Boost asio - multiple client connections to different servers - c++

Question

I am trying to evaluate using async boost udp/tcp socket operations vs synchronous for my application. I have been trying to find an example that is similar to my design but did not find anything which led me to believe I might be trying to fit async ops into my design even though it is not the right path.
I want to connect to multiple (read: between 1-10) servers and communicate with them using different protocols; I have 4-5 threads which are producing data that needs to be communicated to any one of these server connections.
My current design is synchronous and uses an io_service object per server-connection thread and then using a thread safe queue between the producing threads and each connection thread.
This design does not seem scalable in terms of throughput performance, this is something I would like to maximize.
Are there any examples which provide this multiple connections to different servers pattern?

Answer 1

I have written a client to connect to 6 different servers using TCP/IP SSL/TLS which is implemented with ASIO. All 6 use the same protocol. So, if it helps, here is my code:
SSLSocket.H
#pragma once
#include <cstdlib>
#include <iostream>
#include <queue>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/asio/ssl.hpp>
#include <boost/thread.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/shared_ptr.hpp>
using namespace std;
//
#include "BufferManagement.h"
#include "Logger.h"
#include "Common Classes\Locking.h"
#include "Message.h"
class SSLSocket;
class ConcurrentMsgQueue;
#define BOOST_ASIO_ENABLE_HANDLER_TRACKING
typedef void (__stdcall *Callback)(const SSLSocket* pSSLS, const int bytesInMsg, const void* pBuf);
// typedef std::vector<boost::asio::ssl::stream<boost::asio::ip::tcp::socket> SocketVectorType;
enum {MsgLenBytes = 4};
class SSLSocket
{
// This class handles all communications between the client and the server
// using TCP/IP SSL v1. The Boost ASIO (Asynchronous I/O) library is used to accomplish this.
// Initally written by Bob Bryan on 1/21/2013.
//
public:
SSLSocket(const bool logToFile, const bool logToConsole, const bool displayInHex, const LogLevel levelOfLog, const string& logFileName, const int bufMangLen);
~SSLSocket();
void Connect(SSLSocket* psSLS, const string& serverPath, string& port);
void SendToServer(const int bytesInMsg, Byte* pBuf);
void Stop();
static void SetCallback(Callback callbackFunction)
{
// This method is required in order to be able to do a reverse pinvoke from C#.
// This callback function pointer is what is used to communicate back to the C# code.
CallbackFunction = callbackFunction;
}
static Byte* AllocateMem(int length)
{
// Allocate some memory. This method winds up getting called when the C# client needs to allocate some memory for a message.
Byte* pBuf = BufMang.GetPtr(length);
return pBuf;
}
//
static Logger Log; // Object used to log info to a file and/or to the console.
static Callback CallbackFunction; // Callback function object used to communicate with the worker thread in C#.
private:
void InitAsynchIO();
void HandleConnect(const boost::system::error_code& error);
void HandleHandshake(const boost::system::error_code& error);
void HandleFirstWrite(const boost::system::error_code& error, size_t bytes_transferred);
void HandleRead(const boost::system::error_code& error, size_t bytesTransferred);
// void HandleRead(const boost::system::error_code& error, size_t bytes_transferred);
void Terminate();
void static RcvWorkerThread(SSLSocket* sSLS);
void static SendWorkerThread(SSLSocket* psSLS);
void ProcessSendRequests();
void HandleWrite(const boost::system::error_code& error, size_t bytesTransferred);
static void WorkerThread(boost::shared_ptr< boost::asio::io_service > io_service);
//
struct Bytes
{
// Used to convert 4 bytes to an int.
unsigned char B1;
unsigned char B2;
unsigned char B3;
unsigned char B4;
};
union Bytes4ToInt
{
// Converts 4 bytes to an int.
int IntVal;
Bytes B;
};
inline int BytesToInt(const Byte * pBuf)
{
// This method converts 4 bytes from an array of bytes to a 4-byte int.
