Talk 2023-01-18: some WIP talking about consensus

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Alex Auvolat 2023-01-12 16:27:02 +01:00
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@ -3,6 +3,7 @@ ASSETS=assets/consistent_hashing_1.pdf \
assets/consistent_hashing_3.pdf \
assets/consistent_hashing_4.pdf \
assets/garage_tables.pdf \
assets/consensus.pdf_tex \
assets/deuxfleurs.pdf
talk.pdf: talk.tex $(ASSETS)
@ -10,3 +11,6 @@ talk.pdf: talk.tex $(ASSETS)
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@ -8,6 +8,7 @@
\usepackage{multirow}
\usetheme{boxes}
\usepackage{graphicx}
\usepackage{import}
\usepackage{adjustbox}
%\useoutertheme[footline=authortitle,subsection=false]{miniframes}
%\useoutertheme[footline=authorinstitute,subsection=false]{miniframes}
@ -479,33 +480,185 @@
\section{Problem 2: ensuring consistency}
%\begin{frame}
% \frametitle{Garage's architecture}
% \begin{center}
% \includegraphics[width=.35\linewidth]{assets/garage.drawio.pdf}
% \end{center}
%\end{frame}
\begin{frame}
\frametitle{Garage is \emph{coordination-free}:}
\begin{itemize}
\item No Raft or Paxos
\vspace{1em}
\item Internal data types are CRDTs
\vspace{1em}
\item All nodes are equivalent (no master/leader/index node)
\end{itemize}
\vspace{2em}
$\to$ less sensitive to higher latencies between nodes
\frametitle{Consensus vs weak consistency}
\hspace{1em}
\begin{minipage}{7cm}
\textbf{Consensus-based systems:}
\vspace{1em}
\begin{itemize}
\item \textbf{Leader-based:} a leader is elected to coordinate
all reads and writes
\vspace{1em}
\item \textbf{Linearizability} of all operations\\
(strongest consistency guarantee)
\vspace{1em}
\item \textbf{Replicated state machines} that can implement
any sequential specification
\vspace{1em}
\item \textbf{Costly}, the leader is a bottleneck;
leader elections on failure take time
\end{itemize}
\end{minipage}
\hfill
\begin{minipage}{7cm} \visible<2->{
\textbf{Weakly consistent systems:}
\vspace{1em}
\begin{itemize}
\item \textbf{Nodes are equivalent}, any node
can originate a read or write operation
\vspace{1em}
\item \textbf{Read-after-write consistency} with quorums,
eventual consistency without
\vspace{1em}
\item \textbf{Operations have to commute}, i.e.~we
can only implement CRDTs
\vspace{1em}
\item \textbf{Fast}, no node is a bottleneck;\\
works the same with offline nodes
\end{itemize}
} \end{minipage}
\hspace{1em}
\end{frame}
\begin{frame}
\frametitle{Consistency model}
\frametitle{Consensus vs weak consistency}
\begin{center}
\textbf{The same objects cannot be implemented in both models.}
\end{center}
\vspace{2em}
\hspace{1em}
\begin{minipage}{7cm}
\underline{Consensus-based systems:}
\vspace{1em}
\textbf{Any sequential specification}\\~
\end{minipage}
\hfill
\begin{minipage}{7cm}
\underline{Weakly consistent systems:}
\vspace{1em}
\textbf{CRDTs only}\\(conflict-free replicated data types)
\end{minipage}
\hspace{1em}
\vspace{3em}
\begin{center}
Part of the complexity is \textbf{reported to the consumer of the API}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Consensus vs weak consistency}
\begin{center}
\textbf{From a theoretical point of view:}\\
\end{center}
\vspace{2em}
\hspace{1em}
\begin{minipage}{6.5cm}
\underline{Consensus-based systems:}
\vspace{1em}
Require \textbf{additionnal assumptions} such as a fault detector or a strong RNG\\~
\end{minipage}
\hfill
\begin{minipage}{6.5cm}
\underline{Weakly consistent systems:}
\vspace{1em}
Can be implemented in \textbf{any asynchronous message passing distributed system}
\end{minipage}
\hspace{1em}
\vspace{3em}
\begin{center}
They represent \textbf{different classes of computational capability}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Understanding the power of consensus}
\textbf{Consensus:} an API with a single operation, $propose(x)$
\begin{enumerate}
\item nodes all call $propose(x)$ with their proposed value;
\item nodes all receive the same value as a return value, which is one of the proposed values
\end{enumerate}
\vspace{1em}
\visible<2->{
\textbf{Equivalent to} a distributed algorithm that gives a total order on all requests
}
\vspace{1em}
\visible<3->{
\textbf{Implemented by} this simple replicated state machine:
\vspace{.5em}
\begin{figure}
\centering
\def\svgwidth{.5\textwidth}
\large
\import{assets/}{consensus.pdf_tex}
\end{figure}
\vspace{1em}
}
\end{frame}
\begin{frame}
\frametitle{Can my object be implemented without consensus?}
\underline{Given the specification of an API:}
\vspace{2em}
\begin{itemize}
\item Not ACID (not required by S3 spec) / not linearizable
\item \textbf{Using this API, we can implement the consensus object} (the $propose$ function)\\
$\to$ the API is equivalent to consensus/total ordering of messages\\
$\to$ the API cannot be implemented in a weakly consistent system
\vspace{2em}
\item \textbf{This API can be implemented using only weak primitives}\\
(e.g. a bunch of atomic registers)\\
$\to$ the API is strictly weaker than consensus\\
$\to$ we can implement it in Garage!
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Why avoid consensus?}
Consensus can be implemented reasonably well in practice, so why avoid it?
\vspace{2em}
\begin{itemize}
\item \textbf{Software complexity:} RAFT and PAXOS are complex beasts;\\
harder to prove, harder to reason about
\vspace{1.5em}
\item \textbf{Performance issues:}
\vspace{1em}
\item \textbf{Read-after-write consistency}\\
{\footnotesize (stronger than eventual consistency)}
\begin{itemize}
\item The leader is a \textbf{bottleneck} for all requests
\vspace{1em}
\item Particularly \textbf{sensitive to higher latency} between nodes
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{What can we implement without consensus?}
\begin{itemize}
\item Any \textbf{conflict-free replicated data type} (CRDT)
\vspace{1em}
\item Non-transactional key-value stores such as S3 are equivalent to a simple CRDT:\\
a \textbf{last-writer-wins registry}
\vspace{1em}
\item \textbf{Read-after-write consistency} can be implemented
using quorums on read and write operations
\vspace{1em}
\item \textbf{Monotonicity of reads} can be implemented with repair-on-read\\
(makes reads more costly, not implemented in Garage)
\end{itemize}
\end{frame}