Replaying Replicated Redux

Fri 10 November 2017 by Jim Purbrick

While property based tests proved to be a powerful tool for finding and fixing problems with ReactVR pairs, the limitations of the simplistic clientPredictionConstistenty mechanism remained.

It’s easy to think of applications where one order of a sequence of actions is valid, but another order is invalid. Imagine an application which models a door which can be locked: an unlock action followed by an open action should be valid, but an open action followed by unlock should be invalid given a starting state where the door is locked. It’s a lot more difficult to imagine how every ordering of this simple sequence of actions can be made either valid or invalid.

The limitation of clientPredictionConsistency is caused by the master client having to see an invalid action before it notices that clients need resyncing. An obvious way to avoid this limitation would be to have all other clients let the master know if they have seen an invalid action, but this solution becomes more complicated when you want to avoid the master sending duplicate sync actions if multiple clients report invalid actions simultaneously.

At this point, I took a step back: even if clients could report conflicts without duplicate resyncs, the improved clientPredictionConsistency would centralise conflict resolution in the master. Clients receiving a state sync action would have no context on the conflict and so would be unable to do anything more than reset their local state. Reusing the state sync mechanism which allows late joining is simple, but doesn’t allow anything more than effectively rejoining.

One of the nice things about Redux actions is that they are more meaningful than either UI events or state updates: it would be nice if clients could use the context they have in the actions to resolve conflicts and reconcile optimistic updates with authoritative actions gracefully. This made me think of the optimistic update mechanism used by Half Life which keeps a list of actions which have been predicted locally and reapplies the predictions to new states received by the server. Redux was built to easily support this kind of time travel through application history, so I wondered whether something similar could be built for replicated redux.

Some hacking on these ideas produced replayConsistency: a generalisation of the Half Life optimistic update ideas applied to arbitrary Redux actions. When a non-master client generates a valid local action it is sent to the master, immediately reduced locally, but also appended to a list of predicted actions. When the client recieves a new action from the master it rewinds the state back to the start of the prediction, applies the new master validated action and then reapplies the predicted actions if they are still valid. Eventually every predicted action becomes part of the total ordering defined by the series of actions validated by the master and is sent back to the client, or the state which caused the prediction to be invalid on the master is reached on the client. In either case the prediction is discarded. In the case where a prediction becomes invalid, the client has the state before the prediction, the master validated action and the list of predicted actions available when updatePredictions is called. This context allows the client to do something significantly more sophisticated to fix the local state than simply reseting the entire local state. In fact replayConsistency does not need to send state syncs at all, making it significantly more efficient than clientPredictionConsistency which I renamed resyncConsistency to make the differences between the two optimistic consistency policies clear.

Switching out resyncConsistency for replayConsistency and eyeballing several games of ReactVR Pairs suggested that the new consistency mechanism was working as intended, but finding all of the kinks in resyncConsistency had required testing thousands of sequences of actions using property based tests. My existing tests didn’t apply here: they made sure that an application would work given the limitations of resyncConsistency. The property I really wanted to ensure held for all consistency mechanisms is that regardless of the predictions made at each client, eventually all clients would be consistent.

This test generates a sequence of pairs actions which might be sent by the master or one of two non-master clients and then checks that all clients are eventually consistent even in the pathological case where each non-master predicts all of its actions before getting any actions from the master. A nice feature of this test is that it is independent of the consistency mechanism and so the same test can be run to ensure that both resyncConsistency and replayConsistency result in all clients being eventually consistent for thousands of sequences of actions.

With my tests passing I had high confidence that replayConsistency was working and didn’t impose any limitations on event ordering making it a much more general and efficient solution than resyncConsistency as well as much easier to use as it doesn’t require complicated reasoning about application event ordering. The potential to perform sophisticated application specific state reconciliation when predictions are invalidated is also interesting and I’m excited to see what we can do with it in future.

If you’d like to play the ReactVR version of pairs or see the rest of the code, it’s available on github here.

All code in this post is made available under the ReactVR examples license.

Building Safety in to Social VR

Thu 26 October 2017 by Jim Purbrick

Last year I hosted a panel on creating a safe environment for people in VR with Tony Sheng and Darshan Shankar at OC3. I commented at the time that the discussion reminded me of the story of LambdaMOO becoming a self-governing community told by Julian Dibbell in My Tiny Life ...

read more

Testing Replicated Redux

Mon 31 July 2017 by Jim Purbrick

Opening a couple of browser windows and clicking around was more than sufficient for testing the initial version of ReactVR pairs. Implementing a simple middleware to log actions took advantage of the Redux approach of reifying events to allow a glance at the console to reveal precisely which sequence of ...

read more

ReactVR Redux Revisited

Tue 04 July 2017 by Jim Purbrick

There were a couple of aspects of my previous experiments building networked ReactVR experiences with Redux that were unsatisfactory: there wasn’t a clean separation between the application logic and network code and, while the example exploited idempotency to reduce latency for some actions, actions which could generate conflicts used ...

read more

Generation JPod

Sat 03 June 2017 by Jim Purbrick

I’ve just got back from Kaş where I spent a lovely few days celebrating Pinar and Simon’s wedding and while there spent a few hours reading Now We Are 40: a thoughtful and entertaining look at everything from house music to house prices from the perspective of Generation ...

read more

2² Decades

Thu 20 April 2017 by Jim Purbrick

Several years ago when we were in 100 robots together, Max was celebrating his 40th birthday. When I said that mine would be in 2017, it felt like an impossibly far future date, but, after what feels like the blink of an eye, here we are.

