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Network-Ordered Paxos

This is an implementation of the Network-Ordered Paxos (NOPaxos) protocol, as described in the paper "Just Say NO to Paxos Overhead: Replacing Consensus with Network Ordering" from OSDI 2016.

NOPaxos is a state machine replication protocol based on the idea of co-designing distributed systems with the datacenter network. NOPaxos divides replication responsibility between the network and protocol layers. The network orders requests but does not ensure reliable delivery -- using a new network primitive, Ordered Unreliable Multicast (OUM). The replication protocol exploits network ordering to provide strongly consistent replication without coordination.

In normal case, NOPaxos avoids coordination entirely by relying on the network to deliver messages in the same order. It requires application-level coordination only to handle dropped packets. The resulting protocol is simple, achieving near-optimal throughput and latency, and remains robust to network-level failures. It not only outperforms both latency- and throughput-optimized protocols on their respective metrics, but also yields throughput within 2% and latency within 16 us of an unreplicated system -- providing replication without the performance cost.


This repository contains implementations of 5 replication protocols:

  1. Network Ordered Paxos, including normal operation, gap agreement, view change, and synchronization protocols.

  2. Speculative Paxos, including normal operation, synchronization, and reconciliation protocols.

  3. Viewstamped Replication (VR), aka Multi-Paxos, as described in the paper "Viewstamped Replication Revisited", including an optional batching optimization

  4. Fast Paxos, although only the normal case is implemented.

  5. A simple unreplicated RPC protocol for comparison well as an endhost implementation of the sequencer.

Building and Running

NOPaxos and can be built using make. It has been tested on Ubuntu 14.04, 16.04 and Debian 8. Regression tests can be run with make check

Dependencies include (Debian/Ubuntu packages): protobuf-compiler pkg-config libunwind-dev libssl-dev libprotobuf-dev libevent-dev libgtest-dev

You will need to create a configuration file with the following syntax:

f <number of failures tolerated>
replica <hostname>:<port>
replica <hostname>:<port>
multicast <multicast addr>:<port>

Multicast address is optional. However, the Ordered Unreliable Multicast (OUM) implementation uses the multicast address as the OUM group address. For testing, a reasonable option is to use the broadcast address as multicast address.

In order to run NOPaxos, you need to configure the network to route OUM packets first to the sequencer, and then multicast to all OUM receivers. How this is implemented would depend on network design. The easiest way is to use OpenFlow and install rules that match on the multicast address.

Our OUM implementation expects a custom header at the beginning of the UDP data, which contains the session number and message number described in the paper. If you need to write your own sequencer, please refer to sequencer/ for the custom header format. If you need to modify the header format, you will have to modify ProcessPacket and DecodePacket in lib/

The endhost sequencer also requires a configuration file with the following syntax:

interface <network interface name>
groupaddr <multicast addr>

The endhost sequencer uses raw sockets, so you will need to provide the network interface name. groupaddr is the same as the multicast address used in the previous configuration. You can start the endhost sequencer with sudo ./sequencer/sequencer -c <path to sequencer config file> (sudo is required because of the use of raw sockets)

You can then start a replica with ./bench/replica -c <path to config file> -i <replica number> -m <mode>, where mode is either:

  • nopaxos for Network Ordered Paxos
  • spec for Speculative Paxos
  • vr for Viewstamped Replication
  • fastpaxos for Fast Paxos
  • unreplicated for no replication (uses only the first replica)

The vr mode also accepts a -b option to specify the maximum batch size.

To run a single client, use ./bench/client -c <path to config file> -m <mode>.

For performance measurements, you will likely want to add -DNASSERT and -O2 to the CFLAGS in the Makefile, and run make PARANOID=0, which disables complexity-changing assertions.


NOPaxos is a product of the UW Systems Lab. Please email Jialin Li at with any questions.


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