WIP: finishing first draft of inner projection description
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Scott Lystig Fritchie
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1 changed files with 94 additions and 37 deletions
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@ -814,7 +814,7 @@ is used by the flowchart and throughout this section.
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In Hibari's implementation of Chain Replication
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\cite{cr-theory-and-practice}, the chain members between the
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``head'' and ``official tail'' (inclusive) are what Machi calls the
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UPI server list.
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UPI server list. See also Section~\ref{sub:upi}.
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\item[Repairing] The ordered list of nodes that are in repair mode,
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i.e., synchronizing their data with the UPI members of the chain.
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@ -829,7 +829,7 @@ is used by the flowchart and throughout this section.
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node. It is also the projection with largest
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epoch number in the local node's private projection store.
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\item[$\mathbf{P_{newprop}}$] A new projection proposal, as
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\item[$\mathbf{P_{newprop}}$] A new projection suggestion, as
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calculated by the local server
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(Section~\ref{sub:humming-projection-calculation}).
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@ -861,7 +861,7 @@ right:
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\item[Column B] Do I act?
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\item[Column C] How do I act?
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\begin{description}
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\item[C1xx] Save latest proposal to local private store, unwedge,
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\item[C1xx] Save latest suggested projection to local private store, unwedge,
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then stop.
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\item[C2xx] Ping author of latest to try again, then wait, then iterate.
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\item[C3xx] The new projection appears best: write
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@ -933,7 +933,8 @@ detector such as the $\phi$ accrual failure detector
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\cite{phi-accrual-failure-detector} can be used to help mange such
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situations.
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\paragraph{Flapping due to asymmetric network partitions} TODO revise
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\paragraph{Flapping due to asymmetric network partitions} TODO needs
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some polish
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The simulator's behavior during stable periods where at least one node
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is the victim of an asymmetric network partition is \ldots weird,
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@ -986,20 +987,87 @@ new and nearly-identical projection) is lower with staggered timer.
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\subsection{Writing a new projection}
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\label{sub:humming-proj-storage-writing}
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The actions described in this section are executed in the bottom part of
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Column~A, Column~B, and the bottom of Column~C of
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Figure~\ref{fig:flowchart}.
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See also: Section~\ref{sub:proj-storage-writing}.
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To focus very specifically about writing a projection,
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Figure~\ref{fig:flowchart} shows that writing a private projection is
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done by state $C110$ and that writing a public projection is done by
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states $C300$ and $C310$.
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Broadly speaking, there are a number of decisions made in all three
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columns of Figure~\ref{fig:flowchart} to decide if and when any type
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of projection should be written at all. Sometimes, the best action is
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to do nothing.
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\subsubsection{Column A: Any reason to change?}
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The main tasks of the flowchart states in Column~A is to calculate a
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new projection $P_{new}$ and perhaps also the inner projection
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$P_{new2}$ if we're in flapping mode. Then we try to figure out which
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projection has the greatest merit: our current projection
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$P_{current}$, the new projection $P_{new}$, or the latest epoch
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$P_{latest}$. If $P_{current}$ is best, then there's nothing more to
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do.
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\subsubsection{Column B: Do I act?}
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The main decisions that states in Column B need to make are:
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\begin{itemize}
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\item Is the $P_{latest}$ projection written unanimously (as far as we
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call tell right now)? If yes, then we out to seriously consider
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using it for our new internal state; go to state $C100$.
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\item Is some other server's $P_{latest}$ projection better than my
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$P_{new}$? If so,
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then we wait for a while. The waiting loop is broken by a local
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retry counter. If the counter is small enough, we wait (via state
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$C200$). While we wait, the author of the better projection will
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hopefully have an opportunity to re-write it in a newer epoch
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unanimously. If the counter is too big, then we break out and go to
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$C300$.
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\item Otherwise we go to state $C300$, where we try to write our
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$P_{new}$ to all public projection stores because, as far as we can
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discern, our projection is best and everyone else ought to know it.
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\end{itemize}
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It's notable that if $P_{new}$ is truly the best projection available
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at the moment, it must always be written unanimously to everyone's
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public projection stores and then processed through another
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monitor-calculate loop through the flowchart before it can be adopted
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via state $C120$.
