Defining Aliases for a User Catalog

To use a catalog, the system must be able to determine which data sets should be defined in that catalog. The simplest way to accomplish this is to define aliases for the catalog. Choosing Aliases and an Alias Search Level discusses general considerations for choosing aliases. Before defining an alias, carefully consider the effect the new alias will have on old data sets. A poorly chosen alias could make some data sets inaccessible.

You can define aliases for the catalog in the same job in which you define the catalog by including DEFINE ALIAS commands after the DEFINE USERCATALOG command. You can use conditional operators to ensure the aliases are only defined if the catalog is successfully defined. After the catalog is defined, you can add new aliases or delete old aliases.

Catalog aliases are defined in the master catalog, which contains an entry for the user catalog.

By default, the number of aliases a catalog can have is limited by the maximum record size for the master catalog. If the master catalog is defined with the default record sizes, there is a practical maximum of 3000 aliases per catalog, assuming the aliases are only for high-level qualifiers. If you use multilevel aliases, fewer aliases per catalog can be defined.
On a system at a z/OS® V1R13 level or higher, you can increase the number of aliases possible from 3000 to a theoretical limit of over 500,000 aliases depending on the alias name length. You can exploit this increased number of aliases by specifying the EXTENDEDALIAS enable feature on the MODIFY CATALOG command as follows:
```
F CATALOG,ENABLE(EXTENDEDALIAS)
```
Once a user catalog connector extension record is created, it will exist for the rest of the life of the user catalog connector. For example, when all the association entries in the user catalog connector extension record are deleted, the user catalog connector extension record will still exist with an empty association cell. Also, there will be no associated order for the association entries of the user catalog connector to be returned.

By default, EXTENDEDALIAS is disabled. You should only enable EXTENDEDALIAS to increase the number of possible catalog aliases when all systems in the sysplex are z/OS V1R13 or greater.
Note that you can disable this function using the MODIFY CATALOG command as follows:
```
 F CATALOG,DISABLE(EXTENDEDALIAS)
```

You cannot define an alias if a data set cataloged in the master catalog has the same high-level qualifier as the alias. The DEFINE ALIAS command fails with a "duplicate data set name" error. For example, if a catalog is named PAYROLL.TESTSYS.ICFCAT, you cannot define the alias PAYROLL for any catalog.