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.
Consider for example this modified Simple TCP sample program:
How can I display the current state of the program like
Wait for Connection
Connected
Connection terminated
on the frontpanel, depending on where the "data flow" currently is.
The easiest way to do this is to place a string indicator on your front panel and write messages to a local variable of this indicator at each point where you want to see a status update.
You need to keep in mind how LabVIEW dataflow works: code will execute as soon as the data it depends on becomes available. Sometimes you can use existing structures to enforce this - for example, if you put a string constant inside your loop and wire it to a local variable terminal outside the loop, the write will only happen after the loop exits. Sometimes you may need to enforce that dataflow artificially, for example by placing your operation inside a sequence frame and connecting a wire to the border of the sequence: then what's inside the sequence will only happen after data arrives on that wire. (This is about the only thing you should use a sequence for!)
This method is not guaranteed to be deterministic, but it's usually good enough for giving a simple status indication to the user.
A better version of the above would be to send the status messages on a queue or notifier which you read, and update the status indicator, in a separate loop. The queue and notifier write functions have error terminals which can help you to enforce sequence. A notifier is like the local variable in that you will only see the most recent update; a queue keeps all the data you write to it in the right order so would be more suitable if you want to log all the updates to a scrolling list or log file. With this solution you could add more features: for example the read loop could add a timestamp in front of each message so you could see how recent it was.
A really good solution to this general problem is to use a design pattern based on a state machine. Now your program flow is clearly organised into different states and it's very easy to add in functionality like sending a different message from each state. There are good examples and project templates for these design patterns included with recent versions of LabVIEW.
You should be able to find more information on any of the terms in bold in the LabVIEW help or on the NI website.
I have created a database/app where a report is created when a particular button is clicked. just now, two people managed to hit the button at exactly the same time, which caused all sorts of not-good.
Is there a way to make a button invisible across instances once it's clicked by one person? Or some way to lock the database so nothing can be done until the person who clicked first is done?
I have a solution (basically, a global check variable that stops the report creation) but now I want to know if either of the other two options can be done.
It would really help to know more about your architecture here. What database? What language have you written your application in? Concurrent reading is usually an important and basic feature of most multi-user databases.
Seconding Daniel Cook's general notion, maybe explicating a bit: don't have the button run the report directly. Have it run a little subroutine that first checks a special purpose table where you represent report "runs" with a new record that has a start date-time and an end date-time. If there is a record sitting in the table with no (null) end-date, then the report must still be running, therefore, do NOT begin report, turn off button instead. Else, insert into that same table and then start running the report. Add to this a periodic, not-too-frequent callback on that button to perform the same check, and you've got something that comes close, but isn't "realtime", but should work in most architectures (not knowing anything about session management capabilities).
Here's what I did:
If DLookup("PayLock", "table", "pkID=1") Then 'it's locked - exit
MsgBox "Someone else has already started the pay process.", vbOKOnly
Exit Sub
Else
blah blah blah......
The "PayLock" field in the table holds the check variable. After "Else" comes the actual code to run when the button is clicked.
Just FYI, since they were asked:
it is split database
there are multiple users
yes, the report just reads data and exports it into an excel spreadsheet.
It looks like this is the only solution, which works, but seems inelegant. I keep discovering that the way I get around my lack of knowledge is the actual way to do it...
I'm doing a db in access 2002 and I've had some problems with the locking shared mode. I have an app that has a lot of programs and almost 10 users logged on. And one of things that the app do is open a report, that user choose the program and the number (usually has 4 numbers for each program), and before the report open, I open it in hidden mode and I edit the report for current program/number, with this rotine I just have one report that may turn in hundreds... saving memory and optimizing it. Then I save it and open it again in view mode.
But the problem is when has other person using the app, so the access can't edit and save report... just in exclusive mode!
Has a cmd in vba to allow momentarily changes in shared mode? I don't know, like freeze all user, save and then unfreeze them?
Or any other suggestion?
About create a Front-End/Back-End I think that is impracticable, cause is a beta version and I have to update it often and I already tried to do this also, but it's became too slow... I splitted then in a database(just tables) into network and front-end with all querys, forms, reports and linked tables in local PC, but it really became tooo slow. If someone can help me let it faster would solve my problems too
I splitted the db and I'm trying optimize it. I read a lot about it on the web and I changed all Access setting that I saw that need to be changed and now I get a faster program. But slower than with a single app.
But now there are just fell Forms that making my app slow.
For example I have some Forms that always when I close it I spends a long time waiting it to close ;(
So I realised that this Forms are always saving before close. And always that a form need to save(with linked tables taht are in the network) waste a lot of time, so I need to avoid this.
But I didn't get it so far...
I realise that this forms are saving because in form_open I hidde some columns(that are different for each program) and edit it caption. And then if I need to close form, it saves and waste this such time!
How could I hidde/edit this columns withou need to save form? Or how could I close form without save structure changes?
I know how to do that with just a button, but these forms are datasheets and I can close it only in "X" Form button. And unfortunattely Access dont have BeforeClose event, and in a OnClose event it save before go to this sub!
See if you can make use of a WhereCondition with the DoCmd.OpenReport Method to avoid the need to modify your report's design at run time.
The WhereCondition is applied to your report's existing record source query as if it where written into that query's WHERE clause.
So if the record source for YourReport is ...
SELECT program_id, some_number, another_field
FROM YourTable;
... then this ...
DoCmd.OpenReport "YourReport", _
WhereCondition:="program_id = 7 AND some_number = 22"
would give you the same set of rows as would revising the record source to this ...
SELECT program_id, some_number, another_field
FROM YourTable
WHERE
program_id = 7
AND some_number = 22;
This advantage of this approach, if you can make it work for your situation, is that you would no longer need exclusive access to the db since you're not actually changing the report's design.
The related issue about beta status making it impractical to split the application is something you should re-consider carefully. Splitting ensures you can easily preserve the data in the BE when you roll out changes to the FE application. Even if you've come up with another method to avoid losing data when you change versions, that method can not be simpler than segregating the data into a BE file.
And when you split the application, each user should get their own copy of the FE file which is stored locally on their machine's hard drive; those FE files will contain links to the tables in the BE file which is stored on a file share.
Keeping the users' FE applications updated as you release new versions is a problem which has been solved. For example, see Tony Toews' Auto FE Updater. And you can find other approaches by searching the web.
If your concern is performance with a split application, check Tony's Microsoft Access Performance FAQ.
Merged with Kick users out of an application.
I have written an application that is used company wide. A lot of users have the tendency to leave the application open 24-7. This application is under development constantly as the Marketing department's needs change. Most changes are minor but often times need to be implemented quickly. Salesmen are often away from their desks or on the phone, so an intercom announcement does not always do the trick. The application is housed on a network file share and shortcutted to.
What I envision is something like a message popping up something like "The program will exit for an update in five minutes, please save your changes now." Is there a way to display this message and kick them from the exe remotely? I thought of maybe just having a database value that it checks periodically to see if an update is pending, but that seems like a cheap hack to me.