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From Łukasz Dywicki <>
Subject Re: AW: Source Time of PLC Value?
Date Thu, 11 Mar 2021 18:13:41 GMT
Hey Matthias and Chris,
I agree with you about complexity and alignment. I am far from trying to
cover each and every case. At the same time I am aware that making thing
fine in first run is crucial to avoid major changes afterwards. That's
also why I did throw pseudo-code upfront to brainstorm. Leaving it
undiscussed could lead to complicated implementation and also usage.

While I do appreciate work made to bring all (C/Python/C#?) languages
under Apache PLC4X umbrella I also do see that sometimes they are being
a baggage we carry on for no real use. When I say no use I mean that
these are not officially adopted by anyone except Chris who made most of
the work. Chris can't keep up all these alone. If theese are adopted
then obviously users can and should take care of them given very limited
resources we have.
Above leads us to situation where Java which is simplest (or most
crowded) part of our project can't move much forward. Java API can not
extend its functionality because there are 2 or 3 other languages to
carry on, with very few people who can do it. Any bigger step involving
MSpec is a trouble forcing update of generators for few languages,
holding us hostage of undisclosed use.

Isn't it a time to make sure that C/C#/Python can still compile with 0.8
release?  I feel it is a time to move forward with features we could
ship in 0.9.
And yes, effectively what I do propose is making unused
languages/runtimes a lower priority. We never received external inquiry
of these (at least here), never had external contribution and barely
have docs for these. Despite of huge investment made they are not
picking up to speed.
If we focus on languages which been adopted and do not divide ourselves
too much we can still attract people. When time comes we will help
adopters who use different programming languages to close the gap.

These are my 0.02€.


On 11.03.2021 14:44, Matthias Milan Strljic wrote:
> Hi Lukasz,
> I see your point there and I am not against "fatter" data structure either.
> My problem is just that we should not make it complicated to use, but
> mainly to implement. And if we add to go the route with additional
> metadata, I would consider the need for a policy to homogenize certain
> properties. I hope I wasn't a bit rude in my reply earlier and I'm not
> against the idea of metadata either. Because when we start to make deeper
> features optional on the one hand and different between protocols or
> programming platforms on the other hand, the charm of using PLC4X instead
> of a native driver gets increasingly lost. So I'm actually +1 for the idea,
> but critical about the implementation. I just love simplicity :D
> Greetings the simple boy Matthias
> Am Mi., 10. März 2021 um 12:58 Uhr schrieb Łukasz Dywicki <
>> 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
>>>>>>> duplicate PLcValue classes and give us a lot of flexibility in
>>>>> 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 the
>>>>>>>> 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
>>>>>>>> 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
>>>>>>>> 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
>>>>> types.
>>>>>>>> If a driver supports this quality-of-service type of data,
>>>>>>>> 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
>>>>>>>> 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
>>>>>>>> Yet looking at present state of API we are unlikely to make
>>>>>>>> and every PlcValue in each and every variant (qualified and
>>>>>>>> timestamped, timestamped, qualified). It would force quite
a lot of
>>>> wrapping.
>>>>>>>> Safest option I see is to extend subscription/response API
>>>>>>>> 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
>>>>>>>>> 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 be
>>>>>>>>>> extended a bit. I don't think we have one for BAD
(we still
>>>>>>>>>> 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.).
>>>>>>>>>>> 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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