Using the Eclipse Paho “Test” Broker to Help Test MQTT

I may not have finished all my goals for the test material in the Eclipse Paho project, as outlined in this blog post of mine, but some components are still useful in their current state.

Recently my IBM and Paho colleague, James Sutton, needed to check the behaviour of the Java and Android clients when receiving an error code in response to an MQTT subscribe request. This error code, returned in the MQTT suback packet, was introduced in the last, and current, version of MQTT, 3.1.1. It takes the form of an 0x80 value in the granted QoS (Quality of Service) field, for which only 0, 1 or 2 are valid values – the set of integers which QoS can take in MQTT.

Now you could fire up a broker like Eclipse Mosquitto and configure it to disallow a subscription to a certain topic. If you know the broker well enough, this may be quick and easy. It’s possible you might have to do some fishing around, and figure out whether that broker is actually returning 0x80 as you wanted.

So James turned to the Paho test broker ( in this repo.) (You must use Python 3 to run it, not Python 2). As it stands, this broker will return 0x80 if you try to subscribe to the topic “test/nosubscribe” (see line 322 in That’s pretty easy. If you checked out that file, you will notice another two topics: “test/QoS 1 only” and “test/QoS 0 only”, which will return granted QoSs of a maximum of 1 and 0 respectively. These behaviours can be hard to elicit out of a standard MQTT broker.

Ultimately I guess I should make the specific topics which these responses are attached to configurable, but they aren’t right now.

There are some other characteristics of this broker, which single it out as a “test” broker rather than a product:

  • the goal of the coding is clarity rather than performance. You can tell me whether I succeeded
  • there is no persistence: if you want to simulate stopping and restarting a broker, this broker can just remain running, and disconnect all clients
  • MQTT specification conformance statements are embedded in the code, so that when a test suite is run against the broker, it can tell you which statements were encountered, and which weren’t.
  • the broker has parameters to choose behaviours which can vary but still conform to the MQTT specification:
    1. whether to publish QoS 2 messages on PUBREL or not
    2. whether multiple matching subscriptions result in one publication, or more than one
    3. whether queued QoS 0 messages are dropped if the client is disconnected, or not
    4. whether zero_length_clientids are allowed

    it’s possible more might be added in the future.

The main missing feature of this broker is TLS – but it does have WebSocket support. TLS is on my to do list, as will be updates for the next version of MQTT, 5, which we are working on, and is tentatively scheduled for completion next year.

As outlined in the blog post, this broker is meant to help with broker testing as well, as an oracle.

For some more information, see the Eclipse Paho website for the test tools.

You can use issues on this project to ask for new features or identify bugs, or pull requests to offer your own contributions.

Why doesn’t MQTT have a payload format?

I was talking to Andy Stanford-Clark on Friday, and asked him if he and Arlen ever considered using a payload format in MQTT.  I was interested to know this because we are discussing approaches to MQTT metadata in the standardization process.

Many of the industrial protocols that MQTT was intended to replace, enhance or work alongside go to great lengths to specify the details of the payload.  Data types, metadata, objects, all of these things may be defined in the data.  MQTT does none of these things – the payload is completely undefined.

Andy’s answer follows.

MQTT is intended as a transport, not a protocol.   It’s job is to get the data from one machine to another, not to define what that data is.   Industrial protocols have often conflated the two jobs, so that they are inseparable.  This makes them more complex and difficult to understand than they need to be.

MQTT can be used as a basis for a protocol which also defines the data to be exchanged, but the job of getting the data from one machine to another is already done.  Andy gave the example of a company who used MQTT to defined a new protocol for the first time.  Because MQTT took care of the connection between client and server and transporting the data between them, the process of defining the new protocol took half a day.  Andy asked them how long defining a protocol would have previously taken: the answer was “two weeks”.

Since MQTT was first defined in 1998, data serialization formats have come into and gone out of fashion.  XML was the “obvious” choice for a while.  JSON and Google Protocol Buffers may be the favourite options today.  MQTT is agnostic to them all, it can carry any of them, so it has not outlived its usefulness.

