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From Andreas Vogler <>
Subject Re: Source Time of PLC Value?
Date Wed, 10 Mar 2021 14:38:15 GMT
Just a side note: it hurts to hear that OPC UA sucks badly performance-wise. I have done tests
with Milo OPC UA Client and SCADA OPC UA servers, and they perform pretty fine. 200 MQTT clients
writing values via a Gateway with OPC UA Milo (Client) to two OPC UA servers, each OPC UA
server handles 50000 values changes per second:

But I fully agree with MQTT - it’s simple - and I think that’s the reason why it is so

> On 10.03.2021, at 14:34, Christofer Dutz <> wrote:
> Yes,
> OPC-UA has all of this ... and it's probably one of the reasons so many implementations
differ, and you have to again interpret results. It's also probably a reason why OPC-UA sucks
that badly performance-wise.
> Just saying that we should be careful what we choose as reference. In contrast typical
MQTT doesn't have all of this and it seems to be gaining more and more traction, just because
it doesn't have all of this overhead.
> Just my thoughts on this.
> Chris
> -----Ursprüngliche Nachricht-----
> Von: Łukasz Dywicki <> 
> Gesendet: Mittwoch, 10. März 2021 12:58
> An:
> Betreff: Re: AW: Source Time of PLC Value?
> Matthias,
> I do believe there are several points which are relevant and still not possible to be
implemented using available PLC4X API. Metadata might not be most fortunate way, but it can
enable or ease analytical and troubleshooting scenarios.
> Two major points I see:
> - Verification of device condition - ie. how does caller know response latency? This
is especially relevant with frequent polling. Currently caller has no measures to know that
and can not measure that in reliable way due to async nature of API.
> There is a way to implement backpressure technics with callback chains (we can automatically
delay execute call), but first we need to bring measures for that.
> - Caching of values - when we do cyclic subscriptions or emulation of these through passive
connections. There is no way to make caller code aware of nature of data received. Unless
he code everything on his end.
> - Per operation timeout settings/automatic retry are another functionalities which are
relevant in case of querying and writing. We do not want every driver to implement it, so
making a metadata and/or decorator would unify this layer between drivers.
> I speak of above from my own perspective cause I did implement an experimental decorator
API for PLC4X [1] to cope with CANopen arbitration issues I found in devices we interface
with. It allows to catch read/write failures [2] and re-attempt entire operation. It is proven
to work [3] independently of driver. Sadly it has no measures to signal its execution back
to caller. In the end, traceability is pretty tough.
> Obviously OPC UA has everything sorted out, but device specific protocols often do not.
Even a limited set of metadata can give us a way to start bridging the gap and making Ben's
Milo/OPC UA server much closer to a real thing.
> Best,
> Łukasz
> [1]
> [2]
> [3]
> On 08.03.2021 23:15, Matthias Milan Strljic wrote:
>> Hi all,
>> i see there a point of how useful this meta information could be in 
>> some scenarios. But i would rather go the route of chris. Because I 
>> see there is more of a risk of an overcomplicated inhomogeneous api 
>> structure. I would not say that all API should have the same java 
>> structure on each platform but if you have for all the different 
>> platforms other featuresets you could also just use partially the code 
>> generation attempt to allow a separate meta info layer.
>> And there would be the question if that is worth the additional feature?
>> I mean I know we could add especially on the OPC UA side some meta 
>> information but is there more than the  actual source timestamp? And 
>> if there is none, would it be not better to use it as the actual 
>> timestamp of the value?
>> Einen schönen Abend ;)
>> Greetings Matthias
>> Am Mo., 8. März 2021 um 14:29 Uhr schrieb Christofer Dutz <
>>> Hi,
>>> I'm generally more concerned about users expecting us to deliver 
>>> feature that driver X has for driver Y too.
>>> I won't object, if you think it's worth doing.
>>> Perhaps if you could whip up an example in a feature branch? I think 
>>> perhaps I was still not understanding what you propose.
>>> Would that be ok?
>>> Chris
