From VSAM and IMS to relational
Moving data from VSAM files or IMS databases into relational tables means redesigning record layouts as tables, keys and relationships, and changing how programs read and write data. It is done to make data easier to query, share and govern.
Why move the data
VSAM files and IMS databases are fast and reliable, but their structure lives in the programs: the record layout is a COBOL copybook, and the meaning of each byte is known only to the code. Relational databases such as Db2 describe their structure in a catalog, so any authorised tool can query the data with SQL. Moving makes reporting, integration and governance easier. It is also a large change, so it is usually done application by application, not all at once.
Mapping VSAM records to tables
| VSAM or COBOL feature | Typical relational design |
|---|---|
| KSDS key | Primary key of the table |
| Alternate index | Secondary index on the table |
| Fixed fields in the record | Columns with proper data types (DECIMAL, DATE, CHAR) |
| OCCURS (repeating group) | A child table with one row per occurrence |
| REDEFINES or record-type byte | Separate tables per record type, or nullable columns |
| Dates held as numbers or text | DATE columns, after cleaning invalid values |
The mapping is a design job, not a mechanical copy. A record type indicator in byte 1 usually means the file actually holds several kinds of entity, and each deserves its own table.
Mapping IMS hierarchies
An IMS database is a tree of segments: a root such as CUSTOMER with children such as ACCOUNT and TRANSACTION. Each segment type normally becomes a table. The parent's key becomes a foreign key in the child table, because a child row no longer sits physically under its parent. Features such as logical relationships and secondary indexes need individual design decisions.
Changing the programs
The data move is only half the work. Every program that reads the file or issues DL/I calls must change. Common approaches:
- Rewrite access to SQL: programs use embedded SQL directly. Cleanest result, most code changed.
- I/O module: put all access to a file behind one called module first. Then only that module changes when the data moves. This is often a good refactoring step on its own.
- Transparency or emulation products: some vendors offer layers that let programs keep their VSAM or DL/I calls while the data sits in a relational database. They reduce code change but add a component to support and tune; evaluate performance carefully.
Performance and operations
Keyed VSAM reads are very cheap. The same access through SQL adds overhead, so design indexes for the real access paths and test with production-like volumes. Batch jobs that read a whole file sequentially may need redesign rather than row-by-row SQL. Backup, recovery, REORG and RUNSTATS processes also change, and operations teams need to be ready before cutover.
Common mistakes
Repeating groups and multiple record types end up as an unusable table. Design proper tables, keys and data types.
Sequential VSAM processing is very fast. Row-by-row SQL over millions of records can miss the batch window; test at production volumes.
Put data access behind one I/O module first, so the data move touches as little code as possible.
What you will see at work
- Data modellers and developers work together to turn copybooks into table designs.
- Developers often build an I/O module around a file as the first, low-risk step of a migration.
- DBAs set up indexes, REORG and RUNSTATS for the new tables and compare batch run times with the old file-based jobs.
Key terms
Check your understanding.
Take this lesson's quiz and save your progress. Free.