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Comment by abadid | Hacker News Reader

No. Each process is independently checking the value of a non-changing snapshot of the state of the input data and we therefore do not run into the impossibility result.

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So then whatever process maintains the snapshots of input data must absorb all of the complexity eliminated from the workers, no?

Multi-versioned systems can read from a snapshot at negligible complexity (just read the correct version).

And is the snapshot system also distributed and highly available? If so, wouldn't it take on the challenges of committing changes?

There is no separate snapshot system. Each partition just maintains multiple versions of all data it stores locally.

Do I understand then correctly that the:

      temp = Do_Remote_Read(Y)
      if (temp > 0)
         X = 42

Is actually

      temp = Do_Remote_Read(Y_i)
      if (temp > 0)
         X = 42

where Y_i is the value of Y in the version i that matches the version for which the coordinator created the code?

Otherwise Y can be something else? And isn't then there a need for a synchronization with the keeper of Y, if Y_i is still not available?

Yes, but it is possible to architect the system such that Y_i is available.

> it is possible to architect the system such that Y_i is available.

I'd really like to know how? How can we be sure that the keeper of Y is ready (or even online) at the Read moment?

(As you see I'm not familiar with the systems you worked on but I'd really like to understand the context in which you present the solution).

I think I misunderstood what you were saying. By "available" I mean that the version exists at the time of the read. As far as available wrt failure, this is no different than any other system. If you don't synchronously replicate data, you can cannot process transactions over data that is offline (and still guarantee ACID). The transaction cannot proceed whether or not you are using 2PC.

That's exactly where I got stuck on the example. (Saved me editing my comment. Thanks!)

> (just read the correct version)

Famous last words!

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