We have talked previously about the flows of information in
systems. This time, we’ll look into the flows of material.
- Systems concerning transport of material consist essentially of flows and
current counts in various locations. “Transport” and “material” are to be
interpreted extremely broadly: a bug tracker transports material: bug
tickets are transported in and out of the open state. - Rates of flow are more important than current counts. It’s great that you
have money in your bank account, but if you spend more than you earn, it’s
going to run out eventually. - In the (often not-so-)long run, one-time changes are drowned out by ongoing
flows. If your well is drying up, you might get someone to refill it, but if
you change nothing else, it will dry up again soon enough. - On a limited budget, it pays to quickly get to a point where flows are
rearranged so that time is on your side. After that, you can work as
slowly as whatever remaining budget allows. - In any imbalance of flows, there’s two sides, and you can leverage that to
find counter-intuitive solutions. Maybe the problem of expensive meat is not
to produce more meat, but to encourage people to consume less of it.
This article pretends to be about dishwashers, but it’s really about something
much more general.
After your dishwasher has finished, when it contains clean dishes, you don’t
want to put dirty dishes in there.1 At least that’s how every I’ve met does
it – if you have a process that allows dirty and clean dishes coexist in the
dishwasher, you need to tell me about it! However, your household will continue
produce dirty dishes at a rate which you cannot directly control – these will
collect at the counter, waiting for the dishwasher to be emptied.
The one thing you can control is how much effort you put into maintaining the
dishwasher system. This work either has the effect of transferring clean dishes
out of the dishwasher into cabinets (if the dishwasher contains clean dishes),
or transferring dirty dishes from the counter to the dishwasher (in the other
case).
Let’s visualise these two states of the system. First up is the emptying
state, when the dishwasher contains clean dishes. The grey arrow indicates a
transfer of dishes you don’t have direct control over: your household dirties
dishes at the rate they are used to. The black arrow is the one you can control:
how fast you refill the cabinets with clean dishes from the dishwasher.2 The
attentive reader will note that there are really two transfers from the
dishwasher going on in this state: the household is likely taking clean dishes
from the dishwasher in addition to the cabinets, and this is independent of your
effort. I have excluded this from the model with no loss of utility because it
just complicates the analysis for no qualitative difference.
Then we have the filling state, when the dishwasher contains dirty dishes:
If you haven’t read this type of system diagram before, it should be quite
intuitive: the arrows represent flow of materials, and the box represents
containers that accumulate materials. So the houshold has some dishes, and they
use them, at which point they either end up on the counter or in the dishwasher.
The ones that end up on the counter eventually flow to the dishwasher. Once the
dishwasher has run, the dishes flow back into the household.
What I want to look at here is what happens to the system behaviour as we adjust
the black arrow, i.e. the effort we spend on transporting materials through this
link of the system. Let’s start analysis just after the dishwasher has run, i.e.
the system is in the emptying state.
Resting Behaviour
If we invest no effort in emptying the dishwasher, we get the resting behaviour
of the system: dishes accumulate on the counter. Assuming the household has
access to infinite clean dishes, there will eventually be an infinite mountain
of dishes on the counter. This is a bad place to be in.
Zero To One
The first significant threshold might be clear from above: it happens when we
take the first clean dish out of the dishwasher.
This does not change anything about the system behaviour: we will still
eventually end up with an infinite mountain of dishes on the counter, because we
are pitching a one-time action against a rate of flow. The rate of flow will
almost always win out in the long run. You often see people celebrating one-time
changes, even though the rates of flow have not changed and it’s only a matter
of time before things are as bad as they were before; people underestimate the
power of ongoing flow.3 Typical example is a bug bankruptcy where one
closes all bug reports older than some date. This is a one-time change, and
eventually the flow of new bugs will catch up and you’re back at the same
position again. If we want permanent change, we need to do something about the
flows of bugs, not apply a one-time fix to the current count.,4 Another
example is the “we need to rewrite this entire subsystem to improve it!”
Generally, no. A steady stream of incremental improvements, over time, beats out
the big-bang effort. Especially when there’s a limited budget.
If the first clean dish does not change the dynamics of the system, then why is
it important? That has more to do with human reasons: it indicates we have gone
from not caring about the problem to at least trying to do something about it,
even if it’s insufficient.
Emptied Dishwasher
This is the point I really wanted to get to. If there’s only one thing you
remember from this article, make it this one. The Threshold is passed when we
have managed to transfer enough dishes out of the dishwasher that it is now
empty. When that happens, the systems transitions into the filling state, at
which point our household stops building on the counter mountain.
The reason this is so significant is that we have moved from a condition in
which we had time as our enemy (waiting grows the counter mountain) into a
condition where time is neutral to us.
If you have a system like the dishwasher one, and you have limited budget to
improve the flows under your control, it pays off to spend a lot of that budget
up front to get to the point where the system transitions into a state where
time is no longer your enemy.
Once we’ve transitioned into the filling state, we can work as slowly as we
want on moving dishes from the counter to the dishwasher, assured in the
knowledge that the counter mountain is no longer growing, meaning it will
eventually disappear with even the smallest amount of ongoing work.
If our household also helps out transporting dishes from the counter into the
dishwasher once it’s emptied, then it becomes even more powerful to get to this
state. In that case, time is actually our friend: once we get the dishwasher
emptied, we can sit back and relax and do nothing else, and trust that
eventually the counter mountain will work itself off.5 The example from
software development I have here is when customers resist an improvement. Even
if you can’t spend the money to convince customers to accept the improvement, it
is worth spending whatever budget you have to get the improvement to become the
default for new customers as soon as possible. Once the improvement is the
default for new customers, time is on your side: natural customer churn will
ensure that eventually, the improvement is in place across the board.
Another way to look at this is that when we get the dishwasher to empty, we
shouldn’t think of that as achieving some goal for the current count of dishes in
the dishwasher. We have done something much more fundamental: we have rearranged
the flows of material in the system!
Matching Rates
Matching rates is not a significant threshold in the dishwasher system we have
seen so far. But let’s imagine for a moment that we have a lazy household, and
the filling state looks like this6 In other words, our household doesn’t
put dirty dishes in the dishwasher even when it contains dirty dishes, they
continue to put them on the counter, for us to transfer into the dishwasher.:
In this system, nothing significant happens when we have emptied the dishwasher:
the counter mountain will still keep growing. In fact, there’s no magical
threshold like in the previous system. The closest we get in this new system is
the threshold that we pass when we start to transfer dishes from the counter to
the dishwasher at the same rate as our household produces dirty dishes on the
counter. That is the point at which the mountain on the counter stops
growing.7 This is similar to the bug bankruptcy situation mentioned earlier.
People often focus too much on the current counts, and not the flows. You might
look at how much work you have to do and think that you need to employ more
people. But the alternative interpretation is that you’re promising to do too
much work, and that’s the rate that needs to be reduced. Any time there’s an
imbalance, people tend to forget there are two things that need to be balanced,
and focus on just one of the sides of the scale.
This might sound like we have time on our side, but it’s not quite the same
thing because if we stop working at this high pace, the counter mountain will
start growing again. For it to truly count as having time on our side, we should
be able to stop working without making the situation worse.
Not Significant
Perhaps curiously missing from this list is the point when we have emptied the
counter. The reason it’s not on there is that there’s not much that
fundamentally changes about any of the systems we’ve discussed when the counter
is empty. The time-dependent behaviour of the systems remains almost exactly the
same.
This echoes the general sentiment in this article: the current counts of material don’t
matter as much as the flows of material. This applies broadly to any systems.
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