B2I.B.B1 = *pBuf++;
B2I.B.B2 = *pBuf++;
B2I.B.B3 = *pBuf++;
B2I.B.B4 = *pBuf;
int Value = B2I.IntVal;
return Value;
}
//
boost::thread_group WorkerThreads; // Used to handle creating threads.
CRITICAL_SECTION SocketLock; // Used in conjuction with the Locking object to handle single threading the code.
boost::asio::ssl::stream<boost::asio::ip::tcp::socket>* pSocket; // Pointer to the socket object.
Bytes4ToInt B2I; // Used to translate 4 bytes in the buffer to an int representing the number of bytes in the msg.
std::string sClientIp; // Client IP address. Used for logging.
unsigned short uiClientPort; // Port number. Used for logging.
// static MessageList* pRepMsgs; // Link list of the msgs to send to the server.
Byte* pDataBuf; // Pointer to the data for the current message to be read.
static boost::shared_ptr< boost::asio::io_service > IOService; // Object required for use by ASIO to perform certain functions.
static bool RcvThreadCreated; // Set when the rcv thread is created so that it won't try to create it again.
static int StaticInit; // Indicates whether or not the static members have been initialized or not.
static bool DisplayInHex; // Specifies whether to display a buffer in hex or not.
static BufferManagement BufMang; // Smart pointer to the buffer used to handle requests coming to and from the server for all sockets.
volatile static bool ReqAlive; // Used to indicate whether the request thread should die or not.
// static bool RepAlive; // Used to indicate whether the response thread should die or not.
static ConcurrentMsgQueue SendMsgQ; // Holds the messages waiting to be sent to the server.
static HANDLE hEvent; // Used for signalling between threads.
};
SSLSocket.cpp
#include "StdAfx.h"
#include "SSLSocket.h"
boost::shared_ptr< boost::asio::io_service > SSLSocket::IOService;
int SSLSocket::StaticInit = 0;
Callback SSLSocket::CallbackFunction;
BufferManagement SSLSocket::BufMang;
volatile bool SSLSocket::ReqAlive = true;
Logger SSLSocket::Log;
HANDLE SSLSocket::hEvent;
bool SSLSocket::DisplayInHex;
ConcurrentMsgQueue SSLSocket::SendMsgQ;
bool SSLSocket::RcvThreadCreated = 0;
BufferManagement* Message::pBufMang;
SSLSocket::SSLSocket(const bool logToFile, const bool logToConsole, const bool displayInHex,
const LogLevel levelOfLog, const string& logFileName, const int bufMangLen) : pSocket(0)
{
// SSLSocket Constructor.
// If the static members have not been intialized yet, then initialize them.
if (!StaticInit)
{
DisplayInHex = displayInHex;
BufMang.Init(bufMangLen);
Message::SetBufMang(&BufMang);
// This constructor enables logging according to the vars passed in.
Log.Init(logToFile, logToConsole, levelOfLog, logFileName);
// Create the crit section object
// Locking::InitLocking(ReadLock);
// Locking::InitLocking(WriteLock);
StaticInit++;
hEvent = CreateEvent(NULL, false, false, NULL);
// Define the ASIO IO service object.
// IOService = new boost::shared_ptr<boost::asio::io_service>(new boost::asio::io_service);
boost::shared_ptr<boost::asio::io_service> IOServ(new boost::asio::io_service);
IOService = IOServ;
}
}
SSLSocket::~SSLSocket(void)
{
delete pSocket;
if (--StaticInit == 0)
CloseHandle(hEvent);
}
void SSLSocket::Connect(SSLSocket* psSLS, const string& serverPath, string& port)
{
// Connects to the server.
// serverPath - specifies the path to the server. Can be either an ip address or url.
// port - port server is listening on.
//
try
{
Locking CodeLock(SocketLock); // Single thread the code.
// If the user has tried to connect before, then make sure everything is clean before trying to do so again.
if (pSocket)
{
delete pSocket;
pSocket = 0;
}
// If serverPath is a URL, then resolve the address.
// Note that this code expects the first server to always have a url.
if ((serverPath[0] < '0') || (serverPath[0] > '9')) // Assumes that the first char of the server path is not a number when resolving to an ip addr.
