5 ms·

TCP does deliver data more than once though. Sure within a TCP session you are guaranteed not to get the same byte twice out of your socket, bytes are delivere

by YZF 9y ago

TCP does deliver data more than once though. Sure within a TCP session you are guaranteed not to get the same byte twice out of your socket, bytes are delivered exactly once, in order. Now if your application that uses TCP pulls data out of the socket and then dies the data will need to be delivered again and the TCP protocol is unable to help us there, it's application level logic at that point. So everyone is talking about the same thing here, data can get delivered twice, and the application must handle it in an idempotent way. For TCP this happens to be looking at some counter and throwing away anything that's already been seen. With a single client/server connection maintaining sequence numbers like TCP does solve the problem but it's harder to use this same technique in multi-client, multi-server, "stateless" requests that are common in the web world. EDIT: To clarify, what I mean by TCP "does deliver data more than once", is that one side of the connection can send the same data twice. It's true that it's then discarded by the other end but this is what people are talking about when they talk about the theoretical impossibility of never sending anything twice. The rest is basically the idem-potency thing of ensuring that data received twice somehow doesn't cause anything abnormal.

rusanu 9y ago
Not when your 'socket' is a persisted, durable, transacted medium (ie. a database). Sure, applications can 'pull data out of the socket and then die', but this is a common scenarios on databases which is handled with transaction and post-crash recovery. The application comes back after the crash and find the same state as before the crash (the 'socket' still has the data ready to pull off), it pull again, process, and then commit. This is not duplicate delivery, since we're talking about an aborted and rolled back attempt, followed later by a successful processing. Again, databases and database apps have been dealing with this kind of problems for decades and know how handle them. I've been living in this problem space for many years now and seen the wheel reinvented many times. Whenever the plumbing does not guarantees EOIO but the business demands it, it gets pushed into the app layer where TCP (retries and acks) is reimplemented, to various success levels.
- YZF 9y ago
  > The application comes back after the crash and find the same state as before the crash (the 'socket' still has the data ready to pull off), it pull again, process, and then commit. Isn't this the same app layer stuff that has to get reimplemented? I can see how this is often pushed back to a human (oops, my money transfer didn't go through, I'll have to redo it) but it's still something that has to be dealt with somewhere.
  - rusanu 9y ago
    > it's still something that has to be dealt with somewhere Database programmers have the means to deal with it off-the-shelf: BEGIN TRANSACTION ... COMMIT. When your queues are in the database, this becomes trivial. Even without the system I'm talking about (Service Broker) that has the queues stored in the database, most regular messaging systems do support enrolling into a distributed transaction and achieve an atomic dequeue/process sequence, is just that many apps/deployments don't bother to do it because the ops overhead (XA coordinator), reduced throughput and/or simply not understanding the consequences. Point is that durable, persisted, transacted 'sockets' are behaving very differently from a TCP socket. Is a whole lot harder to simply lose a message in the app layer when interacting with a database.
    - wbl 9y ago
      But the application still has to deal with processing the same message twice if it dies before ACK.
      - eternalban 9y ago
        He did say XA. The transaction boundary defined in your consumer covers the interactions of that consumer with other XA aware nodes who all participate in a "distributed transaction". So you can process this message N times without committing, and thus possibly N times telling other systems to do M' side-effect of that message, but until you commit the world has not changed. https://en.wikipedia.org/wiki/X/Open_XA https://en.wikipedia.org/wiki/X/Open_XA
bmelton 9y ago
Right. If your definition of "delivered" is "showed a message to the user", then exactly once is a fairly trivial goal to attain. If it's defined "data over the wire", then it's nearly impossible to avoid some form of ACK/RETRY mechanism on real networks, which means that messages will need to sometimes have to be 'delivered' many times to ensure a single display to the user.

by YZF 9y ago

rusanu 9y ago

Not when your 'socket' is a persisted, durable, transacted medium (ie. a database). Sure, applications can 'pull data out of the socket and then die', but this is a common scenarios on databases which is handled with transaction and post-crash recovery. The application comes back after the crash and find the same state as before the crash (the 'socket' still has the data ready to pull off), it pull again, process, and then commit. This is not duplicate delivery, since we're talking about an aborted and rolled back attempt, followed later by a successful processing. Again, databases and database apps have been dealing with this kind of problems for decades and know how handle them. I've been living in this problem space for many years now and seen the wheel reinvented many times. Whenever the plumbing does not guarantees EOIO but the business demands it, it gets pushed into the app layer where TCP (retries and acks) is reimplemented, to various success levels.

YZF 9y ago
> The application comes back after the crash and find the same state as before the crash (the 'socket' still has the data ready to pull off), it pull again, process, and then commit. Isn't this the same app layer stuff that has to get reimplemented? I can see how this is often pushed back to a human (oops, my money transfer didn't go through, I'll have to redo it) but it's still something that has to be dealt with somewhere.
- rusanu 9y ago
  > it's still something that has to be dealt with somewhere Database programmers have the means to deal with it off-the-shelf: BEGIN TRANSACTION ... COMMIT. When your queues are in the database, this becomes trivial. Even without the system I'm talking about (Service Broker) that has the queues stored in the database, most regular messaging systems do support enrolling into a distributed transaction and achieve an atomic dequeue/process sequence, is just that many apps/deployments don't bother to do it because the ops overhead (XA coordinator), reduced throughput and/or simply not understanding the consequences. Point is that durable, persisted, transacted 'sockets' are behaving very differently from a TCP socket. Is a whole lot harder to simply lose a message in the app layer when interacting with a database.
  - wbl 9y ago
    But the application still has to deal with processing the same message twice if it dies before ACK.
    - eternalban 9y ago
      He did say XA. The transaction boundary defined in your consumer covers the interactions of that consumer with other XA aware nodes who all participate in a "distributed transaction". So you can process this message N times without committing, and thus possibly N times telling other systems to do M' side-effect of that message, but until you commit the world has not changed. https://en.wikipedia.org/wiki/X/Open_XA https://en.wikipedia.org/wiki/X/Open_XA

bmelton 9y ago

Right. If your definition of "delivered" is "showed a message to the user", then exactly once is a fairly trivial goal to attain. If it's defined "data over the wire", then it's nearly impossible to avoid some form of ACK/RETRY mechanism on real networks, which means that messages will need to sometimes have to be 'delivered' many times to ensure a single display to the user.