When people first learn about MQTT, they often ask about the lack of payload format, the implicit question being, what use is MQTT then?   The answer being, whatever you want to use it for.  Combine it with some data formatting and exchange sequences, then you have a protocol.  Maybe we will see some standardized protocols in the future which will be based on MQTT, using it for the data transport.

They Buried her Last Tuesday

You may recall my mentioning another acquaintance, “Shakespeare” Smith, an unsuccessful playwright who lived at Horton Bank Top. He told me he had given lots of thought to his name, and wanting to try something different – song writing, he would adopt another. Many passed through his mind like Irving Australia, Amadeus Wagner, but nothing seemed right until he finally decided on Sullivan O’Gilbert. Anyway, this is his latest attempt. He is just trying to find someone to write the music.

‘Twas by the old Rustic Bridge
on the day I met you
You’d broken your crutches
And your good eye was blue.

You’d a wart on your nose
And a boil on your chin
Your front teeth were missing
And you were incredibly thin.

Your hair somewhat sparse
And that was dyed black
But not quite as noticeable
As the hump on your back

Tears on your cheeks
I asked why you cried
“I can’t help it” you said
“My pet dog just died”.

But I married her anyway
Though she was sick and unhealthy
And I’m very happy
She’s exceedingly wealthy

She was buried last Tuesday
And now is no more
She was carried out feet first
Through the front door.

Maybe I’ll miss her
Now she’s gone off
And in my dreams hear
That terrible cough

I’ll remember her fondly
For all that I got –
Two houses in London
A Rolls Royce and a yacht

Some oil wells in Texas
A villa in Crete
A stud farm in Ireland
And a fishing boat fleet

I can’t stay here talking
All’s done and said
I’ve my packing to do
I’m crusing the Med.


© Colin Flanagan, 1998

What Will The Internet of Things Ever Do For Us?

There have been any number of warnings in recent years about the potential negative impacts of what has come to be known as the Internet of Things. Warnings about security:

Their own devices
Hacking the planet

about privacy and the control of global corporations:

Against the Smart City – Adam Greenfield
The Epic Struggle of the Internet of Things – Bruce Sterling

and user interfaces amongst others.

These ideas have a long history in fiction, Philip K. Dick’s Ubik’s argument with his front door, the discussions with toasters in Hitchhiker’s Guide to the Galaxy and Red Dwarf, for instance. Now these warnings and stories have particular relevance because we are on the cusp of seeing them turn into reality.

I am conscious of the pitfalls we face, but I also have a positive outlook on the Internet of Things. When I started working with embedded devices and MQTT, it wasn’t called that – we were just getting data and command from and to devices. As time went on, the “Internet of” prefix became a shorthand phrase that we would use to describe connected objects of any sort — see “The Internet of Cows”. The “Internet of Things” phrase has caught on, though, for better or worse.

These are some of the obvious positive potential benefits:

  • medical monitoring – of pacemakers for instance
  • environmental monitoring and prediction – flooding, snowfall,
  • infrastructure monitoring – power, water, fuel

but I also have an image of a world where other things are possible, important or trivial:

  • opening the curtains with wave of my hand
  • nano machines that can cure disease from the inside
  • street furniture that changes colour to match my outfit (Only Forward)
  • painted portraits that respond to me and my questions
  • an active view of the sky on my bedroom ceiling
  • a car which can change colour each day to match my mood or fancy

which have all appeared in fiction before, whether as science or magic. I will probably be explaining the unnecessary if I recall Arthur C. Clarke’s aphorism “any sufficiently advanced technology is indistinguishable from magic”.

So I have an image of a world where magical acts are possible, where books I have read come to life. Where the intrusive technology we have today can fade into the background if we want it to. Where art, science and society and can find new ways to influence and improve our daily lives.

Of course we do need to be aware of the dangers that can potentially accrue. But whatever activities we are involved in have dangerous implications, from accidents or intentional criminal behaviour. So those dangers in themselves are no reason to dismiss the entire field Technology may also have awkward interfaces that cause more problems than they solve, which stems from a lack of forethought, of design.

Adam Greenfield, in his book Everyware: The dawning age of ubiquitous computing has considered just about all of the possibilities. He set out some principles for development and use of “Everyware”, which I quote directly.

