TDBGrid doesn't update when multiple users are editing it - delphi

Question

I am developing an application which has a simple database. All of the functions are going well but when a user is editing the database from the program, the other user cannot see the content immediately. The other user needs to close the program and reopen it for the data to appear and its DBGrid be updated with those changes form the other computers. I am using Delphi 7 for this and ZeosLib to access my Firebird database. I tried using the refresh button on the DBNavigator but it doesn't work.
The components I used to connect to the database are:
ZConnection
ZQuery
DataSource
DBGrid
DBNavigator
This is the code for my ZConnection and ZQuery.
object ZConnection1: TZConnection
ControlsCodePage = cGET_ACP
UTF8StringsAsWideField = False
Connected = True
Port = 3051
Database = '192.168.254.254:test'
User = 'test'
Password = 'test'
Protocol = 'firebird-2.5'
Left = 96
Top = 8
end
object ZQuery1: TZQuery
Connection = ZConnection1
Active = True
SQL.Strings = (
'select * from "test"')
Params = <>
Left = 128
Top = 8
object ZQuery1ID: TStringField
FieldName = 'ID'
Required = True
Size = 8
end

Answer 1

Sounds like you're running afoul of ACID. This is a basic guarantee of SQL-style databases, that all database updates will be Atomic, Consistent, Isolated, and Durable, and is accomplished through transactions.
Specifically, you're having trouble with the Consistency and the Isolation, which ensure that an external viewer never sees an update before it's finished, even if that update contains more than one change. (The classic example is a bank transfer, which requires subtracting money from one account and adding it to another. If you only see one of these two actions but not the other one, you have bad data.)
You can think of a transaction as an independent view of the state of the database. Every database connection has its own transaction, and any changes it makes are invisible to anyone else (Isolated) until they Commit (finalize) the transaction. Depending on the transactions' isolation settings, they may remain invisible to other users even after that, if they have an ongoing transaction, until they commit their transaction and begin a new one. It sounds like your code isn't taking this into account.
If you need updates to become visible immediately, you'll want to ensure that the transaction's isolation mode is READ COMMITTED, and set up database events to send notifications to connected clients when various things get updated, so the clients can perform refreshes of their data. You'll also want to ensure that the user updates result in a Commit action right away, so that the isolated data will become available.
Since I don't use ZeosLib, I can't explain all the details of how you'll need to set this all up, but this is enough to get you on the right track.

Answer 2

I suggest that you add a timer to the form which displays the grid. Set the timer so that it fires its OnTimer event once a minute (or longer). In this event, close the query then reopen it. This way, everyone always gets current information (albeit a minute late).
with qWhatever do // this is the query which is connected to the grid
try
disablecontrols;
close;
open
finally
enablecontrols
end;

Answer 3

For a multi-user application, where clients need to receive notifications, one option is to use Firebird events to send a 'broadcast' message for every data change (SQL INSERT, UPDATE or DELETE).
Clients can 'register' (listen) for a specific message type, and whenever the Firebird server sends a message with this type, they will receive it, and run client application code, which in your case would refresh the user interface (grid).
While this can be a sufficient solution in many simple use cases, there are also some restrictions. I recently blogged about this topic here:
Firebird Database Events and Message-oriented Middleware
(I am author of middleware libraries for Delphi and Free Pascal)

Answer 4

I solved this problem by adding this before query.
IBDatabase1.Close;
IBDatabase1.Open;