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Re: Alternative algorithm for timeouts

From: Enrique Arizon Benito <enrique.arizon.benito@everis.com>
Date: Wed, 19 Jul 2017 08:02:25 +0000
To: David Fuelling <dfuelling@sappenin.com>, "public-interledger@w3.org" <public-interledger@w3.org>
Message-ID: <C23179174213FF4A8B0ADF823E0D2DD0C93EA0FD@MBXEUR04.usersad.everis.int>
 I'll try to make it more visual.

 Let's suppose senders initiating the transfers are on ledger1 and receivers are on ledger2.

 Suppose "randomly" ledger1 initiates next transactions, ordered according to its *local* time.

 In this diagram the dot '.' represents a unit of time of 1/15/... secs:

  timeline tx created@ledger1:
    |.........|.........|.........|.........|....
     ^     ^    ^   ^            ^       ^
    tx1   tx2  tx3 tx4          tx5     tx6


  timeline default timeouts@ledger1:
    |.........|.........|.........|.........|....
            ^     ^    ^   ^            ^       ^
           tx1   tx2  tx3 tx4          tx5     tx6


  timeline txs received@ledger2:
    |.........|.........|.........|.........|....
      ^     ^     ^   ^            ^       ^
     tx1   tx2   tx3 tx4          tx5     tx6


  timeline unavailability@ledger2:
    |.........|.........|.........|.........|....
                       ^^^^^^^^^^^
                       unavailable


  timeline txs fulfilled@ledger2:
    |.........|.........|.........|.........|....
      ^     ^    ^                   ^   ^   ^
     tx1   tx2  tx3                 tx5 tx4 tx6


  timeline fulfillment received@ledger1:
    |.........|.........|.........|.........|....
       ^     ^     ^                   ^   ^  ^
      tx1   tx2   tx3                 tx5 tx4tx6
                                           ^
                                          race
                                        condition

  In the previous diagram "unavailable" time represent any internal state in
ledger2 that doesn't allow to process/fulfill the received ILP transfer (a
reset, overload, ...).  In the blockchain case it could also the case that
temporally miners fees are higher than normal, that will make the system
"unavailable" to TXs sent with lower fees.

  The problem with race-conditions arise with transfers arriving just before
the unavailable window time in ledger2 (tx4 in this case).

  tx4 will be fulfilled by ledger2 but when the fulfillment arrives to the
originating ledger, the transfer is already expired (race condition).

  Now imagine that the timeouts in ledger1 are established as explained in the
previous mail.

  Let's take K = 2, that means that timeout for tx4 doesn't start to count
until tx6 or tx7 or ... has arrived. That is, until tx_4+K has arrived.

  In this case, tx6 is processed quickly as expected when the system is
running and not overloaded. tx6 fulfillment arrives to ledger1, and is at that
moment that the timeout for tx4 starts to run. Since the fulfillment for tx4
has already arrived shortly after tx5, the race condition dissapears.

  Obviously there are other problems that can arise, like what happen if
ledger2 is unresponsive "for long time" and the queue of prepared transfers in
ledger1 to ledger2 starts to acumulate. In this case we can define another
timeout, related to how much time we consider any ledger can be un-responsive.
That is a timeout for ledger responses (versus a timeout for transfer fulfillments).

  When such timeout passes, we can rollback transfers in ledger1 and ledger2 (and
ledger2 will ignore valid fulfillments since it's aware that ledger1 will not
accept them anyway).

 It also doesn't solve the case in which ledger2 fulfills the transfer and the
fulfillment is lost in the way back to ledger1 due to some misbehaving
connector, but, I think, this could be considered a "weird" scenario.


Regards,

Enrique


________________________________
De: David Fuelling [dfuelling@sappenin.com]
Enviado: miércoles, 19 de julio de 2017 3:42
Para: Enrique Arizon Benito; public-interledger@w3.org
Asunto: Re: Alternative algorithm for timeouts

Hey Enrique, can you clarify the meaning of M, N, and K?

Thanks!
David
On Fri, Jun 30, 2017 at 6:08 AM Enrique Arizon Benito <enrique.arizon.benito@everis.com<mailto:enrique.arizon.benito@everis.com>> wrote:
At this moment the algorithm to establish a timeout is something like:

  1.  start transaction
  2.  set timeout as "NOW" + Constant_Timeout_time
  3.  wait
  4.  "NOW" > timeout
     *   YES -> timeout transaction
     *   NOW -> Got to 3

This introduces random false timeouts due to race-conditions. It could be possible for the receiving ledger to execute the transfer and for the originating ledger to time out due to the finite time to propagate the fulfillment back through all connectors.


I think next alternative algorithm minimized (nearly avoids) all false timeouts due to race conditions?


- The sending ledger keeps a list of txs not yet executed (that can timeout)  *for each* destination ledger.

   Those txs are stored in an ordered list acording to the sending time [tx1, tx2, tx3, tx4, ...]


- When tx"M" with M = N + K arrives, a timeout is established for tx"N" for each "N" < M - K


- Finally after a given timeout,  tx"N" is cancelled


Said it otherway:

-  transaction in the list does NOT time-out unless two next condition happens:

  - tx"M" fulfillment with M > N + K has arrived

  - tx"N" has timed out.



The logic is next:

 - It's possible that at some point there is an overload in the destination ledger. At this point the average time to process the incomming transfer, executing and returning the fulfillment will increase approaching the initial timeout. The closer it is to the timeout the more probable for race condition during the "travel back".


- Due to the destination ledger system overload, is quite possible that all TXs are delayed and it makes sense for the originating ledger to wait more than ussual.
- Suppose now that tx"M" arrives. At this moment sending ledger setup a timeout for each tx"N" with N < M - K.


  It's quite sensible to think that if there was a system overload in the destination ledger once it's back to normal all pending transfer will arrive shortly after.

  It's also quite sensible to think that if tx"M" arrives, and timeout passes, most probably tx"N" has really timeout (it never reached the destinantion ledger) and that no race condition will arise since they never reached the destination ledger.



Regards,

Enrique


________________________________

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Received on Wednesday, 19 July 2017 08:02:58 UTC

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