This is a lightweight, header-only, Boost-based library providing client-side network primitives to easily organize and implement data transmission with remote endpoints.

This library:

• Inspired by and built around Boost.Asio.
• Provides high-level constructs as connector and connection pool.
• Supports TCP/UDP/SSL/HTTP/HTTPS protocols.
• Uses sockets with the same interface as boost::asio::ip ones.
• Allows transparent timeout/deadline control for all operations.
• Requires boost 1.65+ or 1.66+ if you are planning to use HTTP protocol.

If you are writing software on C++, which communicates with other services as a client, you probably already came-across with the problem - need to implement connectivity layer to provide network transport to all related services. This is exactly what this library's aim to solve.

Sockets are implemented on top of boost::asio::basic_socket and provide classes with timeout control, so in most cases it's enough to just call send()/receive()/write_some()/read_some() with a deadline or timeout. Data supplied to I/O operations should be wrapped into boost::asio::buffer. Basically these clients are timeout-wrapped boost::asio::ip sockets and have the same interface.

Client streams classes:

• stream_client::tcp_client - plain TCP stream socket client. Supports send()/rececive() for admitted transfers of whole buffer along with write_some()/read_some() to transfer at least something.
• stream_client::udp_client - plain UDP socket client. Supports only send()/receive() without any acknowledgment as per UDP specs.
• stream_client::ssl::ssl_client - SSL-encrypted TCP client. Have the same functions as tcp_client plus SSL handshake and context control.
• stream_client::http::http_client - HTTP client. Wraps tcp_client with boost::beast::http::parser and boost::beast::http::serializer and have perform() function to make request-response calls.
• stream_client::http::https_client - HTTPS client. Same as http_client but uses ssl_client client underneath.

const boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::address::from_string("127.0.0.1"), 12345);
const std::chrono::milliseconds connect_timeout(1000);
const std::chrono::milliseconds io_timeout(100);

// this makes connected socket and may take up to connect_timeout time
stream_client::tcp_client client(endpoint, connect_timeout, io_timeout);

// send() operation will transmit all contents of send_data or throw an error;
// also it will throw boost::asio::error::timed_out after io_timeout.
const std::string send_data = "test data";
client.send(boost::asio::buffer(send_data.data(), send_data.size()));

// receive() operation will read send_data.size() size or throw an error;
// also it will throw boost::asio::error::timed_out after io_timeout.
std::string recv_data(send_data.size(), '\0');
client.receive(boost::asio::buffer(&recv_data[0], send_data.size()));

// there are also non-throw overloads to these functions which
// will set boost::system::error_code instead of exception;

Resolver is implemented around the same idea - make sync, timed DNS resolver, therefore it uses boost::asio::ip::basic_resolver to perform actual resolution within specified timeouts.

It is the class template with a single parameter - result protocol, also you can specify which type of IP endpoints you are interested in - IPv4, IPv6 or both. Resolution performed with resolve() call which has a similar signature as sockets operations - you are free to specify timeout/deadline or use default one.

Resolver classes:

• stream_client::resolver::tcp_resolver - resolver which returns iterator of TCP endpoints.
• stream_client::resolver::udp_resolver - resolver which returns iterator of UDP endpoints.

const std::chrono::milliseconds resolve_timeout(5000);

stream_client::resolver::tcp_resolver resolver("localhost", "12345",
                                               resolve_timeout,
                                               stream_client::resolver::ip_family::ipv4);

// resolve() guaranteed to return at least one endpoint or will throw an error;
// it will throw boost::asio::error::timed_out after resolve_timeout.
auto endpoints_it = resolver.resolve();

// returned iterator is an instance of boost::asio::ip::basic_resolver::iterator<Protocol>

Connector uses resolver to perform DNS resolution and update the list of endpoints and uses them if requested with new_session(), to create a new socket of the specified protocol. The work of DNS resolver is wrapped into background thread which triggered upon creation or if there was an error in opening a new socket.

