[ros-users] Another question about policy based synchronizer

Mon Oct 18 17:51:07 UTC 2010

Hi Ken,

I apologize that my earlier email was not clear. I have attached the two 
source files again - "test_policy_synchronizer_publisher.cpp" implements 
a simple node that publishes two messages at different frequencies and 
"test_policy_synchronizer.cpp" uses the approximate time based 
synchronizer. I did not comment out any code in the core ROS libraries. 
I commented out a couple of lines in my publisher node that incremented 
the sequence number before publishing messages (lines 40 and 50 in 
test_policy_synchronizer_publisher.cpp). So now no sequence number is 
manipulated in the publisher node. When received messages are filtered 
through the synchronizer, I see that the sequence numbers in their 
headers are set appropriately. Following is the log of a run:

Publisher log:
[ INFO] [1287423975.468743146]: Starting 
test_policy_synchronizer_publisher node.
[ INFO] [1287423975.476386994]: Publishing AHRS data: time = 
1287423975.476339, seq = 0
[ INFO] [1287423976.476551394]: Publishing AHRS data: time = 
1287423976.476421, seq = 0
[ INFO] [1287423976.476866797]: Publishing Alasca data: time = 
1287423976.476793, seq = 0
[ INFO] [1287423977.476562926]: Publishing AHRS data: time = 
1287423977.476435, seq = 0
[ INFO] [1287423978.476785581]: Publishing AHRS data: time = 
1287423978.476639, seq = 0
[ INFO] [1287423978.477132762]: Publishing Alasca data: time = 
1287423978.477045, seq = 0

Synchronizer log:
[ INFO] [1287423977.477327968]: ----------
[ INFO] [1287423977.477628006]: Received message set
[ INFO] [1287423977.477812526]: AHRS: timestamp = 1287423976.476421, seq = 1
[ INFO] [1287423977.477975048]: Alasca: timestamp = 1287423976.476793, 
seq = 0
[ INFO] [1287423979.477320422]: ----------
[ INFO] [1287423979.477590568]: Received message set
[ INFO] [1287423979.477731158]: AHRS: timestamp = 1287423978.476639, seq = 3
[ INFO] [1287423979.477852403]: Alasca: timestamp = 1287423978.477045, 
seq = 1

  I now understand that I should not modify/use the sequence field; 
however I am curious about knowing how people have used the sequence 
field in the past.

I really appreciate your help.

Thanks,
- Aditya

On 10/18/2010 01:32 PM, Ken Conley wrote:
> Hi Aditya,
>
> The subscriber does not modify the sequence number. There's not enough
> detail in your response as to what code you're referring to expand o
> that.
>
> In general, you shouldn't be using the seq field, and commenting out
> code that manipulates it will cause problems for others who wish to
> use your libraries as you are modifying the core ROS libraries.
>
> HTH,
> Ken
>
> On Mon, Oct 18, 2010 at 10:03 AM, Aditya Gadre<agadre at vt.edu>  wrote:
>    
>> Hi Ken,
>>
>> Thank you very much for your email. That explains the discrepancy.
>> However, when I comment out the code that increments the sequence
>> number, it stays 0. Does this mean the the sequence number is actually
>> only usable on the subscriber side where it gets set to appropriate value?
>>
>> Thanks,
>> - Aditya
>>
>> On 10/18/2010 12:57 PM, Ken Conley wrote:
>>      
>>> Hi Aditya,
>>>
>>> The sequence number is not a user-settable field. We hope to make it
>>> user-settable in the future, but in our previous attempt to do so, the
>>> feedback from ros-users was not to.
>>>
>>> HTH,
>>> Ken
>>>
>>> On Mon, Oct 18, 2010 at 9:28 AM, Aditya Gadre<agadre at vt.edu>    wrote:
>>>
>>>        
>>>> I have a working test code for policy based (approximate time) synchronizer
>>>> in which one node publishes two data messages and the other node runs a
>>>> synchronizer. One message is published at 1Hz and the other at 0.5Hz. For
>>>> each message, I increment the relevant sequence number in the header.
>>>> However, when these messages are received by the node running the policy
>>>> based synchronizer, the sequence numbers are off by 1, although the time
>>>> stamps match perfectly.
>>>>
>>>> For example, following output shows a simple run -
>>>>
>>>> Publisher log:
>>>> [ INFO] [1287418571.114515026]: Starting test_policy_synchronizer_publisher
>>>> node.
>>>> [ INFO] [1287418571.122496486]: Publishing AHRS data: time =
>>>> 1287418571.122447, seq = 1
>>>> [ INFO] [1287418572.122643642]: Publishing AHRS data: time =
>>>> 1287418572.122516, seq = 2
>>>> [ INFO] [1287418572.122956042]: Publishing Alasca data: time =
>>>> 1287418572.122883, seq = 1
>>>> [ INFO] [1287418573.122685477]: Publishing AHRS data: time =
>>>> 1287418573.122559, seq = 3
>>>> [ INFO] [1287418574.122935718]: Publishing AHRS data: time =
>>>> 1287418574.122809, seq = 4
>>>> [ INFO] [1287418574.123219204]: Publishing Alasca data: time =
>>>> 1287418574.123158, seq = 2
>>>> [ INFO] [1287418581.122753362]: Shutting down
>>>> test_policy_synchronizer_publisher node.
>>>>
>>>> Synchronizer node log:
>>>> [ INFO] [1287418573.123472169]: ----------
>>>> [ INFO] [1287418573.123744969]: Received message set
>>>> [ INFO] [1287418573.123947997]: AHRS: timestamp = 1287418572.122516, seq = 1
>>>> [ INFO] [1287418573.124106258]: Alasca: timestamp = 1287418572.122883, seq =
>>>> 0
>>>> [ INFO] [1287418575.122839197]: ----------
>>>> [ INFO] [1287418575.122900518]: Received message set
>>>> [ INFO] [1287418575.122951782]: AHRS: timestamp = 1287418574.122809, seq = 3
>>>> [ INFO] [1287418575.123003883]: Alasca: timestamp = 1287418574.123158, seq =
>>>> 1
>>>>
>>>> Comparing time stamps of sent and received messages, I see that the sequence
>>>> numbers in the synchronizer node are off by 1 (if sent message seq = 3 then
>>>> received message seq = 2). I am not sure why this discrepancy is popping up.
>>>> I would appreciate any help in this regard! I have attached source code for
>>>> both nodes.
>>>>
>>>> Thank you.
>>>> - Aditya
>>>>
>>>> _______________________________________________
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>>>> ros-users at code.ros.org
>>>> https://code.ros.org/mailman/listinfo/ros-users
>>>>
>>>>
>>>>
>>>>          
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>>>        
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