kanta/docs/database.md

Kanta Database Format and Design Principles

This document describes the on-disk format and design principles of Kanta. It is intentionally focused on the current standalone package behavior.

Core Principles

1. Append-only durability

• State changes are persisted as appended JSON lines.
• Existing lines are never edited in place.

2. Differential persistence

• Kanta stores diffs (patches), not full state, for normal writes.
• This keeps write volume small and preserves a clear change history.

3. Deterministic replay

• Current state is reconstructed by replaying log records in order.
• Snapshot records accelerate replay while preserving deterministic results.

4. Transactional in-memory writes

• Application code mutates in-memory data inside kanta.transaction(...).
• On success, Kanta computes and queues a diff record.
• On failure, in-memory data is rolled back.

5. Explicit schema evolution

• Schema migration functions are versioned (migrate_vN).
• Migrations run at open time and advance the stored version.

On-Disk Record Types

Kanta uses a newline-delimited stream where each line is either a change record or a snapshot record.

Change record

One JSON object per line:

{"ts":"2026-06-10T02:55:00Z","a":"update","v":5,"u":"user-id","m":"2026-06-10T02:55:00Z","diff":{"users":{"alice":{"age":31}}}}

Fields:

• ts: UTC timestamp of the record.
• a: action name.
• v: schema version after this change.
• u: optional actor identifier.
• m: optional domain modification timestamp.
• diff: jsondiff patch payload.

Snapshot record

Snapshot lines are prefixed with SNAPSHOT , followed by JSON:

SNAPSHOT {"ts":"2026-06-10T00:00:00Z","v":5,"state":{"users":{}},"m":"2026-06-10T00:00:00Z"}

Fields:

• ts: snapshot creation time.
• v: schema version represented by the snapshot.
• state: full state dictionary.
• m: optional domain modification timestamp.

Replay Model

1. Find the last snapshot in the file, if present.
2. Initialize replay state from snapshot state (or {} if none).
3. Replay subsequent change records in order using patch application.
4. The final replay state becomes in-memory kanta.data.

This model provides fast startup for large logs while retaining append-only history.

Serialization Semantics

• In-memory data is defined by an application msgspec.Struct type.
• Kanta round-trips through plain builtins for persistence and diffing.
• Dict keys are serialized as strings (str_keys=True) for stable JSON form.
• Normalization changes introduced by struct decode/encode are logged as migrate:msgspec when they produce a diff.

Transaction Semantics

• kanta.transaction(action=...) captures a pre-transaction snapshot dict.
• By default a transaction updates the modification time m to the current UTC time.
• mtime=True|False|datetime controls the modification time m:
  • True (default) sets m to the current UTC time.
  • False omits m, leaving the previous modification time in effect.
  • A datetime sets m to that explicit value.
• System operations such as migrate:msgspec use mtime=False so they are not considered modifications and do not advance m.
• On success:
  • compute diff between previous builtins and current builtins,
  • queue a ChangeRecord if non-empty,
  • update kanta.mtime when the change carries an m value.
• On exception:
  • restore in-memory data from snapshot,
  • re-raise the exception.

Nested transactions are rejected.

Modification Time

kanta.mtime exposes the last modification time carried forward from change records. It is updated by normal transactions and preserved across snapshots and reloads, while system operations such as migrations leave it unchanged.

Flush and Lifecycle

• Writes are queued in memory.
• kanta.flush() appends queued records to disk.
• A background async task can flush periodically.
• kanta.close() performs final flush and releases file resources.
• async with Kanta(...) guarantees open/close lifecycle management.

Open Modes

• await kanta.open() (default) creates the database file if missing.
• await kanta.open(create=False) fails when the file is missing or empty.
• await kanta.open(readonly=True) opens an existing database read-only.
  • The file is opened without acquiring a lock and without a background flush task.
  • Existing records are replayed and migrations are still applied in memory.
  • Transactions and explicit flushes are rejected.
  • The file is never created if missing.

Callbacks

All callbacks are registered via decorators and receive arguments by their annotation types. Parameters without a supported annotation are only allowed when they have a default value.

Bootstrap Callbacks

• When open() creates a new database, it always writes a single bootstrap ChangeRecord.
• The simplest bootstrap is the initial data object passed to Kanta(...); bootstrap callbacks are optional and only needed when you want to modify or enrich that object at creation time.
• Register callbacks via:
  • @kanta.bootstrap
  • @kanta.bootstrap(action=..., user=..., mtime=...)
• Bootstrap callbacks may be sync or async. The live root data object is injected by annotating a parameter with the struct type passed to Kanta, and the Kanta instance itself can be injected by annotating a parameter with Kanta.
• Multiple bootstrap callbacks are supported:
  • callbacks execute in registration order,
  • exactly one bootstrap ChangeRecord is queued,
  • bootstrap metadata (action, user, mtime) is taken from the last callback registration.
• If no bootstrap callbacks are registered, the bootstrap record still uses action="bootstrap" and contains the initial data object.
• If any bootstrap callback raises, Kanta closes and removes the database file, then re-raises the exception.

Fatal Error Handlers

• Fatal background persistence errors can be handled with @kanta.fatal_error.
• Handlers may be sync or async. The DatabaseError is injected by annotating a parameter with DatabaseError; Kanta may also be injected.
• Multiple handlers are supported and invoked in registration order. A failing handler is logged and does not prevent subsequent handlers from running.

