Portable MQTT C/C++ clients for resource-limited platforms

When I started writing the Paho MQTT C client as long ago as 2008, it looked to me like the world of embedded operating systems was converging on Linux and Windows variants. This was based on my (limited) experience with embedded systems such as the Arcom Viper and various WinCE hand held devices. Consequently I wrote the Paho MQTT C client using Posix and Windows system APIs.

Now that decision doesn’t look so appropriate. There is a proliferation of cheap and small embedded platforms – some can run Linux, some can’t. In my last two posts I starting describing a new, more portable MQTT C client at its simplest. That first layer simply takes C data and serializes it into a buffer ready for sending across the network using any network APIs you choose:

len = MQTTSerialize_connect(buf, buflen, &data); /* 1 */
len += MQTTSerialize_publish(buf + len, buflen - len, 0, 0, 0, 0, topicString, payload, payloadlen); /* 2 */
rc = write(mysock, buf, len);

– there are corresponding API calls for deserializing buffers received from the network into C data structures.

API for mbed

This works fine, but I was also asked to write an API that would work well in the mbed world (The mbed development platform is the fastest way to create products based on ARM microcontrollers). C++ is the standard language here, but non-Posix networking APIs. What I’ve come up with so far (after some discussion) is an API that has an outline like this:

template<class Network, class Timer, class Thread, class Mutex>
class Client
    /* connectionLost and default messageArrived callbacks to be added */
    struct Result
        /* success or failure result data */
        Client<Network, Timer, Thread, Mutex>* client;
        int connack_rc;
    typedef void (*resultHandler)(Result*); 

    Client(Network* network, const Limits limits = Limits()); 

    int connect(MQTTPacket_connectData* options = 0, resultHandler fn = 0);     
    int publish(const char* topic, Message* message, resultHandler rh = 0);
    int subscribe(const char* topicFilter, enum QoS qos, messageHandler mh, resultHandler rh = 0);
    int unsubscribe(const char* topicFilter, resultHandler rh = 0);
    int disconnect(int timeout, resultHandler rh = 0);

    void yield(int timeout);

This API will look familiar to anyone who is acquainted with the Paho MQTT APIs for C and Java. The resultHandler parameters are optional callback functions which give the outcome of the standard calls. If these are not set, then the MQTT operation calls will block until complete, using no background thread. If resultHandler callback functions are set, then a background thread will be started and will handle MQTT networking while the application can continue with other business.

I am considering splitting the API into two completely separate units, one blocking and single-threaded, the second non-blocking and multi-threaded. This would make the intended use clearer, and reduce the size of both APIs (resultHandlers would be unnecessary in the single-threaded API for instance). Other points to note:

  • C++ Standard Template Library (STL) not used – too heavyweight
  • messageHandler callback per subscription (see discussion) – there will also be a default messageHandler
  • system APIs for networking, timing and threading passed in as template parameters (for the synchronous API threading and mutex classes are not needed)
  • heap memory allocation minimized (I hope to avoid it altogether)
  • memory use limited at object construction time by the limits parameter (see below)
  • network connection handled outside of this API
  • persistence, if/when added, as another template parameter

The limits parameter looks like this:

typedef struct limits
    int MAX_MQTT_PACKET_SIZE;       // two buffers, for sending and receiving, will be allocated of this size
    int MAX_MESSAGE_HANDLERS;       // each subscription requires a message handler
    int MAX_CONCURRENT_OPERATIONS;  // each command which runs concurrently can have a result handler, when we are in multi-threaded mode
    int command_timeout;            // how long to wait for any command to complete, in seconds
        MAX_MQTT_PACKET_SIZE = 100;
        MAX_CONCURRENT_OPERATIONS = 1; // 1 indicates single-threaded mode - set to >1 for multithreaded mode
        command_timeout = 30;
} Limits;

Here are example programs to use this API on mbed and Linux. These demonstrate the portability of the API – it’s not just for mbed, or for one networking API.

I’ve not finished this API yet, and it will be pulled back into Paho.

Three Layers

I’ve been thinking that a third layer supplying offline message buffering would complete the picture. (I’m hoping that I will never have to write another MQTT C client API after this effort 🙂 ) This simply means the ability to send and receive messages from the API when there is no connection to the MQTT server. This feature is frequently asked for, but does increase the internal complexity of an API and thus its memory requirements. Some of the questions that arise when considering a buffered client API are:

  • what if messages are “sent” by the application, but the client library never connects successfully?
  • how many messages should we buffer? what size?
  • should the messages be sent/received when the application is not running?
  • when and how often should the API try to (re-)connect to the server?
  • how should buffered messages be stored? for how long?

There are reasonable answers to these questions which will make such an API suitable for embedded platforms, and certainly for a portable C/C++ MQTT API. The family of MQTT C/C++ APIs for portability/embedded systems would then look like this:

  1. serialization/deserialization API – currently C only
  2. blocking and non-blocking unbuffered MQTT client in a style similar to current C and Java APIs, but portable across system libraries – currently C++ only (on top of library 1)
  3. buffered MQTT API, portable across system libraries (on top of library 2).

Whether each of these layers should be in both C and C++, I’m not sure. I think it depends on the demand. I am relishing using C++ again (without STL) and am reluctant to go back to C for layers 2 and 3 unless there’s a real need.

Comments or suggestions on any of this are welcome.