I am working on migrating a MS Access Database over to a newer SQL platform.
But, with all of the users who are currently using it, we're migrating slowly/carefully.
The first step is that we are re-writing the VBA code into C#, which is then deployed in a .dll along with the database.
Now, the VBA code calls into the C# to do the business logic, then the VBA continues to do the displays/UI, while Access still hosts the database.
The problem comes in that I have a report that is being run after the business logic from the C# in one place, and apparently MS Access has a cache, which clears every 5 seconds. So, the transaction that occurs in the C# code writes to the database, but the VBA code is still using the cache. This is causing errors, as the records added to the database (which the VBA report is trying to report on) don't exist in the cache yet...
I'm guessing that the C# .dll must be getting treated as a "second connection" to the MS Access database, which is what seems to typically cause this error in my searches (thinks that one process is writing, and the other is reading).
Since the cache is cleared out every 5 seconds, we can just put the process to sleep, and wake it up after 5 seconds, and then run the report, but that's pretty terrible for an end user.
And, making things difficult, the cache seems like it only gets used in the deployed version (so, when running from source / in debug mode, the error never happens).
Doing some searches, there seems to be plenty of people who have said "just refresh the cache." But, the question is: within VBA, how do you refresh the cache?
Any advice would be welcome.
Thanks
I've been fighting the same issue for years as I write a lot of tools around an old Powerbuilder application that has an Access MDB back end.
The cache does exist and it is VERY real. When data is inserted on a different connection than it is queried on, the cache can be directly observed and measured. It was also documented by Microsoft before they blackholed a bunch of their old articles...
Microsoft Jet has a read-cache that is updated every PageTimeout milliseconds (default is 5000ms = 5 seconds). It also has a lazy-write mechanism that operates on a separate thread to main processing and thus writes changes to disk asynchronously. These two mechanisms help boost performance, but in certain situations that require high concurrency, they may create problems.
I've found a couple workarounds that are not the best, but somewhat make due until I find something better or can re-write the app with a better back end database.
The seemingly best answer I've found (that may actually work for you since you say you need VBA) is to use JRO.RefreshCache. I've been trying to figure out how to implement this using C# or VB.net without any luck. Below is a link to a code example where you execute the RefreshCache method on your 2nd connection that needs to pull the data. I have not tested this myself.
https://documentation.help/MSJRO/jrmthrefreshcachex.htm
A workaround I've found that will deliver the query results within 500ms to 1000ms of insert time (instead of anywhere between 500 and 5000 ms - or more):
Use System.Data.ODBC instead of OleDB, with connection string: Driver={Microsoft Access Driver (*.mdb, *.accdb)};Dbq=;
If someone knows how to use the JRO.RefreshCache method with OLEDB and C# or VB.net, I'd be forever grateful. I believe the issue is it's looking for an ADO connection to be passed in, not an OLEDB connection.
I not aware of ANY suggesting that some 5 second cache exits? Where did this idea come from????
Furthermore, if you have 5 users, then you not going to be able to update their cache, are you?
In other words, the issue of some cache for one user still not going to solve or work with mutli-users anyway, is it?
The simple matter is if you load up a form with 100 reocrds, and then other users are ALSO working on that 100 rows, then all users will not see other changes until such time you tell access to re-load the form.
You can do this with a me.Refresh in the form, and then it will show changes made by other users (or even your c# code!!!).
However, that not really the soluion here.
How does near EVERY system deal with this issue?
Answer:
You don't, you "design" the software to take the user work flow into account.
So, in place of loading up a form with 100 rows of data? (which you should not, unless SUPER DUPER reason exists for doing that).
The you provide a UI in which the user FIRST searches for whatever it is they want to work on.
In other words, say you just booked a user on a tour. Now, they call the office back, and want to change some details of that tour. But, a different tour staff might pick up the phone. So, now a 2nd user opens the tour?
So, you solve that issue by NOT loading all the tours into that form in the first place.
you provide a search screen, so they can search for the user, find the user, maybe type in a invoice number or whatever.
You display the results in a pick list, and then launch the form to the ONE record (and perhaps detail records from child tables).
So there no concpet of a cache in Access anymore then there is in c#.
However, if you load up a datatable in c#, and then display that data?
Well, what about the other users on that system. They will not see changes to that data ANY MORE then the current access form.
So, if you want to update some data in c#? Then fine, but you need/want to do two things:
First, before you call any c# code that may update the current form reocrd? You need to FORCE a data save of that current record BEFORE you call any code, be it VBA code, or c# code that going to update that current reocrd the user is working on.
You can in Access save the current reocrd in MANY different ways, but the typical approach is:
' single record save - current record
if me.dirty then me.dirty = false
' VBA or c# code goes here.