>>> -----Ursprüngliche Nachricht-----
>>> Von: Lukas Ott <>
>>> Gesendet: Montag, 8. März 2021 14:20
>>> An:
>>> Betreff: Re: AW: Source Time of PLC Value?
>>> +1 to Lukasz
>>> Am Mo., 8. März 2021 um 14:09 Uhr schrieb Łukasz Dywicki <
>>>> Hold on for a second.
>>>> Making options pushed over connection string is a long term recipe 
>>>> for disaster. Apache Camel is a prime example of what could happen 
>>>> if you starting with configuration with URIs and dynamic parameters. 
>>>> Most of components distinguish producer (writer) and consumer 
>>>> (receiver) options, most complicated components ended up with 40+ 
>>>> options settable via URI. Having a mix of PlcValues which are 
>>>> timestamped and
>>> 'qualified'
>>>> and not will complicate encoding logic (how we push above info to 
>>>> IEC encoder/handler or decorate it?) and how we keep custom types 
>>>> compatible with above?
>>>> That is why I rather opt for extending API in a way which does not 
>>>> force existing drivers to do anything. When any of community 
>>>> members, regardless if its developer or user, will find it necessary 
>>>> to be ported in another driver, she or he have right to do it. It is 
>>>> not only yours duty to keep all drivers and languages the same.
>>>> Speaking of which I am also not concerned about making all languages 
>>>> having the same functionality as languages tend to offer different 
>>>> options which might be hard to be ported. Lets stick to basics which 
>>>> are same (connection strings, field definitions, 
>>>> read/write/subscribe
>>>> syntax) and let various runtime lead their own ways toward
>>> implementation.
>>>> Key point is simple - we won't be able to maintain all languages 
>>>> equally. We should not hold moving ie. go or java forward because we 
>>>> can't port new API functionality to C/Python/C#/whenever. If there 
>>>> is someone who *does* use above in production and *wish* to have new 
>>>> API parts then that person/organization duty is to do it.
>>>> Best,
>>>> Łukasz
>>>> On 08.03.2021 13:17, Christofer Dutz wrote:
>>>>> Hi,
>>>>> I must admit that I would be in favor of keeping it as simple as
>>>> possible (max age in the connection string) and to implement more of 
>>>> the missing parts in plc4x (like the subscription simulation layer) 
>>>> and hereby getting the drivers we have a bit more aligned, than to 
>>>> implement more and more special API features, that only one or two 
>>>> drivers support. Cause this way we're running away form the 
>>>> project-promise of running a given program at any type of PLC with 
>>>> any
>>> protocol.
>>>>> I mean ... at the current velocity the project is underway, I 
>>>>> wouldn't
>>>> really want to add a new MetaData layer that probably in the end 
>>>> only
>>>> 1-2 people will be implementing and which we must not only find out 
>>>> how to fill with life in the first place, but also port to all of 
>>>> our supported languages.
>>>>> If someone's willing to bring in some serious man/womenpower, I'm 
>>>>> fine
>>>> with that ... or we add it to some wish-list for future extensions.
>>>> But I wouldn't want to start something like this right now.
>>>>> Chris
>>>>> -----Ursprüngliche Nachricht-----
>>>>> Von: Łukasz Dywicki <>
>>>>> Gesendet: Montag, 8. März 2021 13:07
>>>>> An:
>>>>> Betreff: Re: Source Time of PLC Value?
>>>>> I have not mentioned metadata term in my answer, so credits for 
>>>>> mining
>>>> it API go to you. :-)
>>>>> Now since you brought it I remember that JDBC has a "result set
>>>> metadata". That might be thing we still miss. The result code is 
>>>> most important and available instantly, other information is 
>>>> supplemental and can be read additionally.
>>>>> Best,
>>>>> Łukasz
>>>>> On 08.03.2021 13:01, Ben Hutcheson wrote:
>>>>>> +1 for Łukasz‘s metadata approach, it would avoid having to create
>>>>>> duplicate PLcValue classes and give us a lot of flexibility in the
>>>> future.
>>>>>> On Mon, Mar 8, 2021 at 5:14 AM Christofer Dutz 
>>>>>> <>
>>>>>> wrote:
>>>>>>> Hi all,
>>>>>>> sorry for being late to the party ... KNX is currently consuming

>>>>>>> all my cycles.
>>>>>>> Just wanted to add my thoughts to the discussion.
>>>>>>> Initially I thought about adding this sort of information to
>>>>>>> API, but then I thought that we have so little protocols 
>>>>>>> supporting this sort of concept that blowing up every PLCValue

