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\title{Garage, the low-tech storage platform for geo-distributed clusters}
\author{Alex Auvolat, Deuxfleurs}
\date{FOSDEM'24, 2024-02-03}
\begin{document}
\begin{frame}
\centering
\includegraphics[width=.3\linewidth]{../../sticker/Garage.png}
\vspace{1em}
{\large\bf Alex Auvolat, Deuxfleurs Association}
\vspace{1em}
\url{https://garagehq.deuxfleurs.fr/}
Matrix channel: \texttt{\#garage:deuxfleurs.fr}
\end{frame}
\begin{frame}
\frametitle{Who I am}
\begin{columns}[t]
\begin{column}{.2\textwidth}
\centering
\adjincludegraphics[width=.4\linewidth, valign=t]{../assets/alex.jpg}
\end{column}
\begin{column}{.6\textwidth}
\textbf{Alex Auvolat}\\
PhD; co-founder of Deuxfleurs
\end{column}
\begin{column}{.2\textwidth}
~
\end{column}
\end{columns}
\vspace{2em}
\begin{columns}[t]
\begin{column}{.2\textwidth}
\centering
\adjincludegraphics[width=.5\linewidth, valign=t]{../assets/deuxfleurs.pdf}
\end{column}
\begin{column}{.6\textwidth}
\textbf{Deuxfleurs}\\
A non-profit self-hosting collective,\\
member of the CHATONS network
\end{column}
\begin{column}{.2\textwidth}
\centering
\adjincludegraphics[width=.7\linewidth, valign=t]{../assets/logo_chatons.png}
\end{column}
\end{columns}
\end{frame}
\begin{frame}
\frametitle{Our objective at Deuxfleurs}
\begin{center}
\textbf{Promote self-hosting and small-scale hosting\\
as an alternative to large cloud providers}
\end{center}
\vspace{2em}
\visible<2->{
Why is it hard?
}
\visible<3->{
\vspace{2em}
\begin{center}
\textbf{\underline{Resilience}}\\
{\footnotesize (we want good uptime/availability with low supervision)}
\end{center}
}
\end{frame}
\begin{frame}
\frametitle{How to make a \underline{stable} system}
Enterprise-grade systems typically employ:
\vspace{1em}
\begin{itemize}
\item RAID
\item Redundant power grid + UPS
\item Redundant Internet connections
\item Low-latency links
\item ...
\end{itemize}
\vspace{1em}
$\to$ it's costly and only worth it at DC scale
\end{frame}
\begin{frame}
\frametitle{How to make a \underline{resilient} system}
\only<1,4-5>{
Instead, we use:
\vspace{1em}
\begin{itemize}
\item \textcolor<2->{gray}{Commodity hardware (e.g. old desktop PCs)}
\vspace{.5em}
\item<4-> \textcolor<5->{gray}{Commodity Internet (e.g. FTTB, FTTH) and power grid}
\vspace{.5em}
\item<5-> \textcolor<6->{gray}{\textbf{Geographical redundancy} (multi-site replication)}
\end{itemize}
}
\only<2>{
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/neptune.jpg}
\end{center}
}
\only<3>{
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/atuin.jpg}
\end{center}
}
\only<6>{
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/inframap_jdll2023.pdf}
\end{center}
}
\end{frame}
\begin{frame}
\frametitle{Object storage: a crucial component}
\begin{center}
\includegraphics[height=6em]{../assets/Amazon-S3.jpg}
\hspace{3em}
\includegraphics[height=5em]{../assets/minio.png}
\hspace{3em}
\includegraphics[height=6em]{../../logo/garage_hires_crop.png}
\end{center}
\vspace{1em}
S3: a de-facto standard, many compatible applications
\vspace{1em}
MinIO is self-hostable but not suited for geo-distributed deployments
\vspace{1em}
\textbf{Garage is a self-hosted drop-in replacement for the Amazon S3 object store}
\end{frame}
\begin{frame}
\frametitle{CRDTs / weak consistency instead of consensus}
Consensus can be implemented reasonably well in practice, so why avoid it?
