# Perspective Project Script Persistance **URL:** \ **Category:** Ignition\ **Tags:** perspective, scripting\ **Created:** [July 11, 2024, 6:50pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103 "2024-07-11T18:50:33Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 11, 2024, 6:50pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/1 "2024-07-11T18:50:33Z")
Ignition v8.1.35 I have a project for Zebra mobile scanners. Each scanner has it's own respective UDT that it reads/writes to using the `scannerNumber`. The `scannerNumber` is set by an `onActionPerformed` event on a `Begin` button that is on the starting page of the project. This starting page is used to either get a `:scannerNumber` URL parameter if it were present, if not an entry field appears allowing the user to manually enter a number. I was asked if I could get/display the scanners gps coords on a map, and I figured out how to do that. I have a timer script that sends a message to all sessions, causing them to grab their current gps coords, and write them to the respective tag. I would like to instead save the gps coords in a class instance and persist the `scanners` dictionary across script restarts. Reasons being: - Having multiple sessions call `system.tag.writeBlocking()` once they receive the message from the gateway, does not sound efficient. There could be at least 20 scanners online, and possibly more in the future. - Allowing a new session to then create it's own `Scanner` instance automatically, essentially removes the need to have a UDT for each scanner and not have to create/delete tags manually or via scripting. Although it can be done. Now, I made two classes that I create an instance of in Perspective Project Scripts. The `newScanner()` function is called from the same `onActionPerformed` event on the `Begin` button. I also have a `removeScanner(scannerNumber)` function that will delete the scanner from the `scanners` dictionary when a session is closed. How could I (or should I) persist the `scanners` dictionary across script restarts? ```python class Gps: def __init__ (self, lat, lng, tstamp): self.lat = lat self.lng = lng self.tstamp = tstamp class Scanner: def __init__ (self, session): self.session = session self.name = 'Scanner #{0}'.format(str(session.custom.scannerNumber)) self.gps = Gps(session.props.geolocation.data.latitude, session.props.geolocation.data.longitude, session.props.geolocation.data.timestamp) def __str__ (self): return 'Scanner# {0}, Session ID:{1}, IP: {2}'.format(self.session.custom.scannerNumber, self.session.props.id, self.session.props.address) def __eq__ (self, scannerToCompare): if isinstance(scannerToCompare, Scanner): return self.session.custom.scannerNumber == scannerToCompare.session.custom.scannerNumber return False def getName(self): return self.name def getSession(self): return self.session def getScannerNumber(self): return self.session.custom.scannerNumber scanners = {} def newScanner(session): if session: new_scanner = Scanner(session) scanners[new_scanner.getScannerNumber()] = new_scanner refreshScannerTable() refreshScannerGps() def removeScanner(scannerNumber): if scannerNumber in scanners: del scanners[scannerNumber] refreshScannerTable() refreshScannerGps() def getScanner(scannerNumber = None): if scannerNumber is None: return list(scanners.values()) else: return scanners.get(scannerNumber, None) def updateScannerGps(scannerNumber, lat, lng, tstamp): if scannerNumber in scanners: scanner = scanners[scannerNumber] scanner.gps.lat = lat scanner.gps.lng = lng scanner.gps.tstamp = tstamp refreshScannerGps() def getScannerGps(): scannersData = [] for scanner in scanners.values(): scannerData = { "name": scanner.getName(), "properties": {}, "enabled": True, "lat": scanner.gps.lat, "lng": scanner.gps.lng, "opacity": 1, "icon": { "path": "material/location_on", "color": "#4190F7", "rotate": 0, "size": { "width": 30, "height": 30 }, "style": { "classes": "" } }, "event": { "stopPropagation": False }, "tooltip": { "content": { "text": scanner.getScannerNumber(), "view": { "path": "", "params": {} } }, "direction": "top", "permanent": False, "sticky": False, "opacity": 0.7 }, "popup": { "enabled": False, "content": { "text": scanner.getName(), "view": { "path": "", "params": {} } }, "width": { "max": 300, "min": 50 }, "height": { "max": None }, "pan": { "auto": True }, "closeButton": True, "autoClose": True, "closeOnEscapeKey": True, "closeOnClick": None } } scannersData.append(scannerData) return scannersData def refreshScannerGps(): local.util.sendMessageToSessions('refreshScannerGps', payload = None) def getScannersTable(): scannersData = [] for scanner in scanners.values(): data = { "scannerNumber": scanner.getScannerNumber(), "sessionId": scanner.getSession().props.id, } scannersData.append(data) return scannersData def refreshScannerTable(): local.util.sendMessageToSessions('refreshScannerTable', payload = None) ``` ---
**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:** [July 11, 2024, 7:01pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/2 "2024-07-11T19:01:24Z")
You cannot persist class instances across script restarts in Ignition. That leaks huge amounts of memory, as the instances hold onto the old code versions, which holds onto the entire script interpreter and all of its old code modules. Store only native python or java collection types in persistent JVM objects. At any nesting depth. ---
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 11, 2024, 7:19pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/3 "2024-07-11T19:19:44Z")
Thanks, Phil. Now that you say that, I don't _have_ to save the class instances. I just need the data to be saved. Technically, I don't need classes at all. Just a dictionary containing each scanner's dictionary of values. I can save that to `getGlobals()` and reinitialize my `scanners` dictionary after restart. Any update to a scanner or new scanners coming online, will update the `getGlobals()` variable. Are the variables in `getGlobals()` there forever? Is it a good idea to clean it up manually? ---
