The word flow made out of pebbles

5 useful #Devops lessons about flow you can learn while getting off a plane.

One of our favourite books here at DevOpsGuys is Donald Reinertsen’s “Principles of Product Development Flow”. It’s a compelling argument about the importance of “flow” – small batch sizes, frequent releases, rapid feedback, all the key tenets of continuous delivery and DevOps.

It is, sadly, not the sort of easily consumed management book that might appeal to your ADD-afflicted manager (should you be able to tear his/her attention away from his/her iPhone or iPad).

@TheDevMgr and myself (@TheOpsMgr) were discussing this last week as we landed at Munich airport to attend the @Jetbrains partner conference (we’re huge fans of TeamCity for continuous integration).

As we went thru the process of disembarking from the flight we realised we were in the middle of a real-world analogy for benefits of flow – an analogy that might be readily understandable to those senior managers we mentioned earlier.

Let’s walk thru it.

The plane lands and reaches the stand. Immediately everyone in the aisles seats un-clicks and stands up, congesting the aisle. Once the aisle is congested (=high utilisation) the ability of people to collect luggage from the overhead lockers is significantly reduced – everything becomes less efficient.

At high rates of utilisation of any resource waiting times, thrashing between tasks and the potential for disruption are likely to go up. So that’s Useful lesson #1.

All this activity and jamming of the aisle is, ultimately, futile because no-one is going anywhere until the stairs get there and the cabin doors are opened. This is the next constraint in the pipeline.

Useful lesson #2 – until you understand the constraints you might be just rushing headlong at the next bottleneck.

Eventually they arrive and we all start shuffling off like sheep (or zombies) and walk down the stairs… to the waiting buses on the tarmac.

Useful lesson #3 – if you’re trying to optimise flow you need to look beyond the immediate constraint.

In this, case the cabin door & stairs, and look at the entire system (this is the essential message of systems thinking and the “1st way of DevOps”).

The buses were fairly large and held about 60+ people each (=large batch size), and everyone tries to cram onto the first bus… which then waits until the second bus is full before both buses head across the tarmac. When we reach the terminal the buses park up… and the second bus is actually closer to the door we need to enter.

Useful lesson #4 – don’t assume that the batch size is what you think it is (i.e. 1 bus load). It might be more (2 buses!). Also, just because something looks FIFO doesn’t mean it is…

Once we enter the terminal we then hit another constraint – clearing Immigration control. Luckily we were able to go thru the EU Resident’s queue, which flowed at a fairly constant rate due to the minimal border control. But looking at the non-EU Residents queue that the flow was turbulent – some passengers went thru quickly but others took much longer to process due to their different nationality, visa requirements or whatever had caught the Border Control officer’s attention. Unfortunately, the people who could have been processed faster were stuck in the queue behind the “complex” processing.

Useful lesson #5 – If you can break your “unit of work” down to a relatively standard size the overall flow through the system is likely to improve. This is why most Scrum teams insist that a given user story shouldn’t’ take more than 2-3 days to complete, and if it would it then it should be split up into separate stories until it does.

Luckily we avoided the queue for checked luggage as we only had carry-on and we were able to get in the queue for the taxis instead… so that’s the end of our analogy for now.

So let’s think of some theoretical optimisations to improve the flow.

Firstly, why not only let the people on ONE side of the aisle stand to collect their overhead luggage and prepare to disembark, thereby avoiding the congestion in the aisle? You can then alternate sides until everyone’s off.

Secondly, why not see if you can get a second set of stairs so you can disembark from the forward AND rear cabin doors, and alleviate that bottleneck?

Thirdly, why not have smaller buses, and dispatch them immediately they are full, and thereby reduce the batch size that arrives at Immigration?

Fourthly, why not have more agents at Border Control to alleviate that bottleneck, or create different queue classes to try to optimise flow e.g. EU Residents, “Other countries we vaguely trust and generally wave through” and “countries we generally give a hard time to”. You could even have a special queue for “dodgy looking people from whatever nationality that are about to spend some quality time with a rubber glove”. Or why not create totally new categories like “those we hand luggage” versus “those with checked luggage who are only going to have to wait at the luggage carousel anyway so you might as well wait here anyway”.

These proposed optimisations might be simplistic. For example the reason the two buses leave together is probably because ground traffic at airports is strictly controlled (in fact there is a “ground traffic controller” just the same as an “air traffic controller”). So there are often constraints we might not be aware of until we do more investigation BUT the goal of any DevOps organisation should be to try and identify the constraints, and experiment with different ways to alleviate that constraint. Try something, learn from it, iterate around again.

Hopefully by using a common, real-world analogy for product development flow you’ll be able to convince your Boss to let you apply these principles to your DevOps delivery pipeline and improve flow within your organisation!
Photo credit: aselundblad / Foter / Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic (CC BY-NC-SA 2.0)

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