This Tech Tip discusses a way to keep users from being able to login multiple times when connecting to 4D Server.

Three methods will be created:
1. isLoggedIn
2. trackUser
3. removeUser

Each of the three project methods must have the "Execute on Server" attribute enabled in the method properties.

isLoggedIn checks if the current user is already logged in. If the user is already logged in, the method returns True, otherwise False.

`Method: isLoggedIn `Parameters: `$1 - current username C_TEXT($1;$userName) C_BOOLEAN($0) C_LONGINT($found_at) If (Count parameters=1)   $userName:=$1   $found_at:=Find in array(<>usersLoggedIn;$userName)   $0:=($found_at#-1) End if

trackUser adds the current username to the interprocess array <>usersLoggedIn. This is done to keep track of all users logged in.

`Method: trackUser `Parameters: `$1 - current user C_TEXT($1;$userName) If (Count parameters=1)   $userName:=$1   APPEND TO ARRAY(<>usersLoggedIn;$userName) End if

Once a user quits the application, they need to be removed from the array <>usersLoggedIn. This can be done using the method removeUser.

`Method: removeUser `Parameters: `$1 - current user C_TEXT($1;$userName) If (Count parameters=1)   $userName:=$1   $found_at:=Find in array(<>usersLoggedIn;$userName)   If ($found_at#-1)     DELETE FROM ARRAY(<>usersLoggedIn;$found_at)   End if End if

Initialize the interprocess variable, <>usersLoggedIn, inside the database method On Server Startup.

`Database Method: On Server Startup ARRAY TEXT(<>usersLoggedIn;0)

isLoggedIn should be called in the database method On Startup. If the user is already logged in, quit the application, otherwise add the current user's username to the array <>usersLoggedIn by making a call to trackUser. Also, if the user is already logged in, <>userConflict is set to true to make sure the user is not removed in the database method On Exit.

`Database Method: On Startup C_TEXT($user) C_BOOLEAN(<>userConflict) $user:=Current user If (isLoggedIn ($user))   <>userConflict:=True   ALERT($user+" is already logged on!")   QUIT 4D Else   trackUser ($user) End if

When a user quits the application and <>userConflict is set to False, they need to be deleted from the array <>usersLoggedIn by calling the method removeUser.

`Database Method: On Exit If (Not(<>userConflict))   removeUser (Current user) End if

Note: The above code does not handle unexpected log offs, such as a crash.