DDS 1.5 RTF Avatar
  1. OMG Issue

DDS15 — History and Reliability should be orthogonally independent concerns

  • Key: DDS15-25
  • Legacy Issue Number: 17284
  • Status: open  
  • Source: ZettaScale Technology ( Angelo Corsaro, PhD.)
  • Summary:

    The current DDSv1.2 specification provides a slightly different semantics for the history on the writer side and on the reader.

    In addition, the DDS v1.2 specification allows to use the data-writer history as the "reliability-send-queue" – this is combined with the fact that History is (correctly) not an RxO policy leads to a serious bug in the spec (luckily not all DDS implementations follow this path) . Let's try to understand why.

    Firs, it is important to comprehend that unless a data writer uses KeepAll history, reliable data delivery is not guaranteed to matching data readers. This can't be guaranteed even when using Reliable communication (obviously ignoring data-writer crashes for the time being).

    Essentially, under this scheme a DataWriter is allowed send/re-send only what is in its history cache. If a sample is out of history a DataWriter won't do anything, thus leading to potential inconsistencies in case some reader has lost the message.

    Things get very messy when a DataReader with KeepAll History is matching a DataWriter that does uses KeepLast(n). In this case, the poor DataReader which might legitimally expect to have on his history all the samples written by the writers – without holes – might finds him-/her-self surprised by the fact that some samples might be missing.

  • Reported: DDS 1.4 — Thu, 29 Mar 2012 04:00 GMT
  • Updated: Thu, 10 Oct 2019 00:03 GMT