>>>>>>> with this time information just would waste CPU time and Memory.
>>>>>>> When it comes to simulating subscriptions by polling in the 
>>>>>>> background, I thought it would be a valid solution to provide
>>>>>>> default age a value can have in the cache and to make it possible

>>>>>>> to override this in the connection string options. If a value
>>>>>>> too old,
>>>> a return code of:
>>>>>>> STALE_DATA could be returned.
>>>>>>> Also, we do have an option of adding an additional set of 
>>>>>>> PlcValue
>>>> types.
>>>>>>> If a driver supports this quality-of-service type of data, we

>>>>>>> could return special versions of PlcValues, that have these
>>> properties?
>>>>>>> This way we don't have to waste the CPU time and Memory for 
>>>>>>> protocols that don't support this concept (We could even 
>>>>>>> implement the methods on the default PlvValues to simply return

>>>>>>> some constants to the PlcValues all have the same API.
>>>>>>> Chris
>>>>>>> -----Ursprüngliche Nachricht-----
>>>>>>> Von: Łukasz Dywicki <>
>>>>>>> Gesendet: Montag, 8. März 2021 10:43
>>>>>>> An:
>>>>>>> Betreff: Re: Source Time of PLC Value?
>>>>>>> Looking at issue we could utilize marker interfaces.
>>>>>>> At least in Java we could define Qualified and Timestamped types.
>>>>>>> Yet looking at present state of API we are unlikely to make each

>>>>>>> and every PlcValue in each and every variant (qualified and 
>>>>>>> timestamped, timestamped, qualified). It would force quite a
>>>>>>> of
>>> wrapping.
>>>>>>> Safest option I see is to extend subscription/response API with

>>>>>>> additional method to retrieve all markers. For most of drivers
>>>>>>> would return just an empty set. It leaves us also an open door

>>>>>>> for future drivers (or devices) to ship additional piece of 
>>>>>>> metadata which
>>>> does not fit into present API.
>>>>>>> Best,
>>>>>>> Łukasz
>>>>>>> On 08.03.2021 04:30, Otto Fowler wrote:
>>>>>>>> So, maybe the extension is timestamp *and* timestamp source.
>>>>>>>> uint64_t timestamp;
>>>>>>>> typedef enum {
>>>>>>>>      /* timestamp was generated by the source device */
>>>>>>>>      SOURCE,
>>>>>>>>      /* timestamp was generated by plc4x on receiving */
>>>>>>>>      GENERATED,
>>>>>>>> } TimestampQuality;
>>>>>>>>> On Mar 5, 2021, at 18:04, Ben Hutcheson <>
>>>> wrote:
>>>>>>>>> We do have some error codes, but it would just need to
>>>>>>>>> extended a bit. I don't think we have one for BAD (we
>>>>>>>>> receive data but the source has marked it bad).
>>>>>>>>> Timestamps I think are definitely a good idea, especially
if we 
>>>>>>>>> eventually DNP3.
>>>>>>>>> On Fri, Mar 5, 2021 at 5:39 PM Łukasz Dywicki 
>>>>>>>>> <>
>>>>>>> wrote:
>>>>>>>>>> You have response code for each individual field
which allows 
>>>>>>>>>> you to determine state (OK, REMOTE_ERROR etc.). Same

>>>>>>>>>> information should be available also for subscriptions.
>>>>>>>>>> Best,
>>>>>>>>>> Łukasz
>>>>>>>>>> On 05.03.2021 21:04, Andreas Vogler wrote:
>>>>>>>>>>> Ah, forgot: and is there a status available?
>>>>>>>>>>> Valid/invalid/good/bad
>>>>>>>>>> indicator of a plc value?
>>>>>>>>>>>> On 05.03.2021, at 21:02, Andreas Vogler 
>>>>>>>>>>>> <>
>>>>>>>>>> wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> How can I get the source time of a PlcValue?
>>>>>>>>>>>> a) from a subscription PlcSubscriptionEvent
>>>>>>>>>>>> b) from a read request  PlcReadResponse
>>>>>>>>>>>> Best regards,
>>>>>>>>>>>> Andreas

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