C++ ASIO: Asynchronous sockets and threading - c++

Question

My application is based on the asio chat example and consists of a client and a server:
- Client: Connect to the server, receive requests and respond to it
- Server: Has a QT GUI (main thread) and a network service (separate thread) listening for connections, sending requests to particular clients and interprets the response from/in the GUI
I want to achieve this in an asynchronous way to avoid a seperate thread for each client connection.
In my QT window, I have one io_service instance and one instance of my network service:
io_service_ = new asio::io_service();
asio::ip::tcp::endpoint endpoint(asio::ip::tcp::v4(), "1234");
service_ = new Service(*io_service_, endpoint, this);
asio::io_service* ioServicePointer = io_service_;
t = std::thread{ [ioServicePointer](){ ioServicePointer->run(); } };
I want to be able to send data to one client, like this:
service_->send_message(selectedClient.id, msg);
And I am receiving and handling the responses via the observer pattern (the window implements the IStreamListener interface)
Service.cpp:
#include "Service.h"
#include "Stream.h"
void Service::runAcceptor()
{
acceptor_.async_accept(socket_,
[this](asio::error_code ec)
{
if (!ec)
{
std::make_shared<Stream>(std::move(socket_), &streams_)->start();
}
runAcceptor();
});
}
void Service::send_message(std::string streamID, chat_message& msg)
{
io_service_.post(
[this, msg, streamID]()
{
auto stream = streams_.getStreamByID(streamID);
stream->deliver(msg);
});
}
Stream.cpp:
#include "Stream.h"
#include <iostream>
#include "../chat_message.h"
Stream::Stream(asio::ip::tcp::socket socket, StreamCollection* streams)
: socket_(std::move(socket))
{
streams_ = streams; // keep a reference to the streamCollection
// retrieve endpoint ip
asio::ip::tcp::endpoint remote_ep = socket_.remote_endpoint();
asio::ip::address remote_ad = remote_ep.address();
this->ip_ = remote_ad.to_string();
}
void Stream::start()
{
streams_->join(shared_from_this());
readHeader();
}
void Stream::deliver(const chat_message& msg)
{
bool write_in_progress = !write_msgs_.empty();
write_msgs_.push_back(msg);
if (!write_in_progress)
{
write();
}
}
std::string Stream::getName()
{
return name_;
}
std::string Stream::getIP()
{
return ip_;
}
void Stream::RegisterListener(IStreamListener *l)
{
m_listeners.insert(l);
}
void Stream::UnregisterListener(IStreamListener *l)
{
std::set<IStreamListener *>::const_iterator iter = m_listeners.find(l);
if (iter != m_listeners.end())
{
m_listeners.erase(iter);
}
else {
std::cerr << "Could not unregister the specified listener object as it is not registered." << std::endl;
}
}
void Stream::readHeader()
{
auto self(shared_from_this());
asio::async_read(socket_,
asio::buffer(read_msg_.data(), chat_message::header_length),
[this, self](asio::error_code ec, std::size_t /*length*/)
{
if (!ec && read_msg_.decode_header())
{
readBody();
}
else if (ec == asio::error::eof || ec == asio::error::connection_reset)
{
std::for_each(m_listeners.begin(), m_listeners.end(), [&](IStreamListener *l) {l->onStreamDisconnecting(this->id()); });
streams_->die(shared_from_this());
}
else
{
std::cerr << "Exception: " << ec.message();
}
});
}
void Stream::readBody()
{
auto self(shared_from_this());
asio::async_read(socket_,
asio::buffer(read_msg_.body(), read_msg_.body_length()),
[this, self](asio::error_code ec, std::size_t /*length*/)
{
if (!ec)
{
// notify the listener (GUI) that a response has arrived and pass a reference to it
auto msg = std::make_shared<chat_message>(std::move(read_msg_));
std::for_each(m_listeners.begin(), m_listeners.end(), [&](IStreamListener *l) {l->onMessageReceived(msg); });
readHeader();
}
else
{
streams_->die(shared_from_this());
}
});
}
void Stream::write()
{
auto self(shared_from_this());
asio::async_write(socket_,
asio::buffer(write_msgs_.front().data(),
write_msgs_.front().length()),
[this, self](asio::error_code ec, std::size_t /*length*/)
{
if (!ec)
{
write_msgs_.pop_front();
if (!write_msgs_.empty())
{
write();
}
}
else
{
streams_->die(shared_from_this());
}
});
}
Interfaces
class IStream
{
public:
/// Unique stream identifier
typedef void* TId;
virtual TId id() const
{
return (TId)(this);
}
virtual ~IStream() {}
virtual void deliver(const chat_message& msg) = 0;
virtual std::string getName() = 0;
virtual std::string getIP() = 0;
/// observer pattern
virtual void RegisterListener(IStreamListener *l) = 0;
virtual void UnregisterListener(IStreamListener *l) = 0;
};
class IStreamListener
{
public:
virtual void onStreamDisconnecting(IStream::TId streamId) = 0;
virtual void onMessageReceived(std::shared_ptr<chat_message> msg) = 0;
};
/*
streamCollection / service delegates
*/
class IStreamCollectionListener
{
public:
virtual void onStreamDied(IStream::TId streamId) = 0;
virtual void onStreamCreated(std::shared_ptr<IStream> stream) = 0;
};
StreamCollection is basically a set of IStreams:
class StreamCollection
{
public:
void join(stream_ptr stream)
{
streams_.insert(stream);
std::for_each(m_listeners.begin(), m_listeners.end(), [&](IStreamCollectionListener *l) {l->onStreamCreated(stream); });
}
// more events and observer pattern inplementation
First of all: The code works as intended so far.
My question:
Is this the way ASIO is supposed to be used for asynchronous programming? I'm especially unsure about the Service::send_message method and the use of io_service.post. What is it's purpose in my case? It did work too when I just called async_write, without wrapping it in the io_service.post call.
Am I running into problems with this approach?

