OpenDDS and notification of publisher presence - c++

Problem: How can I get liveliness notifications of booth publisher connect and disconnect?
Background:
I'm working with a OpenDDS implementation where I have a publisher and a subscriber of a data type (dt), using the same topic, located on separate computers.
The reader on the subscriber side has overridden implementations of on_data_available(...)and on_liveliness_changed(...). My subscriber is started first, resulting in a callback to on_liveliness_changed(...) which says that there are no writers available. When the publisher is started I get a new callback to telling me there is a writer available, and when the publisher publishes, on_data_available(...) is called. So far everything is working as expected.
The writer on the publisher has a overridden implementation of on_publication_matched(...). When starting the publisher, on_publication_matched(...) gets called since we already have a subscriber started.
The problem is that when the publisher disconnects, I get no callback to on_liveliness_changed(...) on the reader side, nor do I get a new callback when the publisher is started again.
I have tried to change the readerQos by setting the readerQos.liveliness.lease_duration.
But the result is that the on_data_available(...) never gets called, and the only callback to on_liveliness_changed(...) is at startup, telling me that there are no publishers.
DDS::DataReaderQos readerQos;
DDS::StatusKind mask = DDS::DATA_AVAILABLE_STATUS | DDS::LIVELINESS_CHANGED_STATUS | DDS::LIVELINESS_LOST_STATUS ;
m_subscriber->get_default_datareader_qos( readerQos );
DDS::Duration_t t = { 3, 0 };
readerQos.liveliness.lease_duration = t;
m_binary_Reader = static_cast<binary::binary_tdatareader( m_subscriber->create_datareader(m_Sender_Topic,readerQos,this, mask, 0, false) );
/Kristofer

Ok, guess there aren't many DDS users here.
After some research I found that a reader/writer match occurs only if this compatibility criterion is satisfied: offered lease_duration <= requested lease_duration
The solution was to set the writer QoS to offer the same liveliness. There is probably a way of checking if the requested reader QoS could be supplied by the corresponding writer, and if not, use a "lower" QoS, all thou I haven't tried it yet.
In the on_liveliness_changed callback method I simply evaluated the alive_count in the from the LivelinessChangedStatus.
/Kristofer

Related

MismatchingMessageCorrelationException : Cannot correlate message ‘onEventReceiver’: No process definition or execution matches the parameters

We are facing an MismatchingMessageCorrelationException for the receive task in some cases (less than 5%)
The call back to notify receive task is done by :
protected void respondToCallWorker(
#NonNull final String correlationId,
final CallWorkerResultKeys result,
#Nullable final Map<String, Object> variables
) {
try {
runtimeService.createMessageCorrelation("callWorkerConsumer")
.processInstanceId(correlationId)
.setVariables(variables)
.setVariable("callStatus", result.toString())
.correlateWithResult();
} catch(Exception e) {
e.printStackTrace();
}
}
When i check the logs : i found that the query executed is this one :
select distinct RES.* from ACT_RU_EXECUTION RES
inner join ACT_RE_PROCDEF P on RES.PROC_DEF_ID_ = P.ID_
WHERE RES.PROC_INST_ID_ = 'b2362197-3bea-11eb-a150-9e4bf0efd6d0' and RES.SUSPENSION_STATE_ = '1'
and exists (select ID_ from ACT_RU_EVENT_SUBSCR EVT
where EVT.EXECUTION_ID_ = RES.ID_ and EVT.EVENT_TYPE_ = 'message'
and EVT.EVENT_NAME_ = 'callWorkerConsumer' )
Some times, When i look for the instance of the process in the database i found it waiting in the receive task
SELECT DISTINCT * FROM ACT_RU_EXECUTION RES
WHERE id_ = 'b2362197-3bea-11eb-a150-9e4bf0efd6d0'
However, when i check the subscription event, it's not yet created in the database
select ID_ from ACT_RU_EVENT_SUBSCR EVT
where EVT.EXECUTION_ID_ = 'b2362197-3bea-11eb-a150-9e4bf0efd6d0'
and EVT.EVENT_TYPE_ = 'message'
and EVT.EVENT_NAME_ = 'callWorkerConsumer'
I think that the solution is to save the "receive task" before getting the response for respondToCallWorker, but sadly i can't figure it out.
I tried "asynch before" callWorker and "Message consumer" but it did not work,
I also tried camunda.bpm.database.jdbc-batch-processing=false and got the same results,
I tried also parallel branches but i get OptimisticLocak exception and MismatchingMessageCorrelationException
Maybe i am doing it wrong
Thanks for your help
This is an interesting problem. As you already found out, the error happens, when you try to correlate the result from the "worker" before the main process ended its transaction, thus there is no message subscription registered at the time you correlate.
This problem in process orchestration is described and analyzed in this blog post, which is definitely worth reading.
Taken from that post, here is a design that should solve the issue:
You make message send and receive parallel and put an async before the send task.
By doing so, the async continuation job for the send event and the message subscription are written in the same transaction, so when the async message send executes, you already have the subscription waiting.
Although this should work and solve the issue on BPMN model level, it might be worth to consider options that do not require remodeling the process.
First, instead of calling the worker directly from your delegate, you could (assuming you are on spring boot) publish a "CallWorkerCommand" (simple pojo) and use a TransactionalEventLister on a spring bean to execute the actual call. By doing so, you first will finish the BPMN process by subscribing to the message and afterwards, spring will execute your worker call.
Second: you could use a retry mechanism like resilience4j around your correlate message call, so in the rare cases where the result comes to quickly, you fail and retry a second later.
Another solution I could think of, since you seem to be using an "external worker" pattern here, is to use an external-task-service task directly, so the send/receive synchronization gets solved by the Camunda external worker API.
So many options to choose from. I would possibly prefer the external task, followed by the transactionalEventListener, but that is a matter of personal preference.

