Two questions about MCM’s article
Fundamentals of Multi-User Programming in PowerBASIC
1. MCM describes two forms of record locking: physical and logical. In physical locking the network O/S locks the record (per program command). The lock is detected by other users when they try to access the record and get an Error. In logical locking each record has an extra one byte field. The program sets this to “L” to lock the record. Other users check this byte to determine when the record is locked.
Question: In logical locking if the “locker” user terminates abnormally his lock stays in place. In case a lock gets stuck like that, as pointed out in MCM’s article, there needs to be an administrative way of removing locks. How about physical locks: what happens to a physical lock if the computer of the user who set it crashes? If the lock stays (for any great length of time) how would another program remove it?
Question: Why provide two different methods of locking records? What are the advantages and disadvantages of each?
2. Per MCM’s article, the data file, say Customer.txt, is opened with
OPEN "Customer.txt" FOR RANDOM ACCESS READ WRITE LOCK SHARED AS CustFileNo LEN = LEN(DataRecord)
When is the file open and closed? Can the program open it when the user starts the program and close it only when the user closes the program, keeping the file open all day long? (After all, it’s SHARED.) Or should the program, each time a user wants to access a record, open the file then close it right after the user is done with the record?
MCM’s example code just does one edit of a given record, but you could put a do-loop around it. So the above question is: put OPEN before DO and CLOSE after LOOP, or OPEN after DO and CLOSE before LOOP ?
End of questions.
MCM’s example code doesn’t check if the record got changed by someone else while the user was typing. This check has been added in a new version of MCM’s code below. If another user had changed the field by the time he tries to save his work, he gets a message that it cannot be saved. (Of course that situation could be handled another way.) Other changes include:
1. PBCC instead of PBDOS.
2. A loop so you can edit several records without restarting the program.
3. A more persistent GET, trying several times each fifth of a second instead of just twice after half a second.
4. Closing the file. (MCM’s original code is missing a CLOSE statement.)
I put OPEN outside the do-loop.
Fundamentals of Multi-User Programming in PowerBASIC
1. MCM describes two forms of record locking: physical and logical. In physical locking the network O/S locks the record (per program command). The lock is detected by other users when they try to access the record and get an Error. In logical locking each record has an extra one byte field. The program sets this to “L” to lock the record. Other users check this byte to determine when the record is locked.
Question: In logical locking if the “locker” user terminates abnormally his lock stays in place. In case a lock gets stuck like that, as pointed out in MCM’s article, there needs to be an administrative way of removing locks. How about physical locks: what happens to a physical lock if the computer of the user who set it crashes? If the lock stays (for any great length of time) how would another program remove it?
Question: Why provide two different methods of locking records? What are the advantages and disadvantages of each?
2. Per MCM’s article, the data file, say Customer.txt, is opened with
OPEN "Customer.txt" FOR RANDOM ACCESS READ WRITE LOCK SHARED AS CustFileNo LEN = LEN(DataRecord)
When is the file open and closed? Can the program open it when the user starts the program and close it only when the user closes the program, keeping the file open all day long? (After all, it’s SHARED.) Or should the program, each time a user wants to access a record, open the file then close it right after the user is done with the record?
MCM’s example code just does one edit of a given record, but you could put a do-loop around it. So the above question is: put OPEN before DO and CLOSE after LOOP, or OPEN after DO and CLOSE before LOOP ?
End of questions.
MCM’s example code doesn’t check if the record got changed by someone else while the user was typing. This check has been added in a new version of MCM’s code below. If another user had changed the field by the time he tries to save his work, he gets a message that it cannot be saved. (Of course that situation could be handled another way.) Other changes include:
1. PBCC instead of PBDOS.
2. A loop so you can edit several records without restarting the program.
3. A more persistent GET, trying several times each fifth of a second instead of just twice after half a second.
4. Closing the file. (MCM’s original code is missing a CLOSE statement.)
I put OPEN outside the do-loop.
Code:
' MULTIA.bas
' Bare-bones multiuser customer file maintenance.
' Original PDOS by Michael Mattias, October 1995.
' Here converted to PBCC with minor improvements.
#Compile Exe
#Dim All
'------------------------------------------------------------------
' Constants
%PhysicalLocking = -1 'true (compiler option)
%LogicalLocking = Not %PhysicalLocking
%PermissionDenied = 70 'error code
'------------------------------------------------------------------
Type CustomerRecord ' three fields, not counting AcctNo & Record Status
AcctNo As Long
AcctName As String * 30
StreetAddress As String * 30
CityStateZip As String * 30
RecordStatus As String * 1 ' used if %LogicalLocking: "L" or " "
End Type
'------------------------------------------------------------------
Global ReadDelay As Single
Global CustFileNo As Long
'==================================================================
' The general purpose subroutines from here to the next double line
' separator could be placed in an include file.
