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 actions caused a problem.

Adding support for optimistic consistency made testing more challenging. In order to test conflict resolution, conflicting actions needed to be generated on multiple clients almost simultaneously. After a couple of sessions testing broken versions of pairs with friends it was clear that a more efficient process was required. Fortunately, Redux actions are independent of the UI events which generate them. This separation of concerns made it trivial to randomly generate and dispatch actions without driving the UI. Opening clients dispatching several randomly generated actions per second made it easy to generate conflicts to test optimistic consistency policies while watching games play out made it easy to eyeball the results to check that they were correct. This random action generation mechanism can be enabled by adding ?random as the query string when opening the Pairs example in a browser.

One of the problems found by this approach was that clients didn’t always end up eventually consistent. One client would end up with all squares shown and all pairs scored, while another would have some squares hidden. After some digging it turned out that in these cases the master would be reducing a hide action followed by a score action, while the other client would reduce the actions in the reverse order, causing the hide acton to be invalid. Without a way for a non-master client to let the master know about the conflict the master would not send a sync action and the clients would not end up eventually consistent.

This problem identified a limitation with the optimistic clientPredictionConsistency policy: if any sequence of actions causes a conflict then every ordering of those actions must also cause a conflict in order for the clients to end up eventually consistent. The fix for the hide-score case seemed clear: if the score action was only valid if the pair was shown then both orderings of those actions would generate a conflict and so the master would generate a sync action regardless of the order in which it reduced the actions. Some more eyeballing seemed to suggest that the problem had been solved, but a better way to test the property that sync action generation is independent of action order was to write a property based test.

Some Googling revealed that my Facebook colleague Lee Byron had written a JavaScript property based testing framework called test-check which was compatible with the Jest framework used by ReactVR tests, so I started hacking. I soon had a test which could generate an arbitrary sequence of actions, dispatch them and check that if the sequence of actions generated a sync action then dispatching the same sequence of actions in reverse would also generate a sync action.

I could now test that the property held for thousands of action sequences in a few seconds and so I found the next bug almost immediately. While my change to make any ordering of test then hide generate a sync had fixed one problem it had introduced another. The validity of score events was now dependent on the preceding show events, so it was possible for show-show-score to be valid but for every other order of those events to cause the score event to be invalid and so not reduced.

At this point I took a step back. The only situation that should cause a conflict that needs to be resolved is when more than one player tries to score the same pair. In this situation clients don’t have enough information to resolve the conflict and a master client needs to pick an ordering and communicate the result to the other clients. In the case of hide and score actions every client can do the right thing. Hide actions can be made to not hide scored squares and score actions can be made to show pairs. With the reducers changed to work in this way hide actions can always be reduced and score actions are only invalid when they conflict with each other. With these changes in place the validation logic becomes dramatically simpler to reason about and the property based tests were unable to find any more cases which would not be eventually consistent even after thousands of runs.

Testing distributed systems is hard, but combining replicated Redux with property based tests has proved to be a powerful way to gain a high degree of confidence that applications will work correctly despite limitations in the current simplistic clientPredictionConsistency mechanism. The same property based tests will enable new optimistic consistency mechanisms without those limitations to be developed far more quickly 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.

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

Parse By The Sea

Sat 19 October 2013 by Jim Purbrick


A few weeks ago Facebook London hosted the Parse By The Sea hackathon at the Brighton Dome as part of the Brighton Digital Festival. The idea was to take one of our internal hackathons on the road and invite members of the public to join us, turning a hackathon in ...

read more

Facebook Hackathons

Mon 16 September 2013 by Jim Purbrick

I’ve been a big fan of hackathons since one of the first Yahoo! Hack Days I attended at Alexandra Palace was struck by lightning. The lightning caused the fire alarms to go off which opened the roof to let the torrential rain pour on to hundreds of geeks and ...

read more
Fork me on GitHub