\vspace{1em}
\begin{itemize}
\item \textbf{Software complexity}
\vspace{1em}
\item \textbf{Performance issues:}
\vspace{.5em}
\begin{itemize}
\item The leader is a \textbf{bottleneck} for all requests\\
\vspace{.5em}
\item \textbf{Sensitive to higher latency} between nodes
\vspace{.5em}
\item \textbf{Takes time to reconverge} when disrupted (e.g. node going down)
\end{itemize}
\end{itemize}
\vspace{2em}
$\to$ Garage uses only CRDTs internally (conflict-free replicated data types)
\end{frame}
\begin{frame}
\frametitle{The data model of object storage}
Object storage is basically a key-value store:
\vspace{1em}
{\footnotesize
\begin{center}
\begin{tabular}{|l|p{8cm}|}
\hline
\textbf{Key: file path + name} & \textbf{Value: file data + metadata} \\
\hline
\hline
\texttt{index.html} &
\texttt{Content-Type: text/html; charset=utf-8} \newline
\texttt{Content-Length: 24929} \newline
\texttt{<binary blob>} \\
\hline
\texttt{img/logo.svg} &
\texttt{Content-Type: text/svg+xml} \newline
\texttt{Content-Length: 13429} \newline
\texttt{<binary blob>} \\
\hline
\texttt{download/index.html} &
\texttt{Content-Type: text/html; charset=utf-8} \newline
\texttt{Content-Length: 26563} \newline
\texttt{<binary blob>} \\
\hline
\end{tabular}
\end{center}
}
\vspace{1em}
Simple interface, compatible with many existing applications
\vspace{1em}
Maps well to CRDT data types
\end{frame}
\begin{frame}
\frametitle{Performance gains in practice}
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/endpoint_latency_0.7_0.8_minio.png}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Timeline}
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/timeline-22-24.pdf}
\end{center}
\end{frame}
% ====================== v0.7.0 ===============================
\begin{frame}
\frametitle{April 2022 - Garage v0.7.0}
Focus on \underline{observability and ecosystem integration}
\vspace{2em}
\begin{itemize}
\item \textbf{Monitoring:} metrics and traces, using OpenTelemetry
\vspace{1em}
\item Alternative replication modes with 1 or 2 copies,\\
modes with weaker consistency
\vspace{1em}
\item Kubernetes integration
\vspace{1em}
\item Admin API (v0.7.2)
\vspace{1em}
\item Experimental K2V API (v0.7.2)
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Metrics (Prometheus + Grafana)}
\begin{center}
\includegraphics[width=.9\linewidth]{../assets/grafana_dashboard.png}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Traces (Jaeger)}
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/jaeger_listobjects.png}
\end{center}
\end{frame}
% ====================== v0.8.0 ===============================
\begin{frame}
\frametitle{November 2022 - Garage v0.8.0}
Focus on \underline{performance}
\vspace{2em}
\begin{itemize}
\item \textbf{Alternative metadata DB engines} (LMDB, Sqlite)
\vspace{1em}
\item \textbf{Performance improvements:} block streaming, various optimizations...
\vspace{1em}
\item Bucket quotas (max size, max \#objects)
\vspace{1em}
\item Quality of life improvements, observability, etc.
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{About metadata DB engines}
Issues with Sled:
\vspace{2em}
\begin{itemize}
\item Huge files on disk
\vspace{.5em}
\item Unpredictable performance, especially on HDD
\vspace{.5em}
\item API limitations
\vspace{.5em}
\item Not actively maintained
\end{itemize}
\vspace{2em}
LMDB: very stable, good performance, reasonably small files on disk
\vspace{1em}
Sled will be removed in Garage v1.0
\end{frame}
\begin{frame}
\frametitle{DB engine performance comparison}
\begin{center}
\includegraphics[width=.6\linewidth]{../assets/db_engine.png}
\end{center}
NB: Sqlite was slow due to synchronous journaling mode, now configurable
\end{frame}
\begin{frame}
\frametitle{Block streaming}
\begin{center}
\only<1>{\includegraphics[width=.8\linewidth]{../assets/schema-streaming-1.png}}
\only<2>{\includegraphics[width=.8\linewidth]{../assets/schema-streaming-2.png}}
\end{center}
\end{frame}
\begin{frame}
\frametitle{TTFB benchmark}
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/ttfb.png}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Throughput benchmark}
\begin{center}
