# Custom Tag History

**URL:** <https://forum.inductiveautomation.com/t/custom-tag-history/87202>\
**Category:** Ignition\
**Tags:** ignition81, scripting\
**Created:** [March 12, 2024, 4:53pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202 "2024-03-12T16:53:47Z")\
**Posts on this page:** 14\
**Page:** 1

<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 4:53pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/1 "2024-03-12T16:53:48Z")

</div>

I have a script that exists in my project library that I use to log tag values to a database. I've tried to incorporate features similar to IA's Tag Historian.

One of these features is the min sample rate and dead band.

To implement these, of course we would need to compare that current time and tag value to the last entry for the tag.

Right now I'm doing this by querying the database to get the last row for the tag and comparing the now time and current value of the tag to the t\_stamp and val columns, respectively, to determine if a new row should be added.

It just occurred to me that instead of checking against the database I could use a `lastTime` and `lastVal` custom property on the tag to record these values so I'm not querying the database every time I need to determine if the tag change should be logged. I would basically be trading a `system.db.runNamedQuery()` call each time the tag value changes for a `system.tag.writeAsync()` call each time the value has changed enough to log the change.

So, something like this

```auto
def logTagChange(tag, currentValue, initialChange):

    #logging enabled and not initial change
    if (tag.LogEN and not initialChange):
    
        #trim the OPC path to get the tag name
        PlcTag = tag.OpcItemPath.replace('ns=1;s=['+tag.Device+']','')
        
        #get the value of the last entry for this tag
        lastDBvalue = system.db.runNamedQuery('Project','Last_Tag_Value',{ 'tag_name' : PlcTag })
        numRows = lastDBvalue.rowCount()
        
        #at least one row was retrieved
        if numRows > 0:
            
            #access the most recent time and value
            lastVal = lastDBvalue.getValueAt(0, 'val')
            lastTime = lastDBvalue.getValueAt(0, 't_stamp')
            
            #calculate the time between now and the last row
            deltaT = system.date.secondsBetween(lastTime, system.date.now())
            
        #no rows in database yet, or large enough change in tag value, or min. sample rate has elapsed
        if (not numRows > 0) or ( abs(currentValue.value - lastVal) >= tag.LogDB ) or ( tag.LogSR > 0 and deltaT >= tag.LogSR ):
        
            #log row in the database
            system.db.runNamedQuery('Project', 'Store_Tag_Change', {'tag_name' : PlcTag, 'val' : currentValue.value)

```

would change to something like this:

```auto
def logTagChange(tag, tagPath, currentValue, initialChange):

    #logging enabled and not initial change
    if (tag.LogEN and not initialChange):
    
        #trim the OPC path to get the tag name
        PlcTag = tag.OpcItemPath.replace('ns=1;s=['+tag.Device+']','')
        
        #access the most recent time and value
        lastVal = tag.lastVal
        lastTime = tag.lastTime
        now = system.date.now()
        
        deltaT = system.date.secondsBetween(lastTime, now)
            
        #no rows in database yet, or large enough change in tag value, or min. sample rate has elapsed
        if ( abs(currentValue.value - lastVal) >= tag.LogDB ) or lastTime == 0 or ( tag.LogSR > 0 and deltaT >= tag.LogSR ):
        
            #log row in the database
            system.db.runNamedQuery('Project', 'Store_Tag_Change', {'tag_name' : PlcTag, 'val' : currentValue.value)

            #update tag values
            system.tag.writeAsync([tagPath+'.lastVal',tagPath+'.lastTime'],[currentValue.value, now])

```

Thoughts on how the performance might compare?

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<div class="post-metadata">

**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [March 12, 2024, 4:58pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/2 "2024-03-12T16:58:48Z")

</div>

Unless we're doing something pathological, performance should absolutely be better, because you're eliminating a network hop.

That said, be _very_ careful with your locking and order of operations. You're trading out a lot of battle-tested guarantees for something entirely homegrown.

---

<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 5:00pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/3 "2024-03-12T17:00:33Z")

</div>

The first `logTagChange` script you see above has been running for a year with okay performance. This improvement just came to me as I was reviewing the code again.

