Elegant way of reconnecting loop with boost::asio? - c++

Question

I am trying to write a very elegant way of handling a reconnect loop with boost async_connect(...). The problem is, I don't see a way how I could elegantly solve the following problem:
I have a TCP client that should try to connect asynchronously to a server, if the connection fails because the server is offline or any other error occurs, wait a given amount of time and try to reconnect. There are multiple things to take into consideration here:
Avoidance of global variables if possible
It has to be async connect
A very basic client is instantiated like so:
tcpclient::tcpclient(std::string host, int port) : _endpoint(boost::asio::ip::address::from_string(host), port), _socket(_ios) {
logger::log_info("Initiating client ...");
}
Attempt to connect to the server:
void tcpclient::start() {
bool is_connected = false;
while (!is_connected) {
_socket.async_connect(_endpoint, connect_handler);
_ios.run();
}
// read write data (?)
}
The handler:
void tcpclient::connect_handler(const boost::system::error_code &error) {
if(error){
// trigger disconnect (?)
logger::log_error(error.message());
return;
}
// Connection is established at this point
// Update timer state and start authentication on server ?
logger::log_info("Connected?");
}
How can I properly start reconnecting everytime the connection fails (or is dropped)? Since the handler is static I can not modify a class attribute that indicates the connection status? I want to avoid using hacky global variable workarounds.
How can I solve this issue in a proper way?
My attempt would be something like this:
tcpclient.h
enum ConnectionStatus{
NOT_CONNECTED,
CONNECTED
};
class tcpclient {
public:
tcpclient(std::string host, int port);
void start();
private:
ConnectionStatus _status = NOT_CONNECTED;
void connect_handler(const boost::system::error_code& error);
boost::asio::io_service _ios;
boost::asio::ip::tcp::endpoint _endpoint;
boost::asio::ip::tcp::socket _socket;
};
tcpclient.cpp
#include "tcpclient.h"
#include <boost/chrono.hpp>
#include "../utils/logger.h"
tcpclient::tcpclient(std::string host, int port) : _endpoint(boost::asio::ip::address::from_string(host), port),
_socket(_ios) {
logger::log_info("Initiating client ...");
logger::log_info("Server endpoint: " + _endpoint.address().to_string());
}
void tcpclient::connect_handler(const boost::system::error_code &error) {
if(!error){
_status = CONNECTED;
logger::log_info("Connected.");
}
else{
_status = NOT_CONNECTED;
logger::log_info("Failed to connect");
_socket.close();
}
}
void tcpclient::start() {
while (_status == NOT_CONNECTED) {
std::this_thread::sleep_for(std::chrono::milliseconds(2000));
_socket.close();
_socket.async_connect(_endpoint, std::bind(&tcpclient::connect_handler, this, std::placeholders::_1));
_ios.run();
}
}
The problem is that the reconnect is not working properly and the application seems to freeze for some reason? Aside from that reconnecting also seems problematic once a connection was established and is then dropped (e.g. due to the server crashing/closing).

Answer 1

std::this_thread::sleep_for(std::chrono::milliseconds(2000)); will freeze program for 2 seconds. What can you do here is to launch async timer when connection attempt fails:
::boost::asio::steady_timer m_timer{_ios, boost::asio::chrono::seconds{2}};
void tcpclient::connect_handler(const boost::system::error_code &error)
{
if(!error)
{
_status = CONNECTED;
logger::log_info("Connected.");
}
else
{
_status = NOT_CONNECTED;
logger::log_info("Failed to connect");
_socket.close();
m_timer.expires_from_now(boost::asio::chrono::seconds{2});
m_timer.async_wait(std::bind(&tcpclient::on_ready_to_reconnect, this, std::placeholders::_1));
}
}
void tcpclient::on_ready_to_reconnect(const boost::system::error_code &error)
{
try_connect();
}
void tcpclient::try_connect()
{
m_socket.async_connect(_endpoint, std::bind(&tcpclient::connect_handler, this, std::placeholders::_1));
}
void tcpclient::start()
{
try_connect();
_ios.run();
}
There is also no need for while (_status == NOT_CONNECTED) loop, because io service will be busy and _ios.run(); won't return until connection is established.