Overview

This document covers the core functionality of Eclipse iceoryx and is intended to provide a quick introduction to setting up iceoryx applications.

General

To set up a collection of applications using iceoryx (an iceoryx system), the applications need to initialize a runtime and create communication participants like publishers and subscribers or clients and servers. Publishers send data of a specific topic which can be received by subscribers of the same topic. Servers wait on a topic for requests from clients and respond to these requests. To enable the operation of these participants, the middleware daemon, called RouDi, must be running.

But before we get into more details, let's start with a simple publish-subscribe example.

A first example

We need to create a runtime with a unique name along all applications for each application to enable communication with RouDi.

iox::runtime::PoshRuntime::initRuntime("some_unique_name");

Now this application is ready to communicate with RouDi and we can define the data type we want to send.

struct CounterTopic
{
    uint32_t counter;
};

Then we create a publisher that offers our CounterTopic.

iox::popo::Publisher<CounterTopic> publisher({"Group", "Instance", "CounterTopic"});

Now we can use the publisher to send the data.

auto result = publisher.loan();
if(!result.has_error())
{
    auto& sample = result.value();
    sample->counter = 30;
    sample.publish();
}
else
{
    // handle the error
}

Here result is an expected and hence we may get an error. This can happen if we try to loan too many samples and exhaust memory. We have to handle this potential error since the expected class has the nodiscard keyword attached. This means we get a warning (or an error when build in strict mode) when we don't handle it. We could also explicitly discard it with IOX_DISCARD_RESULT which is discouraged. If you want to know more about expected, take a look at How optional and error values are returned in iceoryx.

Let's create a corresponding subscriber.

iox::popo::Subscriber<CounterTopic> subscriber({"Group", "Instance", "CounterTopic"});

Now we can use the subscriber to receive data. For simplicity, we assume that we periodically check for new data. It is also possible to explicitly wait for data using the WaitSet or the Listener. The code to receive the data is the same, the only difference is the way we wake up before checking for data.

while (keepRunning)
{
    // wait for new data (either sleep and wake up periodically or by notification from the waitset)

    auto result = subscriber.take();

    if(!result.has_error())
    {
        auto& sample = result.value();
        uint32_t counter = sample->counter;
        //process the data
    }
    else
    {
        //handle the error
    }

    std::this_thread::sleep_for(std::chrono::milliseconds(100));
}

By calling take we get an expected and hence we have to handle the potential error.

And that's it. We have created our first simple iceoryx example. Here you can find further examples which demonstrate how iceoryx can be used and describe our API in more detail.

Now that we have applications capable of sending and receiving data, we can run the complete iceoryx system.

Note

RouDi and all Apps have to be built with the same compiler and the same compiler flags.

First we need to start RouDi.

# If installed and available in PATH environment variable
iox-roudi
# If build from scratch with script in tools
$ICEORYX_ROOT/build/iox-roudi

Afterwards, we can start the applications which immediately connect to the RouDi via their runtime.

When the application terminates, the runtime cleans up all resources needed for communication with RouDi. This includes all memory chunks used for the data transmission which may still be held by the application.

Before going into the details in the next sections, the following animation depicts the course of events.

We now briefly define the main entities of an iceoryx system which were partially already used in the example above.

RouDi

RouDi is an abbreviation for Routing and Discovery. RouDi takes care of the communication setup but does not actually participate in the communication between the publisher and the subscriber or the client and the server. RouDi can be thought of as the switchboard operator of iceoryx. One of its other major tasks is the setup of the shared memory, which the applications use for exchanging payload data. Sometimes referred to as daemon, RouDi manages the shared memory and is responsible for the service discovery, i.e. enabling subscribers/clients to find topics offered by publishers/servers. It also keeps track of all applications which have initialized a runtime and are hence able to create publishers, subscribers, servers or clients. It provides facilities for applications to query this information.