\includegraphics[width=.7\linewidth]{../assets/io-0.7-0.8-minio.png}
\end{center}
\end{frame}
% ====================== v0.9.0 ===============================
\begin{frame}
\frametitle{October 2023 - Garage v0.9.0}
Focus on \underline{streamlining \& usability}
\vspace{2em}
\begin{itemize}
\item Support multiple HDDs per node
\vspace{1em}
\item S3 compatibility:
\vspace{1em}
\begin{itemize}
\item support basic lifecycle configurations
\vspace{.5em}
\item allow for multipart upload part retries
\end{itemize}
\vspace{1em}
\item LMDB by default, deprecation of Sled
\vspace{1em}
\item New layout computation algorithm
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Layout computation}
\begin{center}
\includegraphics[width=\linewidth]{../assets/location-aware.png}
\end{center}
\vspace{2em}
Garage replicates data on different zones when possible
\end{frame}
\begin{frame}
\frametitle{Layout computation}
\begin{center}
\includegraphics[width=.8\linewidth]{../assets/map.png}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Optimal layout computation}
\begin{center}
\includegraphics[width=.6\linewidth]{../assets/geodistrib_paper.png}
\end{center}
\end{frame}
\begin{frame}
\frametitle{What a "layout" is}
\textbf{A layout is a precomputed index table}
\vspace{2em}
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
\textbf{Partition} & \textbf{Node 1} & \textbf{Node 2} & \textbf{Node 3} \\
\hline
\hline
Partition 0 & Io (jupiter) & Drosera (atuin) & Courgette (neptune) \\
\hline
Partition 1 & Datura (atuin) & Courgette (neptune) & Io (jupiter) \\
\hline
Partition 2 & Io(jupiter) & Celeri (neptune) & Drosera (atuin) \\
\hline
\hspace{1em}$\vdots$ & \hspace{1em}$\vdots$ & \hspace{1em}$\vdots$ & \hspace{1em}$\vdots$ \\
\hline
Partition 255 & Concombre (neptune) & Io (jupiter) & Drosera (atuin) \\
\hline
\end{tabular}
\end{center}
\vspace{1em}
The index table is built centrally using an optimal algorithm,\\
then propagated to all nodes
\end{frame}
\begin{frame}
\frametitle{The relationship between \emph{partition} and \emph{partition key}}
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
\textbf{Partition key} & \textbf{Partition} & \textbf{Sort key} & \textbf{Value} \\
\hline
\hline
\texttt{website} & Partition 12 & \texttt{index.html} & (file data) \\
\hline
\texttt{website} & Partition 12 & \texttt{img/logo.svg} & (file data) \\
\hline
\texttt{website} & Partition 12 &\texttt{download/index.html} & (file data) \\
\hline
\hline
\texttt{backup} & Partition 42 & \texttt{borg/index.2822} & (file data) \\
\hline
\texttt{backup} & Partition 42 & \texttt{borg/data/2/2329} & (file data) \\
\hline
\texttt{backup} & Partition 42 & \texttt{borg/data/2/2680} & (file data) \\
\hline
\hline
\texttt{private} & Partition 42 & \texttt{qq3a2nbe1qjq0ebbvo6ocsp6co} & (file data) \\
\hline
\end{tabular}
\end{center}
\vspace{1em}
\textbf{To read or write an item:} hash partition key
\\ \hspace{5cm} $\to$ determine partition number (first 8 bits)
\\ \hspace{5cm} $\to$ find associated nodes
\end{frame}
% ====================== v0.10.0 ===============================
\begin{frame}
\frametitle{October 2023 - Garage v0.10.0 beta}
Focus on \underline{consistency}
\vspace{2em}
\begin{itemize}
\item Fix consistency issues when reshuffling data
\end{itemize}
\end{frame}
% --------------------------------------------------------------------
% =================================================================================
% =================================================================================
% --------------------------------------------------------------------
% --------------------------------------------------------------------
% --------------------------------------------------------------------
% =================================================================================
% =================================================================================
% --------------------------------------------------------------------
\begin{frame}
\frametitle{Two big problems}
\begin{enumerate}
\item \textbf{How to place data on different nodes?}\\
\vspace{1em}
\underline{Constraints:} heterogeneous hardware\\
