I am developing a GUI app that will be used supposedly by mutliple users. In my app, I use QAbstractTableModel to display a MS Access Database (stored on a local server, accessed by several PCs) in a QTableView. I developped everything I needed for unique user interaction. But now I'm moving to the step where I need to think about multi-user interaction.
For exemple, if user A changes a specific line, the instance of the app on user's B PC needs to update the changed line. Another example, if user A is modifying a specific line, and user B also wants to modify it, it needs to be notified as "already being modified, wait please", and once the modification from user A is done, the user B needs to see this modification updated before he has any interaction.
Today, because of the local nature of the MS Access database, I have to update the table view a lot of time, based on user interaction, in order to not miss any database modification from other potential users. It is kinda greedy in terms of performance and resources.
I was thinking about using Django in order make the different app instances communicate with each other, but maybe I'm overthingking it and may be there is other solutions.
Dunno if it's clear, I'm available for more informations !
Perhaps you could simply store a "lastUpdated" timestamp on the row. With each update, you update that timestamp.
Now, when you submit an update, you include that timestamp, and if the timestamps don't match, you let the user know, and handle the conflict on the frontend (Perhaps a simple "overwrite local copy, or force update and overwrite server copy" option).
That's a simple and robust solution, but if you don't want users wasting time writing updates for old rows, you could use WebSockets to communicate from a server to any clients with that row open for editing, and let them know that the row has been updated.
If you want to "lock" rows while the row is already being edited, you could simply store a "inUse" boolean and have users check the value before continuing.
Usually, when using a MVC pattern (which is what QAbstractTableModel + QTableView is) the responsibility of updating the view should lie on the model itself. I.e. it's the model that should notify the view that something changed.
It seems that QAbstractTableModel has a dataChanged signal that gets emitted on data changes.
I suggest you to connect it to your view refresh slot as done here.
In this way you avoid the need of another moving part/infrastructure component (django).
Related
I dont have much knowledge in dbs, but wanted to know if there is any technique by which when i update or insert a specific entry in a table, it should notify my python application to which i can then listen whats updated and then update that particular row, in the data stored in session or some temporary storage.
I need to send data filter and sort calls again n again, so i dont want to fetch whole data from sql, so i decided to keep it local, nd process it from there. But i was worried if in the mean time the db updates, and i could have been passing the same old data to filter requests.
Any suggestions?
rdbs only will be updated by your program's method or function sort of things.
you can just print console or log inside of yours.
if you want to track what updated modified deleted things,
you have to build a another program to able to track the logs for rdbs
thanks.
i have written MicroServices like for auth, location, etc.
All of microservices have different database, with for eg location is there in all my databases for these services.When in any of my project i need a location of user, it first looks in cache, if not found it hits the database. So far so good.Now when location is changed in any of my different databases, i need to update it in other databases as well as update my cache.
currently i made a model (called subscription) with url as its field, whenever a location is changed in any database, an object is created of this subscription. A periodic task is running which checks for subscription model, when it finds such objects it hits api of other services and updates location and updates the cache.
I am wondering if there is any better way to do this?
I am wondering if there is any better way to do this?
"better" is entirely subjective. if it meets your needs, it's fine.
something to consider, though: don't store the same information in more than one place.
if you need an address, look it up from the service that provides address, every time.
this may be a performance hit, but it eliminates the problem of replicating the data everywhere.
another option would be a more proactive approach, as suggested in comments.
instead of creating a task list for changes, and doing that periodically, send a message across rabbitmq immediately when the change happens. let every service that needs to know, get a copy of the message and update it's own cache of info.
just remember, though. every time you have more than one copy of the information, you reduce the "correctness" of the system, as a whole. it will always be possible for the information found in one of your apps to be out of date, because it did not get an update from the official source.
I want to use caching in Django and I am stuck up with how to go about it. I have data in some specific models which are write intensive. records will get added continuously to the model. Each user has some specific data in the model similar to orders table.
Since my model is write intensive I am not sure how effective caching frameworks in Django are going to be. I tried Django view specific caching and I am try to develop a view where first it will pick up data from the cache. Then I will have another call which will bring in data which was added to the model after the caching was done. What I want to do is add the updated data to the original cache data and store it again.
It is like I don't want to expire my cache, I just want to keep adding to my existing cache data. may be once in 3 hrs I can clear it.
Is what I am doing right. Are there better ways than this. Can I really add to items in existing cache.
I will be very glad for your help
You ask about "caching" which is a really broad topic, and the answer is always a mix of opinion, style and the specific app requirements. Here are a few points to consider.
If the data is per user, you can cache it per user:
from django.core.cache import cache
cache.set(request.user.id,"foo")
cache.get(request.user.id)
The common practice it to keep a database flag that tells you if the user's data changed since it was cached. So before you fetch the data from cache, check only this flag from the DB. If the flag says nothing changed, get the data from cache. If it did change, pull from DB, replace the cache, and set the flag again.
The flag check should be fast and simple: one table, indexed by user.id, and a boolean flag field. This will squeeze a lot of index rows into a single DB page, and enables a fast fetching of a single one field row. Yet you still get a persistent updated main storage, that prevents the use of not updated cache data. You can check this flag in a middleware.
You can run expiry in many ways: clear cache when user logs out, run a cron script that clears items, or let the cache backend expire items. If you use a flag check before you use the cache, there is no issue in keeping items in cache except space, and caching backends handle that. If you use the django simple file cache (which is easy, simple and zero config), you will have to clear the cache. A simple cron script will do.
I want all registered users to have the ability to change existing data..
But to make sure they don't mess with it, I want the ability to check the new data before commiting it to my db.
How can I do it as nice as possible, that me and all admins only need one click to reject/accept updated data?
This job can implement by workflow systems similar viewflow or GoFlow and others, in this way added/changed data saves on database but waiting to confirm by workflow master actors.
I want to load info from another site (this part is done), but i am doing this every time the page is loaded and that wont do. So i was thinking of having a variable in a table of settings like 'last checked bbc site' and when the page loads it would check if its been long enough since last check to check again. Is there anything silly about doing it that way?
Also do i absolutely have to use tables to store 1 off variables like this setting?
I think there are 2 options that would work for you, besides creating a entity in the datastore to keep track of "last visited time".
One way is to just check the external page periodically, using the cron api as described by jldupont.
The second way is to store the last visited time in memcache. Although memcache is not permanent, it doesn't have to be if you are only storing last refresh times. If your entry in memcache were to disappear for some reason, the worst that would happen would be that you would fetch the page again, and update memcache with the current date/time.
The first way would be best if you want to check the external page at regular intervals. The second way might be better if you want to check the external page only when a user clicks on your page, and you haven't fetched that page yourself in the recent past. With this method, you aren't wasting resources fetching the external page unless someone is actually looking for data related to it.
You could also use Scheduled Tasks.
Also, you don't absolutely need to use the Datastore for configuration parameters: you could have this in a script / config file.
If you want some handler on your GAE app (including one for a scheduled task, reception of messages, web page visits, etc) to store some new information in such a way that some handler in the future can recover that information, then GAE's storage is the only good general way (memcache could expire from under you, for example). Not sure what you mean by "tables" (?!), but guessing that you actually mean GAE's storage the answer is "yes". (Under very specific circumstances you might want to put that data to some different place on the network, such as your visitor's browser e.g. via cookies, or an Amazon storage instance, etc, but it does not appear to me that those specific circumstances are appliable to your use case).