Ubiquitous systems must:

  1. default to harmlessness
  2. be self-disclosing
  3. be conservative of face (not take actions to unduly embarrass users)
  4. be conservative of time (must not introduce undue complications into ordinary operations)
  5. be deniable (users must be able to opt out at any time)

I think it is integral to the above aims that the ethos driving these systems is openness:

so that control of our technological environment is not entirely left to our governments and corporations. The more transparent the whole process is, the better. (I do like to remember from time to time, however, that governments and corporations are people too.) Essential infrastructure needs to be secure from attack, from malevolent intent, but each of us should be able to choose what happens to data we own. I see reason to be optimistic that the advance of technology is empowering the individual as well as the organization.

Today we allow organizations to obtain data about us in exchange for free services we value. Maybe we should consider whether a service which charges but gives us control over our data is a better bargain.

Paho 1.1 Released

Finally I have produced the remaining pieces, and am pleased to be able to say that Paho 1.1 is released.

What’s new:

1) Paolo Patierno’s .Net and WinRT MQTT client, M2Mqtt, version 4.0.0
2) An Android service – a wrapper around the existing Java client. Version 1.0.2
3) Embedded C and C++ MQTT client libraries, version 1.0.0

Updated versions of the existing client libraries:

1) Roger Light’s Python MQTT client, version 1.1
2) The full fat Posix/Windows/MacOS C client, updated to version 1.0.3
3) The Java MQTT client, updated to version 1.0.2
4) The JavaScript MQTT client, updated to version 1.0.1

The downloads page is updated, and the formal release documentation is available.

Thanks to everyone who has contributed, with code, bugs, suggestions or advice — anything 🙂

I’ve updated some essentials on the web site, including references to the MQTT 3.1.1 standard, but there are updates that are still needed. If you find anything, please let me know.

An Example of Using TLS with the Paho MQTT Embedded C++ Client

Understandably, I’ve been asked a few times whether the Paho embedded client library will work with TLS. It will, but the only platform where I’ve written the code to do it so far is on mbed mbed. The reason why it hasn’t been widely publicised is that it uses the CyaSSL TLS library, which is licensed under the GPL. This means binaries linked with CyaSSL also have to be GPL, rather than the Apache license usually used on mbed, or the Eclipse licenses used for Paho.

But I can show you the code I did write. Now the client library is structured to be portable to any network library without changing the core code. It seems I need to explain this better, and further, certainly on the main Paho website as well as this blog. (Soon, I promise.) The following module is one I wrote to create a network class for CyaSSL which can be used by the embedded client class, MQTTClient. The two core methods which are needed are read and write; these are called whenever the library needs data, or has data to send. This is a basic connect — no client certificates are involved, but should serve as a model for what can be done. Here is the complete module — skip afterwards to see how to use it.

#if !defined(MQTTSSL_H)
#define MQTTSSL_H

#include "MQTT_mbed.h"
#include "mbed.h"
#include "EthernetInterface.h"


static TCPSocketConnection mysock; 

static int SocketReceive(CYASSL* ssl, char *buf, int len, void *ctx)
    int rc = mysock.receive(buf, len);
    if (rc == -1)
        rc = -2;  // -2 is WANT_READ
    return rc;
static int SocketSend(CYASSL* ssl, char *buf, int len, void *ctx)
    int rc = mysock.send(buf, len);
    return rc;

    MQTTSSL() : eth()
        ssl = 0;
        ctx = 0;
        eth.init();                          // Use DHCP
        method = CyaTLSv1_2_client_method();
    int connect(char* hostname, int port, int timeout=1000)
        int rc = -1;
        /* Initialize CyaSSL Context */
        if ( (ctx = CyaSSL_CTX_new(method)) == NULL)
            WARN("unable to get ctx");
            goto exit;
        CyaSSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, 0);
        CyaSSL_SetIORecv(ctx, SocketReceive); 
        CyaSSL_SetIOSend(ctx, SocketSend);
        mysock.set_blocking(false, timeout);    
        if ( (rc = mysock.connect(hostname, port)) != 0)
            goto exit;
        if ( (ssl = CyaSSL_new(ctx)) == NULL)
            ERROR("unable to get SSL object");
            rc = -1; goto exit;
        CyaSSL_set_using_nonblock(ssl, 1);
        if ( (rc = CyaSSL_connect(ssl)) != SSL_SUCCESS)
            rc = CyaSSL_get_error(ssl, 0);
            WARN("err = %d, %s\n", rc, CyaSSL_ERR_error_string(rc, "\n"));
            WARN("SSL Connection Error\n");
            rc = -1;
            LOG("SSL Connected\n") ;
            rc = 0;
        return rc;