' Read a record, test if locked.
Sub ReadCustomerRecord (RecordNo As Long, DataRecord As CustomerRecord, Stat As Long)
Local count As Long
On Error Resume Next
count = 0
Do
Stat = 0
ErrClear
Incr count
Get CustFileNo, RecordNo, DataRecord
Stat = Err ' record the error
If Stat = 0 _ ' good read
And %LogicalLocking _ ' logical locking
And DataRecord.RecordStatus = "L" Then ' locked by another user
Stat = %PermissionDenied
End If
If Stat = %PermissionDenied Then ' wait and retry
Sleep ReadDelay
End If
Loop Until stat = 0 Or count > 3
' now Stat contains success (0) or failure.
End Sub
'------------------------------------------------------------------
' Lock a record.
Sub LockCustomerRecord (RecordNo As Long, Stat As Long)
Local ReadStatus As Long
Local DataRecord As CustomerRecord
On Error Resume Next
If %PhysicalLocking Then ' use PB and O/S services
Lock CustFileNo, RecordNo
Stat = Err
Else
ReadCustomerRecord RecordNo, DataRecord, ReadStatus
' get current copy of the record
If ReadStatus Then ' some error in the read - maybe locked!
Stat = ReadStatus ' return it to the main program
Else
DataRecord.RecordStatus = "L" ' mark the disk record "locked"
Put CustFileNo, RecordNo, DataRecord ' and store it
Stat = Err ' could get error on PUT!
End If
End If
End Sub
'------------------------------------------------------------------
' Unlock a Record.
Sub UnlockCustomerRecord (RecordNo As Long, Stat As Long)
Dim DataRecord As CustomerRecord
On Error Resume Next
If %PhysicalLocking Then ' use PB and O/S services
UnLock CustFileNo, RecordNo
Stat = Err
Else
ReadCustomerRecord RecordNo, DataRecord, Stat
If Stat = 0 Then
DataRecord.RecordStatus = " " ' clear out the "L"
Put CustFileNo, RecordNo, DataRecord
Stat = Err
End If
End If
End Sub
'==================================================================
Function PBMain
Local DataRecord As CustomerRecord
Local SaveRecord As CustomerRecord ' not used here
Local CustNum As Long
Local Stat As Long ' error code
Local NewAcctName As String
Print " Records are identified by Customer number,"
Print " starting at 1. Each record has three fields,"
Print " but here you can only update one of them:"
Print " AcctName."
Print
ReadDelay = .2 ' global constant, in seconds
CustFileNo = FreeFile ' global constant
Open "Customer.txt" For Random Access Read Write Lock Shared _
As CustFileNo Len = Len(DataRecord)
Do ' loop through given customer numbers
Input " Enter Customer Number to view/update ", CustNum
If CustNum = 0 Then Exit Do
' read it
ReadCustomerRecord Custnum, DataRecord, Stat
If Stat = %PermissionDenied Then
' this will never happen using instances of this program,
' but it might happen in a version that had to lock the
' whole file for a time
Print " Can't update this customer - record in use"
ElseIf Stat <> 0 Then
Print " Error on GET, # "; Stat
Else
Print " AcctName: "; DataRecord.AcctName
Print " StreetAddress: "; DataRecord.StreetAddress
Print " CityStateZip: "; DataRecord.CityStateZip
Print
' save the image used by operator
SaveRecord = DataRecord
' get the new data from the user
Line Input " Enter new AcctName: ", NewAcctName
If NewAcctName = "" Then Iterate Do
' lock the record
LockCustomerRecord CustNum, Stat
' get a current copy of it
If Not Stat Then
ReadCustomerRecord Custnum, DataRecord, Stat
End If
' check if this is the same as it was when we started
If Not Stat Then ' OK on the LOCK and READ
If DataRecord.AcctName = SaveRecord.AcctName Then
' store the record back to disk with new data
DataRecord.AcctName = NewAcctName
Put CustFileNo, CustNum, DataRecord
' unlock the record
UnlockCustomerRecord CustNum, Stat
' tell user successful
Print " Customer file updated."
Else
' unlock the record
UnlockCustomerRecord CustNum, Stat
' tell user he didn't type fast enough
Print " Someone else edited this record while you were typing."
End If
End If
If Stat Then
Print " Error on file access."
End If
Print
End If
Loop
Close CustFileNo
End Function
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