4. RTPS Layer

The lower level RTPS Layer of eprosima Fast DDS serves an implementation of the protocol defined in the RTPS standard. This layer provides more control over the internals of the communication protocol than the DDS Layer, so advanced users have finer control over the library’s functionalities.

4.1. Relation to the DDS Layer

Elements of this layer map one-to-one with elements from the DDS Layer, with a few additions. This correspondence is shown in the following table:

DDS Layer

RTPS Layer

Domain

RTPSDomain

DomainParticipant

RTPSParticipant

DataWriter

RTPSWriter

DataReader

RTPSReader

4.2. How to use the RTPS Layer

We will now go over the use of the RTPS Layer like we did with the DDS Layer one, explaining the new features it presents.

We recommend you to look at the two examples describing how to use the RTPS layer that come with the distribution while reading this section. They are located in examples/C++/RTPSTest_as_socket and examples/C++/RTPSTest_registered

4.2.1. Managing the Participant

Creating a RTPSParticipant is done with RTPSDomain::createParticipant(). RTPSParticipantAttributes structure is used to configure the RTPSParticipant upon creation.

RTPSParticipantAttributes participant_attr;
participant_attr.setName("participant");
RTPSParticipant* participant = RTPSDomain::createParticipant(0, participant_attr);

4.2.2. Managing the Writers and Readers

As the RTPS standard specifies, RTPSWriters and RTPSReaders are always associated with a History element. In the DDS Layer, its creation and management is hidden, but in the RTPS Layer, you have full control over its creation and configuration.

Writers are created with RTPSDomain::createRTPSWriter() and configured with a WriterAttributes structure. They also need a WriterHistory which is configured with a HistoryAttributes structure.

HistoryAttributes history_attr;
WriterHistory* history = new WriterHistory(history_attr);
WriterAttributes writer_attr;
RTPSWriter* writer = RTPSDomain::createRTPSWriter(participant, writer_attr, history);

Similar to the creation of Writers, Readers are created with RTPSDomain::createRTPSReader() and configured with a ReaderAttributes structure. A HistoryAttributes structure is used to configure the required ReaderHistory. Note that in this case, you can provide a specialization of ReaderListener class that implements your callbacks:

class MyReaderListener : public ReaderListener
{
    // Callbacks override
};
MyReaderListener listener;
HistoryAttributes history_attr;
ReaderHistory* history = new ReaderHistory(history_attr);
ReaderAttributes reader_attr;
RTPSReader* reader = RTPSDomain::createRTPSReader(participant, reader_attr, history, &listener);

4.2.3. Using the History to Send and Receive Data

In the RTPS Protocol, Readers and Writers save the data about a topic in their associated Histories. Each piece of data is represented by a Change, which eprosima Fast DDS implements as CacheChange_t. Changes are always managed by the History.

You can add a new CacheChange_t to the History of the Writer to send data. The procedure is as follows:

  1. Request a CacheChange_t from the Writer with RTPSWriter::new_change(). In order to allocate enough memory, you need to provide a callback that returns the maximum number bytes in the payload.

  2. Fill the CacheChange_t with the data.

  3. Add it to the History with WriterHistory::add_change().

The Writer will take care of everything to communicate the data to the Readers.

//Request a change from the writer
CacheChange_t* change = writer->new_change([]() -> uint32_t
                {
                    return 255;
                }, ALIVE);
//Write serialized data into the change
change->serializedPayload.length = sprintf((char*) change->serializedPayload.data, "My example string %d", 2) + 1;
//Insert change into the history. The Writer takes care of the rest.
history->add_change(change);

If your topic data type has several fields, you will have to provide functions to serialize and deserialize your data in and out of the CacheChange_t. Fast DDS-Gen does this for you.

You can receive data from within the ReaderListener::onNewCacheChangeAdded callback, as we did in the DDS Layer:

  1. The callback receives a CacheChange_t parameter containing the received data.

  2. Process the data within the received CacheChange_t.

  3. Inform the Reader’s History that the change is not needed anymore.

class MyReaderListener : public ReaderListener
{
public:

    MyReaderListener()
    {
    }

    ~MyReaderListener()
    {
    }

    void onNewCacheChangeAdded(
            RTPSReader* reader,
            const CacheChange_t* const change)
    {
        // The incoming message is enclosed within the `change` in the function parameters
        printf("%s\n", change->serializedPayload.data);
        // Once done, remove the change
        reader->getHistory()->remove_change((CacheChange_t*)change);
    }

};

4.3. Configuring Readers and Writers

One of the benefits of using the RTPS Layer is that it provides new configuration possibilities while maintaining the options from the DDS layer. For example, you can set a Writer or a Reader as a Reliable or Best-Effort endpoint as previously:

writer_attr.endpoint.reliabilityKind = BEST_EFFORT;

4.3.1. Setting the data durability kind

The Durability parameter defines the behavior of the Writer regarding samples already sent when a new Reader matches. eProsima Fast DDS offers three Durability options:

  • VOLATILE (default): Messages are discarded as they are sent. If a new Reader matches after message n, it will start received from message n+1.