Can you explain more about "locking" and order of operations? Not familiar enough with those terms to know what they mean in this context.

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<div class="post-metadata">

**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [March 12, 2024, 5:08pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/4 "2024-03-12T17:08:30Z")

</div>

This all sort of applies to both versions, but for instance:  
What happens if two value changes happen in rapid succession, and the first script has a pending write operation out and the second one adds another? I don't think (someone will surely correct me) that we strictly guarantee `writeAsync` operations to happen in the order they're submitted.  
What happens to your script if you weren't able to retrieve a last value or last time for the tag?  
What happens to your logging based on system timestamps if the system time suddenly jumps backwards (can happen with DST events)?

Admittedly, my first post was somewhat kneejerk from the idea of implementing your own 'historian'. What you've got here is nicely narrow in scope, so it will probably be fine.

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<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 5:10pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/5 "2024-03-12T17:10:43Z")

</div>

Ya, daylight savings... man I miss AZ. At worst, we are "missing" an hour of time in the spring and doubling up an hour of time in the fall. I can live with that for the headache it would cause to program around it.

As for the async call order. I'm guessing blocking would just freeze everything up?

---

<div class="post-metadata">

**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [March 12, 2024, 5:20pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/6 "2024-03-12T17:20:56Z")

</div>

Blocking would, by definition, block the tag event script thread handling this. Bad idea. Probably the most "proper" way to do things would be to hold your own concurrent-safe queue of values to process, and 'process' those values separately - but then you're at a significantly greater level of complexity to implement and maintain.

---

<div class="post-metadata">

**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [March 12, 2024, 5:22pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/7 "2024-03-12T17:22:21Z")

</div>

> [@paul-griffith](#):
>
> complexity to implement

Well, except that there's some sample code to get one started:

> [@Java concurrent queue for tag change scripts](https://forum.inductiveautomation.com/t/java-concurrent-queue-for-tag-change-scripts/82044/38):
>
> Okay. Draft for your edification: [queueUtil.py](https://www.automation-pros.com/ignition/queueUtil.py) Includes Google docstrings. Utterly untested. grimacing Maybe later. Edit: docstring typos fixed and ambiguities addressed. Heh: if you delete all comments, docstrings, and blank lines, it is 29 lines of code.

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<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 8:31pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/8 "2024-03-12T20:31:13Z")

</div>

That's way over my head... will probably not get into that (_yet_, anyways)

---

<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 8:35pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/9 "2024-03-12T20:35:35Z")

</div>

since `lastTime` will be a number and `system.date.now()` would return a date, what type casting do I need to do to make that comparison work?

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<div class="post-metadata">

**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [March 12, 2024, 8:58pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/10 "2024-03-12T20:58:46Z")

</div>

`system.date.now().time` will be milliseconds since epoch.  
Or if you want to save a few nanoseconds, you could import `java.lang.System` and use `System.currentTimeMillis()`.

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<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 9:00pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/11 "2024-03-12T21:00:58Z")

</div>

Ok, but seconds between needs two dates, so probably get rid of that and just use epoch time everywhere?

```auto
now = system.date.now().time / 1000
deltaT = lastTime - now

```

---

<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 9:12pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/12 "2024-03-12T21:12:06Z")

</div>

Also, when I pass the entire tag reference to the project script and access via `tag.lastVal`, is that a qv?

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<div class="post-metadata">

**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\
**Post date:** [March 12, 2024, 9:30pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/13 "2024-03-12T21:30:06Z")

</div>

I have no idea, to be honest.

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<div class="post-metadata">

**Author:** ![amarks](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/amarks/32/45610_2.png) [@amarks](https://forum.inductiveautomation.com/u/amarks)\
**Post date:** [March 12, 2024, 9:32pm UTC](https://forum.inductiveautomation.com/t/custom-tag-history/87202/14 "2024-03-12T21:32:39Z")

</div>

I don't think it is. Seems to be working fine just using `tag.lastVal`