When an application crashes, RouDi cleans up all resources. Due to our mostly lock-free inter-process mechanisms (only one last lock; we are working to remove it), iceoryx-based communication is much more reliable compared to traditional mechanism using locking.

To view the available command line options for RouDi call $ICEORYX_ROOT/build/iox-roudi --help.

Shared memory

To enable zero-copy inter-process communication, iceoryx uses the shared memory approach, i.e. publishers and subscribers or clients and servers can communicate via shared memory resulting in zero-copy communication.

Shared memory is physical memory that is made accessible to multiple processes via a mapping to a memory area in their virtual address spaces.

For further information have a look at our shared memory concept article.

Runtime

Each application that wants to use iceoryx has to instantiate its runtime, which essentially enables communication with RouDi.

To do so, the following lines of code are required

iox::runtime::PoshRuntime::initRuntime("some_unique_application_name");

The runtime is an object inside the user application that maps the shared memory into the user application's address space.

Note

Only one runtime object per user application is allowed.

Creating service descriptions for topics

A ServiceDescription in iceoryx represents a topic under which publishers and subscribers or clients and servers can exchange data and is uniquely identified by three string identifiers.

1. Group name
2. Instance name
3. Topic name

A triple consisting of such strings is called a ServiceDescription. Two ServiceDescriptions are considered matching if all these three strings are element-wise equal, i.e. group, instance and topic names are the same for both of them. This means the group and instance identifier can be ignored to create different ServiceDescriptions. They will be needed for advanced filtering functionality in the future.

The service model of iceoryx is derived from AUTOSAR and is still used in the API with these names (Service, Instance, Event). The so called canonical protocol is implemented in the namespace capro.

The following table gives an overview of the different terminologies and the current mapping:

	Group	Instance	Topic
rmw_iceoryx	Type	Namespace/Topic	-
AUTOSAR	Service	Instance	Event
DDS Gateway	-	-	/Group/Instance/Topic
Cyclone DDS	-	Type Name	Topic Name

Service is related to instance like classes are related to objects in C++. A service describes an abstract topic and an instance is one instantiation of that abstraction, like an object is an instantiated class. Events are in this context like members of a class.

Example:

class MyRadarService {
   public:
      bool hasObstacleDetected;
      float distanceToObstacle;
};

MyRadarService frontLeftRadarInstance;
std::cout << frontLeftRadarInstance.hasObstacleDetected << std::endl;

In the iceoryx world, we would for instance subscribe to the service ("MyRadarService", "frontLeftRadarInstance", "hasObstacleDetected") and would receive a sample whenever an obstacle was detected. Or we would subscribe to distanceToObstacle and would receive a constant stream of data which presents the distance to the obstacle.

Restrictions

The data type of the transmitted data can be any C++ class, struct or plain old data type as long as it satisfies the following conditions:

• no heap is used
• the data structure is entirely contained in the shared memory - no pointers to process local memory, no references to process local constructs, no dynamic allocators
• the data structure has to be relocatable and therefore must not internally use pointers/references
• no virtual member functions

Note

Most of the STL types cannot be used, but we reimplemented some of them so that they meet the conditions above. You can find an overview here.

Publisher

A publisher is tied to a topic and needs a service description to be constructed. If it is typed, one needs to additionally specify the data type as a template parameter. Otherwise, the publisher is only aware of raw memory and the user has to ensure that it is interpreted correctly.

Once it has offered its topic, it is able to publish (send) data of the specific type. Note that the default is to have multiple publishers for the same topic (n:m communication). A compile-time option to restrict iceoryx to 1:n communication is available. Should 1:n communication be used, RouDi checks for multiple publishers on the same topics and raises an error if there is more than one publisher for a topic.

Subscriber

Symmetrically a subscriber also corresponds to a topic and thus needs a service description to be constructed. As for publishers we distinguish between typed and untyped subscribers.

