Re: concurrency problem

sathish kumar shanmugavelu wrote:
> INSERT INTO rcp_patient_visit_monitor (
> entry_no, patient_id, visit_date, is_newpatient,
> visit_type, is_medical,
> is_review, is_labtest, is_scan, is_scopy, is_xray,
> weight, height)
> VALUES ((SELECT coalesce(max(entry_no)+1, 1) FROM
> rcp_patient_visit_monitor),?,current_timestamp,?,?,?,?,?,?,?,?,?,?)
>
> The point to note here is the select statement which gets the max
> entry_no and adds one to it and save the new value. entry_no is the primary
> key of the above table.
>
> Now i run the same program (different instance) from two systems, save
> the form simultaneously, only one entry is saved, in the other system the
> error says - duplicate key violates.

BEGIN;
LOCK TABLE ...
INSERT ...
COMMIT;

You'll need to handle possible errors where one client fails to get a
lock and times out. It won't happen often, but you do need to consider
the option.

--
Richard Huxton
Archonet Ltd