    int read(unsigned char* buffer, int len, int timeout)
        int rc = 0;
        mysock.set_blocking(false, timeout);  
        rc = CyaSSL_read(ssl, buffer, len);
        DEBUG("called CyaSSL_read len %d rc %d\n", len, rc);
        return rc;
    int write(unsigned char* buffer, int len, int timeout)
        int rc = 0;
        mysock.set_blocking(false, timeout);  
        rc = CyaSSL_write(ssl, buffer, len);
        DEBUG("called CyaSSL_write len %d rc %d\n", len, rc);
        return rc;
    int disconnect()
        int rc = mysock.close();
        CyaSSL_CTX_free(ctx) ;
        return rc;
    EthernetInterface& getEth()
        return eth;

    EthernetInterface eth;

    CYASSL_METHOD*  method;
    CYASSL_CTX*     ctx;
    CYASSL*         ssl;


To use it, you need to create an instance of MQTTSSL, and pass the class as a template parameter when you create an MQTT client instance.

MQTTSSL ipstack;
MQTT::Client client(ipstack);

Now follows the sort of function you might use to connect. You have to make the network connection first, before calling the MQTT connect.

int connect(MQTT::Client* client, MQTTSSL* ipstack)
    const char* iot_ibm = "";
    // Network connect first
    char hostname[strlen(org) + strlen(iot_ibm) + 1];
    sprintf(hostname, "%s%s", org, iot_ibm);
    int rc = ipstack->connect(hostname, IBM_IOT_PORT);
    if (rc != 0)
        return rc;
    // Construct clientId - d:org:type:id
    char clientId[strlen(org) + strlen(type) + strlen(id) + 5];
    sprintf(clientId, "d:%s:%s:%s", org, type, id);
    // Now MQTT Connect
    MQTTPacket_connectData data = MQTTPacket_connectData_initializer;
    data.clientID.cstring = clientId;    
    if ( (rc = client->connect(&data)) == 0) 
    return rc;

If this is successful, you can now make other MQTT calls, like subscribe and publish.

if ( (rc = client.subscribe("iot-2/cmd/+/fmt/json", MQTT::QOS1, messageArrived)) != 0)
           WARN("rc from MQTT subscribe is %d\n", rc); 

MQTT::Message message;
char* pubTopic = "iot-2/evt/status/fmt/json";
char buf[250];
     "{\"d\":{\"myName\":\"IoT mbed\",\"accelX\":%0.4f,\"accelY\":%0.4f,\"accelZ\":%0.4f,\"temp\":%0.4f,\"joystick\":\"%s\",\"potentiometer1\":%0.4f,\"potentiometer2\":%0.4f}}",
            MMA.x(), MMA.y(), MMA.z(), sensor.temp(), joystickPos,,;
message.qos = MQTT::QOS0;
message.retained = false;
message.dup = false;
message.payload = (void*)buf;
message.payloadlen = strlen(buf);
client.publish(pubTopic, &message);

I will write this up more fully soon. With any luck, this has also given you an idea of what needs to be done to port the client to a different network API.

Paho Embedded C++ MQTT Client on mbed and for Arduino

I’ve written previously about the embedded client libraries in Paho. There’s
MQTTPacket, and then the start of the design for the higher level APIs.

I started the first C++ API in ARM mbed as IBM and ARM were working together on an Internet of Things (IoT) kit. The ARM mbed online tools make it very easy to build and load applications onto an mbed-enabled micro controller. Just download the built application, drag to the USB-connected filesystem, then reset the micro controller. Easy as that!

To fit into mbed API conventions, the API:

  • is written in C++
  • uses templates
  • does not use the STL (uses too much resources)
  • is blocking, for ease of use

and uses no dynamic memory allocations. It’s called MQTTClient, and you can read the API description here.

