Class template asynchronous_sink

boost::log::sinks::asynchronous_sink — Asynchronous logging sink frontend.

Synopsis

// In header: <boost/log/sinks/async_frontend.hpp>

template<typename SinkBackendT, 
         typename QueueingStrategyT = unbounded_fifo_queue> 
class asynchronous_sink :
  public basic_sink_frontend, public QueueingStrategyT
{
public:
  // types
  typedef SinkBackendT           sink_backend_type;   // Sink implementation type. 
  typedef implementation_defined locked_backend_ptr;  // A pointer type that locks the backend until it's destroyed. 

  // member classes/structs/unions

  // Function object to run the log record feeding thread.

  class run_func {
  public:
    // types
    typedef void result_type;

    // construct/copy/destruct
    explicit run_func(asynchronous_sink *) noexcept;

    // public member functions
    result_type operator()() const;
  };

  // A scope guard that implements active operation management.

  class scoped_feeding_opereation {
  public:
    // construct/copy/destruct
    explicit scoped_feeding_opereation(asynchronous_sink &);
    scoped_feeding_opereation(scoped_feeding_opereation const &) = delete;
    scoped_feeding_opereation & 
    operator=(scoped_feeding_opereation const &) = delete;
    ~scoped_feeding_opereation();
  };

  // A scope guard that resets a flag on destructor.

  class scoped_flag {
  public:
    // construct/copy/destruct
    explicit scoped_flag(frontend_mutex_type &, condition_variable_any &, 
                         boost::atomic< bool > &);
    scoped_flag(scoped_flag const &) = delete;
    scoped_flag & operator=(scoped_flag const &) = delete;
    ~scoped_flag();
  };

  // construct/copy/destruct
  explicit asynchronous_sink(bool = true);
  explicit asynchronous_sink(shared_ptr< sink_backend_type > const &, 
                             bool = true);
  template<typename... Args> explicit asynchronous_sink(Args &&...);
  ~asynchronous_sink();

  // public member functions
  locked_backend_ptr locked_backend();
  virtual void consume(record_view const &);
  virtual bool try_consume(record_view const &);
  void run();
  void stop();
  void feed_records();
  virtual void flush();
};

Description

The frontend starts a separate thread on construction. All logging records are passed to the backend in this dedicated thread.

The user can prevent spawning the internal thread by specifying start_thread parameter with the value of false on construction. In this case log records will be buffered in the internal queue until the user calls run, feed_records or flush in his own thread. Log record queueing strategy is specified in the QueueingStrategyT template parameter.

`asynchronous_sink` public construct/copy/destruct

explicit asynchronous_sink(bool start_thread = true);

Default constructor. Constructs the sink backend instance. Requires the backend to be default-constructible.

Parameters:

start_thread

If true, the frontend creates a thread to feed log records to the backend. Otherwise no thread is started and it is assumed that the user will call run, feed_records or flush himself.

explicit asynchronous_sink(shared_ptr< sink_backend_type > const & backend, 
                           bool start_thread = true);

Constructor attaches user-constructed backend instance

Parameters:

`backend`	Pointer to the backend instance.
`start_thread`	If `true`, the frontend creates a thread to feed log records to the backend. Otherwise no thread is started and it is assumed that the user will call `run`, `feed_records` or `flush` himself.

Requires:

backend is not NULL.

```
template<typename... Args> explicit asynchronous_sink(Args &&... args);
```
Constructor that passes arbitrary named parameters to the interprocess sink backend constructor. Refer to the backend documentation for the list of supported parameters.

The frontend uses the following named parameters:
- start_thread - If true, the frontend creates a thread to feed log records to the backend. Otherwise no thread is started and it is assumed that the user will call run, feed_records or flush himself.
```
~asynchronous_sink();
```
Destructor. Implicitly stops the dedicated feeding thread, if one is running.

`asynchronous_sink` public member functions

```
locked_backend_ptr locked_backend();
```
Locking accessor to the attached backend

virtual void consume(record_view const & rec);

Enqueues the log record to the backend

```
virtual bool try_consume(record_view const & rec);
```
The method attempts to pass logging record to the backend
```
void run();
```
The method starts record feeding loop and effectively blocks until either of this happens:
- the thread is interrupted due to either standard thread interruption or a call to stop
- an exception is thrown while processing a log record in the backend, and the exception is not terminated by the exception handler, if one is installed
Requires:

The sink frontend must be constructed without spawning a dedicated thread

void stop();

The method softly interrupts record feeding loop. This method must be called when run, feed_records or flush method execution has to be interrupted. Unlike regular thread interruption, calling stop will not interrupt the record processing in the middle. Instead, the sink frontend will attempt to finish its business with the record in progress and return afterwards. This method can be called either if the sink was created with an internal dedicated thread, or if the feeding loop was initiated by user.

If no record feeding operation is in progress, calling stop marks the sink frontend so that the next feeding operation stops immediately.

Note

	Note
Returning from this method does not guarantee that there are no records left buffered in the sink frontend. It is possible that log records keep coming during and after this method is called. At some point of execution of this method log records stop being processed, and all records that come after this point are put into the queue. These records will be processed upon further calls to `run` or `feed_records`.

Returning from this method does not guarantee that there are no records left buffered in the sink frontend. It is possible that log records keep coming during and after this method is called. At some point of execution of this method log records stop being processed, and all records that come after this point are put into the queue. These records will be processed upon further calls to run or feed_records.

Note

	Note
If the record feeding loop is being run in a user's thread (i.e. `start_thread` was specified as `false` on frontend construction), this method does not guarantee that upon return the thread has returned from the record feeding loop or that it won't enter it in the future. The method only ensures that the record feeding thread will eventually return from the feeding loop. It is user's responsibility to synchronize with the user's record feeding thread.

If the record feeding loop is being run in a user's thread (i.e. start_thread was specified as false on frontend construction), this method does not guarantee that upon return the thread has returned from the record feeding loop or that it won't enter it in the future. The method only ensures that the record feeding thread will eventually return from the feeding loop. It is user's responsibility to synchronize with the user's record feeding thread.

```
void feed_records();
```
The method feeds log records that may have been buffered to the backend and returns

Requires:

The sink frontend must be constructed without spawning a dedicated thread
```
virtual void flush();
```
The method feeds all log records that may have been buffered to the backend and returns. Unlike feed_records, in case of ordering queueing the method also feeds records that were enqueued during the ordering window, attempting to drain the queue completely.

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