Along with many other ...

read more

VR Redux

Wed 04 January 2017 by Jim Purbrick

Mike and I have been talking about how to easily build simple networked social applications with ReactVR for a while, so I spent some time hacking over the Christmas break to see if I could build a ReactVR version of the pairs game in Oculus Rooms. Pairs is simple and ...

read more

Creating A Safe Environment For People In VR

Mon 31 October 2016 by Jim Purbrick

I was very happy that Oculus found time at OC3 to host a panel on creating a safe environment for people in VR. As social VR becomes more popular over the next few years it will quickly have to learn how to keep people safe together in shared environments. Some ...

read more


Sun 21 August 2016 by Jim Purbrick

At the 3rd Party Dev State of the Union at EVE Fanfest 2016 earlier this year, CCP FoxFour drew my attention to a limitation of the current approach used by crestmatic to generate CREST documentation: it only discovers resources always reachable from the API root from the perspective of the ...

read more

Strange Tales From Other Worlds

Tue 10 May 2016 by Jim Purbrick

At the end of last year, Michael Brunton-Spall and Jon Topper asked me if I would like to give the opening keynote at Scale Summit as I had “lots of experience scaling weird things”, by which they meant Second Life and EVE Online. I immediately thought of The Corn Field ...

read more

Towards A Generic Media Type System

Sun 17 April 2016 by Jim Purbrick

The early days of RESTful hypermedia API design tends to involve lots of homogeneous collections. In the case of CREST vnd.ccp.eve.Api-v1 pointed to the logged in vnd.ccp.eve.ccp.Capsuleer-v1 which pointed to a vnd.eve.ccp.CharacterCollection-v1 of contacts which pointed to many vnd.ccp ...

read more


Fri 15 April 2016 by Jim Purbrick


3 weeks ago I spent a few hours with photoshop working on the Story Bird logo that Linda made a while ago to make it suitable for print. 2 weeks ago I spent a few hours researching the best way to convert the 24 bit 48 Khz Story Bird mixes ...

read more


Sat 26 March 2016 by Jim Purbrick

Black barn mixing desk

I love record shops. Whenever I had pocket money it would go on Metallica and Nirvana CDs bought from Our Price or black t-shirts to match. When I lived in Nottingham I bought Boards Of Canada CDs from the same Selectadisc that my Dad bought a rare Fairport Convention single ...

read more


Sun 03 January 2016 by Jim Purbrick

A year ago I gave a talk at EVE Vegas about building RESTful CREST applications. My #1 recommendation was to specify representations in requests, but that’s hard to do when there is little documentation on which representations are available and what they contain.

Fortunately CREST is self describing: send ...

read more

Free Tests For Everyone!

Thu 11 June 2015 by Jim Purbrick

Modern software development is sometimes colourfully described as being similar to firing tracer bullets at a target. Rather than spending time doing a lot of research, design and specification up front, the smallest, simplest version of the software is built and the feedback gathered from its use is used to ...

read more

Investing In Testing

Wed 10 June 2015 by Jim Purbrick

Droidcon London

Last year I was talking to an engineer at Droidcon London who was working on an Android app with 100% test coverage. I immediately asked whether he thought 100% test coverage was worthwhile: many software engineering teams strive to achieve 100% test coverage, but few succeed because it’s an ...

read more


Mon 18 August 2014 by Jim Purbrick


One of the things I’ve been working on since joining Facebook is Buck, an open source Android & Java build tool which is significantly faster than many other Java build tools for a number of reasons.

As well as being fast, Buck gains a lot of power and flexibility by ...

read more

Organisational Structures

Thu 20 March 2014 by Jim Purbrick

There have been a number of blog posts recently about exciting new organisational structures. As Cory points out “Every early stage company thinks it has reinvented management”: a very dangerous belief when betting on a new organisational structure can be much riskier than betting on the wrong product.

It starts ...

read more

Beyond Time Dilation?

Wed 29 January 2014 by Jim Purbrick

The Battle of B-R5RB

EVE online is a remarkable game. On Monday over 2000 people spent over 20 hours destroying virtual spaceships worth 200,000 USD in real money in what was the likely the largest battle in a video game ever. That EVE is capaple of supporting such large engagements is an amazing ...

read more

Osprey Therian

Sun 15 December 2013 by Jim Purbrick

In mid-2004 I first started exploring Second Life. Version 1.4 had just been released and Philip Rosedale had said in the press release “My fantasy is to be Uma Thurman in Kill Bill, and now I can. I’d pay $10 for her yellow jumpsuit and sword moves and ...

read more
Fork me on GitHub