The definitive source for the library is still in Paho, but both MQTTPacket and MQTTClient are also available directly in the mbed online environment. You can find them in the code for the MQTT team along with a sample application HelloMQTT, which uses the mbed Ethernet library.

If you want to use these APIs to connect to the IBM IoT service with MQTT, you can find applications that allow you to do just that in the IBM IoT team code library.

It’s easy to port the MQTTClient library to other networking libraries and other operating systems. So far, we’ve done that for Linux and Arduino.  The Linux port makes it easy to build and test the code in the normal Eclipse build environments.  The Arduino port was started to see if it would work.  It does, and the resulting API is actually quite close to the Arduino conventions too.

The prebuilt Arduino client library is available here. After downloading it, in the Arduino IDE use Sketch -> Import Library… -> Add Library… selecting the downloaded zip file. A sample sketch demonstrating the client is included in the download. I intend to add an entry on the Arduino libraries page sometime soon.

We’ll make ports for other operating systems available as we create them.  But if anyone gets around to a port before we do, we’d love that to be contributed to Paho.  Operating systems such as Contiki, TinyOS, FreeRTOS and RIOT.  If you have a favourite embedded OS that I’ve not mentioned, please suggest it.

More Rigorous Testing for MQTT Servers

I’ve had a sustained interest in a rigorous approach to test design ever since I studied formal methods in the 1990s. This is reflected in the domain name of this blog. ((All the test material referred to in this post is in the Eclipse Paho project.))

Testing both client libraries and servers for adherence to the MQTT specification seems a natural candidate. Any form of rigorous testing will need more investment than a few manual experiments, but the payoff is potentially large (unless you really like a boring, repetitive life). The MQTT standardization process is nearly complete. A major goal of any standardization is to encourage implementations, so there will be plenty of opportunity for a rigorous test suite to recoup the investment made in it.

If a test suite is a haphazard collection of acquired test programs, it tends to suffer from gaps in coverage and a lack of consistent approach and motivation. When the object of that test suite changes, the suite will be hard to maintain because you have to work out the intent of each of the component test programs. Even explaining the exact reasoning behind an individual test program can be hard. With the approach outlined here I hope to make the goals and attainments of the test suite much more transparent, and maintenance easier and more logical.

I start with the MQTT 3.1.1 Specification. The standardized MQTT specification is a great improvement over the previous version because it is has been argued over intensively, with the (planned for) result that many of the ambiguities have been resolved. Each identified conformance statement has been numbered and listed in Appendix B. I use that list as a starting point for measuring test coverage of the complete MQTT specification. ((It has been pointed out to me, by Peter Niblett of IBM, that this list has been constructed by fallible human beings, and thus itself is not rigorous.  I knew that already, but you have to start from somewhere, and this is better than any other specification I have previously tested against. ))

An example statement from the table:

Normative Statement Number Normative Statement
[MQTT-1.5.3-1] The character data in a UTF-8 encoded string MUST be well-formed UTF-8 as defined by the Unicode specification [Unicode] and restated in RFC 3629 [RFC3629]. In particular this data MUST NOT include encodings of code points between U+D800 and U+DFFF. If a Server or Client receives a Control Packet containing ill-formed UTF-8 it MUST close the Network Connection.

What we need first is a model of the expected behaviour of an MQTT implementation, a test oracle. I had previously written an MQTT server in Python, for test purposes, so I updated it for MQTT 3.1.1. I also seeded it with log entries for conformance statements from the specification at appropriate points ((Look for"[MQTT … statements in this example. )) During the execution of this server, any conformance statements encountered are logged. At the end of the test, we can measure the number of conformance statements that that test invoked in the server. We can assess a test suite both by the conformance statements invoked, and by those which are not.

Next we need a model of the test inputs, the MQTT packets that we want to send to the model MQTT server.  We wouldn’t need this model if we could exercise the whole input space without worrying, but for any non-trivial software, this space is so vast that we need to focus.  Even a computer needs to focus.  The first model I’ve created is intended to cover the basic MQTT test scenarios. It responds to QoS 2 publish flows correctly for instance. In the basic scenarios, we want to make sure that the “golden paths” are executed, and some of the error cases. Other input models will be needed for complete coverage — when a large message takes longer to send than the keep alive interval, for instance.