Clock

• @kanta.clock registers a callback () -> datetime that replaces the default UTC clock. Its value is used for all record timestamps (ts, and m when mtime is True) and for snapshot timestamps.
• The clock is only read when a timestamp is actually produced; no-op transactions and skipped snapshot checks do not read it.
• Register before open() so that bootstrap and migration records use the custom clock as well. This is mainly useful for tests and reproducible demos.

Transaction Log Formatting

• Logfmt callbacks prettify identifiers in the change log and are registered with @kanta.logfmt.
• A logfmt callback is called for every value Kanta renders: diff values, path components, and the transaction user. It receives the value as its first parameter and optionally a path: str parameter with the dot-notation path to the value. The special path "$user" is used when rendering the transaction actor, replacing the old user_display parameter.
• The callback returns str | None: a string replaces the default rendering, while None means "fall through to the next formatter".
• State dicts can be injected via DictPre (Annotated[dict, "pre"]) and DictPost (Annotated[dict, "post"]); the Kanta instance can also be injected.
• Alternatively, a logfmt callback can be a class inheriting from LogFmt; the framework instantiates it with the state dicts and calls its resolve(value, path) -> str | None method.
• Multiple logfmt callbacks are stacked in registration order; the first callback to return a non-None result wins. If none handle a value, Kanta falls back to its default formatting.

The decorator accepts an optional path so the callback only runs for values at that exact path:

@kanta.logfmt(path="$user")
def resolve_user(value: str, current: DictPost) -> str | None:
    return current.get("users", {}).get(value, {}).get("name")

@kanta.logfmt(path="users.uuid-1")
def resolve_user_key(value: str) -> str | None:
    return names_by_id.get(value)

Transaction Log Headers

• By default a transaction is logged with an action by user header followed by the diff lines. Added paths are colored green, deleted paths red.
• kanta.transaction(..., extra="...") accepts a display-only string that is appended after the action in the header (colored by Kanta); it is never persisted in the ChangeRecord.
• kanta.transaction(..., logdiff=False) skips building and printing the diff body and logs only the header, which is useful for large or noisy changesets. Diff output can also be disabled globally with configure_logging(diff=False); diff lines are emitted on the kanta.transaction.diff child logger so applications can route or silence them separately from the headers.

Log Emitters

• Every change-related message Kanta emits (transaction/bootstrap/migration changes, Created <file>, migration summaries, aborted transactions) is described by a kanta.logging.LogEvent and dispatched through kanta.logging.emit_event. Kanta's own output goes through the same mechanism: when no logemit callback handles an event, kanta.logging.default_emit renders it with the built-in formatting.
• A LogEvent carries the event kind ("change", "created", "migrated", "aborted"), the preferred logger and level, the kanta instance, and all relevant state: action, user, extra, error (for aborted transactions), diff, previous/current state dicts, the built logfmt chain, and version info for migration events.
• The built-in formatting is assembled from standard blocks that custom emitters can reuse as-is or replace piecemeal:
  • event.header — a lazy property producing the default one-line header for any kind: <action>[ <extra>][ by <user>] for changes, <action>[ by <user>] transaction aborted: <error> for aborts, and the plain Created/Migrated summaries. It is settable: assign event.header = ... and return truthy to restyle the header while keeping the default diff routing.
  • event.diff_lines — a lazy property producing the pretty diff body for change events (built only if accessed).
  • default_emit itself is just header plus the diff_lines routing.
• @kanta.logemit registers a callback receiving the event. The callback decides what is logged and where: it may log one or more messages on event.logger, log somewhere else, or nothing at all. A falsy return value marks the event handled and stops the chain; a truthy return value passes the event — possibly modified — to the next registered callback. When all callbacks pass, default_emit renders the event; a callback may also call default_emit(event) itself to delegate events it does not customize. Operational diagnostics (integrity errors, background flush failures) do not go through this mechanism.
• Logging never breaks functionality: a crashing logemit callback is reported with logger.exception and the event falls back to the built-in formatting; if the built-in formatting itself fails, the error is reported and swallowed. The same applies to logfmt callbacks (a failing one is treated as a fall-through) and logmigr callbacks.

@kanta.logemit
def emit(ev: LogEvent):
    if ev.kind != "change":
        return default_emit(ev)  # delegate, no chaining needed
    actor = ev.current.get("users", {}).get(ev.user, {}).get("name", ev.user)
    line = Line().user(actor, width=20)(" ").action(ev.action)
    ev.logger.log(ev.level, f"{line}\n" + "\n".join(ev.diff_lines))

Terminal Formatting Helpers

• kanta.tty provides the building blocks used by Kanta's own rendering:
  • colors: the mutable color palette. Colors are bare SGR parameter strings (e.g. "1;34", "38;5;226") without escape framing. Attributes are read at render time, so assignments (colors.action = "36") and additions (colors.session = "38;5;226") take effect immediately.
  • Line: builds a terminal string part by part. Calling it appends content (str-converted); .<colorname> arms a palette color for the next call only, and the reset is folded into a single escape sequence with whatever color comes next. width=/align= pad by display width; str(line) finishes the line and restores default colors.
  • strip_ansi, displaywidth (wide chars and emoji count correctly) and pad for working with pre-colored strings.

Migrations

• Migration source is configured on Kanta(...) via migrations=.
• Accepted values:
  • imported module object,
  • import path string.
• Migrations mutate replayed dict state in-place and return the new version.

Safety Invariants

• Any detected out-of-transaction mutation is treated as a fatal consistency violation.
• Flush failures mark the instance as failed and trigger shutdown behavior.
• Object identity of kanta.data is preserved across rollback when possible, minimizing stale-reference hazards for callers.