Qpid Proton C++ API  0.18.1
multithreaded_client_flow_control.cpp

A multithreaded sender and receiver enhanced for flow control.Requires C++11

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*
* http://www.apache.org/licenses/LICENSE-2.0
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//
// C++11 or greater
//
// A multi-threaded client that sends and receives messages from multiple AMQP
// addresses.
//
// Demonstrates how to:
//
// - implement proton handlers that interact with user threads safely
// - block sender threads to respect AMQP flow control
// - use AMQP flow control to limit message buffering for receivers threads
//
// We define sender and receiver classes with simple, thread-safe blocking
// send() and receive() functions.
//
// These classes are also privately proton::message_handler instances. They use
// the thread-safe proton::work_queue and standard C++ synchronization (std::mutex
// etc.) to pass messages between user and proton::container threads.
//
// NOTE: no proper error handling
#include <atomic>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <queue>
#include <sstream>
#include <string>
#include <thread>
// Lock output from threads to avoid scramblin
std::mutex out_lock;
#define OUT(x) do { std::lock_guard<std::mutex> l(out_lock); x; } while (false)
// A thread-safe sending connection that blocks sending threads when there
// is no AMQP credit to send messages.
class sender : private proton::messaging_handler {
// Only used in proton handler thread
proton::sender sender_;
// Shared by proton and user threads, protected by lock_
std::mutex lock_;
proton::work_queue *work_queue_;
std::condition_variable sender_ready_;
int queued_; // Queued messages waiting to be sent
int credit_; // AMQP credit - number of messages we can send
public:
sender(proton::container& cont, const std::string& url, const std::string& address)
: work_queue_(0), queued_(0), credit_(0)
{
cont.open_sender(url+"/"+address, proton::connection_options().handler(*this));
}
// Thread safe
void send(const proton::message& m) {
{
std::unique_lock<std::mutex> l(lock_);
// Don't queue up more messages than we have credit for
while (!work_queue_ || queued_ >= credit_) sender_ready_.wait(l);
++queued_;
}
work_queue_->add([=]() { this->do_send(m); }); // work_queue_ is thread safe
}
// Thread safe
void close() {
work_queue()->add([=]() { sender_.connection().close(); });
}
private:
proton::work_queue* work_queue() {
// Wait till work_queue_ and sender_ are initialized.
std::unique_lock<std::mutex> l(lock_);
while (!work_queue_) sender_ready_.wait(l);
return work_queue_;
}
// == messaging_handler overrides, only called in proton hander thread
void on_sender_open(proton::sender& s) override {
// Make sure sender_ and work_queue_ are set atomically
std::lock_guard<std::mutex> l(lock_);
sender_ = s;
work_queue_ = &s.work_queue();
}
void on_sendable(proton::sender& s) override {
std::lock_guard<std::mutex> l(lock_);
credit_ = s.credit();
sender_ready_.notify_all(); // Notify senders we have credit
}
// work_queue work items is are automatically dequeued and called by proton
// This function is called because it was queued by send()
void do_send(const proton::message& m) {
sender_.send(m);
std::lock_guard<std::mutex> l(lock_);
--queued_; // work item was consumed from the work_queue
credit_ = sender_.credit(); // update credit
sender_ready_.notify_all(); // Notify senders we have space on queue
}
void on_error(const proton::error_condition& e) override {
OUT(std::cerr << "unexpected error: " << e << std::endl);
exit(1);
}
};
// A thread safe receiving connection that blocks receiving threads when there
// are no messages available, and maintains a bounded buffer of incoming
// messages by issuing AMQP credit only when there is space in the buffer.
class receiver : private proton::messaging_handler {
static const size_t MAX_BUFFER = 100; // Max number of buffered messages
// Used in proton threads only
proton::receiver receiver_;
// Used in proton and user threads, protected by lock_
std::mutex lock_;
proton::work_queue* work_queue_;
std::queue<proton::message> buffer_; // Messages not yet returned by receive()
std::condition_variable can_receive_; // Notify receivers of messages
public:
// Connect to url
receiver(proton::container& cont, const std::string& url, const std::string& address)
: work_queue_()
{
// NOTE:credit_window(0) disables automatic flow control.
