APPN checkpointing data sets

These data sets are used when z/OS® Communications Server is defined as a network node or interchange node, and are required for the APPN checkpointing function. These data sets cannot be allowed to span multiple volumes.

SYS1.DSDB1
SYS1.DSDB2
SYS1.DSDBCTRL
SYS1.TRSDB

SYS1.DSDB1 and SYS1.DSDB2 contain APPN directory information that is used to initialize the directory database when z/OS Communications Server is restarted.

Directory database information is stored alternately between SYS1.DSDB1 and SYS1.DSDB2. The directory database information is written to one of the data sets whenever a MODIFY CHKPT TYPE=ALL or TYPE=DIR, HALT, or HALT QUICK command is issued.

Not all of the resources from the directory database are written to the data sets when there is a checkpoint. The resources that are written to the data sets are those that satisfy these requirements:

Targeted by a search
Have a dynamic entry type that is not registered
Updated within a period of time specified by the DIRTIME start option

The resources that are registered to the database at startup through resource registration and definition are not included in the checkpointed information.

SYS1.DSDBCTRL contains the current status of SYS1.DSDB1 and SYS1.DSDB2. It is read by z/OS Communications Server during initialization to determine whether SYS1.DSDB1 or SYS1.DSDB2 will be used to load the APPN directory database.

SYS1.TRSDB is required for checkpointing the network topology database. The information in this data set is used to initialize the network topology database whenever z/OS V2R1 Communications Server is restarted. The network topology database is written to this file whenever a MODIFY CHKPT TYPE=TOPO or TYPE=ALL, HALT, or HALT QUICK command is issued.

The APPN checkpointing data sets should be allocated and cataloged prior to z/OS Communications Server initialization. To prepare the APPN checkpointing data sets, do these tasks:

Specify the DD name for SYS1.DSDB1 as DSDB1, for SYS1.DSDB2 as DSDB2, for SYS1.DSDBCTRL as DSDBCTRL, and SYS1.TRSDB as TRSDB.
Specify these DCB subparameters for SYS1.DSDB1, SYS1.DSDB2, and SYS1.TRSDB:
```
RECFM=FB,LRECL=1000,BLKSIZE=any multiple of 1000,DSORG=PS
```
Specify these DCB subparameters for SYS1.DSDBCTRL:
```
RECFM=FB,LRECL=20,BLKSIZE=20,DSORG=PS
```

Rule: Do not modify any of the foregoing data sets.

Guidelines:

The DSDBCTRL is a fixed, 20-byte file; it requires a 20-byte block.
Regarding DSDB1 and DSDB2: Every thousand resources to be checkpointed occupies 35 logical records, or six 6KB blocks of space; the only resources to be checkpointed are the cache DLU entries found during the search.
z/OS Communications Server fails the initial load of the network topology database if the checkpointed data set of another node is used, or the SSCPNAME operand is changed between the two IPLs. Should the initial load fail, z/OS Communications Server can acquire the information dynamically using TDUs.