The restore dialog asked GET /api/admin/backups/{id}/summary for the
contents of a copy, and the route read the whole stored copy to answer:
every piece opened, every measurement parsed, the rest validated. On an
account of 1.8 million readings on slow hardware that took about two
minutes. The dialog stopped waiting after 15 s and said the contents
could not be read, and its automatic retry made the server read the
whole copy a second time.
The copy passes through the process once anyway while it is written, so
the preview is worked out there and kept on the row (DataBackup.preview,
migration 0364): the summarizeBackup counts, the schema version, and
the inner key ids with the sections they sit in and one sample value
each. Every creation path goes through storeBackupBlob (the weekly and
manual pass, and an upload), so every new copy carries one. The column
is plain JSON: counts and dates only, and the samples are ciphertext
that stands for what is inside the copy, which a key rotation must not
re-seal.
The route answers from it without opening the copy. Both verdicts are
taken again on every read, from what the preview keeps: whether this
release restores the schema version, and whether this server holds the
keys the copy needs. The pieces are still counted against the row, so
a copy that lost one is refused as the restore and the download refuse
it. A preview names the copy it describes and is ignored once the copy
changes; converting a single stored value into pieces carries it over.
A copy written before this has no preview. It is read whole once, as
before, and the result is kept on the row, so it is slow only the first
time; concurrent requests for the same copy share the one read.
Only a file that fails the schema answers 422 "Backup payload failed
schema validation". Read refusals keep their codes; anything else is
the server failing and answers 500.
The dialog no longer retries the preview, waits up to 55 s, and when it
stops waiting on an older copy it says the copy is still being read and
to open the dialog again in a few minutes.
Refs #1031
Key rotation re-sealed a single-value backup in place, which holds the
stored string, its bytes, the plaintext and the new ciphertext at once; a
copy can be over a hundred megabytes and the in-app rotation walked 25 of
them per page, enough to run a 1 GB container out of memory. Rotation now
converts each such copy into pieces under the active key instead, one row
at a time. The ~hlgcm1. stream of v1.38.6 to v1.39.1 is read a megabyte
at a time with its tag checked at the end, inside one transaction, so a
copy that fails the check is left untouched; the copy keeps its date. The
scan reads only the first characters of each value. Measured on a copy of
a 2.6 million-reading account (126 MB stored): converted in 25 s with the
heap flat at 57 MB under a 200 MB limit, where re-sealing in place ran out
of heap.
The weekly and manual backup leave an account alone while a restore of
its current copy is queued or running, and count it as skipped, not
failed. A reader whose copy is replaced mid-read answers 409
backup_changed, "replaced by a newer one while it was being read", rather
than the undecryptable error; a restore rolls back with the same code.
Pieces are sealed with a fixed associated-data label, so a value the same
key sealed for another purpose never opens as a piece; rotation re-seals
under the same label. A row holding both a single value and pieces, which
only an older release writing after a downgrade produces, is refused with
a message saying so, and the next weekly run replaces it. Rotation treats a
row deleted between its read and its write as gone rather than an error.
A stored backup was one value in data_backups.data. v1.39.1 wrote it a
piece at a time but still joined the pieces into that one value, and one
value has to pass through the app whole, so a copy was capped at a fifth of
the heap: 105 MB in the default 1 GB container. An account with 1.75
million readings is past that and could not be backed up at all (#1031).
A copy is now kept as ordered pieces of about a megabyte in the new
data_backup_chunks table (migration 0352). Each piece is sealed on its own
with AES-256-GCM, and its sealed header names the copy (a random stream id
kept on the row), its position and whether it is the last. A reader checks
every piece before it releases a byte, so a piece that is missing, moved,
altered, taken from another copy, or a copy cut short is refused with
backup.payload.undecryptable before a restore deletes anything, and the
second read during the restore checks each piece again.
The write is one transaction, so the previous copy stays whole and readable
until the new one commits. Pieces are written through the model API: a raw
query with a large parameter is kept alive by Prisma's query plan cache,
which left 160 MB of heap behind per hundred one-megabyte inserts.
Restore, preview, download, the restore job and the admin list read both
forms; copies written before this change keep their single value and still
restore. The heap-derived limit is gone. The one limit left is on storage,
BACKUP_MAX_STORED_MB (default 2048), and its message names the setting. Key
rotation covers the pieces, the wipe and account deletion take them with
their backup, and the backup plan lists the table as not carried.
Measured on an account of 2.6 million readings in a 1 GB container with web
and worker in one process: backup 87 s, 97 MiB in 96 pieces, preview 20 s,
download 25 s, restore 264 s with every reading back exactly, peak 567 MiB,
no restart.
Marc-André Bombeck