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docs/server-server/versioning.rst

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+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.