Target endpoint to open a new socket selected randomly from the DNS results, which makes it balanced connector to a remote host. Also, the connector allows specifying separate timeouts for resolution, connection, and I/O operation of new sockets.

Connector classes:

• stream_client::connector::tcp_connector - TCP connector, to obtain stream_client::tcp_client sockets.
• stream_client::connector::udp_connector - UDP connector, to obtain stream_client::udp_client sockets.
• stream_client::connector::ssl_connector - TCP connector, to obtain stream_client::ssl::ssl_client sockets.
• stream_client::connector::http_connector - HTTP connector, to obtain stream_client::http::http_client sockets.
• stream_client::connector::https_connector - HTTPS connector, to obtain stream_client::http::https_client sockets.

const std::chrono::milliseconds resolve_timeout(5000);
const std::chrono::milliseconds connect_timeout(1000);
const std::chrono::milliseconds io_timeout(100);

// this will return immediately, starting background thread for name resolution
stream_client::connector::tcp_connector connector("localhost", "12345",
                                                  resolve_timeout, connect_timeout, io_timeout,
                                                  stream_client::resolver::ip_family::ipv4);

// this will acquire new socket or throw;
// it will throw boost::asio::error::timed_out after connect_timeout
std::unique_ptr<stream_client::tcp_client> client = connector.new_session();

// use the client
const std::string send_data = "test data";
std::string recv_data(send_data.size(), '\0');
client->send(boost::asio::buffer(send_data.data(), send_data.size()));
client->receive(boost::asio::buffer(&recv_data[0], send_data.size()));

Represents container occupied with opened sockets. Uses connector to open new sockets in the background thread which is triggered once there are vacant places in the pool. User can call get_session() to obtain a socket from the pool and return_session() to give it back.

There are two strategies to refill the pool:

• greedy (stream_client::connector::greedy_strategy). If there are vacant places it will try to fill them with new sessions simultaneously. This is the default one.
• conservative (stream_client::connector::conservative_strategy). Will try to fill up to 2/3 of vacant places in the poll. If failed will back of for some time and retry later. Also, after failures it will create only one new session.

Both of them are defined in terms of stream_client::connector::pool_strategy interface, so you are free to implement new one.

Limitations:

1. Sockets that are already in the pool are not checked or maintained in any way. Hence, the pool doesn't guarantee that all sockets are opened at an arbitrary point in time due to the complexity of such checks for all supported protocols.
2. Nothing specific done with sockets upon their return within return_session(). Therefore, if they have or will have pending data to read, it will stay there until reading.

Considering this, the best strategy to use a connection pool is such:

1. Create it, specifying all timeouts as you need.
2. Once created, use get_session() to obtain opened socket.
3. Do needed I/O operations on the socket.
4. If 2-3 succeed, return it back with return_session(), else repeat point 2.

It is important to discard sockets on failure and not reuse them, or request-response management will get nasty.

Connection pools:

• stream_client::connector::tcp_pool - pool of stream_client::tcp_client sockets.
• stream_client::connector::udp_pool - pool of stream_client::udp_client sockets.
• stream_client::connector::ssl_pool - pool of stream_client::ssl::ssl_client sockets.
• stream_client::connector::http_pool - pool of stream_client::http::http_client sockets.
• stream_client::connector::https_pool - pool of stream_client::http::https_client sockets.

There are also aliases for the same pools but using conservative reconnection strategy:

