No daemon, on purpose

Nx runs a daemon by default. Turborepo shipped one and, since 2.8.11,
no longer uses it for turbo run. Both exist for the same reason: a warm run needs to
know what changed since the last one, and walking the filesystem to
find out is slow. So a background process watches the tree and keeps
the answer ready.

vx has no daemon and will not grow one. Here is the reasoning, and
what replaced it.

What a daemon costs

A daemon is a second copy of the truth. It holds a model of the tree
that is correct exactly as long as every change went through a
filesystem event it received and processed. The failure modes are
familiar to anyone who has typed nx reset: a socket left behind by a
crashed process, a stale graph after a branch switch, an event dropped
on a busy machine, a watcher that started after the edit. Each one
turns a fast answer into a wrong one, and "wrong" for a build cache
means a stale hit under a green check.

It also costs the thing it is meant to save. A daemon must be started,
must warm up, must be restarted after an upgrade, and holds memory for
the whole session. The first run of the day pays a cold start either
way.

What replaced it

The question a daemon answers is "which files' hashes do I need to
recompute?" vx made the recomputation cheap enough that the question
stopped mattering:

• The hashes are in git's index. One git ls-files -s returns the file list and every clean file's blob id. A concurrent git status names the dirty ones. A clean tree of thousands of files is keyed with no read of a source file (the details).
• Config evaluation is cached where it is provably safe, keyed by the git blob ids of the import closure, and evaluated live where it is not.
• The graph algorithms are bitsets, so building priorities over 3,270 tasks is milliseconds, not seconds.
• A warm hit on a current tree is N stats and zero writes, because strict output ownership means vx knows what the tree should contain.

The result is a fully cached run of 3,270 tasks in 393ms with no
process left behind, against Turborepo's 463ms (vx 18% faster) and
Nx's 6.45s (vx 16× faster). vx has
no daemon to turn on.

Benchmark workload: a synthetic monorepo of 1,090 packages and 3,270 tasks in 100 dependency layers, every build and test taking 1 s; real repos with uneven task times will differ.

The invariant, stated plainly

There is exactly one source of truth for what a task depends on: the
working tree as git sees it, read fresh on every run. There is no
staleness window because there is no second copy that could be stale.
vx run in a terminal, in CI, in a container, on a machine whose
watcher limits are exhausted, all take the same path and give the same
answer.

That is worth more than a daemon's best case, and it costs less than a
daemon's average case.

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Originally published on the vx blog. vx is an MIT task runner and build cache for JS monorepos: github.com/vznjs/vx.

_{Written with AI assistance.}