Chapar

A framework for modular verification of causal consistency for replicated key-value store implementations and their client programs in Coq. Includes proofs of the causal consistency of two key-value store implementations and a simple automatic model checker for the correctness of client programs.

Meta

• Author(s):
  • Mohsen Lesani (initial)
  • Christian J. Bell (initial)
  • Adam Chlipala (initial)
• Coq-community maintainer(s):
  • Karl Palmskog (@palmskog)
• License: MIT License
• Compatible Coq versions: 8.14 or later
• Additional dependencies: none
• Coq namespace: Chapar
• Related publication(s):
  • Chapar: Certified Causally Consistent Distributed Key-value Stores doi:10.1145/2837614.2837622

Building and installation instructions

The easiest way to install the latest released version of Chapar is via OPAM:

opam repo add coq-released https://coq.inria.fr/opam/released
opam install coq-chapar

To instead build and install manually, do:

git clone https://github.com/coq-community/chapar.git
cd chapar
make   # or make -j <number-of-cores-on-your-machine> 
make install

Chapar Key-value Stores

Three key-value stores, verified to be causally consistent in the Coq proof assistant and extracted to executable code.

• Compatible OCaml versions: 4.05.0 or later
• Additional dependencies:
  • OCamlbuild
  • Batteries Included 2.8.0 or later

Building

To extract the code and compile manually, use the following command:

make stores

Documentation

Coq Framework

The Coq definitions and proofs are located in the coq directory. The code location of the definitions and lemmas presented in the paper are listed below.

Semantics and the Proof Technique

• Section 2, Figure 3 (Program): KVStore.v, Section ValSec
• Section 2, Figure 4 (Key-value Store Algorithm Interface): KVStore.v, Module Type AlgDef
• Section 2, Figure 5 (Concrete Operational Semantics): KVStore.v, Module ConcExec
• Section 3, Figure 6 (Abstract Operational Semantics): KVStore.v, Module AbsExec
• Section 4, Figure 8 (Concrete Instrumented Operational Semantics): KVStore.v, Module InstConcExec
• Section 4, Figure 10 (Correctness Condition WellRec): KVStore.v, Module Type CauseObl
• Section 4, Figure 11 (Causal relation): KVStore.v, Definition cause_step and Inductive cause
• Section 4, Figure 12 (Sequential Operational Semantics): KVStore.v, Module SeqExec
• Section 4, Definition 2 (Causal Consistency) and Theorem 2 (Sufficiency of Well-reception): KVStore.v, Theorem CausallyConsistent. Note that (CauseObl: CauseObl AlgDef) is a parameter of the module ExecToAbstExec.
• Section 4, Lemma 1: KVStore.v, Lemma FaultFreedom. Note that (CauseObl: CauseObl AlgDef) is a parameter of the module ExecToAbstExec.

Algorithms

• Section 5, Figure 13 (Algorithm 1): KVSAlg1.v, Module Type KVSAlg1
• Section 5, Theorem 3: KVSAlg1.v, Module KVSAlg1CauseObl (SyntaxArg: SyntaxPar) <: CauseObl KVSAlg1 SyntaxArg
• Section 5, Corollary 1: KVSAlg1.v, Lemma CausallyConsistent
• Section 5, Lemma 2 (Clock Monotonicity): KVSAlg1.v, Lemma cause_clock
• Section 5, Lemma 3 (CauseCond): KVSAlg1.v, Lemma cause_rec
• Section 5, Figure 14 (Algorithm 2): KVSAlg2.v, Module Type KVSAlg2
• Section 5, Theorem 3: KVSAlg1.v, Module KVSAlg2CauseObl (SyntaxArg: SyntaxPar) <: CauseObl KVSAlg2 SyntaxArg
• Secton 5, Corollary 2: KVSAlg2.v, Lemma CausallyConsistent
• Secton 5, Lemma 4 (Update Dependency Transitivity): KVSAlg2.v, Lemma cause_dep
• Secton 5, Lemma 5: KVSAlg2.v, Lemma cause_received_received
• Secton 5, Lemma 6 (CauseCond): KVSAlg2.v, Lemma cause_rec
• Section 10, Figure 16 (Algorithm 3): KVSAlg3.v, Module Type KVSAlg3
• Section 10, Theorem 5: KVSAlg3.v, Module KVSAlg3CauseObl (SyntaxArg: SyntaxPar) <: CauseObl KVSAlg3 SyntaxArg
• Secton 5, Corollary 3: KVSAlg3.v, Lemma CausallyConsistent
• Section 5, Lemma 7 (Clock Monotonicity): KVSAlg3.v, Lemma cause_clock
• Section 5, Lemma 8 (Dep less than equal Rec): KVSAlg3.v, Lemma dep_leq_rec
• Section 5, Lemma 9 (CauseCond): KVSAlg3.v, Lemma cause_rec

