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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: SharedValue, how does it work?
Date Wed, 01 Oct 2014 20:11:44 GMT
SharedValue keeps a copy of the last version that was read. trySetValue() uses the version
to set the value. This is a feature of ZooKeeper. It allows for optimistic locking. If the
version matches, the update succeeds. Otherwise it fails.

-JZ


On October 1, 2014 at 3:08:04 PM, Scott Blum (dragonsinth@gmail.com) wrote:

I've been staring at SharedValue for the last 15 minutes and I can't figure out how it works.

Naively, I don't see how it's possible to safely implement trySetValue(newValue).  Wouldn't
it have to be compareAndSetValue(oldValue, newValue)?

I'm imagining client code that looks like this:

byte[] currentValue = sharedValue.getValue();
byte[] newValue = operationOn(currentValue);
sharedValue.compareAndSetValue(currentValue, newValue);

The only way to write this right now (unless I'm missing something) is:

byte[] currentValue = sharedValue.getValue();
byte[] newValue = operationOn(currentValue);
sharedValue.trySetValue(newValue);

The problem is, an update can happen in between the call to getValue() and trySetValue(),
and the way the code's written, SharedValue doesn't record read accesses, so it has no idea
if any user code actually called getValue() "recently".

Am I missing something?
Scott


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