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Embeddable multi-Paxos For The JVM

Build Status

Trex: An embeddable Paxos engine for the JVM

Checkout the blog posts for a description of this implementation of Paxos Made Simple.


  • trex-library is the core protocol that is meant for reuse.
  • trex-core is a testable scaffolding that is built using Akka primarily as Akka has excellent test tooling for async messaging. If you are not using Akka in your project you would provide your own alternatives and not use these classes.
  • trex-demo contains an example java app that implements a stack replicated across a paxos cluster.


# Kick the tires
sbt clean test it:test
sbt coverageReport

Java Clustered Stack Demo

This implements a stack service as a paxos cluster with the interface:

package com.github.trex_paxos.javademo;

public interface StringStack {
    String push(String item);
    String pop();
    String peek();
    boolean empty();
    int search(Object o);

The concrete implementation class is com.github.trex_paxos.javademo.StringStackImpl which is a simple wrapper over java.util.Stack<String> where operations are also serialized to disk so that the processes can be killed and restarted to recover the state of the in-memory stack.

In order to run this service as a Paxos cluster the trex-core package is used to wrap the StringStackImpl as create a TrexServer that listens on a unique TCP socket for client connections. The set of servers use UDP to broadcast messages to elect a leader and run the Paxos algorithm.

The class com.github.trex_paxos.javademo.StackClient obtains a dynamic proxy that implements the StringStack interface. The dynamic proxy uses DynamicClusterDriver that makes TCP connections to the all the TrexServer and sends ClientCommandValue messages to current leader.

To run the demo first make some state folders with:

mkdir -p /private/tmp/2552
mkdir -p /private/tmp/2562
mkdir -p /private/tmp/2572

Note the folder will not be automatically cleaned up so that you can experiment with killing and restarting severs.

Then run three versions of com.github.trex_paxos.javademo.StackClusterNode passing the following arguments:

  • server3.conf 2552
  • server3.conf 2562
  • server3.conf 2572

Once the cluster is up run the client main method with:

com.github.trex_paxos.javademo.StackClient clustered client3.conf

This will appear to do nothing after logging a lot of debug as it is awaiting user input. You can push, pop and peek some integers such as push 99. The values will be replicated across the nodes. An example session might look like:

push 1
push 2
push 3

You can experiment with killing nodes and restarting them to see what happens. As long as two out of three nodes are running the client should get responses.


Create a release first edit version.sbt, commit, tag, push and then:

sbt> publishSigned
sbt> sonatypeReleaseAll

Tentative Roadmap

0.5.0 - JPickle for Java Journal (latest Scala 2.12 support)

  • Wrapper that acts as sugar to make it easier to pickle Accepts and Prepares from Java

0.5.1 - Scala 2.13 support

  • Bump to Scala 2.13 and fix all deprecation warnings in preparation for Scala 3. Unfortunately my test dependencies do not yet support Scala 3.

0.5.2 - Bug fixes

  • Issue #33 pickle of Seq[Accept] is broken

0.6.0 - MVStore as Journal with backup method

  • Replace MapDB with H2 MVStore

0.7.0 - Move Akka to be testing only hand upgrade core to RSocket

  • Implement RSocket

0.a - practical

  • dynamic cluster membership with UPaxos

0.b - enhanced

  • learners / scale-out multicast
  • timeline reads
  • noop heartbeats (less duels and partitioned leader detection)
  • snapshots and out of band retransmission
  • metrics/akka-tracing
  • binary tracing
  • jumbo UDP packets
  • complete the TODOs

0.c - performance

  • strong reads
  • outdated reads
  • optimised journal
  • batching
  • remove remote actor from client driver
  • replica strong reads
  • compression
  • journal truncation by size
  • periodically leader number boosting


  • final API
  • hand-off reads?


  • transaction demo
  • ???


The TRex icon is Tyrannosaurus Rex by Raf Verbraeken from the Noun Project licensed under CC3.0