# hive-log

One shared log-sink chooser for the hyperhive daemons: `hive_log::init()`
builds the `EnvFilter` every binary built before, then installs **exactly
one** sink.

## Why one sink and not both

Under a systemd unit the process's stdout already **is** the journal.
Adding a journald layer on top of the `fmt` layer therefore stores every
record twice — once as the native journal entry, once as the text line
`fmt` wrote to a descriptor that leads to the same journal.

Writing to stdout also throws the severity away. A stdout line carries no
priority field, so journald files the whole stream at one level and the
swarm log store shows `info` whatever level `tracing` gave the record.
Logging natively carries the level across. That is the reason to prefer
journald where a journal exists, and the reason never to run both layers.

## How the choice happens

systemd sets `$JOURNAL_STREAM` to the device and inode numbers of the
journal-connected descriptor, in decimal, separated by a colon.

Checking that the variable exists is **not** enough. A child process
inherits `$JOURNAL_STREAM` from its parent even when the parent redirected
that child's stdout somewhere else, so presence alone reports a journal
that the process does not actually write to. `hive-log` instead stats
stdout — the descriptor the `fmt` layer writes to by default, and the one
a journald layer would double up on — and compares both numbers against
the parsed pair.

A match installs the native journald layer alone. No match, an unset
variable, or a value that does not parse installs the `fmt` layer alone.

## When the journald layer fails to build

`tracing_journald::layer()` validates the journal socket up front and
returns an `io::Result`, so a missing journal surfaces as an error rather
than a silent no-op. On that error `hive-log` installs the `fmt` layer and
emits one warning through it. A process must never fail to start because
of its logger, and a logger that drops every record in silence is worse
than a degraded one that says so.

## Scope

The sink chooser and its tests. Nothing else belongs here: the crate
exists so three binaries share one copy of this decision, not as a place
for general utilities.