The writer added in v1.38.6 compared the process's live heap usage against
80 % of V8's heap limit and aborted above it. That reading is not the
backup's footprint; it is the whole process's, garbage included. A
long-running app sits at 400 MB of largely collectable heap, so on a 1 GB
container (a 419 MB budget) the weekly pass aborted the first flush of every
account: four out of four in seven seconds, the smallest of them a demo
record whose entire stored copy is 1.2 MB, each told its record was too large
and its operator to buy memory.
Drop the gauge. What the writer holds is bounded by its shape and needs no
reading to prove: the three unbounded tables are pulled a page at a time,
every other section is released as soon as its JSON is in the sink, and at
most one flush of text is pending. The one copy that does grow with the
record is the stored blob, so the bound moves there — counted in the
ciphertext bytes actually produced, against a fifth of this process's heap
limit, which is what two copies of the answer plus a live set have to fit
inside. The message names both numbers.
A pass that wrote nothing for anybody now fails its job instead of reporting
`ok: true` with `backed: 0`, so the failing-queue panel and the backups page
read the same thing an operator would. A pass where some accounts were
written keeps passing with the failures as counts.
Measured under --max-old-space-size=450 from a heap deliberately dirtied
first: 445 000 measurements peak at 28 MB held by the backup, a one-row
record at under 1 MB.
The compression that landed last release was real and was measured in the
wrong place. On a development machine with a multi-gigabyte heap the pass
finished; the container the job runs in is capped at 1 GB, which gives V8
a 524 MB limit, and there the same pass died of memory about fifteen
seconds in. Because the job shares the application process, that was not
one failed backup — it was a restart, and every signed-in session on the
host went with it.
Reproduced before touching anything, under `--max-old-space-size=450`
against a seeded account of 445 000 measurements, 30 000 mood entries and
60 000 intake events: `FATAL ERROR: Reached heap limit` at 34 s. The same
fixture doubled to 890 000 measurements now completes inside a 296 MB
limit, holding 16 MB of live heap where the old writer held 168 MB.
The document is written incrementally now. `buildFullBackupPayload` takes
a `deferBulk` mode that declares the three unbounded tables instead of
reading them, and `streamFullBackupJson` walks the payload in key order,
pulls those tables through page by page, and releases every other section
as soon as its JSON reaches the sink. `packBackupBlobStreaming` feeds that
into gzip and an AES-256-GCM stream, so nothing between the database rows
and the stored value ever exists whole. Both writers go through the same
per-row serialisers, and the round-trip suite pins their output byte for
byte rather than trusting that they agree.
The intake and mood reads are ordered totally now. A keyset cursor needs
it, and a partial order also let two exports of one account differ over a
tie, which every multi-dose day has.
The streamed ciphertext is a new shape at rest, because GCM only produces
its tag once the last block is in and a leading tag is what makes the old
layout unstreamable. It carries a marker no older writer can produce, so
the two families are disjoint by construction rather than by sniffing.
All three stored shapes — plain, compressed, streamed — restore, proved
both ways.
The writer also watches its own live heap and stops at 80 % of V8's
limit. An account too large for its host is now one failed backup,
counted in the run's meta, instead of a restart for everybody on it.
The stored column is the ceiling that is left: `data_backups.data` is a
single `text` value, so the finished artifact has to exist as one string,
and that is the only part of the job still growing with the record. The
operator runbook says so, with the number at which it starts to matter.
Marc-André Bombeck