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+date = "2017-01-22T00:00:00+01:00"
+title = "Don't let it crash"
+
++++
+
+We have a specific mindset when writing Erlang
+programs. We focus on the normal execution of the
+program and don't handle most of the errors that may
+occur. We sometimes call this normal execution the
+'happy path'.
+
+The general pattern behind writing only for the
+'happy path', letting the VM catch errors (writing
+them to a log for future consumption) and then
+having a supervisor restart the processes that
+failed from a clean state, has a name. We call it
+'let it crash'; and it drives many of our design
+decisions.
+
+It's a really great way to program and the results
+are fantastic compared to most other programming
+languages. And yet, 'let it crash' barely convinced
+anyone that they should use Erlang. Why would that
+be?
+
+You may already know that Cowboy is capable of
+handling at least 2 million Websocket connections
+on a single server. This is in large part thanks
+to the capabilities of the VM. Still, 2 million
+is good, much better than most other servers can
+do.
+
+Cowboy is not just a Websocket server; it's also
+an HTTP and HTTP/2 server, and it handles many
+related features like long polling or the parsing
+of most request headers.
+
+Can you guess how large the Cowboy codebase is,
+without looking at the source?
+
+Do make sure you have a clear answer in your mind
+before you go check.
+
+Good, you are back. Now what were the results? If
+I am correct, you overestimated the size of Cowboy.
+Cowboy is in fact about five thousand lines of code.
+You probably thought it was at least ten thousand.
+About eighty percent of readers will have
+overestimated the size of Cowboy. And you did only
+because I mentioned it can handle millions of
+Websocket connections.
+
+Numerous studies show this effect. Just mentioning
+the large number already prepared your mind to think
+in that direction. Repeating the number made you
+focus even more on it. Then the question asked for
+a number, which ended up larger than the reality.
+
+The same effect can be applied to negotiation for
+example. You generally want to start by giving your
+offer (and not let the other party initiate) and
+you want to give a really large number first. You
+can also prepare your customer by mentioning an even
+larger number in the previous discussion.
+
+And it's not just numbers either. An experiment
+showed that just by looking at an image of clouds,
+customers of a pillow store were buying pillows
+more comfortable (and more expensive) than those
+who didn't see that image.
+
+This is the power of associations. It is covered in
+much larger detail in the books
+https://www.amazon.com/Influence-Psychology-Persuasion-Robert-Cialdini/dp/006124189X[Influence]
+and
+https://www.amazon.com/Pre-Suasion-Revolutionary-Way-Influence-Persuade/dp/1501109790[Pre-suasion].
+I highly recommend reading those and applying what
+you learn to your daily life. I'm definitely not
+a professional psychologist so take this post with
+a grain of salt.
+
+When selling Erlang, whether we are selling it to
+a customer or trying to convince a developer friend
+to start using it, we often talk about how Erlang
+'lets you sleep at night', that it is auto healing
+and always gets fantastic uptimes.
+
+And then we talk about 'let it crash'.
+
+And we describe what it means.
+
+We might as well just say that Erlang crashes a lot
+and then take the door. It would have the same effect.
+It doesn't even stop at programs crashing. You know
+what else crashes? Cars, planes, trains. Often with
+disastrous consequences. Is that really the message
+we want to convey?
+
+They even https://img.youtube.com/vi/oEUBW2lCkIk/0.jpg[printed it on a t-shirt]!
+Keep calm and let it crash. It's the kind of t-shirt
+you probably shouldn't wear in an airport, and for good
+reasons. A few people did, then realized what they were
+wearing and were not too smug about it.
+
+And yet this is how we sell Erlang.
+
+A better way would be to focus on the positives, of
+course, but also to make sure that those positives
+are phrased in a way that prevents bad associations
+to be formed in people's minds.
+
+Instead of 'let it crash', you can say that Erlang
+has 'auto healing mechanisms'. Healing is a good
+thing and accurately describes what happens in the
+system.
+
+Should you need to go into more details, you will
+probably want to avoid 'recover from crashes' and
+instead say 'recover from exceptions'. Exceptions
+are a pretty neutral word and, should you explain
+what you mean by that, you can talk about exceptions
+that occur for reasons unrelated to Erlang, like
+hardware failure or network instability.
+
+The trick is to always use positive words and
+phrases to describe Erlang, and to use external
+factors to explain how Erlang deals with failures.
+Never mention the failures internal to Erlang
+systems unless you are asked specifically, in
+which case you can say that the auto healing
+applies to all exceptions.
+
+The 'let it crash' philosophy is great when
+learning Erlang or when writing fault-tolerant
+systems. But it's not going to convince anyone
+to use it unless they were already looking for
+it.
+
+Do you like this post? Tell me on Twitter. I might
+make more.