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 work with VBA in MS Access databases. I'd like to be able to log when files are saved, modified or deleted without having to update the existing code to do the logging when the pertinent events take place. I want the time, location and the name of the file.
I found a good example here: when file modified
However, it only allows for monitoring a particular location (path). I want to be able to log regardless of where the save, modify or delete takes place. I'm only allowed to program in the MS Office environment in this situation. It seems as though using the Windows API is going to be how this task will be achieved. However, I don't have much experience working with the API. Is there an easier way to achieve what I want that doesn't involve using the API?
Have you worked with After_Updates or After_Insert macros? Also, is your application split? Meaning there's a front-end and a back-end of the database. You can create a separate table that mirrors that table that you need to track changes for. Every time a table is updates, run a macro that inserts a row to that table.
I'm assuming you're saving files to the database. If that's the case, add a after_update or after_insert macro that can keep track of when then files are being modified or added to the table.
I created a time recording program in vb.net with a sql-server as backend. User can send there time entries into the database (i used typed datasets functionality) and send different queries to get overviews over there working time.
My plan was to put that exe in a folder in our network and let the user make a link on their desktops. Every user writes into the same table but can only see his own entries so there is no possibility that two user manipulate the same dataset.
During my research i found a warning that "write contentions between the different users" can be occur. Is that so in my case?
Has anyone experience with "many user using the same exe" and where that is using datasets and could give me an advice whether it is working or what i should do instead?
SQL Server will handle all of your multi-user DB access concerns.
Multiple users accessing the same exe from a network location can work but it's kind of a hack. Let's say you wanted to update that exe with a few bug fixes. You would have to ensure that all users close the application before you could release the update. To answer you question though, the application will be isolated to each user running it. You won't have any contention issues when it comes to CRUD operations on the database due to the network deployment.
You might consider something other than a copy/paste style publishing of your application. Visual Studio has a few simple tools you can use to publish your application to a central location using ClickOnce deployment.
http://msdn.microsoft.com/en-us/library/31kztyey(v=vs.110).aspx
My solution was to add a simple shutdown-timer in the form, which alerts users to saving their data before the program close att 4 AM.
If i need to upgrade, i just replace the .exe on the network.
Ugly and dirty, yes... but worked like a charm for the past 2 years.
good luck!
I would like to maintain a list of all of the strings entered into a ComboBox across all uses of an application on a given PC, for use as the AutoCompleteSource for that ComboBox, i.e., I enter in "Fred" in the ComboBox, commit the data, close the application, reopen the application, reopen the ComboBox, type "F", receive the suggestion "Fred".
Assuming I already have in place code to create and maintain such a list in memory and add new elements to it as they are entered into the ComboBox, etc., what do you suggest as the best way to save/load this data in between sessions?
Possible complicating factors: This application will be deployed via OneClick deployment, and will search for updates online every time the application starts. I would prefer that if the application updates, the list still exists after the update has completed.
Thanks for your help.
Depending on your access level, storing them in a flat file seems the easiest choice.
To read the list of options, read the file and return an empty list if it isn't there.
To add an item, create the file if it isn't there and then append the option.
That means you don't send an empty file with your updates, so the data survives.
It appears that you could put the file in the Data Directory or use Isolated Storage.
I would simply use the built in User Settings (My Project > Settings > Pick "User" for the scope of the setting); they are extremely easy to retrieve and update. And although I haven't done extensive testing, I believe they remain through application updates.
Does your application use a database? If so, that would be another option for permanent storage.
Edit
If you decide to go with the User Settings option, here is a new question that addresses an issue with those settings and ClickOnce.
Part of the setup routine for the product I'm working on installs a database update utility. The utility checks the current version of the users database and (if necessary) executes a series of SQL statements that upgrade the database to the current version.
Two key features of this routine:
Once initiated, it runs without user interaction
SQL operations preserve the integrity of the users data
The goal is to keep the setup/database routine as simple as possible for the end user (the target audience is non-technical). However, I find that in some cases, these two features are at odds. For example, I want to add a unique index to one of my tables - yet it's possible that existing data already breaks this rule. I could:
Silently choose what's "right" for the user and discard (or archive) data; or
Ask the user to understand what a unique index is and get them to choose what data goes where
Neither option sounds appealing to me. I could compromise and not create a unique index at all, but that would suck. I wonder what others do in this situation?
Check out SQL Packager from Red-Gate. I have not personally used it, but these guys make good tools overall and this seems to do what you're looking for. It let's you modify the script to customize the install:
http://www.red-gate.com/products/SQL_Packager/index.htm
You never throw a users data out. One possible option is to try and create the unique index. If the index creation fails, let them know it failed, tell them what they need to research, and provide them a script they can run if they find they have a data error that they choose to fix up.