Once a subscriber is subscribed to some topic, it is able to receive data of the type tied to this topic. In the untyped case this is raw memory and the user must take care that it is interpreted in a way that is compatible to the data that was actually sent.

When multiple publishers have offered the same topic the subscriber will receive the data of all of them (but in indeterminate order between different publishers). Note that the subscriber will not receive data from servers or clients, even when they use the same topic.

Client

Similar to publishers and subscribers, clients are tied to a topic and need a service description to be constructed. If the client is typed, one needs to specify the request and response data types as template parameters. In the untyped case, the client is only aware of raw memory and the user has to take care of its correct interpretation.

Once a client is connected to a server, it can send requests to and receive responses from the server. A sequence ID is used to match a response to a specific request. It must be set on request and checked on response by the user.

Server

Like a client, a server needs a service description to be constructed and can be typed or untyped. In the typed case, the user has to provide the request and the response data types as template parameters. Otherwise, the server handles raw memory and the user has to ensure that it is interpreted correctly.

Once connected, the server can receive requests from clients and send the corresponding responses.

Avoid polling

The easiest way to receive data is to periodically poll whether data is available as depicted on the left side of the picture below for the publish-subscribe messaging pattern. This is sufficient for simple use cases but inefficient in general, as it often leads to unnecessary latency and wake-ups without data. An alternative approach to receive data is to wait for user-defined events to occur. This is provided by our WaitSet and Listener which are introduced in the following sections.

WaitSet

The WaitSet can be used to relinquish control by putting the thread to sleep with a non-busy wait and wait for user-defined events to occur. Usually, these events correspond to the availability of data at specific subscribers or clients. This way we can immediately wake up when data is available and avoid unnecessary wake-ups if no data is available.

One typical use case is to create a WaitSet, attach multiple subscribers and/or clients and user triggers and then wait until one or many of the attached objects signal an event. If this happens one receives a list of all occured events called notificationVector. This makes it possible to collect data directly from the subscriber or client when it signals the WaitSet that new data or a new response is available.

The WaitSet uses the reactor pattern and is informed with a push strategy that one of the attached events occured at which it informs the user.

For more information on how to use the WaitSet see our WaitSet examples.

Listener

The Listener can be used to connect custom callbacks to user-defined events. Unlike the WaitSet, it reacts to those events by executing the connected custom callbacks in a background thread, that will be created by the Listener. As with the WaitSet the background thread waits non-busy on the reception of new data.

Note

The Listener is completely thread-safe but please be aware that most of the objects which can be attached to the Listener are not thread-safe! This means either the object is handled solely by the Listener, which should be the most common use case, or the user has to ensure the thread safety with other means like encapsulating the object in a thread-safe class.

One use case could be that one creates a Listener and attaches multiple subscribers. Every time new data is available, the corresponding connected callback will be executed, e.g. print something to the console or calculate an algorithm. Another use case could be that a server is attached to the Listener and every time a request is received, the connected callback that creates and sends a response, is executed.

Like the WaitSet, the Listener uses the reactor pattern.

For more information about the Listener see our callbacks examples.

API

The API is offered in two languages, C++ and C. Detailed information can be found in the C++ example and C example.

Many parts of the C++ API follow a functional programming approach which is less error-prone. This requires using the monadic types cxx::expected and cxx::optional which are introduced here.

With the C++ API, we distinguish between the typed API and the untyped API. In the typed API, the underlying data type is made apparent by typed pointers or references to some data type T (often a template parameter). This allows working with the data in a C++ idiomatic and type-safe way and should be preferred whenever possible. The typed API is mainly used when iceoryx is used stand-alone, i.e. not integrated into a third party framework.

The untyped API provides opaque (i.e. void) pointers to data, which is flexible and efficient but also requires that the user takes care to interpret received data correctly, i.e. as a type compatible to what was actually sent. This is required for interaction with other lower level APIs and integration into third party frameworks such as ROS. For further information see the respective header files.