Under the standard catalog search order, when you are using aliases, you might find that certain catalogs become over used. For instance, if the catalog becomes too large, recovery might become excessively disruptive, or the catalog might be required to process an excessive number of requests, resulting in frequent enqueues. If these catalogs contain large numbers of data sets with the same high-level qualifier, you can alleviate the problem with the multilevel alias facility. Using multilevel aliases, you can have data sets with the same high-level qualifier cataloged in different catalogs.

For example, your installation might have a project, PROJECT1, which requires many data sets and applications. Perhaps there are large collections of PROJECT1 data sets that have the same second qualifier: TEST or PROD. There are also some miscellaneous PROJECT1 data sets with neither TEST nor PROD as the second qualifier.

If the alias search level is 2, then data sets are cataloged as follows:

In this example, programs being tested (TEST) are isolated from production programs and data (PROD) and other miscellaneous files. This isolation is desirable for data protection and availability. Backup and recovery of one catalog would not affect projects using the other catalogs.

The alias search level is specified in the SYSCATxx member of SYS1.NUCLEUS or LOADxx member of SYS1.PARMLIB (see Identifying the Master Catalog and Initial Configuration (SYSCATxx)). It can also be changed without an IPL using the MODIFY CATALOG,ALIASLEVEL operator command (see Changing the Multilevel Alias Search Level).

When you use an alias to select catalogs, the use of a single level catalog name can also affect the selection. When a catalog has a single level name, that name behaves as an alias. For example, if a catalog has the name of ICFCAT, data sets beginning with ICFCAT, such as ICFCAT.DATA1, ICFCAT.DATA2, and ICFCAT.catalog.DATA are found in the ICFCAT catalog if the CATALOG parameter is not used to limit the search. See Using LISTCAT in Examples for an example using the CATALOG parameter.

With the multilevel alias facility, this concept is extended to catalogs whose names have multiple levels. For instance, if MULTILEVELALIAS=2, and a catalog ICF.CAT exists, ICF.CAT will also behave as an alias. Data sets such as ICF.CAT.DATA1, ICF.CAT.DATA2, and ICF.CAT.catalog.DATA are found in the ICF.CAT catalog if no CATALOG parameter is used.

Using the multilevel alias facility and having short catalog names can influence generic searches for catalogs. The list of catalogs to be searched is more complex when a generic entry is not fully qualified. The generic portion of the name is searched first before all alias levels are satisfied. For example, if the name is SYS1.* and MULTILEVELALIAS=2, catalog management does not know which specific catalogs to select and selects all aliases and catalogs beginning with SYS1 and having two levels. For example, SYS1.CAT and SYS1.ALIAS could be selected. This list is then used to select the catalogs to be searched. Duplicates on the list are eliminated and then each catalog is searched for entries that match the search parameter. If SYS1.ALIAS points to ICF.CAT, then catalogs SYS1.CAT and ICF.CAT will be searched. SYS1.CAT is selected because its name matches the generic SYS1.*. ICF.CAT is selected because an alias that points to it, has a name that matches the generic SYS1.*.

Duplicate entries can appear if this alternate selection occurs. For example, if both SYS1.CAT and ICF.CAT contain an entry for SYS1.PROCLIB, SYS1.PROCLIB will be returned twice. SYS1.PROCLIB is returned once for SYS1.CAT where it was found, and once for ICF.CAT where it also occurs. You should determine which catalogs are involved because it might appear that you are receiving duplicate entries, but you can be receiving different entries with the same name from different catalogs.

In an environment with a multilevel alias level greater than 1, specifying a search such as ABC or ABC* may cause Catalog Management to orient a request to a different catalog. Currently, if catalog management fails to orient to a first catalog, it fails the request, even though it may have been a request for multiple catalogs.

For example, if a user specifies ABC, as in LISTCAT LEVEL(ABC) or ABC on ISPF option 3.4 with this search level, his request may be eligible to search 2 or more catalogs. If he doesn't have authority to access the first catalog, Catalog management fails the request because it can't orient to the first catalog.