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\subsubsection{Column C: How do I act?}
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This column contains three variations of how to act:
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\begin{description}
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\item[C1xx] Try to adopt the $P_{latest}$ suggestion. If the transition
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between $P_{current}$ to $P_{latest}$ is completely safe, we'll use
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it by storing it in our local private projection store and then
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adopt it as $P_{current}$. If it isn't safe, then jump to $C300$.
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\item[C2xx] Do nothing but sleep a while. Then we loop back to state
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$A20$ and step through the flowchart loop again. Optionally, we
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might want to poke the author of $P_{latest}$ to try again to write
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its proposal unanimously.
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\item[C3xx] We try to replicate our $P_{new}$ suggestion to all local
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projection stores, because it seems best.
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\end{description}
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\subsection{Adopting a new projection}
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\label{sub:humming-proj-adoption}
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See also: Section~\ref{sub:proj-adoption}.
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TODO finish
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A new projection $P_E$ is adopted by a Machi server at epoch $E$ if
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two requirements are met:
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the following two requirements are met:
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\paragraph{\#1: All available copies of $P_E$ are unanimous/identical}
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@ -1160,7 +1228,7 @@ One of the intriguing features of humming consensus's reaction to
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asymmetric partition: flapping behavior continues for as long as
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an any asymmetric partition exists.
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\subsubsection{Leaving flapping state}
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\subsubsection{Leaving flapping state and discarding inner projectino}
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There are two events that can trigger leaving flapping state.
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@ -1182,7 +1250,9 @@ There are two events that can trigger leaving flapping state.
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When either event happens, server $S$ will exit flapping state. All
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new projections authored by $S$ will have all flapping diagnostic data
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removed. This includes stopping use of the inner projection.
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removed. This includes stopping use of the inner projection: the UPI
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list of the inner projection is copied to the outer projection's UPI
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list, to avoid a drastic change in UPI membership.
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\subsubsection{Stability in symmetric partition cases}
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@ -1729,7 +1799,7 @@ This property may also be referred to by its acronym, ``UPI''.
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\subsection{Chain Replication and strong consistency}
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The three basic rules of Chain Replication and its strong
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The basic rules of Chain Replication and its strong
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consistency guarantee:
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\begin{enumerate}
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@ -1831,9 +1901,7 @@ then no other chain member can have a prior/older value because their
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respective mutations histories cannot be shorter than the tail
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member's history.
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\section{TODO: orphaned text}
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\subsection{Additional sources for information humming consensus}
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\section{Additional sources for information about humming consensus}
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\begin{itemize}
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\item ``On Consensus and Humming in the IETF'' \cite{rfc-7282}, for
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@ -1845,6 +1913,16 @@ for an allegory in homage to the style of Leslie Lamport's original Paxos
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paper.
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\end{itemize}
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\section{Acknowledgements}
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We wish to thank everyone who has read and/or reviewed this document
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in its really-terrible early drafts and have helped improve it
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immensely: Justin Sheehy, Kota Uenishi, Shunichi Shinohara, Andrew
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Stone, Jon Meredith, Chris Meiklejohn, Mark Allen, and Zeeshan
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Lakhani.
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\section{TODO: orphaned text}
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\subsection{Aside: origin of the analogy to composing music (TODO keep?)}
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The ``humming'' part of humming consensus comes from the action taken
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when the environment changes. If we imagine an egalitarian group of
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@ -1871,27 +1949,6 @@ By analogy, if the rules of the musical score are obeyed, then the
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Chain Replication invariants that are managed by humming consensus are
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obeyed. Such safe management of Chain Replication metadata is our end goal.
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\subsection{1}
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For any key $K$, different projection stores $S_a$ and $S_b$ may store
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nothing (i.e., {\tt error\_unwritten} when queried) or store different
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values, $P_a \ne P_b$, despite having the same projection epoch
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number. The following ranking rules are used to
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determine the ``best value'' of a projection, where highest rank of
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{\em any single projection} is considered the ``best value'':
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\begin{enumerate}
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\item An unwritten value is ranked at a value of $-1$.
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\item A value whose {\tt author\_server} is at the $I^{th}$ position
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in the {\tt all\_members} list has a rank of $I$.
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\item A value whose {\tt dbg\_annotations} and/or other fields have
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additional information may increase/decrease its rank, e.g.,
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increase the rank by $10.25$.
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\end{enumerate}
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Rank rules \#2 and \#3 are intended to avoid worst-case ``thrashing''
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of different projection proposals.
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\subsection{ranking}
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\label{sub:projection-ranking}
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