Existing by coincidence, programming deliberately
Recently I wrote
about how Rust macros
make it easy to refactor repetitive code
that might otherwise become annoying
in a strongly-typed language.
Continuing the theme from that post,
I’ve noticed another use case
where macros can be beneficial:
writing custom assertions in tests.
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About a month ago,
we had an outage
caused by a slow-running query in MySQL.
This particular slow query
wasn’t spotted when it was deployed
because it depended on data
inserted by client browsers
and the related preference in Firefox
was not enabled at that point.
A few weeks after it shipped,
the client pref was flipped on
and as the table grew,
it slowed down MySQL increasingly
until the whole of Firefox Accounts
became unresponsive.
And of course,
because Sod’s Law
is one of the fundamental forces of nature,
this happened late on a Friday night.
There were some complicating factors
that slowed down diagnosis,
but it’s also fair to say
we could have caught it at source
with an EXPLAIN of the offending query
during code review.
Because of that,
I decided to try and automate
EXPLAIN checks for our MySQL queries.
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Refactoring boilerplate code is always easy
in dynamically-typed languages,
but sometimes takes a bit more effort
when constrained by strong typing.
This is something I was puzzling over recently,
when the penny dropped for me
about how Rust’s macros
can be used to bridge the gap.
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Here’s three little Rust tricks
that I learned in the last week or so.
Each struck me as being an elegant approach
to working cleanly within the confines
of a strongly-typed language.
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Let’s say you have a huge amount of JSON data
and you want to parse values from it in Node.js.
Perhaps it’s stored in a file on disk
or, more trickily,
it’s on a remote machine
and you don’t want to download the entire thing
just to get some data from it.
And even if it is on the local file system,
the thing is so huge that reading it in to memory
and calling JSON.parse
will crash the process
with an out-of-memory exception.
Today I implemented a new method
for my async JSON-parsing lib, BFJ,
which has exactly this type of scenario in mind.
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“Bad performance hurts user engagement”
is a sentiment that feels intuitively true
but can be hard to sell in product conversations.
The best way to persuade a non-believer
is to point them at hard evidence
and for that you need
to do a few things
with your performance data.
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