**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:** [July 11, 2024, 7:35pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/4 "2024-07-11T19:35:43Z")
> [@csjohnson](#): > > forever For the life of the JVM itself. Empty on gateway restart. When initializing, be sure to use python dictionaries' `.setdefault()` method to place items in a race-free way. Be aware that script module initialization can run in parallel across projects and scopes--locking (also prepped with `.setdefault()`) is sometimes needed. Weird errors are possible (especially failed name lookups for things like `_25_15` that appear to be line and character positions). Arguably, this is a bug, but appears to be intractable. ---
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 11, 2024, 8:11pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/5 "2024-07-11T20:11:43Z")
Phil, Would you agree that storing/retrieving the scanner data from a dictionary in project scripts would be better than reading/writing to tags? I am not sure how to gauge when "too many is too many" when it comes to read/writing to tags. ---
**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:** [July 11, 2024, 8:23pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/6 "2024-07-11T20:23:16Z")
> [@csjohnson](#): > > Would you agree Always? No. Mostly, yes. Particularly as a custom cache for a database. Memory tags survive gateway restart, and are synchronized to a peer in a redundant peer. No JVM variable does that. But both disk and network traffic are involved, and there's a always a short window where a failure or failover could lose data. Memory tags propagate naturally to all interested parties through bindings. The dictionary from `system.util.getGlobals()` cannot do that. The dictionaries from my own `system.util.globalVarMap()` can do that through the corresponding expression function. (Similar for the Perspective-specific session/page/view VarMaps.) Only actual database operations have long-term persistence. If the life of the JVM suits your needs, a persistent dictionary has huge performance advantages over any other method. ---
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 12, 2024, 12:41pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/7 "2024-07-12T12:41:53Z")
Thanks again, Phil. Updated code: ```python scanners = {} def newScanner(session): if session: newScanner = { 'session': session, 'scannerNumber': session.custom.scannerNumber, 'gps': { 'lat': session.props.geolocation.data.latitude, 'lng': session.props.geolocation.data.longitude, 'tstamp': session.props.geolocation.data.timestamp } } scanners.setDefault(session.custom.scannerNumber, newScanner) refreshScannerTable() refreshScannerGps() def removeScanner(scannerNumber): if scannerNumber in scanners: del scanners[scannerNumber] refreshScannerTable() refreshScannerGps() def getScanner(scannerNumber = None): if scannerNumber is None: return list(scanners.values()) else: return scanners.get(scannerNumber, None) def updateScannerGps(scannerNumber, lat, lng, tstamp): if scannerNumber in scanners: scanner = scanners[scannerNumber] scanner['gps']['lat'] = lat scanner['gps']['lng'] = lng scanner['gps']['tstamp'] = tstamp refreshScannerGps() ``` > [@csjohnson](#): > > I can save that to `getGlobals()` and reinitialize my `scanners` dictionary after restart. > Any update to a scanner or new scanners coming online, will update the `getGlobals()` variable. The `.setDefault()` function is "thread safe" for adding a new scanner to the dict. What would be a "thread safe" method for removing scanners as their sessions shutdown/expire? As in my function `removeScanner()`. What about a session needing to update it's respective scanner's data? Does the below hold true? From: [Python objects and thread safe - Ignition - Inductive Automation Forum](https://forum.inductiveautomation.com/t/python-objects-and-thread-safe/15368/2) > [@Python objects and thread safe](https://forum.inductiveautomation.com/t/python-objects-and-thread-safe/15368/3): > > Jython’s datastructures which are dictionaries, lists and sets are thread safe. So this means that there will not be data corruption of the data structures but there can be race conditions. > > More information about this topic at this link: [http://www.jython.org/jythonbook/en/1.0/Concurrency.html](http://www.jython.org/jythonbook/en/1.0/Concurrency.html) ---
**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:** [July 12, 2024, 12:56pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/8 "2024-07-12T12:56:57Z")
> [@csjohnson](#): > > be a "thread safe" method I usually use `.pop()` to remove items in shared dictionaries. No error if something has already deleted it. > [@csjohnson](#): > > Does the below hold true? Yes, you are responsible for your own locking, if needed. Python objects being thread-safe just means python itself won't crash if two threads access the same object. Your algorithm might crash, though. ---
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 12, 2024, 3:54pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/9 "2024-07-12T15:54:58Z")
Gotcha. The way I understand it now is that I _don't have to_ use a lock in order to safely create/edit/delete scanners in my application, as I have seen used in other threads and examples. A `threading.Lock()` would provide insurance on top of any inherent thread safe ability with the datatypes in my application. Unless, you mean that the `dict` is "thread safe" in that Jython/Python just doesn't care if multiple threads access it at the same time. ---
**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:** [July 12, 2024, 4:22pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/10 "2024-07-12T16:22:10Z")
> [@csjohnson](#): > > just doesn't care if multiple threads access it This, really. Jython implements those base data types with java `ConcurrentMap` and `ConcurrentList` objects. Those will efficiently moderate changes to content from multiple threads, preventing its own crashes, but provide no other guarantee. ---
**Author:** ![csjohnson](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@csjohnson](https://forum.inductiveautomation.com/u/csjohnson)\ **Post date:** [July 12, 2024, 6:59pm UTC](https://forum.inductiveautomation.com/t/perspective-project-script-persistance/93103/11 "2024-07-12T18:59:28Z")
Thanks, Phil. I appreciate you.