inbuxa-server/docs/spec/migration.md

Migrating a Stalwart install to INBUXA

Status: draft, 2026-09-19. Decided in outline (John, 2026-09-19); nothing built.

Two ways in, and they are different workflows:

• Fresh installation. ihasmail-oneshot's deploy: a new stack, new domain, nothing to preserve (SPEC.md §6.1).
• Migration. An existing Stalwart server becomes an INBUXA one, keeping everything. This page is that one.

INBUXA's own cutover is the first run of it (cutover.md). The tool is what that run leaves behind, so the second operator doesn't repeat it by hand.

What the operator is responsible for

The tool moves a running mail server. It says so before it starts, rather than leaving the licence to have said it in a file nobody opens. AGPL-3.0 §15 and §16 already disclaim warranty and liability; nothing here narrows them, and nothing here replaces reading them.

What it does, plainly. It stops the mail server, copies its data store, and starts a different server on the copy. Mail queues at the sending side meanwhile — delayed, not lost. The old install is left standing and is what the service returns to if anything fails.

What can actually go wrong. A general warning helps nobody. These are the specific ways this goes badly, each already seen or measured:

• The window is longer than anyone guessed, because the store is bigger or the disk slower than expected. This is the most common way a planned hour becomes a bad night, and --dry-run turns it from a guess into a number before the window opens.
• The source store is opened by the new server by mistake, after which the old build will not start on it again — not a difficulty, an impossibility (cutover.md). The rollback is gone at that moment. The tool never opens it, and sets it read-only, but a second pair of hands on the same box can still do it.
• The rollback is called after mail has flowed, and returns the service without returning what arrived in the meantime. The tool reports how many messages that is, so it is a decision rather than a discovery.
• Certificates stop renewing, which says nothing for ninety days and then takes the service down quietly. It is the one failure that outlives the maintenance window by a quarter.

Backups are the operator's, and the source install is not one. Keeping the old install is a service safety net, not a data one: it is a single copy, on the same machine, of a single moment. A failed disk, a full filesystem or a mistyped path takes it and the new store together. A backup means a copy somewhere else, that has been restored from at least once. Anyone who cannot say when they last restored one does not yet know whether they have a backup.

Who carries the outcome. The operator does. The tool is careful, it is rehearsable, and it is still software running against a server it has never seen, holding data somebody else depends on.

What the tool does to deserve the trust it is asking for:

• --dry-run first, on a copy, with the timings — and the real run refuses to start until one has succeeded on that host. A warning nobody read is worth less than a rehearsal nobody could skip.
• It never writes to what it migrates from, so the source stays a rollback.
• Verification before mail flows again, and automatic rollback when it fails, because a server that half-works is worse than one that is back where it started.
• The old install is kept until the operator retires it deliberately, and the tool asks before removing anything.
• Every phase timed and reported, so "how long will this take" is answered in the rehearsal rather than during the window.

The confirmation names the host and the store path being moved. --yes skips the prompt, not the dry run.

What "transparent" means here

Everything that defines the server lives in its data store, so a migration that copies the store carries it all: accounts and their passwords, app passwords, OAuth clients and live sessions, aliases and mailing lists, tenants, domains, DNS records and the DNS provider settings, certificates and ACME state, Sieve scripts, the queue, and the mail itself.

Nothing is re-entered and nothing is re-issued. Users are not asked to change a password, re-authorize a client, or re-point a DNS record. That is the promise the tool makes, and the verification below is what checks it rather than assuming it.

The store format is upstream's and unchanged (SPEC.md §2.6), which is what makes this a copy rather than a conversion. A migration that also crosses an upstream version is that upgrade first, then this.

Shape

Side by side. The existing install is stopped but left standing, the fork goes in at its own path with a copy of the data, and the old install is what a rollback returns to. stalwart-migrator upgrades in place and says plainly that it cannot undo one; this tool can, because it never writes to what it migrates from.

It reads the source install and writes only to the new one. The source's store is copied, never moved, never opened for writing.

Phases, each timed

1. Preflight, with nothing stopped yet: the source's version and store backend, disk for a second copy of the store, the target path empty, the ports the fork will want, the front ends' plan, and a snapshot the operator confirms exists. Anything wrong here costs no downtime.

2. Stop: the mail server, then the webmail container. The unit is disabled as well as stopped, so a reboot can't put two servers on one set of ports with two diverging stores.

