SPDLog Sink for "Flush Within Xmsec"

spdlog-extensions.cpp

/*
The code below shows a custom SPDLog sink that uses a separate thread and a C++0x condition_variable
to ensure that a spdlog::sinks::rotating_file_sink_mt is flushed to disk within a second of a new
log message being emitted. When no messages are arriving, the helper thread has very low wake overhead
and sleeps for 30s.
*/

std::mutex spdFlushMtx;
std::condition_variable spdFlushCV;
int spdUnflushed = 0;
bool spdFlushExit = false;

void runFlushLoop() {
    while (true) {
        auto desiredTime = std::chrono::system_clock::now();
        desiredTime += chrono::milliseconds(30000);
        {
            // Wait for a message, or for 30 seconds, whichever happens first
            unique_lock<mutex> lck(spdFlushMtx);
            spdFlushCV.wait_until(lck, desiredTime);
            if (spdFlushExit) {
                return;
            }
            if (spdUnflushed == 0) {
                continue; // detect, avoid spurious wakes
            }
        }
        
        // Debounce 1sec for more messages to arrive
        sleep(1);
        
        {
            // Perform flush
            unique_lock<mutex> lck(spdFlushMtx);
            spdlog::get("logger")->flush();
            spdUnflushed = 0;
        }
    }
}

/*
This is a SPDLog sink that wakes the flush thread,
ensuring that logged messages are flushed to disk
within one second.
*/
class SPDFlusherSink : public spdlog::sinks::sink {
public:
    std::thread * flushThread;
    
    SPDFlusherSink() {
        flushThread = new std::thread(runFlushLoop);
        
    }
    ~SPDFlusherSink() {
        lock_guard<mutex> lck(spdFlushMtx);
        spdFlushExit = true;
        spdFlushCV.notify_one();
    }
    
    void log(const spdlog::details::log_msg& msg) {
        // ensure we have a flush queued
        lock_guard<mutex> lck(spdFlushMtx);
        spdUnflushed += 1;
        spdFlushCV.notify_one();
    }
    
    void flush() {
        // no-op
    }
};


// Example Usage:
string logPath = string(getenv("CONFIG_DIR_PATH")) + "/mailsync.log";

std::vector<shared_ptr<spdlog::sinks::sink>> sinks;
sinks.push_back(make_shared<spdlog::sinks::rotating_file_sink_mt>(logPath, 1048576 * 5, 3));
sinks.push_back(make_shared<SPDFlusherSink>());
spdlog::create("logger", std::begin(sinks), std::end(sinks));