Need information regarding publishing a double variable on to a ROS topic

I am using ROS to publish double variables on to a ROS topic. This topic will be advertising the topic so that any subscribers can access the data.
The following is the code which I have used to publish the data:
ros::NodeHandle n;
ros::Publisher Auc_pub = n.advertise<std_msgs::Float64>("/auc", 1000);
std_msgs::Float64 areaValue;
areaValue.data.push_back(Area)
Auc_pub.publish(areaValue);
Here Area is the double variable which I need to publish it over the topic /auc. I am not able to build this file as don't know how to enter the Area variable into the areaValue.
If you look at the documentation for std_msgs::Float64, it shows that it contains a single data field, which is called data and will be of type double in C++.
So, in your code, you just do:
areaValue.data = Area
assuming that Area is a double.
I suggest that you take a look at the basic Writing a simple Publisher and Subscriber tutorial on the ROS wiki.
EDIT
If what is in the original post is the entirety of your code, then it's probably not going to do exactly what you think it does. If you use the default constructor for a publisher, messages published on them are emitted once immediately. Any nodes subscribed to that topic at the moment of publishing will get the message, and then the topic will be clear. If you want any node that subscribes to the topic to receive the last message published on it, do the following:
ros::Publisher Auc_pub = n.advertise<std_msgs::Float64>("/auc", 1000, true);
That last bool true tells the publisher that it should latch messages that are published.
But you have yet another problem, presuming this is all your code: You never spin, so your node starts up, advertises its topic, publishes one message on it, and then shuts down, taking the topic with it (assuming nothing was subscribed to the topic). You need to add the following before the end of your main:
ros::spin();
This will keep the node active (and thus the topic alive) until ros::ok() returns false, I.E. until the node is killed.
Of course, your message is still only going to be published once, but the topic will at least stay alive. If you want the message to be broadcast periodically, use a ros::Timer and put the pulish call inside the timer's callback.
But seriously, please read the tutorials, they'll walk you through all of this stuff.
I finally found the solution for the problem. I had multiple ros:spinOnce() in the code. And also in the snippet
ros::NodeHandle n;
ros::Publisher Auc_pub = n.advertise<std_msgs::Float64>("/auc", 1000);
std_msgs::Float64 areaValue;
areaValue.data;
Auc_pub.publish(areaValue);
The publisher Auc_pub was created and destroyed ( As I had included the publisher in a function... my bad). Instead, i included the publisher in the main function where the publisher is created only once and stays alive until the execution completes. And, thanks to #aruisdante your suggestion helped.

ActiveMQ-cpp Broker URI with PrefetchPolicy has no effect

I am using activemq-cpp 3.7.0 with VS 2010 to build a client, the server is ActiveMQ 5.8. I have created a message consumer using code similar to the following, based on the CMS configurations mentioned here. ConnClass is a ExceptionListener and a MessageListener. I only want to consume a single message before calling cms::Session::commit().
void ConnClass::setup()
{
// Create a ConnectionFactory
std::tr1::shared_ptr<ConnectionFactory> connectionFactory(
ConnectionFactory::createCMSConnectionFactory(
"tcp://localhost:61616?cms.PrefetchPolicy.queuePrefetch=1");
// Create a Connection
m_connection = std::tr1::shared_ptr<cms::Connection>(
connectionFactory->createConnection());
m_connection->start();
m_connection->setExceptionListener(this);
// Create a Session
m_session = std::tr1::shared_ptr<cms::Session>(
m_connection->createSession(Session::SESSION_TRANSACTED));
// Create the destination (Queue)
m_destination = std::tr1::shared_ptr<cms::Destination>(
m_session->createQueue("myqueue?consumer.prefetchSize=1"));
// Create a MessageConsumer from the Session to the Queue
m_consumer = std::tr1::shared_ptr<cms::MessageConsumer>(
m_session->createConsumer( m_destination.get() ));
m_consumer->setMessageListener( this );
}
void ConnClass::onMessage( const Message* message )
{
// read message code ...
// schedule a processing event for
// another thread that calls m_session->commit() when done
}
The problem is I am receiving multiple messages instead of one message before calling m_session->commit() -- I know this because the commit() call is triggered by user input. How can I ensure onMessage() is only called once before each call to commit()?
It doesn't work that way. When using async consumers the messages are delivered as fast as the onMessage method completes. If you want to consume one and only one message then use a sync receive call.
For an async consumer the prefetch allows the broker to buffer up work on the client instead of firing one at a time so you can generally get better proformance, in your case as the async onMessage call completes an ack is sent back to the broker an the next message is sent to the client.
Yes, I find this too. However, when I use the Destination URI option ( "consumer.prefetchSize=15" , http://activemq.apache.org/cms/configuring.html#Configuring-DestinationURIParameters ) for the asynchronous consumer, It works well.
BTW, I just use the latest ActiveMQ-CPP v3.9.4 by Tim , and ActiveMQ v5.12.1 on CentOS 7.
Thanks!

