forked from Deuxfleurs/garage
Reorder and add a hard problem
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2 changed files with 51 additions and 38 deletions
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@ -506,6 +506,39 @@
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\hspace{1em}
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\end{frame}
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\begin{frame}
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\frametitle{Consensus vs weak consistency}
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\begin{center}
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\textbf{From a theoretical point of view:}\\
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\end{center}
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\vspace{2em}
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\hspace{1em}
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\begin{minipage}{6.5cm}
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\underline{Consensus-based systems:}
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\vspace{1em}
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Require \textbf{additionnal assumptions} such as a fault detector or a strong RNG\\
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(FLP impossibility theorem)
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\end{minipage}
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\hfill
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\begin{minipage}{6.5cm}
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\underline{Weakly consistent systems:}
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\vspace{1em}
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Can be implemented in \textbf{any\\asynchronous message passing\\distributed system} with node crashes
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\end{minipage}
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\hspace{1em}
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\vspace{3em}
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\begin{center}
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They represent \textbf{different classes of computational capability}\\
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\end{center}
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\end{frame}
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\begin{frame}
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\frametitle{Consensus vs weak consistency}
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\begin{center}
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@ -531,7 +564,7 @@
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\vspace{1em}
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\textbf{Limited objects such as CRDTs}\\(conflict-free replicated data types)
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\textbf{Only CRDTs}\\(conflict-free replicated data types)
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\vspace{1em}
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Part of the complexity is \textbf{reported to the consumer of the API}\\~
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@ -539,38 +572,6 @@
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\hspace{1em}
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\end{frame}
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\begin{frame}
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\frametitle{Consensus vs weak consistency}
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\begin{center}
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\textbf{From a theoretical point of view:}\\
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\end{center}
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\vspace{2em}
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\hspace{1em}
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\begin{minipage}{6.5cm}
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\underline{Consensus-based systems:}
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\vspace{1em}
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Require \textbf{additionnal assumptions} such as a fault detector or a strong RNG\\~
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\end{minipage}
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\hfill
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\begin{minipage}{6.5cm}
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\underline{Weakly consistent systems:}
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\vspace{1em}
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Can be implemented in \textbf{any asynchronous message passing distributed system}
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\end{minipage}
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\hspace{1em}
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\vspace{3em}
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\begin{center}
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They represent \textbf{different classes of computational capability}
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\end{center}
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\end{frame}
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\begin{frame}
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\frametitle{Understanding the power of consensus}
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\textbf{Consensus:} an API with a single operation, $propose(x)$
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@ -608,7 +609,7 @@
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$\to$ the API cannot be implemented in a weakly consistent system
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\vspace{2em}
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\item \textbf{This API can be implemented using only weak primitives}\\
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(e.g. a bunch of atomic registers)\\
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(e.g. in the asynchronous message passing model with no further assumption)\\
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$\to$ the API is strictly weaker than consensus\\
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$\to$ we can implement it in Garage!
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\end{itemize}
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@ -625,7 +626,10 @@
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\item \textbf{Performance issues:}
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\vspace{1em}
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\begin{itemize}
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\item The leader is a \textbf{bottleneck} for all requests
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\item Theoretical requirements (RNG, failure detector) translate into \textbf{practical costs}
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\vspace{1em}
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\item The leader is a \textbf{bottleneck} for all requests;\\
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even in leaderless approaches, \textbf{all nodes must process all operations in order}
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\vspace{1em}
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\item Particularly \textbf{sensitive to higher latency} between nodes
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\end{itemize}
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@ -746,10 +750,19 @@
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\end{frame}
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\begin{frame}
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\frametitle{The hard parts we don't address (yet!)}
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\frametitle{A hard problem: layout changes}
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\begin{itemize}
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\item Maintain consistency changes when nodes assigned to a partition change:\\
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\item TODO
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\item We rely on quorums $k > n/2$ within each partition:\\
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$$n=3,~~~~~~~k\ge 2$$
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\item When rebalancing, the set of nodes responsible for a partition can change:\\
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$$\{n_A, n_B, n_C\} \to \{n_A, n_D, n_E\}$$
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\vspace{.01em}
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\item During the rebalancing, $D$ and $E$ don't yet have the data,\\
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~~~~~~~~~~~~~~~~~~~and $B$ and $C$ want to get rid of the data to free up space\\
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\vspace{.2em}
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$\to$ quorums only within the new set of nodes don't work\\
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$\to$ how to coordinate? \textbf{currently, we don't...}
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\end{itemize}
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\end{frame}
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