ESDS
An entry-sequenced dataset: a VSAM dataset where records are kept in the order they were added, with no key.
Also written entry-sequenced dataset, NONINDEXED
An ESDS, or entry-sequenced dataset, is a type of VSAM dataset that stores records in the order they arrive. New records always go at the end. There is no key, so you cannot look a record up by a value such as an account number.
Each record is found by its position, given as a relative byte address: how many bytes it is from the start of the dataset. A program can read the whole dataset in order, or jump straight to a record if it saved that address.
Records cannot be deleted from an ESDS, and a record can be updated only if its length stays the same. Programs that need to mark a record as removed usually set a flag in it instead.
These rules make ESDS a good fit for logs, audit trails and journals, where you only ever add to the end and read back in time order. It is defined with IDCAMS like other VSAM types, using the NONINDEXED option. If you need to find records by key, a KSDS is the usual choice.
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- VSAMA family of dataset types built for fast record access by key, used where applications need to look records up rather than read them in order.
- KSDSThe most common VSAM dataset type, where every record has a unique key used to find it directly.
- IDCAMSThe utility used to create, list, copy and delete datasets, and the standard tool for anything involving VSAM.
- Sequential datasetThe simplest kind of dataset: records stored one after another and normally read from start to finish.
- DEFINE CLUSTERAn IDCAMS command that creates a VSAM file, called a cluster. You give it a name, the kind of file, and the size of the key and the record.