Quill

Asynchronous Low Latency C++ Logging Library

• Introduction
• Documentation
• Features
• Caveats
• Performance
• Quick Start
• Usage
• Design
• License

Quill is a high-performance, cross-platform logging library designed for C++17 and onwards. Quill is a production-ready logging library that has undergone extensive unit testing. It has been successfully utilized in production environments, including financial trading applications, providing high-performance and reliable logging capabilities.

For detailed documentation and usage instructions, please visit the Quill Documentation on Read the Docs. It provides comprehensive information on how to integrate and utilize Quill in your C++ applications.

Additionally, you can explore the examples folder in the Quill repository on GitHub. These examples serve as valuable resources to understand different usage scenarios and demonstrate the capabilities of the library.

• Low Latency Logging: Achieve fast logging performance with low latency. Refer to the Benchmarks for more details.
• Asynchronous logging: Formatting and I/O are all delegated to a background logging thread, effectively offloading the overhead from the critical path.
• Custom Formatters: Customize log formatting based on user-defined patterns. Explore Formatters for further details.
• Flexible Timestamp Generation: Choose between rdtsc, chrono, or custom clocks (useful for simulations) for log message timestamp generation.
• Log Stack Traces: Store log messages in a ring buffer and display them later in response to a higher severity log statement or on demand. Refer to Backtrace Logging for more information.
• Multiple Logging Sinks: Utilize various logging targets, including:
  • Console logging with color support.
  • File logging.
  • Rotating log files based on time or size.
  • JSON logging.
  • Custom sinks.
• Log Message Filtering: Apply filters to selectively process log messages. Learn more about Filters.
• Structured Logging: Generate JSON structured logs. See Structured-Log for details.
• Blocking or Dropping Message Modes: Choose between blocking or dropping message modes in the library. In blocking mode, the hot threads pause and wait when the queue is full until space becomes available, ensuring no message loss but introducing potential latency. In dropping mode, log messages beyond the queue's capacity may be dropped to prioritize low latency. The library provides reports on dropped messages, queue reallocations, and blocked hot threads for monitoring purposes.
• Queue Types: The library supports different types of queues for transferring logs from the hot path to the backend thread: bounded queues with a fixed capacity and unbounded queues that start small and can dynamically grow.
• Wide Character Support: Wide strings compatible with ASCII encoding are supported, applicable to Windows only. Additionally, there is support for logging STL containers consisting of wide strings. Note that chaining STL types, such as std::vector<std::vector<std::wstring>> is not supported for wide strings.
• Ordered Log Statements: Log statements are ordered by timestamp even when produced by different threads, facilitating easier debugging of multithreaded applications.
• Compile-Time Log Level Stripping: Completely strip out log levels at compile time, reducing if branches.
• Clean and Warning-Free Codebase: Ensure a clean and warning-free codebase, even with high compiler warning levels.
• Crash-Safe Behavior: Benefit from crash-safe behavior with a built-in signal handler.
• Type-Safe API: Type safe api using the excellent {fmt} library.
• Huge Pages: Benefit from support for huge pages on the hot path. This feature allows for improved performance and efficiency.

Quill may not work well with fork() since it spawns a background thread and fork() doesn't work well with multithreading.

If your application uses fork() and you want to log in the child processes as well, you should call quill::start() after the fork() call. Additionally, you should ensure that you write to different files in the parent and child processes to avoid conflicts.

For example :

#include "quill/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/FileSink.h"

int main()
{
  // DO NOT CALL THIS BEFORE FORK
  // quill::Backend::start();

  if (fork() == 0)
  {
    quill::Backend::start();
        
    // Get or create a handler to the file - Write to a different file
    auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
      "child.log");
    
    quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(file_sink));

    QUILL_LOG_INFO(logger, "Hello from Child {}", 123);
  }
  else
  {
    quill::Backend::start();
          
    // Get or create a handler to the file - Write to a different file
    auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
      "parent.log");
    
    quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(file_sink));
    
    QUILL_LOG_INFO(logger, "Hello from Parent {}", 123);
  }
}

The results presented in the tables below are measured in nanoseconds (ns).

