Duplicate logons

In z/OS® V1R4, JES2 stopped preventing duplicate TSO/E logons. In z/OS V2R1, JES3 can be configured to allow duplicate TSO/E logons. See the DUPLOGON keyword for the OPTIONS statement in z/OS JES3 Initialization and Tuning Reference.

As a result, duplicate instances of a given userid can log on at the same time to different systems, even in the same JES2 MAS or JES3 JESplex, depending on the scope of the SYSIKJUA enqueue. If the enqueue has a scope of SYSTEM, multiple TSO/E logons are allowed. This configuration is supported, although users should note the following restrictions:

Messages sent to the users who are logged on more than one system at a time might not go to the expected instance of the user.
Logon reconnect will only allow users to reconnect to an instance of their userid on the same system where they attempt to log on.
ISPF profile sharing or changes to default ISPF data set names might be needed to avoid errors in ISPF with multiple logons. See z/OS ISPF Planning and Customizing for more details about configuring ISPF to support multiple logons.

To prevent duplicate logons, take the following steps:

Merge all existing versions of SYS1.UADS into a single version of the data set.
Modify the default RNLs:
1. Delete the SYSDSN entry for SYS1.UADS from the SYSTEMS exclusion RNL. See Global resource serialization for more details on treating SYS1.UADS as a global resource.
2. Add SYSIKJUA as a generic qname in the SYSTEM inclusion RNL.

Alternate ways to prevent duplicate logons are:

Use JES2 Exit 44. See z/OS JES2 Initialization and Tuning Guide for more information about this exit.
Use JES3 Exit IATUX29 where the customer will need to add code to check for duplicate names. See z/OS JES2 Initialization and Tuning Guide for more information about this exit.