\underline{Objective:} $n$ copies of everything, maximize usable capacity, maximize resilience\\
\vspace{1em}
$\to$ the Dynamo model + optimization algorithms
\vspace{2em}
\item<2-> \textbf{How to guarantee consistency?}\\
\vspace{1em}
\underline{Constraints:} slow network (geographical distance), node unavailability/crashes\\
\underline{Objective:} maximize availability, read-after-write guarantee\\
\vspace{1em}
$\to$ CRDTs, monotonicity, read and write quorums
\end{enumerate}
\end{frame}
\section{Problem 1: placing data}
\begin{frame}
\frametitle{Key-value stores, upgraded: the Dynamo model}
\textbf{Two keys:}
\begin{itemize}
\item Partition key: used to divide data into partitions {\small (a.k.a.~shards)}
\item Sort key: used to identify items inside a partition
\end{itemize}
\vspace{1em}
\begin{center}
\begin{tabular}{|l|l|p{3cm}|}
\hline
\textbf{Partition key: bucket} & \textbf{Sort key: filename} & \textbf{Value} \\
\hline
\hline
\texttt{website} & \texttt{index.html} & (file data) \\
\hline
\texttt{website} & \texttt{img/logo.svg} & (file data) \\
\hline
\texttt{website} & \texttt{download/index.html} & (file data) \\
\hline
\hline
\texttt{backup} & \texttt{borg/index.2822} & (file data) \\
\hline
\texttt{backup} & \texttt{borg/data/2/2329} & (file data) \\
\hline
\texttt{backup} & \texttt{borg/data/2/2680} & (file data) \\
\hline
\hline
\texttt{private} & \texttt{qq3a2nbe1qjq0ebbvo6ocsp6co} & (file data) \\
\hline
\end{tabular}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Key-value stores, upgraded: the Dynamo model}
\begin{itemize}
\item Data with different partition keys is stored independently,\\
on a different set of nodes\\
\vspace{.5em}
$\to$ no easy way to list all partition keys\\
$\to$ no cross-shard transactions\\
\vspace{2em}
\item Placing data: hash the partition key, select nodes accordingly\\
\vspace{.5em}
$\to$ distributed hash table (DHT)
\vspace{2em}
\item For a given value of the partition key, items can be listed using their sort keys
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Issues with consistent hashing}
\begin{itemize}
\item Consistent hashing doesn't dispatch data based on geographical location of nodes
\vspace{1em}
\item<2-> Geographically aware adaptation, try 1:\\
data quantities not well balanced between nodes
\vspace{1em}
\item<3-> Geographically aware adaptation, try 2:\\
too many reshuffles when adding/removing nodes
\end{itemize}
\end{frame}
\section{Problem 2: ensuring consistency}
\begin{frame}
\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 Any sequential specification can be implemented as a \textbf{replicated state machine}
\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 single bottleneck;\\
works the same with offline nodes
\end{itemize}
} \end{minipage}
\hspace{1em}
\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{additional assumptions} such as a fault detector or a strong RNG\\
(FLP impossibility theorem)
\end{minipage}
\hfill
\begin{minipage}{6.5cm}
\underline{Weakly consistent systems:}
\vspace{1em}
Can be implemented in \textbf{any\\asynchronous message passing\\distributed system} with node crashes
\end{minipage}
\hspace{1em}
\vspace{3em}
\begin{center}
They represent \textbf{different classes of computational capability}\\
\end{center}
\end{frame}
\begin{frame}
\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}{6.5cm}
\underline{Consensus-based systems:}
\vspace{1em}
\textbf{Any sequential specification}\\~
\vspace{1em}
\textbf{Easier to program for}: just write your program as if it were sequential on a single machine
\end{minipage}
\hfill
\begin{minipage}{6.5cm}
\underline{Weakly consistent systems:}
\vspace{1em}
\textbf{Only CRDTs}\\(conflict-free replicated data types)
\vspace{1em}
Part of the complexity is \textbf{reported to the consumer of the API}\\~
\end{minipage}
\hspace{1em}
\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 \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<2-> \textbf{This API can be implemented using only weak primitives}\\
(e.g. in the asynchronous message passing model with no further assumption)\\
$\to$ the API is strictly weaker than consensus\\
$\to$ we can implement it in Garage!
