Now, consider another situation when both these users (John and Tom) have to update two text files. While an update is in progress, a GT.M LOCK should prevent the other user from LOCKing that file. In some cases, a deadlock occurs when both users cannot move forward because they do not release their current LOCKs before adding additional LOCKs.

A deadlock situation can occur in the following situation:

John           Tom 
LOCK +file_1   LOCK +file_2 
LOCK +file_2   LOCK +file_1

Here both the users are deadlocked and neither can move forward. Note that a deadlock situation does not actually block the underlying resource.

Let us now create this situation.

At the GT.M prompt of John, execute the following commands:

GTM>set file1="file_1.txt"
GTM>lock +file1
GTM>open file1:APPEND 
GTM>use file1 
GTM>write "John",!
GTM>close file1

Note that John has not released the LOCK on resource "file1".

At the GT.M prompt of Tom, execute the following commands:

GTM> set file2="file_2.txt" 
GTM> lock +file2
GTM> open file2:APPEND 
GTM> use file2 
GTM>write "Tom",!
GTM>close file2

Note that Tom has not released the LOCK on resource "file2".

Now, at the GT.M prompt of John, execute the following commands.

GTM>set file2="file_2.txt" 
GTM>lock +file2

The latter command attempts to acquire a lock on resource file2 that is already locked by Tom. Therefore, this results in a deadlock situation. Repeat the same process for Tom and attempt to lock resource file1.

Execute the following command at LKE prompt to view this deadlock situation.

LKE>show -all -wait 
file1 Owned by PID= 2080 which is an existing process 
Request PID= 2089 which is an existing process 
file2 Owned by PID= 2089 which is an existing process 
Request PID=2080 which is an existing process

This shows a deadlock situation where neither user can proceed forward because it is waiting for the other user to release the lock. You can resolve this situation by clearing the locks using the LKE CLEAR -PID command.

	Avoid using the LKE CLEAR command to clear a deadlock in a production environment as it may lead to unpredictable application behavior. Always use the MUPIP STOP command to clear a deadlock situation in your production environment. However, in a debugging environment, you can use LKE to debug LOCKs, analyze the status of the LOCK database and even experiment with LKE CLEAR.