Request/Reply patter for Modern C++

Hi Massimo,

We plan to release the modern C++ request-reply API with our next major release, in 2017.

In the meantime, this solution may help you, although you may already have seen it: How do I use the Traditional C++ and Modern C++ APIs in the same application?

Unfortunately, if you want to publish and/or subscribe to the same types in both APIs, you will have to generate and compile the type plugin separately for each.

I hope this helps.

Alex

As you said, the key element to the request-reply pattern is the correlation between replies and the request that originated them. 

The correlation process is as follows:

• Requester DataWriter is identified by a specific GUID (let's call it requester_guid)
• Requester DataWriter writes a request normally
• Replier DataReader receives the request
• Replier DataWriter writes a reply, using WriteParams::related_sample_identity().guid() set to request.info().original_publication_virtual_guid() (== requester_guid)
• Requester DataReader has a ContentFilteredTopic that only allows samples such that original_publication_virtual_guid == requester_guid.*

* The following code generates the CFT expression you need:

const int GUID_SIZE = 16;
const char * GUID_FIELD_NAME = "@related_sample_identity.writer_guid.value";

void append_guid(std::ostringstream& os, const rti::core::Guid& guid)
{
 for (int i = 0; i < GUID_SIZE; i++) {
 if (guid[i] < 0x10) {
 os << "0";
 }
 os << std::hex << (int) guid[i];
 }
}

std::string create_correlation_filter_expression(
 const rti::core::Guid& correlation_guid)
{
 std::ostringstream expression;
 expression << GUID_FIELD_NAME << " = &hex(";
 append_guid(expression, correlation_guid);
 expression << ")";
 return expression.str();
}

This is enough to make sure the Requester only receives replies directed to it. This is not enough to allow sample-level correlation—that is, waiting for a reply to a specific request.

I hope this helps.

Alex

Hi Massimo,

1) The write overload that receives WriteParams is an RTI extension, so the standard requires to access it with " ->"

writer->write(data, params);

2) Writing with related_sample_identity requires a concrete guid and a concrete sequence number. You can get both packed in a SampleIdentity instance:

SampleIdentity sample_id = sample_info->original_publication_virtual_sample_identity(); // note the use of "->" to access an extension

Dear all,

following the descriptions above, I managed to create a CFT that filters samples according to a given GUID, so this is nice. However, I fail to create a QueryCondition that would allow me to use a WaitSet for a specific sample with a given "sequence_number". My naive approach is as follows:

std::string query_expression = "@related_sample_identity.sequence_number.value = " + std::to_string(sequence_number.value());
 
auto default_read_state = dds::sub::status::DataState();
default_read_state << dds::sub::status::SampleState::not_read();
 
dds::sub::cond::QueryCondition(dds::sub::Query(reader, query_expression), default_read_state);

When I execute this, I get the following error: 

[D0100|Reader(80000504)|T=QueryServer::TextService::ReplyTopic_01015e7d85f75b87425f8a618000a503|CREATE READCONDITION] DDS_TypeCode_dereference_member_name:member starting with [value = 1] not found
[D0100|Reader(80000504)|T=QueryServer::TextService::ReplyTopic_01015e7d85f75b87425f8a618000a503|CREATE READCONDITION] DDS_SqlFilter_compile:SQL compiler failed with error-code: -15 (Invalid symbol)
[D0100|Reader(80000504)|T=QueryServer::TextService::ReplyTopic_01015e7d85f75b87425f8a618000a503|CREATE READCONDITION] PRESPsReader_createReadOrQueryConditionI:content filter compile error 1
[D0100|Reader(80000504)|T=QueryServer::TextService::ReplyTopic_01015e7d85f75b87425f8a618000a503|CREATE READCONDITION] DDS_QueryCondition_createI:!create DDS_QueryCondition

It seems that the ".value" part of the sequence number is not recognized. I also tried to remove the ".value" part in the query expression and then I get this error:

[D0100|Reader(80000004)|T=QueryServer::TextService::ReplyTopic_01010076d3ab36d66bc1b8d480000003|CREATE READCONDITION] DDS_SqlFilter_symbol_lookup:symbol starting with [@related_sample_identity.sequence_number = 2] is not a primitive
[D0100|Reader(80000004)|T=QueryServer::TextService::ReplyTopic_01010076d3ab36d66bc1b8d480000003|CREATE READCONDITION] DDS_SqlFilter_compile:SQL compiler failed with error-code: -15 (Invalid symbol)
[D0100|Reader(80000004)|T=QueryServer::TextService::ReplyTopic_01010076d3ab36d66bc1b8d480000003|CREATE READCONDITION] PRESPsReader_createReadOrQueryConditionI:content filter compile error 1
[D0100|Reader(80000004)|T=QueryServer::TextService::ReplyTopic_01010076d3ab36d66bc1b8d480000003|CREATE READCONDITION] DDS_QueryCondition_createI:!create DDS_QueryCondition

So comparing the sequence_number directly also doesn't seem to work. What am I missing here? Can anybody help me?

Cheers

Alex