• stream_client::connector::tcp_conservative_pool
• stream_client::connector::udp_conservative_pool
• stream_client::connector::ssl_conservative_pool
• stream_client::connector::http_conservative_pool
• stream_client::connector::https_conservative_pool

const std::chrono::milliseconds resolve_timeout(5000);
const std::chrono::milliseconds connect_timeout(1000);
const std::chrono::milliseconds io_timeout(100);

auto connector_pool =
    std::make_shared<stream_client::connector::tcp_pool>("localhost", "12345",
                                                         resolve_timeout, connect_timeout, io_timeout,
                                                         stream_client::resolver::ip_family::ipv4);
const size_t threads_num = 10;
std::vector<std::thread> threads;
const std::string send_data = "test data";
std::mutex stdout_mutex;

for (size_t i = 0; i < threads_num; ++i) {
    threads.emplace_back([&send_data, &stdout_mutex, connector_pool]() {
        std::string recv_data(send_data.size(), '\0');

        auto client = connector_pool->get_session();
        // both these calls will throw exception on error or timeout
        client->send(boost::asio::buffer(send_data.data(), send_data.size()));
        client->receive(boost::asio::buffer(&recv_data[0], send_data.size()));
        // if there was no exception, return it
        connector_pool->return_session(std::move(client));

        const std::lock_guard<std::mutex> lk(stdout_mutex);
        std::cout << std::this_thread::get_id() << ": " << recv_data << std::endl;
    });
}

for (auto& t : threads) {
    t.join();
}

The library uses basic logger interface implemented internally. You can modify logger level using stream_client::set_log_level() or stream_client::get_log_level() functions.

These levels are supported:

enum class log_level : int
{
    trace = 0,
    debug,
    info,
    warning,
    error,
};

By default library prints messages to stdout with decent formatting. If you want to overwrite this behavior you can set you own logger via:

void stream_client::set_logger(std::shared_ptr<stream_client::log_interface> logger);

void stream_client::set_logger(stream_client::log_level level, stream_client::log_func_type log_func);

Which allows to either overwrite logger instance of use a callback of with proper signature. These types are defines as:

class log_interface
{
public:
    virtual void set_level(log_level level) noexcept = 0;
    virtual log_level get_level() const noexcept = 0;
    virtual void message(log_level level, const std::string& location, const std::string& message) const = 0;
};

using log_func_type = std::function<void(log_level level, const std::string& location, const std::string& message)>;

For more information please look inside logger.hpp.

This library supposed to be somewhat multi-platform, however, it was tested and mainly used on ubuntu and macOS.
Prefer out-of-source building.

sudo apt update
sudo apt install build-essential cmake libboost-dev libboost-system-dev libssl-dev

brew install cmake pkg-config icu4c openssl boost

To build:

cmake -H. -Bbuild
cmake --build ./build

To install (sudo may be required):

cmake -H. -Bbuild -DSTREAMCLIENT_BUILD_TESTING=OFF -DSTREAMCLIENT_BUILD_DOCS=OFF -DSTREAMCLIENT_BUILD_EXAMPLES=OFF
cmake --build ./build --target install

Or test:

cmake -H. -Bbuild -DSTREAMCLIENT_BUILD_TESTING=ON -DSTREAMCLIENT_BUILD_DOCS=OFF -DSTREAMCLIENT_BUILD_EXAMPLES=OFF
cmake --build ./build
cmake -E chdir ./build ctest --output-on-failure

All these commands assume you are in stream-client root folder

• CMAKE_BUILD_TYPE - build type. RelWithDebInfo by default.
• BUILD_SHARED_LIBS - build shared or static library. ON by default.
• STREAMCLIENT_BUILD_TESTING - build tests or not. OFF by default
• STREAMCLIENT_BUILD_EXAMPLES - build library examples or not. OFF by default.
• STREAMCLIENT_BUILD_DOCS – build html (sphinx) reference docs. OFF by default.
• OPENSSL_USE_STATIC_LIBS - link statically or dynamically against found openssl. If BUILD_SHARED_LIBS is OFF then this options is set.
• OPENSSL_ROOT_DIR - folder where to look for openssl. Set by pkg-config of brew by default.

Developed at Tinkoff.ru in 2020.
Distibuted under Apache License 2.0 LICENSE. You may also obtain this license at https://www.apache.org/licenses/LICENSE-2.0.

Author - i.s.vovk@tinkoff.ru
Current maintainers - i.s.vovk@tinkoff.ru, n.suboch@tinkoff.ru