  • TRANSIENT_LOCAL: The Writer saves a record of the last k messages it has sent. If a new reader matches after message n, it will start receiving from message n-k

  • TRANSIENT: As TRANSIENT_LOCAL, but the record of messages will be saved to persistent storage, so it will be available if the writer is destroyed and recreated, or in case of an application crash.

To choose your preferred option:

writer_attr.endpoint.durabilityKind = TRANSIENT_LOCAL;

Because in the RTPS Layer you have control over the History, in TRANSIENT_LOCAL and TRANSIENT modes the Writer sends all changes you have not explicitly released from the History.

4.4. Configuring the History

The History has its own configuration structure, the HistoryAttributes.

4.4.1. Changing the maximum size of the payload

You can choose the maximum size of the Payload that can go into a CacheChange_t. Be sure to choose a size that allows it to hold the biggest possible piece of data:

history_attr.payloadMaxSize  = 250;//Defaults to 500 bytes

4.4.2. Changing the size of the History

You can specify a maximum amount of changes for the History to hold and an initial amount of allocated changes:

history_attr.initialReservedCaches = 250; //Defaults to 500
history_attr.maximumReservedCaches = 500; //Defaults to 0 = Unlimited Changes

When the initial amount of reserved changes is lower than the maximum, the History will allocate more changes as they are needed until it reaches the maximum size.

4.5. Using a custom Payload Pool

A Payload is defined as the data the user wants to transmit between a Writer and a Reader. RTPS needs to add some metadata to this Payload in order to manage the communication between the endpoints. Therefore, this Payload is encapsulated inside the SerializedPayload_t field of the CacheChange_t, while the rest of the fields of the CacheChange_t provide the required metadata.

WriterHistory and ReaderHistory provide an interface for the user to interact with these changes: Changes to be transmitted by the Writer are added to its WriterHistory, and changes already processed on the Reader can be removed from the ReaderHistory. In this sense, the History acts as a buffer for changes that are not fully processed yet.

During a normal execution, new changes are added to the History and old ones are removed from it. In order to manage the lifecycle of the Payloads contained in these changes, Readers and Writers use a pool object, an implementation of the IPayloadPool interface. Different pool implementations allow for different optimizations. For example, Payloads of different size could be retrieved from different preallocated memory chunks.

Writers and Readers can automatically select a default Payload pool implementation that best suits the configuration given in HistoryAttributes. However, a custom Payload pool can be given to RTPSDomain::createRTPSWriter() and RTPSDomain::createRTPSReader() functions. Writers and Readers will use the provided pool when a new CacheChange_t is requested or released.

4.5.1. IPayloadPool interface

  • IPayloadPool::get_payload overload with size parameter:

    Ties an empty Payload of the requested size to a CacheChange_t instance. The Payload can then be filled with the required data.

  • IPayloadPool::get_payload overload with SerializadPayload parameter:

    Copies the given Payload data to a new Payload from the pool and ties it to the CacheChange_t instance. This overload also takes a pointer to the pool that owns the original Payload. This allows certain optimizations, like sharing the Payload if the original one comes form this same pool, therefore avoiding the copy operation.

  • IPayloadPool::release_payload:

    Returns the Payload tied to a CacheChange_t to the pool, and breaks the tie.

Important

When implementing a custom Payload pool, make sure that the allocated Payloads fulfill the requirements of standard RTPS serialization. Specifically, the Payloads must be large enough to accommodate the serialized user data plus the 4 octets of the SerializedPayloadHeader as specified in section 10.2 of the RTPS standard.

For example, if we know the upper bound of the serialized user data, we may consider implementing a pool that always allocates Payloads of a fixed size, large enough to hold any of this data. If the serialized user data has at most N octets, then the allocated Payloads must have at least N+4 octets.

Note that the size requested to IPayloadPool::get_payload already considers this 4 octet header.

4.5.2. Default Payload pool implementation

If no custom Payload pool is provided to the Writer or Reader, Fast DDS will automatically use the default implementation that best matches the memoryPolicy configuration of the History.

