From 44f6077705c3c43a2d96707e15d5f9d9e77975ba Mon Sep 17 00:00:00 2001 From: Matthew Hodgson Date: Tue, 11 Nov 2014 21:11:47 +0200 Subject: [PATCH] what was versioning.rst --- drafts/ancient_federated_versioning_design_notes.rst | 11 +++++++++++ 1 file changed, 11 insertions(+) create mode 100644 drafts/ancient_federated_versioning_design_notes.rst diff --git a/drafts/ancient_federated_versioning_design_notes.rst b/drafts/ancient_federated_versioning_design_notes.rst new file mode 100644 index 00000000..ffda6063 --- /dev/null +++ b/drafts/ancient_federated_versioning_design_notes.rst @@ -0,0 +1,11 @@ +Versioning is, like, hard for backfilling backwards because of the number of Home Servers involved. + +The way we solve this is by doing versioning as an acyclic directed graph of PDUs. For backfilling purposes, this is done on a per context basis. +When we send a PDU we include all PDUs that have been received for that context that hasn't been subsequently listed in a later PDU. The trivial case is a simple list of PDUs, e.g. A <- B <- C. However, if two servers send out a PDU at the same to, both B and C would point at A - a later PDU would then list both B and C. + +Problems with opaque version strings: + - How do you do clustering without mandating that a cluster can only have one transaction in flight to a given remote home server at a time. + If you have multiple transactions sent at once, then you might drop one transaction, receive another with a version that is later than the dropped transaction and which point ARGH WE LOST A TRANSACTION. + - How do you do backfilling? A version string defines a point in a stream w.r.t. a single home server, not a point in the context. + +We only need to store the ends of the directed graph, we DO NOT need to do the whole one table of nodes and one of edges.