{
// Create the resolver and query objects to resolve the host name in serverPath to an ip address.
boost::asio::ip::tcp::resolver resolver(*IOService);
boost::asio::ip::tcp::resolver::query query(serverPath, port);
boost::asio::ip::tcp::resolver::iterator EndpointIterator = resolver.resolve(query);
// Set up an SSL context.
boost::asio::ssl::context ctx(*IOService, boost::asio::ssl::context::tlsv1_client);
// Specify to not verify the server certificiate right now.
ctx.set_verify_mode(boost::asio::ssl::context::verify_none);
// Init the socket object used to initially communicate with the server.
pSocket = new boost::asio::ssl::stream<boost::asio::ip::tcp::socket>(*IOService, ctx);
//
// The thread we are on now, is most likely the user interface thread. Create a thread to handle all incoming socket work messages.
if (!RcvThreadCreated)
{
WorkerThreads.create_thread(boost::bind(&SSLSocket::RcvWorkerThread, this));
RcvThreadCreated = true;
WorkerThreads.create_thread(boost::bind(&SSLSocket::SendWorkerThread, this));
}
// Try to connect to the server. Note - add timeout logic at some point.
boost::asio::async_connect(pSocket->lowest_layer(), EndpointIterator,
boost::bind(&SSLSocket::HandleConnect, this, boost::asio::placeholders::error));
}
else
{
// serverPath is an ip address, so try to connect using that.
//
// Create an endpoint with the specified ip address.
const boost::asio::ip::address IP(boost::asio::ip::address::from_string(serverPath));
int iport = atoi(port.c_str());
const boost::asio::ip::tcp::endpoint EP(IP, iport);
// Set up an SSL context.
boost::asio::ssl::context ctx(*IOService, boost::asio::ssl::context::tlsv1_client);
// Specify to not verify the server certificiate right now.
ctx.set_verify_mode(boost::asio::ssl::context::verify_none);
// Init the socket object used to initially communicate with the server.
pSocket = new boost::asio::ssl::stream<boost::asio::ip::tcp::socket>(*IOService, ctx);
//
// Try to connect to the server. Note - add timeout logic at some point.
//pSocket->core_.engine_.do_connect(void*, int);
// pSocket->next_layer_.async_connect(EP, &SSLSocket::HandleConnect)
// pSocket->next_layer().async_connect(EP, &SSLSocket::HandleConnect);
boost::system::error_code EC;
pSocket->next_layer().connect(EP, EC);
if (EC)
{
// Log an error. This worker thread should exit gracefully after this.
stringstream ss;
ss << "SSLSocket::Connect: connect failed to " << sClientIp << " : " << uiClientPort << ". Error: " << EC.message() + ".\n";
Log.LogString(ss.str(), LogError);
}
HandleConnect(EC);
// boost::asio::async_connect(pSocket->lowest_layer(), EP,
// boost::bind(&SSLSocket::HandleConnect, this, boost::asio::placeholders::error));
}
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::Connect: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::SendToServer(const int bytesInMsg, Byte* pBuf)
{
// This method creates a msg object and saves it in the SendMsgQ object.
// sends the number of bytes specified by bytesInMsg in pBuf to the server.
//
Message* pMsg = Message::GetMsg(this, bytesInMsg, pBuf);
SendMsgQ.Push(pMsg);
// Signal the send worker thread to wake up and send the msg to the server.
SetEvent(hEvent);
}
void SSLSocket::SendWorkerThread(SSLSocket* psSLS)
{
// This thread method that gets called to process the messages to be sent to the server.
//
// Since this has to be a static method, call a method on the class to handle server requests.
psSLS->ProcessSendRequests();
}
void SSLSocket::ProcessSendRequests()
{
// This method handles sending msgs to the server.