' optional refresh the current form to reflect changes
me.Refresh
So, in most cases, it is the "design" of your software that will solve this issue.
For example, in the tour example, or in fact ANY system, the user can't work, can't update, and can't do their job UNLESS they first find/search and have a means to bring up that form + record data in the first place.
So, ANY typical good design will:
Ask the user for that name, invoce number or whatever.
Display the results of the search, and THEN allow the user to pick the record/data to work on. When they are done, they close that form and are RIGHT BACK to the search form to do battle with the next customer or task or phone call or whatever.
So, a search form might look like this:
In above, I typed in smi, and then displayed a pick list.
The user can further type in say part of the first name, and thus now get this:
So, maybe they type in a invoice number, customer number, booking number or whatever.
So, you display the results, and then they can select the row or "thing" to work on.
thus, we click on the row (or above glasses button), and then jump to the ONE record.
so, the user does whatever they have to do with the customer. Now, when done, they close the ONE thing, the ONE main reocrd.
This not only saves the data (so others in the office can now use that booking data), but it also means the data is saved. and they are NOW right back at the search screen, ready to do battle with the next customer.
So, not only does this mean we have a VERY bandwith friednly design (we only pull the one main reocrd into that form), but it also is better for work flow.
The Access form's cache thus becomes a non issue, since we only dealing with the one record.
And as I pointed out, if the system is multi-user, then you NOT going to be able to udpate and deal with multiple users cached data anyway, are you?
Think of ANY system you EVER used from a software point of view.
When you use google, does it download the WHOLE internet, and then you use ctrl-f to search megs and megs of data in the browser?
Nope!
you search first, get a list of that search, and THEN pick one!!
And when that list is display, maybe others on the internet are udpateing, and add new data - but if that was cached in your browser, then it would not work!!!
And same goes for a desktop accounting system. You don't load up all accounts, and THEN have the user go ctrl-f to search all the data. You search for the customer, invoice number and PICK ONE to work on.
And it does not make sense to load up a form with 1000 customers, and then go ctrl-f to find that customer. Same goes for a instant banking machine. It does not download ALL customers and THEN let you search. It asks you FIRST to get what you need. So, be it browser based, desktop based, or JUST ABOUT ANY software you use?
You quite much elminate the cache issue, since not pre-loading boatloads of data, but asking and letting the user search for the data they need.
So, in regards to the Access form data and cache?
If you are on a form, and call VBA code, or c# code or whatever?
If that code update the current form, you have NO MORE OR LESS of a issue when calling VBA code, or c# code!!!! If that code updates the current form, and the reocrd is dirty (has pending edits), then you get that message about the current form's reocrd having been udpated by another user!!!
So, your cache issue does NOT IN ANY WAY exist MORE or LESS as a issue in typical Access software.
As a genreal rule, if you are on a form with pending edits, and say want to pop up some form to edit releated data?
You have to ensure that pending edits are SAVED before you launch an form that can edit the same data, or run code that can/may edit that data.
As a result, ZERO cache issues should exist, and they no more or no less exist when calling sql or VBA update code in a form then calling some c# code from that form.
So, write the pending update for that form.
Then run your VBA, SQL, or c# code.
And then do a me.Refresh to display any changes made by those external routines.
there is no documetjion, or ANY article I can find that suggests some kind of 5 seocnd cache or update - it is a urban myth, and your software challenge here in regards to use c# or VBA, or even SQL server stored procedures?
They are all the same issue, and I dare say that often access is used as a front end to SQL server, and ALL OF the SAME issues exist when using SQL server with ms-access.
I have the following issue.
In the past, we have added some fields to transaction IH09.
However last year we migrated to HANA and a lot of programs were updated in the process including the program RIMARA20, which is the program behind IH09.
IH09 has worked fine with the added fields were seen.
Last week I was asked to add another field and I did it however although the field catalog has the new field, it is not shown in the output of IH09.
I have debugged the code countless times trying to figure out what is going on but I don't know what happens.
This program internally uses the function REUSE_ALV_GRID_DISPLAY.
We still have the former SAP environment and I tried to make exactly the same enhancement with this new field and I can see it as expected.
In other words; the same field, the same data element, and the same enhancement in both environments but in the HANA instance I cannot see it.
I'm truly frustrated because I see the new field in the field catalog but I can't make it visible in the report.
Any advice on this issue?
Sounds like the REUSE_ALV_GRID_DISPLAY has "remembered" the old catalog
Did you try
to reset the layout / add the missing field?
We have an issue with a calculation field not always working. (We have about 5 calculation fields that are similar so same applies to all).
See in the attached example.
The Textbook item has a calc field 'Set Text'. One of the values in there is showing 'Null'.
Most of the time this works fine but in about 8% of the cases it has this null error.