With the Python script the input model is used to generate a sequence of inputs which are sent to the model MQTT server.  The outputs from the model MQTT server are recorded along with the inputs, to result in a self-checking test case.  The exact same sequence of inputs are not used more than once in the same test. The resulting test case can be used directly against a real MQTT server with the There are some ‘little’ complications — MQTT packet IDs do not have to be the same on each run, so they have to be remapped. But in general we replay the same recorded scenario against the MQTT server we want to test.

Currently, the generated test suites reach about 55% coverage of the conformance statements. I have to work on this input model, and maybe some others to increase that coverage to as near 100% as I can manage.   Another complication is that some behaviours of MQTT are optional or can vary, so we have to be able to generate test suites for each of these variations. I aim to be working on these and other items in the upcoming weeks and months before EclipseCon NA next year.

Some more information is available on the Paho wiki. I hope this post is helpful in explaining my intentions. Please do ask questions. I expect at least two other follow on posts:

  • More Rigorous Testing for MQTT Clients
  • A “model” implementation of an MQTT server

so this won’t be the last word on the subject.

Porting the Paho synchronous embedded C++ client

The MQTT library on also now exists in Eclipse Paho. I intend Paho to be the master copy.

There are two APIs in this library, the largely complete synchronous API in MQTTClient.h, and the unfinished asynchronous API in MQTTAsync.h. In this post I’ll be talking about the synchronous API. I have discussed the major design points previous posts, now I’ll describe how to use the API. The documentation page on mbed can be found here.

The API implementation is intended to avoid system calls to allow it to be easily ported. Its function is limited purely to MQTT rather than networking setup for instance. This does mean that there is some work to use the API on a new OS, but we plan to collect examples in Paho for as many OSes as we can — contributions welcome!

The API is defined as a class template with these parameters:

template<class Network, class Timer, int MAX_MQTT_PACKET_SIZE = 100, int MAX_MESSAGE_HANDLERS = 5>

There are two class parameters, Network and Timer which are needed to provide platform specific implementations of sending and receiving data from the network, and timing, respectively. The third and fourth parameters are limits on the maximum MQTT packet size to be handled and the number of message handling callbacks. For each subscription, a message handling callback is needed, so you can think of this second limit as the maximum number of subscriptions you can have active at one time. Increasing these limits will increase the storage used when allocating the MQTT client object.

The file mbed.h contains an mbed specific implmentation of the Timer class. The file MQTTSocket.h contains an mbed specific network implementation for the Ethernet interface. The only requirements for the network class that you need to supply are the two methods:

int read(unsigned char* buffer, int len, int timeout);
int write(unsigned char* buffer, int len, int timeout);


is the location to write data to, or read data from

is the number of bytes of that datatimeoutis the maximum number of milliseconds for the operation to complete.

The timer class counts down from a value to zero. I chose this way because it is always used for timeouts, so it checks to see whether the time has expired:

bool expired();                 // has the time expired?
void countdown_ms(int ms);      // start the timer for ms milliseconds
void countdown(int seconds);    // start the timer in seconds
int left_ms();                  // return the time left in milliseconds

Looking at the mbed example HelloMQTT, you see we have to connect the network interface before calling the MQTT connect:

MQTTEthernet ipstack = MQTTEthernet();
MQTT::Client<MQTTEthernet, Countdown> client = MQTT::Client<MQTTEthernet, Countdown>(ipstack);

int rc = ipstack.connect("", 1883);
if (rc != 0)
   lcd.printf("rc from TCP connect is %d\n", rc);
MQTTPacket_connectData data = MQTTPacket_connectData_initializer;       
data.MQTTVersion = 3;
data.clientID.cstring = "mbed-sample";
data.username.cstring = "testuser";
data.password.cstring = "testpassword";
if ( (rc = client.connect(&data)) != 0)
   lcd.printf("rc from MQTT connect is %d\n", rc);

and then we can call subscribe, publish, unsubscribe and disconnect methods on the client in the usual sort of way for MQTT APIs.

Examples of how to use this API on Linux is in this
directory on Paho, in the hello.cpp and stdoutsub.cpp programs. The API is already running on Arduino, for which we’ll add an example, and for other platforms and operating systems later.

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.