//
std::stringstream ss;
DWORD WaitResult;
Log.LogString("SSLSocket::ProcessSendRequests: Worker thread " + Logger::NumberToString(boost::this_thread::get_id()) + " started.\n", LogInfo);
// Loop until the user quits, or an error of some sort is thrown.
try
{
do
{
// If there are one or more msgs that need to be sent to a server, then send them out.
if (SendMsgQ.Count() > 0)
{
Message* pMsg = SendMsgQ.Front();
SSLSocket* pSSL = pMsg->pSSL;
SendMsgQ.Pop();
const Byte* pBuf = pMsg->pBuf;
const int BytesInMsg = pMsg->BytesInMsg;
boost::system::error_code Error;
{
Locking CodeLock(SocketLock); // Single thread the code.
boost::asio::async_write(*pSSL->pSocket, boost::asio::buffer(pBuf, BytesInMsg), boost::bind(&SSLSocket::HandleWrite, this,
boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}
ss << "SSLSocket::ProcessSendRequests: # bytes sent = " << BytesInMsg << "\n";
Log.LogString(ss.str(), LogDebug2);
Log.LogBuf(pBuf, BytesInMsg, DisplayInHex, LogDebug3);
}
else
{
// Nothing to send, so go into a wait state.
WaitResult = WaitForSingleObject(hEvent, INFINITE);
if (WaitResult != 0L)
{
Log.LogString("SSLSocket::ProcessSendRequests: WaitForSingleObject event error. Code = " + Logger::NumberToString(GetLastError()) + ". \n", LogError);
}
}
} while (ReqAlive);
Log.LogString("SSLSocket::ProcessSendRequests: Worker thread " + Logger::NumberToString(boost::this_thread::get_id()) + " done.\n", LogInfo);
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::ProcessSendRequests: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::HandleWrite(const boost::system::error_code& error, size_t bytesTransferred)
{
// This method is called after a msg has been written out to the socket. Nothing to do really since reading is handled by the HandleRead method.
std::stringstream ss;
try
{
if (error)
{
ss << "SSLSocket::HandleWrite: failed - " << error.message() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::HandleHandshake: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::RcvWorkerThread(SSLSocket* psSLS)
{
// This is the method that gets called when the receive thread is created by this class.
// This thread method focuses on processing messages received from the server.
//
// Since this has to be a static method, call a method on the class to handle server requests.
psSLS->InitAsynchIO();
}
void SSLSocket::InitAsynchIO()
{
// This method is responsible for initiating asynch i/o.
boost::system::error_code Err;
string s;
stringstream ss;
//
try
{
ss << "SSLSocket::InitAsynchIO: Worker thread - " << Logger::NumberToString(boost::this_thread::get_id()) << " started.\n";
Log.LogString(ss.str(), LogInfo);
// Enable the handlers for asynch i/o. The thread will hang here until the stop method has been called or an error occurs.
// Add a work object so the thread will be dedicated to handling asynch i/o.
boost::asio::io_service::work work(*IOService);
IOService->run();
Log.LogString("SSLSocket::InitAsynchIO: receive worker thread done.\n", LogInfo);
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::InitAsynchIO: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::HandleConnect(const boost::system::error_code& error)
{
// This method is called asynchronously when the server has responded to the connect request.
std::stringstream ss;
try
{
if (!error)
{
pSocket->async_handshake(boost::asio::ssl::stream_base::client,
boost::bind(&SSLSocket::HandleHandshake, this, boost::asio::placeholders::error));
ss << "SSLSocket::HandleConnect: From worker thread " << Logger::NumberToString(boost::this_thread::get_id()) << ".\n";
Log.LogString(ss.str(), LogInfo);
}
else
{
// Log an error. This worker thread should exit gracefully after this.
ss << "SSLSocket::HandleConnect: connect failed to " << sClientIp << " : " << uiClientPort << ". Error: " << error.message() + ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::InitAsynchIO: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::HandleHandshake(const boost::system::error_code& error)
{
// This method is called asynchronously when the server has responded to the handshake request.
std::stringstream ss;
try
{
if (!error)
{
// Try to send the first message that the server is expecting. This msg tells the server we want to start communicating.
// This is the only msg specified in the C++ code. All other msg processing is done in the C# code.
