Note that bootsnap writes to tmp/cache (or the path specified by ENV['BOOTSNAP_CACHE_DIR']), and that directory must be writable. Rails will fail to boot if it is not. If this is unacceptable (e.g. you are running in a read-only container and unwilling to mount in a writable tmpdir), you should remove this line or wrap it in a conditional.
Note also that bootsnap will never clean up its own cache: this is left up to you. Depending on your deployment strategy, you may need to periodically purge tmp/cache/bootsnap*. If you notice deploys getting progressively slower, this is almost certainly the cause.
Controllable input metrics are exactly as described: they are directly actionable (hence ‘controllable’), and they impact some output metric you care about (hence the name ‘input’). Examples include some of the metrics discussed in the previous section, like “Add 500 new products to the Musical Instruments category of Amazon.com.fr (100 in Q1, 200 in Q2 …)” or “Run at least 20 newsletter ads per month.”
Output metrics are metrics that you ultimately care about. These are things like ‘number of engaged users’ or ‘revenue’, or ‘absolute dollar free cash flow’, or ‘contribution profit’ or ‘DAU/MAU ratio’. You are not allowed to discuss output metrics during the WBR, except in a reporting sense — the way to hit your output metric targets is to seek out controllable input metrics for each of your output metrics and drive those.