\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<2-> Non-transactional key-value stores such as S3 are equivalent to a simple CRDT:\\
a map of \textbf{last-writer-wins registers} (each key is its own CRDT)
\vspace{1em}
\item<3-> \textbf{Read-after-write consistency} can be implemented
using quorums on read and write operations
\vspace{1em}
\item<4-> \textbf{Monotonicity of reads} can be implemented with repair-on-read\\
(makes reads more costly, not implemented in Garage)
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{CRDTs and quorums: read-after-write consistency}
\begin{figure}
\centering
\def\svgwidth{.8\textwidth}
\only<1>{\import{assets/}{lattice1.pdf_tex}}%
\only<2>{\import{assets/}{lattice2.pdf_tex}}%
\only<3>{\import{assets/}{lattice3.pdf_tex}}%
\only<4>{\import{assets/}{lattice4.pdf_tex}}%
\only<5>{\import{assets/}{lattice5.pdf_tex}}%
\only<6>{\import{assets/}{lattice6.pdf_tex}}%
\only<7>{\import{assets/}{lattice7.pdf_tex}}%
\only<8>{\import{assets/}{lattice8.pdf_tex}}%
\end{figure}
\end{frame}
\begin{frame}
\frametitle{CRDTs and quorums: read-after-write consistency}
\textbf{Property:} If node $A$ did an operation $write(x)$ and received an OK response,\\
\hspace{2cm} and node $B$ starts an operation $read()$ after $A$ received OK,\\
\hspace{2cm} then $B$ will read a value $x' \sqsupseteq x$.
\vspace{1em}
\hspace{1em}
\begin{minipage}{6.8cm}
\textbf{Algorithm $write(x)$:}
\begin{enumerate}
\item Broadcast $write(x)$ to all nodes
\item Wait for $k > n/2$ nodes to reply OK
\item Return OK
\end{enumerate}
\end{minipage}
\hfill
\begin{minipage}{6.8cm}
\vspace{1em}
\textbf{Algorithm $read()$:}
\begin{enumerate}
\item Broadcast $read()$ to all nodes
\item Wait for $k > n/2$ nodes to reply\\
with values $x_1, \dots, x_k$
\item Return $x_1 \sqcup \dots \sqcup x_k$
\end{enumerate}
\end{minipage}
\hspace{1em}
\vspace{2em}
\textbf{Why does it work?} There is at least one node at the intersection between the two sets of nodes that replied to each request, that ``saw'' $x$ before the $read()$ started ($x_i \sqsupseteq x$).
\end{frame}
\begin{frame}
\frametitle{CRDTs and quorums: monotonic-reads consistency}
\begin{figure}
\centering
\def\svgwidth{.8\textwidth}
\only<1>{\import{assets/}{latticeB_1.pdf_tex}}%
\only<2>{\import{assets/}{latticeB_2.pdf_tex}}%
\only<3>{\import{assets/}{latticeB_3.pdf_tex}}%
\only<4>{\import{assets/}{latticeB_4.pdf_tex}}%
\only<5>{\import{assets/}{latticeB_5.pdf_tex}}%
\only<6>{\import{assets/}{latticeB_6.pdf_tex}}%
\only<7>{\import{assets/}{latticeB_7.pdf_tex}}%
\only<8>{\import{assets/}{latticeB_8.pdf_tex}}%
\only<9>{\import{assets/}{latticeB_9.pdf_tex}}%
\only<10>{\import{assets/}{latticeB_10.pdf_tex}}%
\end{figure}
\end{frame}
\begin{frame}
\frametitle{CRDTs and quorums: monotonic-reads consistency}
\textbf{Property:} If node $A$ did an operation $read()$ and received $x$ as a response,\\
\hspace{2cm} and node $B$ starts an operation $read()$ after $A$ received $x$,\\
\hspace{2cm} then $B$ will read a value $x' \sqsupseteq x$.
\vspace{1em}
\textbf{Algorithm $monotonic\_read()$:} {\small (a.k.a. repair-on-read)}
\begin{enumerate}
\item Broadcast $read()$ to all nodes
\item Wait for $k > n/2$ nodes to reply with values $x_1, \dots, x_k$
\item If $x_i \ne x_j$ for some nodes $i$ and $j$,\\
\hspace{1cm}then call $write(x_1 \sqcup \dots \sqcup x_k)$ and wait for OK from $k' > n/2$ nodes
\item Return $x_1 \sqcup \dots \sqcup x_k$
\end{enumerate}
\vspace{1em}
This makes reads slower in some cases, and is \textbf{not implemented in Garage}.
