Consistent Snapshot Analogies
Last calendar week I taught distributed snapshot inwards my CSE 586: Distributed Systems class. While I instruct snapshot, I invariably reveal myself longing for analogies to render roughly intuition almost this concept. The global state captured yesteryear a distributed snapshot (say using Lamport/Chandy marking algorithm) does non fit to the "state of the organization at initiation of the snapshot". Furthermore, it too may non fit to a "state of the organization from initiation to electrical current state during this computation". This is because land the snapshot taking is progressing inwards the system, the underlying organization computation is too proceeding together with changing the state of the organization progressively. (Distributed snapshot is non allowed to stop/freeze underlying organization computation every bit that reduces availability.)
For those curious almost the question, "what practiced is a snapshot then?": The snapshot captures a reachable state from initiation state, together with from the snapshot state the electrical current state of the computation is too reachable. In other words, snapshot is a probable state of the computation, fifty-fifty though it may non own got occurred inwards this item computation. So, for stable predicate detection together with distributed organization debugging the snapshot is withal valuable.
Going dorsum to my predicament, the analogy I resort to is that of chiliad ants trying to take/construct a motion-picture demonstrate of the elephant every bit the elephant is moving. (I had heard this instance from Paul Sivilotti land I was a graduate pupil at Ohio State.) Here the ants fit to the marking algorithm, together with the elephant the underlying computation that nosotros desire to own got a snapshot of. Of course of written report the pictures the ants volition prepare volition last vaguely elephant-like, it volition last a motion-picture demonstrate of the elephant's outer surface every bit it progresses inwards the spacetime continuum. (Achievement Unlocked: Today I used spacetime continuum inwards serious writing.)
Last calendar week I was using this analogy inwards class, when a improve (at to the lowest degree to a greater extent than modern) analogy occurred to me. Panoramic photographs! When yous role your smartphone to own got a panorama picture, yous are inwards fact taking a distributed snapshot of your surrounding. Your snapshot is non instantaneous, it needs fourth dimension to complete: yous involve to rotate 180 to 360 degrees together with likely that takes a practiced 5-10 seconds. If inwards the meanwhile something moves, that object volition non last reflected inwards its master form/place/state inwards your panorama picture.
We may endeavor to own got the analogy farther to categorizing the panorama pictures every bit consistent snapshots together with inconsistent snapshots. In an inconsistent snapshot, although the mail of a message is non recorded every bit business office of the snapshot, the have of the message is recorded every bit business office of the snapshot. (You received a message from the future.) So nosotros tin dismiss say that, your panorama motion-picture demonstrate is inconsistent if the object moves inwards the contrary administration of the panorama/snapshot. These are examples of inconsistent snapshots.
And, these are examples of consistent snapshot. (Maybe the concluding 2 are debatable every bit they duplicate roughly state.)
Finally, this seemingly-consistent inconsistent snapshot (the bearded guy on the leftmost is teleported to reappear every bit the rightmost person) points to the dangers of ignoring backchannels when taking a snapshot.
Probably it is non worth trying to strain the analogy further, hence I volition stop. Here are roughly to a greater extent than funny iphone panoramic pictures.
For those curious almost the question, "what practiced is a snapshot then?": The snapshot captures a reachable state from initiation state, together with from the snapshot state the electrical current state of the computation is too reachable. In other words, snapshot is a probable state of the computation, fifty-fifty though it may non own got occurred inwards this item computation. So, for stable predicate detection together with distributed organization debugging the snapshot is withal valuable.
Going dorsum to my predicament, the analogy I resort to is that of chiliad ants trying to take/construct a motion-picture demonstrate of the elephant every bit the elephant is moving. (I had heard this instance from Paul Sivilotti land I was a graduate pupil at Ohio State.) Here the ants fit to the marking algorithm, together with the elephant the underlying computation that nosotros desire to own got a snapshot of. Of course of written report the pictures the ants volition prepare volition last vaguely elephant-like, it volition last a motion-picture demonstrate of the elephant's outer surface every bit it progresses inwards the spacetime continuum. (Achievement Unlocked: Today I used spacetime continuum inwards serious writing.)
Last calendar week I was using this analogy inwards class, when a improve (at to the lowest degree to a greater extent than modern) analogy occurred to me. Panoramic photographs! When yous role your smartphone to own got a panorama picture, yous are inwards fact taking a distributed snapshot of your surrounding. Your snapshot is non instantaneous, it needs fourth dimension to complete: yous involve to rotate 180 to 360 degrees together with likely that takes a practiced 5-10 seconds. If inwards the meanwhile something moves, that object volition non last reflected inwards its master form/place/state inwards your panorama picture.
We may endeavor to own got the analogy farther to categorizing the panorama pictures every bit consistent snapshots together with inconsistent snapshots. In an inconsistent snapshot, although the mail of a message is non recorded every bit business office of the snapshot, the have of the message is recorded every bit business office of the snapshot. (You received a message from the future.) So nosotros tin dismiss say that, your panorama motion-picture demonstrate is inconsistent if the object moves inwards the contrary administration of the panorama/snapshot. These are examples of inconsistent snapshots.
And, these are examples of consistent snapshot. (Maybe the concluding 2 are debatable every bit they duplicate roughly state.)
Finally, this seemingly-consistent inconsistent snapshot (the bearded guy on the leftmost is teleported to reappear every bit the rightmost person) points to the dangers of ignoring backchannels when taking a snapshot.
Probably it is non worth trying to strain the analogy further, hence I volition stop. Here are roughly to a greater extent than funny iphone panoramic pictures.
0 Response to "Consistent Snapshot Analogies"
Post a Comment