// We will use flow control to match AMQP credit to buffer capacity.
cont.open_receiver(url+"/"+address, proton::receiver_options().credit_window(0),
proton::connection_options().handler(*this));
}
// Thread safe receive
proton::message receive() {
std::unique_lock<std::mutex> l(lock_);
// Wait for buffered messages
while (!work_queue_ || buffer_.empty())
can_receive_.wait(l);
proton::message m = std::move(buffer_.front());
buffer_.pop();
// Add a lambda to the work queue to call receive_done().
// This will tell the handler to add more credit.
work_queue_->add([=]() { this->receive_done(); });
return m;
}
void close() {
std::lock_guard<std::mutex> l(lock_);
if (work_queue_) work_queue_->add([this]() { this->receiver_.connection().close(); });
}
private:
// ==== The following are called by proton threads only.
void on_receiver_open(proton::receiver& r) override {
receiver_ = r;
std::lock_guard<std::mutex> l(lock_);
work_queue_ = &receiver_.work_queue();
receiver_.add_credit(MAX_BUFFER); // Buffer is empty, initial credit is the limit
}
void on_message(proton::delivery &d, proton::message &m) override {
// Proton automatically reduces credit by 1 before calling on_message
std::lock_guard<std::mutex> l(lock_);
buffer_.push(m);
can_receive_.notify_all();
}
// called via work_queue
void receive_done() {
// Add 1 credit, a receiver has taken a message out of the buffer.
receiver_.add_credit(1);
}
void on_error(const proton::error_condition& e) override {
OUT(std::cerr << "unexpected error: " << e << std::endl);
exit(1);
}
};
// ==== Example code using the sender and receiver
// Send n messages
void send_thread(sender& s, int n) {
auto id = std::this_thread::get_id();
for (int i = 0; i < n; ++i) {
std::ostringstream ss;
ss << std::this_thread::get_id() << "-" << i;
s.send(proton::message(ss.str()));
OUT(std::cout << id << " sent \"" << ss.str() << '"' << std::endl);
}
OUT(std::cout << id << " sent " << n << std::endl);
}
// Receive messages till atomic remaining count is 0.
// remaining is shared among all receiving threads
void receive_thread(receiver& r, std::atomic_int& remaining) {
auto id = std::this_thread::get_id();
int n = 0;
// atomically check and decrement remaining *before* receiving.
// If it is 0 or less then return, as there are no more
// messages to receive so calling r.receive() would block forever.
while (remaining-- > 0) {
auto m = r.receive();
++n;
OUT(std::cout << id << " received \"" << m.body() << '"' << std::endl);
}
OUT(std::cout << id << " received " << n << " messages" << std::endl);
}
int main(int argc, const char **argv) {
try {
if (argc != 5) {
std::cerr <<
"Usage: " << argv[0] << " MESSAGE-COUNT THREAD-COUNT URL\n"
"CONNECTION-URL: connection address, e.g.'amqp://127.0.0.1'\n"
"AMQP-ADDRESS: AMQP node address, e.g. 'examples'\n"
"MESSAGE-COUNT: number of messages to send\n"
"THREAD-COUNT: number of sender/receiver thread pairs\n";
return 1;
}
const char *url = argv[1];
const char *address = argv[2];
int n_messages = atoi(argv[3]);
int n_threads = atoi(argv[4]);
int count = n_messages * n_threads;
// Total messages to be received, multiple receiver threads will decrement this.
std::atomic_int remaining;
remaining.store(count);
// Run the proton container
proton::container container;
auto container_thread = std::thread([&]() { container.run(); });
// A single sender and receiver to be shared by all the threads
sender send(container, url, address);
receiver recv(container, url, address);
// Start receiver threads, then sender threads.
// Starting receivers first gives all receivers a chance to compete for messages.
std::vector<std::thread> threads;
for (int i = 0; i < n_threads; ++i)
threads.push_back(std::thread([&]() { receive_thread(recv, remaining); }));
for (int i = 0; i < n_threads; ++i)
threads.push_back(std::thread([&]() { send_thread(send, n_messages); }));
// Wait for threads to finish
for (auto& t : threads)
t.join();
send.close();
recv.close();
container_thread.join();
if (remaining > 0)
throw std::runtime_error("not all messages were received");
std::cout << count << " messages sent and received" << std::endl;
return 0;
} catch (const std::exception& e) {
std::cerr << e.what() << std::endl;
}
return 1;
}