understanding RProperty IPC communication

i'm studying this source base. Basically this is an Anim server client for Symbian 3rd edition for the purpose of grabbing input events without consuming them in a reliable way.
If you spot this line of the server, here it is basically setting the RProperty value (apparently to an increasing counter); it seems no actual processing of the input is done.
inside this client line, the client is supposed to be receiving the notification data, but it only calls Attach.
my understanding is that Attach is only required to be called once, but is not clear in the client what event is triggered every time the server sets the RProperty
How (and where) is the client supposed to access the RProperty value?
After Attaching the client will somewhere Subscribe to the property where it passes a TRequestStatus reference. The server will signal the request status property via the kernel when the asynchronous event has happened (in your case the property was changed). If your example source code is implemented in the right way, you will find an active object (AO; CActive derived class) hanging around and the iStatus of this AO will be passed to the RProperty API. In this case the RunL function of the AO will be called when the property has been changed.
It is essential in Symbian to understand the active object framework and quite few people do it actually. Unfortunately I did not find a really good description online (they are explained quite well in Symbian OS Internals book) but this page at least gives you a quick example.
Example
In the ConstructL of your CMyActive subclass of CActive:
CKeyEventsClient* iClient;
RProperty iProperty;
// ...
void CMyActive::ConstructL()
{
RProcess myProcess;
TSecureId propertyCategory = myProcess.SecureId();
// avoid interference with other properties by defining the category
// as a secure ID of your process (perhaps it's the only allowed value)
TUint propertyKey = 1; // whatever you want
iClient = CKeyEventsClient::NewL(propertyCategory, propertyKey, ...);
iClient->OpenNotificationPropertyL(&iProperty);
// ...
CActiveScheduler::Add(this);
iProperty.Subscribe(iStatus);
SetActive();
}
Your RunL will be called when the property has been changed:
void CMyActive::RunL()
{
if (iStatus.Int() != KErrCancel) User::LeaveIfError(iStatus.Int());
// forward the error to RunError
// "To ensure that the subscriber does not miss updates, it should
// re-issue a subscription request before retrieving the current value
// and acting on it." (from docs)
iProperty.Subscribe(iStatus);
TInt value; // this type is passed to RProperty::Define() in the client
TInt err = iProperty.Get(value);
if (err != KErrNotFound) User::LeaveIfError(err);
SetActive();
}

How to display remote email message?

I have been using this code to display IMAP4 messages:
void DisplayMessageL( const TMsvId &aId )
{
// 1. construct the client MTM
TMsvEntry indexEntry;
TMsvId serviceId;
User::LeaveIfError( iMsvSession->GetEntry(aId, serviceId, indexEntry));
CBaseMtm* mtm = iClientReg->NewMtmL(indexEntry.iMtm);
CleanupStack::PushL(mtm);
// 2. construct the user interface MTM
CBaseMtmUi* uiMtm = iUiReg->NewMtmUiL(*mtm);
CleanupStack::PushL(uiMtm);
// 3. display the message
uiMtm->BaseMtm().SwitchCurrentEntryL(indexEntry.Id());
CMsvOperationWait* waiter=CMsvOperationWait::NewLC();
waiter->Start(); //we use synchronous waiter
CMsvOperation* op = uiMtm->OpenL(waiter->iStatus);
CleanupStack::PushL(op);
CActiveScheduler::Start();
// 4. cleanup for example even members
CleanupStack::PopAndDestroy(4); // op,waiter, mtm, uimtm
}
However, in case when user attempts to download a remote message (i.e. one of the emails previously not retrieved from the mail server), and then cancels the request, my code remains blocked, and it never receives information that the action was canceled.
My question is:
what is the workaround for the above, so the application is not stuck?
can anyone provide a working example for asynchronous call for opening remote messages which do not panic and crash the application?
Asynchronous calls for POP3, SMTP and local IMAP4 messages work perfectly, but remote IMAP4 messages create this issue.
I am testing these examples for S60 5th edition.
Thank you all in advance.
First of all, I would retry removing CMsvOperationWait and deal with the open request asynchronously - i.e. have an active object waiting for the CMsvOperation to complete.
CMsvOperationWait is nothing more than a convenience to make an asynch operation appear synchronous and my suspicion is that this is culprit - in the case of download->show message, there are two asynch operations chained.