move(job));
        }
 
        std::this_thread::sleep_for(std::chrono::milliseconds(500));
 
        std::cout << "Stopping gracefully (pending jobs remain in queue)...\n";
        pool->stop(false);  // Graceful shutdown
        std::cout << "Done\n\n";
    }
 
    // Test 2: Immediate shutdown
    {
        std::cout << "--- Immediate Shutdown (immediately_stop = true) ---\n";
        auto pool = std::make_shared<thread_pool>("immediate_pool");
        auto worker = std::make_unique<thread_worker>();
        pool->enqueue(std::move(worker));
        pool->start();
 
        // Enqueue multiple jobs
        for (int i = 0; i < 5; ++i)
        {
            auto job = std::make_unique<cancellable_long_job>(
                formatter::format("immediate_job_{}", i), 20);
            pool->enqueue(std::move(job));
        }
 
        std::this_thread::sleep_for(std::chrono::milliseconds(500));
 
        std::cout << "Stopping immediately (clearing pending jobs)...\n";
        pool->stop(true);  // Immediate shutdown
        std::cout << "Done (pending jobs were cleared)\n\n";
    }
}
 
int main()
{
    std::cout << "╔═══════════════════════════════════════════════════════╗\n";
    std::cout << "║   Thread System - Job Cancellation System Demo       ║\n";
    std::cout << "╚═══════════════════════════════════════════════════════╝\n";
 
    try
    {
        demo_basic_cancellation();
        std::this_thread::sleep_for(std::chrono::seconds(1));
 
        demo_non_cooperative_job();
        std::this_thread::sleep_for(std::chrono::seconds(1));
 
        demo_pool_level_cancellation();
        std::this_thread::sleep_for(std::chrono::seconds(1));
 
        demo_immediate_vs_graceful();
 
        std::cout << "\n========================================\n";
        std::cout << "All demonstrations completed!\n";
        std::cout << "========================================\n\n";
 
        std::cout << "Key Takeaways:\n";
        std::cout << "1. ✅ Jobs MUST check cancellation_token periodically\n";
        std::cout << "2. ✅ Worker stop() propagates cancellation to running job\n";
        std::cout << "3. ✅ Pool stop() cancels all workers simultaneously\n";
        std::cout << "4. ⚠️  Non-cooperative jobs delay shutdown\n";
        std::cout << "5. ✅ Immediate stop clears pending jobs from queue\n\n";
    }
    catch (const std::exception& e)
    {
        std::cerr << "Error: " << e.what() << std::endl;
        return 1;
    }
 
    return 0;
}