Considerations for using the Segment Edit/Compression exit routine

Before using a Segment Edit/Compression exit routine, you should be aware several points.

The points you should be aware of include:

• Because your edit routine is executed as part of a DL/I call, if it abnormally terminates so does the entire IMS region.
• Your routine cannot use the operating system macros LOAD, GETMAIN, SPIE or STAE.
• The name of the Segment Edit/Compression exit routine must not be the same as the DBDNAME.
• Editing and compressing of each segment on its way to or from an application program requires additional processor time.
• If you are using the DEDB Alter utility to add a Segment Edit/Compression exit routine, the Segment Edit/Compression exit routine must be able to handle mixed compressed and non-compressed data in a DEDB database. If data is non-compressed, on read access, it does not expand the non-compressed data, and on write access, it compresses the data and writes out as compressed data. If data is compressed, on read access, it expands the compressed data, and on write access, it compresses the data and writes out as compressed data.

Depending on the options you select, search time to locate a specific segment can increase. If you are fully compressing the segment using key compression, every segment type that is a candidate to satisfy either a fully qualified key or data field request must be expanded or divided. IMS then examines the appropriate field. For key field qualification, only those fields from the start of the segment through the sequence field are expanded during the search. For data field qualification, the total segment is expanded. In the case of data compression and a key field request, little more processing is required to locate the segment than that of non-compressed segments. Only the segment sequence field is used to determine if this segment occurrence satisfies the qualification.

Other considerations can affect total system performance, especially in an online environment. For example, being able to load an algorithm table into storage gives the compression routine a large amount of flexibility. However, this can place the entire IMS control region into a wait state until the requested member is present in storage. It is suggested that all alternatives be explored to lessen the impact of situations such as this.