Talk 2023-01-18: some WIP talking about consensus
This commit is contained in:
Alex Auvolat
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@ -3,6 +3,7 @@ ASSETS=assets/consistent_hashing_1.pdf \
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assets/consistent_hashing_3.pdf \
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assets/consistent_hashing_4.pdf \
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assets/garage_tables.pdf \
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assets/consensus.pdf_tex \
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assets/deuxfleurs.pdf
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talk.pdf: talk.tex $(ASSETS)
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@ -10,3 +11,6 @@ talk.pdf: talk.tex $(ASSETS)
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assets/%.pdf: assets/%.svg
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inkscape -D -z --file=$^ --export-pdf=$@
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assets/%.pdf_tex: assets/%.svg
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inkscape -D -z --file=$^ --export-pdf=$@ --export-latex
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137
doc/talks/2023-01-18-tocatta/assets/consensus.svg
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doc/talks/2023-01-18-tocatta/assets/consensus.svg
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@ -8,6 +8,7 @@
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\usepackage{multirow}
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\usetheme{boxes}
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\usepackage{graphicx}
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\usepackage{import}
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\usepackage{adjustbox}
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%\useoutertheme[footline=authortitle,subsection=false]{miniframes}
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@ -479,33 +480,185 @@
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\section{Problem 2: ensuring consistency}
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%\begin{frame}
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% \frametitle{Garage's architecture}
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% \begin{center}
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% \includegraphics[width=.35\linewidth]{assets/garage.drawio.pdf}
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% \end{center}
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%\end{frame}
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\begin{frame}
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\frametitle{Garage is \emph{coordination-free}:}
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\begin{itemize}
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\item No Raft or Paxos
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\vspace{1em}
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\item Internal data types are CRDTs
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\vspace{1em}
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\item All nodes are equivalent (no master/leader/index node)
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\end{itemize}
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\vspace{2em}
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$\to$ less sensitive to higher latencies between nodes
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\frametitle{Consensus vs weak consistency}
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\hspace{1em}
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\begin{minipage}{7cm}
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\textbf{Consensus-based systems:}
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\vspace{1em}
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\begin{itemize}
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\item \textbf{Leader-based:} a leader is elected to coordinate
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all reads and writes
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\vspace{1em}
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\item \textbf{Linearizability} of all operations\\
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(strongest consistency guarantee)
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\vspace{1em}
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\item \textbf{Replicated state machines} that can implement
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any sequential specification
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\vspace{1em}
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\item \textbf{Costly}, the leader is a bottleneck;
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leader elections on failure take time
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\end{itemize}
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\end{minipage}
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\hfill
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\begin{minipage}{7cm} \visible<2->{
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\textbf{Weakly consistent systems:}
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\vspace{1em}
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\begin{itemize}
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\item \textbf{Nodes are equivalent}, any node
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can originate a read or write operation
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\vspace{1em}
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\item \textbf{Read-after-write consistency} with quorums,
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eventual consistency without
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\vspace{1em}
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\item \textbf{Operations have to commute}, i.e.~we
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can only implement CRDTs
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\vspace{1em}
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\item \textbf{Fast}, no node is a bottleneck;\\
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works the same with offline nodes
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\end{itemize}
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} \end{minipage}
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\hspace{1em}
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\end{frame}
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\begin{frame}
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\frametitle{Consistency model}
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\frametitle{Consensus vs weak consistency}
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\begin{center}
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\textbf{The same objects cannot be implemented in both models.}
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\end{center}
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\vspace{2em}
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\hspace{1em}
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\begin{minipage}{7cm}
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\underline{Consensus-based systems:}
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\vspace{1em}
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\textbf{Any sequential specification}\\~
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\end{minipage}
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\hfill
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\begin{minipage}{7cm}
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\underline{Weakly consistent systems:}
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\vspace{1em}
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\textbf{CRDTs only}\\(conflict-free replicated data types)
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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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Part of the complexity is \textbf{reported to the consumer of the API}
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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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\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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\begin{enumerate}
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\item nodes all call $propose(x)$ with their proposed value;
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\item nodes all receive the same value as a return value, which is one of the proposed values
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\end{enumerate}
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\vspace{1em}
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\visible<2->{
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\textbf{Equivalent to} a distributed algorithm that gives a total order on all requests
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}
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\vspace{1em}
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\visible<3->{
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\textbf{Implemented by} this simple replicated state machine:
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\vspace{.5em}
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\begin{figure}
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\centering
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\def\svgwidth{.5\textwidth}
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\large
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\import{assets/}{consensus.pdf_tex}
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\end{figure}
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\vspace{1em}
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}
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\end{frame}
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\begin{frame}
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\frametitle{Can my object be implemented without consensus?}
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\underline{Given the specification of an API:}
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\vspace{2em}
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\begin{itemize}
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\item Not ACID (not required by S3 spec) / not linearizable
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\item \textbf{Using this API, we can implement the consensus object} (the $propose$ function)\\
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$\to$ the API is equivalent to consensus/total ordering of messages\\
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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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$\to$ 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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\end{frame}
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\begin{frame}
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\frametitle{Why avoid consensus?}
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Consensus can be implemented reasonably well in practice, so why avoid it?
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\vspace{2em}
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\begin{itemize}
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\item \textbf{Software complexity:} RAFT and PAXOS are complex beasts;\\
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harder to prove, harder to reason about
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\vspace{1.5em}
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\item \textbf{Performance issues:}
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\vspace{1em}
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\item \textbf{Read-after-write consistency}\\
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{\footnotesize (stronger than eventual consistency)}
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\begin{itemize}
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\item The leader is a \textbf{bottleneck} for all requests
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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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\end{itemize}
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\end{frame}
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\begin{frame}
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\frametitle{What can we implement without consensus?}
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\begin{itemize}
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\item Any \textbf{conflict-free replicated data type} (CRDT)
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\vspace{1em}
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\item Non-transactional key-value stores such as S3 are equivalent to a simple CRDT:\\
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a \textbf{last-writer-wins registry}
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\vspace{1em}
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\item \textbf{Read-after-write consistency} can be implemented
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using quorums on read and write operations
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\vspace{1em}
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\item \textbf{Monotonicity of reads} can be implemented with repair-on-read\\
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(makes reads more costly, not implemented in Garage)
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\end{itemize}
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\end{frame}
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