//
unsigned char Msg[27] = {0x17, 0x00, 0x00, 0x00, 0x06, 0x00, 0x01, 0x00, 0x00, 0x00, 0x0b, 0x00, 0x41,
0x74, 0x74, 0x61, 0x63, 0x6b, 0x50, 0x6f, 0x6b, 0x65, 0x72, 0x02, 0x00, 0x65, 0x6e};
boost::system::error_code Err;
sClientIp = pSocket->lowest_layer().remote_endpoint().address().to_string();
uiClientPort = pSocket->lowest_layer().remote_endpoint().port();
ReqAlive = true;
// boost::asio::async_write(*pSocket, boost::asio::buffer(Msg), boost::bind(&SSLSocket::HandleFirstWrite, this,
// boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
int Count = boost::asio::write(*pSocket, boost::asio::buffer(Msg), boost::asio::transfer_exactly(27), Err);
if (Err)
{
ss << "SSLSocket::HandleHandshake: write failed - " << error.message() << ".\n";
Log.LogString(ss.str(), LogInfo);
}
HandleFirstWrite(Err, Count);
// boost::asio::async_write(pSocket, boost::asio::buffer(Msg, 27), boost::bind(&SSLSocket::HandleWrite, this,
// boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
ss.str("");
ss << "SSLSocket::HandleHandshake: From worker thread " << boost::this_thread::get_id() << ".\n";
}
else
{
ss << "SSLSocket::HandleHandshake: failed - " << error.message() << ".\n";
IOService->stop();
}
Log.LogString(ss.str(), LogInfo);
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::HandleHandshake: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::HandleFirstWrite(const boost::system::error_code& error, size_t bytesTransferred)
{
// This method is called after a msg has been written out to the socket.
std::stringstream ss;
try
{
if (!error)
{
// boost::asio::async_read(pSocket, boost::asio::buffer(reply_, bytesTransferred), boost::bind(&SSLSocket::handle_read,
// this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
// boost::asio::async_read(pSocket, boost::asio::buffer(reply_, 84), boost::bind(&SSLSocket::handle_read,
// this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
// Locking CodeLock(ReadLock); // Single thread the code.
// Signal the other threads that msgs are now ready to be sent and received.
// boost::asio::async_read(pSocket, boost::asio::buffer(pRepBuf), boost::asio::transfer_exactly(4), boost::bind(&SSLSocket::HandleRead,
// this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
//
// Notify the UI that we are now connected. Create a 6 byte msg for this.
pDataBuf = BufMang.GetPtr(6);
BYTE* p = pDataBuf;
// Create msg type 500
*p = 244;
*++p = 1;
CallbackFunction(this, 2, (void*)pDataBuf);
// Get the 1st 4 bytes of the next msg, which is always the length of the that msg.
pDataBuf = BufMang.GetPtr(MsgLenBytes);
// int i1=1,i2=2,i3=3,i4=4,i5=5,i6=6,i7=7,i8=8,i9=9;
// (boost::bind(&nine_arguments,_9,_2,_1,_6,_3,_8,_4,_5,_7))
// (i1,i2,i3,i4,i5,i6,i7,i8,i9);
// boost::asio::read(*pSocket, boost::asio::buffer(pReqBuf, MsgLenBytes), boost::asio::transfer_exactly(MsgLenBytes), Err);
// boost::asio::async_read(pSocket, boost::asio::buffer(pReqBuf, MsgLenBytes), boost::bind(&SSLSocket::HandleRead, _1,_2,_3))
// (this, pReqBuf, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred);
// boost::asio::async_read(*pSocket, boost::asio::buffer(reply_), boost::asio::transfer_exactly(ByteCount), boost::bind(&Client::handle_read,
// this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
// boost::asio::async_write(*pSocket, boost::asio::buffer(pDataBuf, MsgLenBytes), boost::bind(&SSLSocket::HandleWrite, this,
// boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
Locking CodeLock(SocketLock); // Single thread the code.
boost::asio::async_read(*pSocket, boost::asio::buffer(pDataBuf, MsgLenBytes), boost::bind(&SSLSocket::HandleRead, this,
boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}
else
{
ss << "SSLSocket::HandleFirstWrite: failed - " << error.message() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::HandleFirstWrite: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::HandleRead(const boost::system::error_code& error, size_t bytesTransferred)
{
// This method is called to process an incomming message.