Answer 1

Asio is designed to be a tookit rather than a framework. As such, there are various ways to successfully use it. Separating the GUI and network threads, and using asynchronous I/O for scalability can be a good idea.
Delegating work to the io_service within a public API, such as Service::send_message(), has the following consequences:
decouples the caller's thread from the thread(s) servicing the io_service. For example, if Stream::write() performs a time consuming cryptographic function, the caller thread (GUI) would not be impacted.
it provides thread-safety. The io_service is thread-safe; however socket is not thread-safe. Additionally, other objects may not be thread safe, such as write_msgs_. Asio guarantees that handlers will only be invoked from within threads running the io_servce. Consequently, if only one thread is running the io_service, then there is no possibility for concurrency and both socket_ and write_msgs_ will be accessed in a thread-safe manner. Asio refers to this as an implicit strand. If more than one thread is processing the io_service, then one may need to use an explicit strand to provide thread safety. See this answer for more details on strands.
Additional Asio considerations:
Observers are invoked within handlers, and handlers are running within the network thread. If any observer takes a long time to complete, such as having to synchronize with various shared objects touched by the GUI thread, then it could create poor responsiveness across other operations. Consider using a queue to broker events between the observer and subject components. For instance, one could use another io_service as a queue, that is being ran by its own thread, and post into it:
auto msg = std::make_shared<chat_message>(std::move(read_msg_));
for (auto l: m_listeners)
dispatch_io_service.post([=](){ l->onMessageReceived(msg); });
Verify that the container type for write_msgs_ does not invalidate iterators, pointers and references to existing elements on push_back() and other elements for pop_front(). For instance, using std::list or std::dequeue would be safe, but a std::vector may invalidate references to existing elements on push_back.
StreamCollection::die() may be called multiple times for a single Stream. This function should either be idempotent or handle the side effects appropriately.
On failure for a given Stream, its listeners are informed of a disconnect only in one path: failing to read a header with an error of asio::error::eof or asio::error::connection_reset. Other paths do not invoke IStreamListener.onStreamDisconnecting():
the header is read, but decoding failed. In this particular case, the entire read chain will stop without informing other components. The only indication that a problem has occurred is a print statement to std::cerr.
when there is a failure reading the body.