\end{frame}
\begin{frame}
\frametitle{A hard problem: layout changes}
\begin{itemize}
\item We rely on quorums $k > n/2$ within each partition:\\
$$n=3,~~~~~~~k\ge 2$$
\item<2-> When rebalancing, the set of nodes responsible for a partition can change:\\
$$\{n_A, n_B, n_C\} \to \{n_A, n_D, n_E\}$$
\vspace{.01em}
\item<3-> During the rebalancing, $D$ and $E$ don't yet have the data,\\
~~~~~~~~~~~~~~~~~~~and $B$ and $C$ want to get rid of the data to free up space\\
\vspace{.2em}
$\to$ quorums only within the new set of nodes don't work\\
$\to$ how to coordinate? \textbf{currently, we don't...}
\end{itemize}
\end{frame}
\section{Operating big Garage clusters}
\begin{frame}
\frametitle{Operating Garage}
\begin{center}
\only<1-2>{
\includegraphics[width=.9\linewidth]{assets/scr_garage_status.png}
\\\vspace{1em}
\visible<2>{\includegraphics[width=.85\linewidth]{assets/scr_garage_status_broken.png}}
}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Garage's architecture}
\begin{center}
\only<1>{\includegraphics[width=.45\linewidth]{assets/garage.drawio.pdf}}%
\only<2>{\includegraphics[width=.6\linewidth]{assets/garage_sync.drawio.pdf}}%
\end{center}
\end{frame}
\begin{frame}
\frametitle{Digging deeper}
\begin{center}
\only<1>{\includegraphics[width=.9\linewidth]{assets/scr_garage_stats.png}}
\only<2>{\includegraphics[width=.6\linewidth]{assets/scr_garage_worker_list.png}}
\only<3>{\includegraphics[width=.6\linewidth]{assets/scr_garage_worker_get.png}}
\end{center}
\end{frame}
\begin{frame}
\frametitle{Potential limitations and bottlenecks}
\begin{itemize}
\item Global:
\begin{itemize}
\item Max. $\sim$100 nodes per cluster (excluding gateways)
\end{itemize}
\vspace{1em}
\item Metadata:
\begin{itemize}
\item One big bucket = bottleneck, object list on 3 nodes only
\end{itemize}
\vspace{1em}
\item Block manager:
\begin{itemize}
\item Lots of small files on disk
\item Processing the resync queue can be slow
\item Multi-HDD support not yet released (soon!)
\end{itemize}
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Deployment advice for very large clusters}
\begin{itemize}
\item Metadata storage:
\begin{itemize}
\item ZFS mirror (x2) on fast NVMe
\item Use LMDB storage engine
\end{itemize}
\vspace{.5em}
\item Data block storage:
\begin{itemize}
\item Wait for v0.9 with multi-HDD support
\item XFS on individual drives
\item Increase block size (1MB $\to$ 10MB, requires more RAM and good networking)
\item Tune \texttt{resync-tranquility} and \texttt{resync-worker-count} dynamically
\end{itemize}
\vspace{.5em}
\item Other :
\begin{itemize}
\item Split data over several buckets
\item Use less than 100 storage nodes
\item Use gateway nodes
\end{itemize}
\vspace{.5em}
\end{itemize}
Current deployments: $< 10$ TB, we don't have much experience with more
\end{frame}
\section{Going further than the S3 API}
\begin{frame}
\frametitle{Using Garage for everything}
\begin{center}
\only<1>{\includegraphics[width=.8\linewidth]{assets/slideB1.png}}%
\only<2>{\includegraphics[width=.8\linewidth]{assets/slideB2.png}}%
\only<3>{\includegraphics[width=.8\linewidth]{assets/slideB3.png}}%
\end{center}
\end{frame}
\begin{frame}
\frametitle{K2V Design}
\begin{itemize}
\item A new, custom, minimal API\\
\vspace{.5em}
\begin{itemize}
\item Single-item operations
\item Operations on ranges and batches of items
\item Polling operations to help implement a PubSub pattern
\end{itemize}
\vspace{1em}
\item<2-> Exposes the partitoning mechanism of Garage\\
K2V = partition key / sort key / value (like Dynamo)
\vspace{1em}
\item<3-> Weakly consistent, CRDT-friendly\\
$\to$ no support for transactions (not ACID)
\vspace{1em}
\item<4-> Cryptography-friendly: values are binary blobs
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Handling concurrent values}
\textbf{How to handle concurrency?} Example:
\vspace{1em}
\begin{enumerate}
\item Client $A$ reads the initial value of a key, $x_0$
\vspace{1em}
\item<2-> Client $B$ also reads the initial value $x_0$ of that key
\vspace{1em}
\item<3-> Client $A$ modifies $x_0$, and writes a new value $x_1$
\vspace{1em}
\item<4-> Client $B$ also modifies $x_0$, and writes a new value $x'_1$,\\
without having a chance to first read $x_1$\\
\vspace{1em}
$\to$ what should the final state be?