//
std::stringstream ss;
int ByteCount;
try
{
ss << "SSLSocket::HandleRead: From worker thread " << boost::this_thread::get_id() << ".\n";
Log.LogString(ss.str(), LogInfo);
// Set to exit this thread if the user is done.
if (!ReqAlive)
{
// IOService->stop();
return;
}
if (!error)
{
// Get the number of bytes in the message.
if (bytesTransferred == 4)
{
ByteCount = BytesToInt(pDataBuf);
}
else
{
// Call the C# callback method that will handle the message.
ss << "SSLSocket::HandleRead: From worker thread " << boost::this_thread::get_id() << "; # bytes transferred = " << bytesTransferred << ".\n";
Log.LogString(ss.str(), LogDebug2);
Log.LogBuf(pDataBuf, (int)bytesTransferred, true, LogDebug3);
Log.LogString("SSLSocket::HandleRead: sending msg to the C# client.\n\n", LogDebug2);
CallbackFunction(this, bytesTransferred, (void*)pDataBuf);
// Prepare to read in the next message length.
ByteCount = MsgLenBytes;
}
pDataBuf = BufMang.GetPtr(ByteCount);
boost::system::error_code Err;
// boost::asio::async_read(pSocket, boost::asio::buffer(pDataBuf, ByteCount), boost::bind(&SSLSocket::HandleRead,
// this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
Locking CodeLock(SocketLock); // Single thread the code.
boost::asio::async_read(*pSocket, boost::asio::buffer(pDataBuf, ByteCount), boost::bind(&SSLSocket::HandleRead,
this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
// boost::asio::read(pSocket, boost::asio::buffer(reply_), boost::asio::transfer_exactly(ByteCount), Err);
}
else
{
Log.LogString("SSLSocket::HandleRead failed: " + error.message() + "\n", LogError);
Stop();
}
}
catch (std::exception& e)
{
stringstream ss;
ss << "SSLSocket::HandleRead: threw an error - " << e.what() << ".\n";
Log.LogString(ss.str(), LogError);
Stop();
}
}
void SSLSocket::Stop()
{
// This method calls the shutdown method on the socket in order to stop reads or writes that might be going on. If this is not done, then an exception will be thrown
// when it comes time to delete this object.
ReqAlive = false;
SetEvent(hEvent);
IOService->stop();
}
So, here are the key points:
When connecting to a server for the first time, a new instance of the SSLSocket class is created. The io_service object is static and created just once. It is used by all 6 instances of the SSLSocket class.
There are 2 threads that are used for everything having to do with socket communication across all 6 servers. One thread is for processing messages received from a server. The other thread is used for sending messages to a server.
This code uses SSL/TSL. If you are using straight TCP, then you can simply remove the 3 lines in SSLSocket::Connect method as well as the ssl #include line.
The technique used in HandleRead uses a double read method. The first read gets the number of bytes (since the protocol uses the first 4 bytes as the message length) and the second one obtains the total number of bytes in that message. This may not be the most efficient or even most desirable way to handle reading data off of the socket. But, it is the easiest and simplest to understand. You might consider using a different approach if your protocol is different and/or the message size is much larger and you have the ability to begin processing messages before the entire message has been received.
This code uses Boost 1.52.0 with Visual Studio 2008 for Windows.

Answer 2

There are no direct examples of the one-to-many client-server design included with the Asio examples. If your design is fixed at a maximum of 10 connections, using synchronous communication with a thread for each should be fine. However if you intend this to scale to much more than that, it is obvious to see the diminishing returns from creating a few hundred or thousand threads.
That said, using async_connect combined with async_read and async_write is not difficult to understand or implement. I've used this same concept to manage several thousand connections on the world's fastest supercomputer using only a handful of threads. The async TCP client example is probably the best one to study if you choose this route.
If you are looking for more than just examples, there are several open source projects using Asio that you might find useful.