Random access on sequential file

niks_jude · Posted: Tue Jan 08, 2008 9:56 pm

If we have a file defined earlier as sequential file. Suppose I want to use for dynamic access. Assuming the file is sorted in the key positions. And I define it as organisation is indexed. Can random access be done on it ?

stodolas · Posted: Wed Jan 09, 2008 1:08 am

murmohk1 · Posted: Wed Jan 09, 2008 7:58 am

niks_jude · Posted: Wed Jan 09, 2008 6:59 pm

I tried this on mainframe and I got IEC161I 076-002. Seems the sequential file could not get opened as VSAM.

stodolas · Posted: Wed Jan 09, 2008 7:18 pm

Of course it can't get opened as a VSAM. It isn't a VSAM. Just like you can't open a database like a VSAM because it isn't a VSAM

Anuj Dhawan · Posted: Wed Jan 09, 2008 9:23 pm

niks_jude · Posted: Thu Jan 10, 2008 11:33 am

My idea was if a sequential file is sorted in key positions. then can be random access be done on it. Basically whether physical organisation has anything to do with what we define logically in a COBOL program. Just like we can use VSAM sequentially, is the other way round possible ?

stodolas · Posted: Thu Jan 10, 2008 6:29 pm

Move the data into a VSAM or DB table if you need random access.

Ajay Baghel · Posted: Thu Jan 10, 2008 6:59 pm

The organisation of the file sequential file has the characteristic that allows the records to be accessed in the order in which they are stored in the file. Here I am talking about the PS (physical sequential) file. The random and dynamic accesses are not allowed. Also START and DELETE operations can not be performed on it.

But an ESDS VSAM file can be accessed randomly using RBA in CICS.

Thanks,
Ajay

Anuj Dhawan · Posted: Thu Jan 10, 2008 7:01 pm

niks_jude · Posted: Fri Jan 11, 2008 11:24 am

Thanks it clears my doubt.

niks_jude · Posted: Fri Jan 11, 2008 12:06 pm

One more thing to add which should clear everyone on this topic -----

The main reason is that for random access an index is required. The index is the displacement corresponding to each key. This is not part of the sequential file organization that is why random access cannot be done on the sequential file.

mmwife · Super Moderator Joined: 30 May 2003 Posts: 1592

Hi niks_jude,

The main reason is that seq files were never designed for random access. That's why they call them PS (Physical Sequential) files.

You can define them as "indexed", but that doesn't make it so.

Douglas Wilder · Posted: Tue Jan 15, 2008 8:57 pm

It is too bad that on the mainframe this cannot be done. Some languages on PCs allow random access of sequential file by positioning to a record or byte position within the file and then doing a read. But if it cannot be done, there is not anything we can do about it. (or is there?)

dick scherrer · Posted: Wed Jan 16, 2008 3:55 am

Hello,

This goes back a long, long time and i have no reference material, but i seem to recall that one could issue a BDAM read against a PS (QSAM) file.

Douglas Wilder · Posted: Wed Jan 16, 2008 4:51 am

I found this on the WEB, is it correct?

dick scherrer · Posted: Wed Jan 16, 2008 9:08 pm

Hello,

I believe that is correct.

If one was so iniclined, i believe it could still be done in assembler.

As suggested in the quote, i'd recommend defining a vsam file and using that for random access.