PREALLOCATED_MEMORY_MODE

All payloads will have a data buffer of fixed size, equal to the value of payloadMaxSize, regardless of the size requested to IPayloadPool::get_payload. Released Payloads can be reused for another CacheChange_t. This reduces memory allocation operations at the cost of higher memory usage.

During the initialization of the History, initialReservedCaches Payloads are preallocated for the initially allocated CacheChange_t.

PREALLOCATED_WITH_REALLOC_MEMORY_MODE

Payloads are guaranteed to have a data buffer at least as large as the maximum between the requested size and payloadMaxSize. Released Payloads can be reused for another CacheChange_t. If there is at least one free Payload with a buffer size equal or larger to the requested one, no memory allocation is done.

During the initialization of the History, initialReservedCaches Payloads are preallocated for the initially allocated CacheChange_t.

DYNAMIC_RESERVE_MEMORY_MODE

Every time a Payload is requested, a new one is allocated in memory with the appropriate size. payloadMaxSize is ignored. The memory of released Payloads is always deallocated, so there are never free Payloads in the pool. This reduces memory usage at the cost of frequent memory allocations.

No preallocation of Payloads is done in the initialization of the History,

DYNAMIC_REUSABLE_MEMORY_MODE

Payloads are guaranteed to have a data buffer at least as large as the requested size. payloadMaxSize is ignored.

Released Payloads can be reused for another CacheChange_t. If there is at least one free Payload with a buffer size equal or larger to the requested one, no memory allocation is done.

4.5.3. Example using a custom Payload pool

// A simple payload pool that reserves and frees memory each time
class CustomPayloadPool : public IPayloadPool
{
    bool get_payload(
            uint32_t size,
            CacheChange_t& cache_change) override
    {
        // Reserve new memory for the payload buffer
        octet* payload = new octet[size];

        // Assign the payload buffer to the CacheChange and update sizes
        cache_change.serializedPayload.data = payload;
        cache_change.serializedPayload.length = size;
        cache_change.serializedPayload.max_size = size;

        // Tell the CacheChange who needs to release its payload
        cache_change.payload_owner(this);

        return true;
    }

    bool get_payload(
            SerializedPayload_t& data,
            IPayloadPool*& /* data_owner */,
            CacheChange_t& cache_change) override
    {
        // Reserve new memory for the payload buffer
        octet* payload = new octet[data.length];

        // Copy the data
        memcpy(payload, data.data, data.length);

        // Assign the payload buffer to the CacheChange and update sizes
        cache_change.serializedPayload.data = payload;
        cache_change.serializedPayload.length = data.length;
        cache_change.serializedPayload.max_size = data.length;

        // Tell the CacheChange who needs to release its payload
        cache_change.payload_owner(this);

        return true;
    }

    bool release_payload(
            CacheChange_t& cache_change) override
    {
        // Ensure precondition
        assert(this == cache_change.payload_owner());

        // Dealloc the buffer of the payload
        delete[] cache_change.serializedPayload.data;

        // Reset sizes and pointers
        cache_change.serializedPayload.data = nullptr;
        cache_change.serializedPayload.length = 0;
        cache_change.serializedPayload.max_size = 0;

        // Reset the owner of the payload
        cache_change.payload_owner(nullptr);

        return true;
    }

};

std::shared_ptr<CustomPayloadPool> payload_pool = std::make_shared<CustomPayloadPool>();

// A writer using the custom payload pool
HistoryAttributes writer_history_attr;
WriterHistory* writer_history = new WriterHistory(writer_history_attr);
WriterAttributes writer_attr;
RTPSWriter* writer = RTPSDomain::createRTPSWriter(participant, writer_attr, payload_pool, writer_history);

// A reader using the same instance of the custom payload pool
HistoryAttributes reader_history_attr;
ReaderHistory* reader_history = new ReaderHistory(reader_history_attr);
ReaderAttributes reader_attr;
RTPSReader* reader = RTPSDomain::createRTPSReader(participant, reader_attr, payload_pool, reader_history);

// Write and Read operations work as usual, but take the Payloads from the pool.
// Requesting a change to the Writer will provide one with an empty Payload taken from the pool
CacheChange_t* change = writer->new_change([]() -> uint32_t
                {
                    return 255;
                }, ALIVE);

// Write serialized data into the change and add it to the history
change->serializedPayload.length = sprintf((char*) change->serializedPayload.data, "My example string %d", 2) + 1;
writer_history->add_change(change);