3. Copy the store to the fork's path, in two passes where the filesystem has no snapshot to offer (cutover.md): a first pass with the server still running, deliberately inconsistent, which moves the bulk outside the window; then, inside the window and after the process has fully exited, a delta pass that makes the copy consistent. A RocksDB store is mostly immutable SST files, so the delta is small and the window is the delta, not the store. The tool times both and reports the second as the downtime.

   Then hand the copy to the user the fork runs as, and make the original unwritable before the fork starts: until the old server stopped, its own store lock was the only thing preventing a second process from opening it.

4. Configure: the fork's config and unit, pointing at the copy.

5. Start the fork, and wait for it to answer.

6. Front ends: INBUXA Admin and the webmail, against the same store's OAuth clients.

7. Verify (below). This is the gate, not a formality.

8. Report.

Downtime is phases 2 to 7, and it is the number worth advertising — not the total, which preflight and the first copy pass dominate and which nobody experiences. The first pass happens with the server still serving, which is what keeps the window proportional to the delta rather than to the store. The report gives both, per phase, and writes them as JSON beside the human-readable summary so a release note can quote them honestly. A migration of a small server should be minutes; the number that gets published should be one this tool measured, on a stated store size and account count, not an estimate.

Verification, before mail flows again

The same checks as cutover.md, run by the tool rather than by hand:

• the fork answers on every listener the source had, with the certificate the source was serving;
• an account signs in over IMAP, JMAP and both front ends, with the credential it already had;
• a message arrives from outside and one leaves;
• aliases still receive; a tenant administrator sees its own accounts and domains and no others;
• the queue drains and the task queue gains no permanent failures;
• nothing but the fork is listening on those ports.

A failed check is a failed migration, and the tool rolls back rather than leaving a half-migrated server for the operator to reason about at 2am.

After a migration: the stock client

A migration that passes its own verification has proved the fork works for the fork. Pointing an unmodified public ihasmail at the migrated server proves something else: that a client which knows nothing about INBUXA still works against it. It runs anywhere — JMAP and OAuth over the network — and needs only an OAuth client it may use.

Each difference it turns up is either a regression against upstream's contract or a feature that now expects INBUXA's own front ends. The tool doesn't run this; it belongs in the release notes and the contract (SPEC.md §5.2). It is named here so the first migration doesn't skip it.

Rollback

A first-class path, not an appendix.

• Automatic when verification fails, unless the operator asked for the tool to stop and wait instead. A server that half-works is worse than one that is back where it started.
• On demand afterwards, for as long as the old install is still there: rollback stops the fork, re-enables and starts the original unit, and brings the old webmail container back.
• What it costs: everything the fork accepted while it served. The two stores diverge from the moment the fork starts, so a rollback after mail has flowed leaves that mail in the fork's store. The tool says how many messages that is rather than letting the operator find out.
• What it never does: point the old server at the store the fork has been writing. That is not a recovery path and cannot be made into one — the old build refuses to open a store the fork has touched at all (cutover.md).
• What it undoes first: the read-only flag on the original store. The guard that stops the fork opening it by mistake also stops the old build starting on it.
• The old install is kept until the operator retires it deliberately. The tool does not delete the thing its rollback depends on, and when the operator is done with it the tool asks — keep, or delete — rather than deciding. Nothing the fork runs on may be inside it by then, which verify checks: no open file under the old tree, and no path in the fork's config or unit naming it.

Rehearsal

--dry-run does everything but the stop: copies the store to a scratch path, brings the fork up against it on alternate ports, runs what it can of the verification, reports the timings, and removes the copy. It answers "how long will this take, and will it work" without touching the running server, and it is what the operator should run first.

What the operator inherits

INBUXA is AGPL, and a migrated server is one its users reach over a network, so from its first day its operator owes those users an offer of the Corresponding Source (AGPL section 13). The tool should say so once, at the end of a successful migration, rather than leaving an obligation to be discovered. It is satisfied by a link, and INBUXA's own is published.

Open

• Where it lives: a command in ihasmail-oneshot, a sibling to stalwart-migrator, or the fork's own binary (SPEC.md §8's open question about the installer's repository).
• Whether it handles container installs as well as native ones, which changes "stop the unit" into "stop the stack".
• Whether it migrates a Community-edition install too. Nothing about the copy is Enterprise-specific, and a Community server has fewer objects rather than different ones, so this is likely a documentation question rather than a code one.
• What it does when the source is mid-upgrade, or its store format is older than the fork's base.