\end{enumerate}
\end{frame}
\begin{frame}
\frametitle{Handling concurrent values}
\begin{itemize}
\item If we keep only $x_1$ or $x'_1$, we risk \textbf{loosing application data}
\vspace{1.5em}
\item<2-> Values are opaque binary blobs, \textbf{K2V cannot resolve conflicts} by itself\\
(e.g. by implementing a CRDT)
\vspace{1.5em}
\item<3-> Solution: \textbf{we keep both!}\\
$\to$ the value of the key is now $\{x_1, x'_1\}$\\
$\to$ the client application can decide how to resolve conflicts on the next read
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Keeping track of causality}
How does K2V know that $x_1$ and $x'_1$ are concurrent?
\vspace{1em}
\begin{itemize}
\item $read()$ returns \textbf{a set of values} and an associated \textbf{causality token}\\
\vspace{1.5em}
\item<2-> When calling $write()$, the client sends \textbf{the causality token from its last read}
\vspace{1.5em}
\item<3-> The causality token represents the set of values \textbf{already seen by the client}\\
$\to$ those values are the \textbf{causal past} of the write operation\\
$\to$ K2V can keep concurrent values and overwrite all ones in the causal past
\vspace{1.5em}
\item<4-> Internally, the causality token is \textbf{a vector clock}
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Application: an e-mail storage server}
\begin{center}
\only<1>{\includegraphics[width=.9\linewidth]{assets/aerogramme.png}}%
\end{center}
\end{frame}
\begin{frame}
\frametitle{Aerogramme data model}
\begin{center}
\only<1->{\includegraphics[width=.4\linewidth]{assets/aerogramme_datatype.drawio.pdf}}%
\end{center}
\visible<2->{Aerogramme encrypts all stored values for privacy\\
(Garage server administrators can't read your mail)}
\end{frame}
\begin{frame}
\frametitle{Different deployment scenarios}
\begin{center}
\only<1>{\includegraphics[width=.9\linewidth]{assets/aerogramme_components1.drawio.pdf}}%
\only<2>{\includegraphics[width=.9\linewidth]{assets/aerogramme_components2.drawio.pdf}}%
\end{center}
\end{frame}
\begin{frame}
\frametitle{A new model for building resilient software}
How to build an application using only Garage as a data store:
\vspace{1em}
\begin{enumerate}
\item Design a data model suited to K2V\\
{\footnotesize (see Cassandra docs on porting SQL data models to Cassandra)}
\vspace{1em}
\begin{itemize}
\item Use CRDTs or other eventually consistent data types (see e.g. Bayou)
\vspace{1em}
\item Store opaque binary blobs to provide End-to-End Encryption\\
\end{itemize}
\vspace{1em}
\item<2-> Store big blobs (files) using the S3 API
\vspace{1em}
\item<3-> Let Garage manage sharding, replication, failover, etc.
\end{enumerate}
\end{frame}
\section{Conclusion}
\begin{frame}
\frametitle{Perspectives}
\begin{itemize}
\item Fix the consistency issue when rebalancing
\vspace{1em}
\item Write about Garage's architecture and properties,\\
and about our proposed architecture for (E2EE) apps over K2V+S3
\vspace{1em}
\item Continue developing Garage; finish Aerogramme; build new applications...
\vspace{1em}
\item Anything else?
\end{itemize}
\end{frame}
\begin{frame}
\frametitle{Where to find us}
\begin{center}
\includegraphics[width=.25\linewidth]{../../logo/garage_hires.png}\\
\vspace{-1em}
\url{https://garagehq.deuxfleurs.fr/}\\
\url{mailto:garagehq@deuxfleurs.fr}\\
\texttt{\#garage:deuxfleurs.fr} on Matrix
\vspace{1.5em}
\includegraphics[width=.06\linewidth]{assets/rust_logo.png}
\includegraphics[width=.13\linewidth]{assets/AGPLv3_Logo.png}
\end{center}
\end{frame}
\end{document}
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