Clients

• Section 1, Program 1: Clients.v, Definition prog_photo_upload
• Section 1, Program 2: Clients.v, Definition prog_lost_ring
• Section 10, Program 3: ListClient.v
• Section 2, Theorem 1: Clients.v, Lemma CauseConsistent_Prog1
• Section 3, Definition 1 (Cause-content Program): Definition CausallyContent
• Section 6: ReflectiveAbstractSemantics.v

Experiment Setup

Directory structure

• root (directory): The execution scripts described in the section Running Experiments below

• coq (directory); the Coq verification framework:

  • Framework/KVStore.v: The basic definitions, the semantics and accompanying lemma
  • Framework/ReflectiveAbstractSemantics.v: The client verification definitions and lemmas
  • Algorithms/KVSAlg1.v: The definition and proof of algorithm 1 in the paper
  • Algorithms/KVSAlg2.v: The definition and proof of algorithm 2 in the paper
  • Algorithms/KVSAlg3.v: The definition and proof of algorithm 3 in the appendix
  • Algorithms/ExtractAlgorithm.v: Extraction directives for extracting the algorithms to OCaml
  • Examples/Clients.v: Verified client program
  • Examples/ListClient.v: Verified client program
  • Lib (directory): General purpose Coq libraries

• ml (directory); the OCaml runtime to execute the algorithms:

  • algorithm.ml: Key-value store algorithm shared interface
  • algorithm1.ml, algorithm2.ml, algorithm3.ml: Wrappers for the extracted algorithms
  • benchgen.ml: Benchmark generation and storing program
  • benchprog.ml: Benchmark retrieval program
  • commonbench.ml: Common definitions for benchmarks
  • common.ml: Common definitions
  • configuration.ml: Execution configuration definitions
  • readConfig.ml: Configuration retrieval program
  • runtime.ml: Execution runtime
  • launchStore1.ml, launchStore2.ml, launchStore3.ml: Launchers for the extracted algorithms
  • util.ml: General purpose OCaml functions
  • experiment.ml: Small OCaml programming tests

Running Experiments

Overview

We run with 4 nodes called the worker nodes and a node called the master node that keeps track of the start and end of the runs. The scripts support running with both the current terminal blocked or detached. In the former, the terminal should be active for the entire execution time. To avoid this, we use another node called the launcher node. Repeating and collecting the results of the runs is delegated to the launcher node. The terminal can be closed and the execution results can be retrieved later from the launcher node. The four workers, the master and the launcher can be different nodes. However, to simplify running, the scripts support assigning one host to all of these roles. This is the default setting.

The settings of the nodes can be edited in the file Settings.txt. The following should be noted if other machines are used as the running nodes. The host should have password-less ssh access to the launcher node. The launcher node should have password-less ssh access to the other nodes. This can be done by copying the public key of the accessing machine to the accessed machine by a command like:

cat ~/.ssh/id_dsa.pub | ssh -l remoteuser remote.example.com 'cat >> ~/.ssh/authorized_keys'

The port numbers 9100, 9101, 9102, and 9103 should be open on the worker nodes 1, 2, 3 and 4 respectively. The port number 9099 should be open on the master node.

A simple run

To start the run:

./batchrundetach

To check the status of the run:

./printlauncherout

To get the results once the run is finished:

./fetchresults

The result are stored in the file RemoteAllResults.txt. See fetchresults below for the format of the results.