LOG_INFO(logger, "Logging int: {}, int: {}, double: {}", i, j, d).

LOG_INFO(logger, "Logging int: {}, int: {}, string: {}", i, j, large_string).

The large string used in the log message is over 35 characters to prevent the short string optimization of std::string.

LOG_INFO(logger, "Logging int: {}, int: {}, vector: {}", i, j, v).

Logging std::vector<std::string> v containing 16 large strings, each ranging from 50 to 60 characters. The strings used in the log message are over 35 characters to prevent the short string optimization of std::string.

The benchmark was conducted on Linux RHEL 9 with an Intel Core i5-12600 at 4.8 GHz. The cpus are isolated on this system and each thread was pinned to a different CPU. GCC 13.1 was used as the compiler.

The benchmark methodology involved logging 20 messages in a loop, calculating and storing the average latency for those 20 messages, then waiting around ~2 milliseconds, and repeating this process for a specified number of iterations.

In the Quill Bounded Dropping benchmarks, the dropping queue size is set to 262,144 bytes, which is double the default size of 131,072 bytes.

You can find the benchmark code on the logger_benchmarks repository.

The maximum throughput is measured by determining the maximum number of log messages the backend logging thread can write to the log file per second.

When measured on the same system as the latency benchmarks mentioned above the average throughput of the backend logging thread when formatting a log message consisting of an int and a double is ~4.40 million msgs/sec

While the primary focus of the library is not on throughput, it does provide efficient handling of log messages across multiple threads. The backend logging thread, responsible for formatting and ordering log messages from hot threads, ensures that all queues are emptied on a high priority basis. The backend thread internally buffers the log messages and then writes them later when the caller thread queues are empty or when a predefined limit, backend_thread_transit_events_soft_limit, is reached. This approach prevents the need for allocating new queues or dropping messages on the hot path.

Comparing throughput with other logging libraries in an asynchronous logging scenario has proven challenging. Some libraries may drop log messages, resulting in smaller log files than expected, while others only offer asynchronous flush, making it difficult to determine when the logging thread has finished processing all messages. In contrast, Quill provides a blocking flush log guarantee, ensuring that every log message from the hot threads up to that point is flushed to the file.

For benchmarking purposes, you can find the code here.

Compile times are measured using clang 15 and for Release build.

Below, you can find the additional headers that the library will include when you need to log, following the recommended_usage example

There is also a compile-time benchmark measuring the compilation time of 2000 auto-generated log statements with various arguments. You can find it here. It takes approximately 30 seconds to compile.

#include "quill/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/FileSink.h"

int main()
{
  // Start the backend thread
  quill::Backend::start();

  // Log to file
  auto file_sink = quill::Frontend::create_or_get_sink<quill::FileSink>(
    "example_file_logging.log");

  quill::Logger* logger =
    quill::Frontend::create_or_get_logger("root", std::move(file_sink));

  // set the log level of the logger to trace_l3 (default is info)
  logger->set_log_level(quill::LogLevel::TraceL3);
  
  LOG_INFO(logger, "Welcome to Quill!");
  LOG_ERROR(logger, "An error message. error code {}", 123);
  LOG_WARNING(logger, "A warning message.");
  LOG_CRITICAL(logger, "A critical error.");
  LOG_DEBUG(logger, "Debugging foo {}", 1234);
  LOG_TRACE_L1(logger, "{:>30}", "right aligned");
  LOG_TRACE_L2(logger, "Positional arguments are {1} {0} ", "too", "supported");
  LOG_TRACE_L3(logger, "Support for floats {:03.2f}", 1.23456);
}

#include "quill/Backend.h"
#include "quill/Frontend.h"
#include "quill/LogMacros.h"
#include "quill/Logger.h"
#include "quill/sinks/ConsoleSink.h"

int main()
{
  // Start the backend thread
  quill::Backend::start();

