Why do I need strand per connection when using boost::asio?

The documentation is correct. With a half duplex protocol implementation, such as HTTP Server 3, the strand is not necessary. The call chains can be illustrated as follows:

void connection::start()
{
  socket.async_receive_from(..., &handle_read);  ----.
}                                                    |
    .------------------------------------------------'
    |      .-----------------------------------------.
    V      V                                         |
void connection::handle_read(...)                    |
{                                                    |
  if (result)                                        |
    boost::asio::async_write(..., &handle_write); ---|--.
  else if (!result)                                  |  |
    boost::asio::async_write(..., &handle_write);  --|--|
  else                                               |  |
    socket_.async_read_some(..., &handle_read);  ----'  |
}                                                       |
    .---------------------------------------------------'
    |
    V
void handle_write(...)

As shown in the illustration, only a single asynchronous event is started per path. With no possibility of concurrent execution of the handlers or operations on socket_, it is said to be running in an implicit strand.

Thread Safety

While it does not present itself as an issue in the example, I would like to highlight one important detail of strands and composed operations, such as boost::asio::async_write. Before explaining the details, lets first cover the thread safety model with Boost.Asio. For most Boost.Asio objects, it is safe to have multiple asynchronous operations pending on an object; it is just specified that concurrent calls on the object are unsafe. In the diagrams below, each column represents a thread and each line represents what a thread is doing at a moment in time.

It is safe for a single thread to make sequential calls while other threads make none:

 thread_1                             | thread_2
--------------------------------------+---------------------------------------
socket.async_receive(...);            | ...
socket.async_write_some(...);         | ...

It is safe for multiple threads to make calls, but not concurrently:

 thread_1                             | thread_2
--------------------------------------+---------------------------------------
socket.async_receive(...);            | ...
...                                   | socket.async_write_some(...);

However, it is not safe for multiple threads to make calls concurrently¹:

 thread_1                             | thread_2
--------------------------------------+---------------------------------------
socket.async_receive(...);            | socket.async_write_some(...);
...                                   | ...

Strands

To prevent concurrent invocations, handlers are often invoked from within strands. This is done by either:

Wrapping the handler with strand.wrap. This will return a new handler, that will dispatch through the strand.
Posting or dispatching directly through the strand.

Composed operations are unique in that intermediate calls to the stream are invoked within the handler's strand, if one is present, instead of the strand in which the composed operation is initiated. When compared to other operations, this presents an inversion of where the strand is specified. Here is some example code focusing on strand usage, that will demonstrate a socket that is read from via a non-composed operation, and concurrently written to with a composed operation.

void start()
{
  // Start read and write chains.  If multiple threads have called run on
  // the service, then they may be running concurrently.  To protect the
  // socket, use the strand.
  strand_.post(&read);
  strand_.post(&write);
}

// read always needs to be posted through the strand because it invokes a
// non-composed operation on the socket.
void read()
{
  // async_receive is initiated from within the strand.  The handler does
  // not affect the strand in which async_receive is executed.
  socket_.async_receive(read_buffer_, &handle_read);
}

// This is not running within a strand, as read did not wrap it.
void handle_read()
{
  // Need to post read into the strand, otherwise the async_receive would
  // not be safe.
  strand_.post(&read);
}

// The entry into the write loop needs to be posted through a strand.
// All intermediate handlers and the next iteration of the asynchronous write
// loop will be running in a strand due to the handler being wrapped.
void write()
{
  // async_write will make one or more calls to socket_.async_write_some.
  // All intermediate handlers (calls after the first), are executed
  // within the handler's context (strand_).
  boost::asio::async_write(socket_, write_buffer_,
                           strand_.wrap(&handle_write));
}

// This will be invoked from within the strand, as it was a wrapped
// handler in write().
void handle_write()
{
  // handler_write() is invoked within a strand, so write() does not
  // have to dispatched through the strand.
  write();
}

Importance of Handler Types

Also, within composed operations, Boost.Asio uses argument dependent lookup (ADL) to invoke intermediate handlers through the completion handler's strand. As such, it is important that the completion handler's type has the appropriate asio_handler_invoke() hooks. If type erasure occurs to a type that does not have the appropriate asio_handler_invoke() hooks, such as a case where a boost::function is constructed from the return type of strand.wrap, then intermediate handlers will execute outside of the strand, and only the completion handler will execute within the strand. See this answer for more details.

In the following code, all intermediate handlers and the completion handler will execute within the strand:

boost::asio::async_write(stream, buffer, strand.wrap(&handle_write));

In the following code, only the completion handler will execute within the strand. None of the intermediate handlers will execute within the strand:

boost::function<void()> handler(strand.wrap(&handle_write));
boost::asio::async_write(stream, buffer, handler);

_{1. The revision history documents an anomaly to this rule. If supported by the OS, synchronous read, write, accept, and connection operations are thread safe. I an including it here for completeness, but suggest using it with caution.}

Thread Safety

Strands

Importance of Handler Types

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