Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS 17 encounter data integrity issues. (This is particularly common when developers move from another database to Oracle, or vice versa, and neglect to take the differing concurrency mechanisms into account in their application.) Preventing Lost Updates One of the side effects of Oracle’s non-blocking approach is that if you actually want to ensure that no more than one user has access to a row at once, then you, the developer, need to do a little work yourself. Consider the following example. A developer was demonstrating to me a resourcescheduling program (for conference rooms, projectors, etc.) that he had just developed and was in the process of deploying. The application implemented a business rule to prevent the allocation of a resource to more than one person, for any given period of time. That is, the application contained code that specifically checked that no other user had previously allocated the time slot (as least, the developer thought it did). This code queried the SCHEDULES table and, if no rows existed that overlapped that time slot, inserted the new row. So, the developer was basically concerned with two tables: create table resources ( resource_name varchar2(25) primary key, ... ); create table schedules ( resource_name references resources, start_time date not null, end_time date not null, check (start_time < end_time ), primary key(resource_name,start_time) ); And, before making, say, a room reservation, the application would query: select count(*) from schedules where resource_name = :room_name and (start_time = :new_start_time) It looked simple and bulletproof (to the developer anyway): if the count came back as zero, the room was yours. If it came back as nonzero, you could not reserve the room for that period. Once I knew what his logic was, I set up a very simple test to show him the error that would occur when the application went live—an error that would be incredibly hard to track down and diagnose after the fact. Someone would be convinced it must be a database bug. All I did was get someone else to use the terminal next to him. Both he and the other person navigated to the same screen and, on the count of three, each clicked the Go button and tried to reserve the same room for about the same time—one from 3:00 pm to 4:00 pm and the other from 3:30 pm to 4:00 pm. Both people got the reservation. The logic that worked perfectly in isolation failed in a multiuser environment. The problem in this case was caused in part by Oracle’s non-blocking reads. Neither session ever blocked the other session. Both sessions simply ran the query and then performed the logic to schedule the room. They could both run the query to look for a reservation, even if the other session had already started to
18 CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS modify the SCHEDULES table (the change wouldn’t be visible to the other session until commit, by which time it would be too late). Since they were never attempting to modify the same row in the SCHEDULES table, they would never block each other and, thus, the business rule could not enforce what it was intended to enforce. The developer needed a method of enforcing the business rule in a multiuser environment—a way to ensure that exactly one person at a time made a reservation on a given resource. In this case, the solution was to impose a little serialization of his own. What he did was lock the parent row in the RESOURCES table prior to making a modification in the SCHEDULES table. That way, all modifications to the SCHEDULES table for a given RESOURCE_NAME value would be done one at a time. That is, to reserve a block of time for resource X, he locked the single row in the RESOURCES table for X and then modified the SCHEDULES table. So, in addition to performing the preceding count(*), the developer first performed the following: select * from resources where resource_name = :room_name FOR UPDATE; What he did here was to lock the resource (the room) to be scheduled immediately before scheduling it—in other words, before he queries the SCHEDULES table for that resource. By locking the resource he is trying to schedule, the developer ensures that no one else is modifying the schedule for this resource simultaneously. Everyone else must wait until he commits his transaction, at which point they will be able to see his schedule. The chance of overlapping schedules is removed. Developers must understand that, in the multiuser environment, they must at times employ techniques similar to those used in multithreaded programming. The FOR UPDATE clause is working like a semaphore in this case. It serializes access to the RESOURCES tables for that particular row—ensuring no two people can schedule it simultaneously. My suggestion was to make this logic a transaction API—that is, bundle all of the logic into a stored procedure and only permit applications to modify the data via this API. The code could look like this: create or replace procedure schedule_resource ( p_resource_name in varchar2, p_start_time in date, p_end_time in date ) as l_resource_name resources.resource_name%type; l_cnt number; begin We start by locking the single row in the RESOURCES table for the resource we want to schedule. If anyone else has this row locked, we block and wait for it: select resource_name into l_resource_name from resources where resource_name = p_resource_name FOR UPDATE; Now that we are the only ones inserting into this SCHEDULES table for this resource name, it is safe to look at this table:
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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