Settings and scripts

All of the following files are in the root directory:

• Settings.txt

  • KeyRange: The range of keys in the generated benchmarks is from 0 to this number. For our experiments, it is set to 50.
  • RepeatCount: The number of times that each experiment is repeated. For our experiments, it is set to 5.
  • LauncherNode: The user name and the ip of the launcher node
  • MasterNode: The user name and the ip of the master node
  • WorkerNodes: The user name and the ip of the worker nodes

• batchrun: This is the place where the experiments are listed. Each call to the script run is an experiment. The arguments are:

  • argument 1: The number of nodes. This is 4 for our purpose.
  • argument 2: The number of operations per server. This is 60000 in our experiments.
  • argument 3: The percent of puts. This ranges from 10 to 90 in our experiments.

  This script can be called without using the launcher node. The current terminal is blocked. See batchrundetach below for detached execution of the experiments.

• batchrundetach: To execute using the launcher node. The current terminal is detached.

• printlauncherout: To see the output of the launcher even while the experiments are being run

• printnodesout: To see the output of the worker nodes

• fetchresults: To get the results. The fetched files are:

  • RemoteAllResults.txt: The timing of the replicas
  • RemoteAllOutputs.txt: The outputs of the replicas
  • RemoteLauncherOutput.txt: The output of the launcher node

  The following example output is for the algorithm 2 with 4 worker nodes and 40000 operations per node with 10 percent puts. It shows two runs. Under each run, the time spend by each of the nodes is shown. We compute the maximum of these four numbers to compute the total process time.

  Algorithm: 2
  Server count: 4
   Operation count: 40000
  Put percent: 10
  Run: 1
  1.000000
  3.000000
  1.000000
  1.000000
  Run: 2
  1.000000
  1.000000
  4.000000
  1.000000
  

• clearnodes: To remove the output and result files and the running processes in all the nodes; this is used to start over

The experiments in the paper

The goal of our experimental result section was to show that our verification effort can lead to executable code and also to compare the performance of the two algorithms. As described in the paper, the experiments were done with four worker nodes cluster. Each worker node had an Intel(R) Xeon(R) 2.27GHz CPU with 2GB of RAM and ran Linux Ubuntu 14.04.2 with the kernel version 3.13.0-48-generic#80-Ubuntu. The nodes were connected to a gigabit switch. The keys were uniformly selected from 0 to 50 for the benchmarks. Each experiment was repeated 5 times. (The reported numbers are the arithmetic mean of the five runs.) Each node processed 60,000 requests. The put ratio ranged from 10% to 90%.

Here are the contents of the two configuration files, Settings.txt and batchrun: Note: user and ip should be filled with specific values.

• Settings.txt:

  KeyRange=
  50
  RepeatCount=
  5
  LauncherNode=
  <user>@<ip>
  MasterNode=
  <user>@<ip>
  WorkerNodes=
  <user>@<ip>
  <user>@<ip>
  <user>@<ip>
  <user>@<ip>
  

• batchrun:

  ./run 4 60000 10
  ./run 4 60000 20
  ./run 4 60000 30
  ./run 4 60000 40
  ./run 4 60000 50
  ./run 4 60000 60
  ./run 4 60000 70
  ./run 4 60000 80
  ./run 4 60000 90
  

Interpretation of results from the paper

As expected, the throughput of both of the stores increases as the ratio of the get operation increases. The second algorithm shows a higher throughput than the first algorithm. The reason is twofold. Firstly, in the first algorithm, the clock function of a node keeps an over-approximation of the dependencies of the node. This over-approximation incurs extra dependencies on updates. On the other hand, the second algorithm does not require any extra dependencies. Therefore, in the first algorithm compared to the second, the updates can have longer waiting times, and the update queues tend to be longer. Therefore, the traversal of the update queue is more time consuming in the first algorithm than the second. Secondly, the update payload that is sent and received by the first algorithm contains the function clock. OCaml cannot marshal functions. However, as the clock function has the finite domain of the participating nodes, it can be serialized to and deserialized from a list. Nonetheless, serialization and de-serialization on every sent and received message adds performance cost. On the other hand, the payload of the second algorithm consists of only data types that can be directly marshalled. Therefore, the second algorithm has no extra marshalling cost.