  // Frontend
  auto console_sink = quill::Frontend::create_or_get_sink<quill::ConsoleSink>("sink_id_1");
  quill::Logger* logger = quill::Frontend::create_or_get_logger("root", std::move(console_sink));

  // Change the LogLevel to print everything
  logger->set_log_level(quill::LogLevel::TraceL3);

  LOG_INFO(logger, "Welcome to Quill!");
  LOG_ERROR(logger, "An error message. error code {}", 123);
  LOG_WARNING(logger, "A warning message.");
  LOG_CRITICAL(logger, "A critical error.");
  LOG_DEBUG(logger, "Debugging foo {}", 1234);
  LOG_TRACE_L1(logger, "{:>30}", "right aligned");
  LOG_TRACE_L2(logger, "Positional arguments are {1} {0} ", "too", "supported");
  LOG_TRACE_L3(logger, "Support for floats {:03.2f}", 1.23456);
}

git clone http://github.com/odygrd/quill.git
mkdir cmake_build
cd cmake_build
cmake ..
make install

Note: To install in custom directory invoke cmake with -DCMAKE_INSTALL_PREFIX=/quill/install-dir/

Then use the library from a CMake project, you can locate it directly with find_package()

my_project/
├── CMakeLists.txt
├── main.cpp

# Set only if needed - quill was installed under a custom non-standard directory
set(CMAKE_PREFIX_PATH /test_quill/usr/local/)

find_package(quill REQUIRED)

# Linking your project against quill
add_executable(example main.cpp)
target_link_libraries(example PUBLIC quill::quill)

To embed the library directly, copy the source folder to your project and call add_subdirectory() in your CMakeLists.txt file

my_project/
├── quill/            (source folder)
├── CMakeLists.txt
├── main.cpp

cmake_minimum_required(VERSION 3.1.0)
project(my_project)

set(CMAKE_CXX_STANDARD 17)

add_subdirectory(quill)

add_executable(my_project main.cpp)
target_link_libraries(my_project PUBLIC quill::quill)

To build quill for Android NDK add the following flags when configuring the build:

-DQUILL_NO_THREAD_NAME_SUPPORT:BOOL=ON

Meson's wrapdb includes a quill package, which repackages quill to be built by meson as a subproject.

• Install quill subproject from the wrapdb by running from the root of your project

  meson wrap install quill

If you prefer not to use WrapDB, you can manually integrate Quill into your project by following these steps:

• Copy the contents of this repository under the subprojects directory in your project.

Once the library is integrated into your Meson project, follow these steps to ensure proper usage:

• In your project’s meson.build file, add an entry for the new subproject

  quill = subproject('quill')
  quill_dep = quill.get_variable('quill_dep')

• Include the new dependency object to link with quill

  my_build_target = executable('name', 'main.cpp', dependencies : [quill_dep], install : true)

The library is available on BLZMOD, allowing for easy integration into your project.

If you prefer manual integration, you can add the library as a dependency in your BUILD.bazel file. Below is a sample cc_binary rule demonstrating how to include the library. Ensure to replace //quill_path with the actual path to the directory containing the BUILD.bazel file for the quill library within your project structure.

cc_binary(name = "app", srcs = ["main.cpp"], deps = ["//quill_path:quill"])

When invoking a LOG_ macro:

1. Creates a static constexpr metadata object to store Metadata such as the format string and source location.

2. Pushes the data SPSC lock-free queue. For each log message, the following variables are pushed

Consumes each message from the SPSC queue, retrieves all the necessary information and then formats the message. Subsequently, forwards the log message to all Sinks associated with the Logger.

Quill is licensed under the MIT License

Quill depends on third party libraries with separate copyright notices and license terms. Your use of the source code for these subcomponents is subject to the terms and conditions of the following licenses.

• (MIT License) {fmt} (http://github.com/fmtlib/fmt/blob/master/LICENSE.rst)
• (MIT License) doctest (http://github.com/onqtam/doctest/blob/master/LICENSE.txt)