See the other screenshot of the Aggregator item. The field that is being referenced there is correct.
To fix it we have to manually remove the relationship between the two items and then put it back again, to force the calculation to recalculate. This fixes the issue. So the problem does not seem to be the formula but rather the calculation being performed incorrectly.
The Aggregator items are created and joined to the Textbook item through the API.
This is a big issue for us as we use this value to post to our website so the manual fix is not a practical solution.
Podio support please advice what the issue is here
Aggregator
Null error
Without knowing too many details about your specific workflow and structure, let me offer some ideas:
Podio calculation fields can begin to act in strange ways if the original field is being referenced by calculation fields too many levels deep. What I mean by this is that if the original field that shows the Paper Code (or whatever that int showing null is) is being referenced through too many calculation fields, it can begin to cause problems in cases of updating and adding new items. This is most definitely a bug I've encountered with large-scale systems, but there are workarounds.
First off, I see that you have all of the pertinent non-concatenated fields at the bottom of your Aggregator app. Try manually concatenating those fields in the Textbooks app (where you're doing the result.push) instead of referencing the Web course text calc field exclusively.
If that doesn't work and you're proficient with the API, a longer workaround could be that after the automation runs, you wait a sufficient time (say 30 seconds) until the calculation field updates, have the API check for the text "null" in the calculation field, and refreshes the relationship fields if null is found.
I have list of records in TIBDataSet (Embarcadero Delphi) and I need to locate and modify one record in this list. There is chance that underlying database record has been changed by other queries and operations since TIBDataSet had been opened. Therefor I would like to call RefreshSQL for this one record (to get the latest data) before making any changes and before making post. Is it possible to do so and how?
I am not concerned about state of other records and I am sure that the record under consideration will always be updated and those updates will be commited before I need to changes this record from TIBDataSet.
As far as I understand then RefreshSQL is used for automatic retrieve of changes after TIBDataSet has posted upates to database. But I need manual (explicit) retrieval of the latest state before doing updates.
Try adding a TButton to your form and add the following code to its OnClick handler:
procedure TForm1.btnRefreshClick(Sender: TObject);
begin
IBQuery1.Refresh; // or whatever your IBX dataset is called
end;
and set a breakpoint on it.
Then run your app and another one (e.g. 2nd instance of it) and change a row in the second app, and commit it back to the db.
Navigate to the changed row in your app and click btnRefresh and use the debugger to trace execution.
You'll find that TDataSet.Refresh calls its InternalRefresh which in turn calls TIBCustomDataSet.InternalRefresh. That calls inherited InternalRefresh, which does nothing, followed by TIBCustomDataSet.InternalRefreshRow. If you trace into that, you'll find that it contructs a temporary IB query to retrieve the current row from the server, which should give you what you want before making changes yourself.
So that should do what you want. The problem is, it can be thoroughly confusing trying to monitor the data in two applications because they may be in different transaction states. So you are rather dependent on other users' apps "playing the transactional game" with you, so everyone sees a consistent view of the data.
I have just done a quick search and nothing too relevant came up so here goes.
I have released the first version of an app. I have made a few changes to the SQLite db since then, in the next release I will need to update the DB structure but retain the user's data.
What's the best approach for this? I'm currently thinking that on app update I will never replace the user's (documents folder, not in bundle) database file but rather alter its structure using SQL queries.
This would involve tracking changes made to the database since the previous release. Script all these changes into SQL queries and run these to bring the DB to the latest revision. I will also need to keep a field in the database to track the version number (keep in line with app version for simplicity).
Unless there are specific hooks, delegate methods that are fired at first run after an update I will put calls for this logic into the very beginning of the appDelegate, before anything else is run.
While doing this I will display "Updating app" or something to the user.
Next thing, what happens if there is an error somewhere along the line and the update fails. The DB will be out of date and the app won't function properly as it expects a newer version?
Should I take it upon myself to just delete the user's DB file and replace it with the new version from the app bundle. OR, should I just test, test, test until everything is solid on my side and if an error occurs on the user's side it's something else, in which case I can't do anything about it only discard the data.
Any ideas on this would be greatly appreciated. :)
Thanks!
First of all, the approach you are considering is the correct one. This is known as database migration. Whenever you modify the database on your end, you should collect the appropriate ALTER TABLE... etc. methods into a migration script.
Then the next release of your app should run this code once (as you described) to migrate all the user's data.
As for handling errors, that's a tough one. I would be very weary of discarding the user's data. Better would be to display an error message and perhaps let the user contact you with a bug report. Then you can release an update to your app which hopefully can do the migration with no problems. But ideally you test the process well enough that there shouldn't be any problems like this. Of course it all depends on the complexity of the migration process.