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POImplement pessimistic locking
 

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    <p>I'm interested in how I can implement pessimistic locking, with very specific behavior. (The reason I tagged the question with Sybase+Oracle+MSSQL, is because I'd be happy with a solution or "that's impossible!" for any one of them)</p> <p>What I want is this: 1 - be able to lock a row (so that process can later do update, but no other process can lock the row) 2 - when another process tries to lock same row it should get notification that record is locked - I don't want this process to hang (I believe simple timeout can be used here) 3 - when another process tries to read record, it should be able to read it the way it is currently in database (but I don't want to use dirty reads).</p> <p>The above 3 requirements are currently solved by application using shared memory - and performing record-locking outside database. I'd like to move the locking into the database.</p> <p>So far, I'm having conflicts between #1 and #3 - if I lock record by doing 'update ...' by updating a field to same value, than 'select' from another process hangs.</p> <p>Edit: I'm having some luck now with snapshot isolation level on MSSQL. I can do both the locking, and reads without using dirty reads.</p> <p>The reason I don't want to use dirty-reads, is that if a report is running, it might read multiple tables, and issue multiple queries. Snapshot gives me a consistent snapshot of the datatabase. With a dirty read, I'd have mismatching data - if there were any updates in the middle.</p> <p>I think Oracle has snapshot as well, so now I'm most interested in Sybase.</p>
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    1. COCan you explain in more detail why you don't want to use dirty reads? That's kind of the definition of what you're asking for in #3.
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