SessionStore & EventLog

Mecatl's persistence layer exposes two ports: port.SessionStore and port.EventLog. Both live in engine/port/ and have no dependency on adapters or infrastructure — they are pure Go interfaces the engine consumes through injection.

The two are distinct by design. The store is snapshot-based: each Save captures the full current state; Load restores a live *session.Session from the most recent snapshot. The event log is append-only: the relay records every event the loop emits in chronological order, and Read streams them back. Neither depends on the other; they are wired independently in composition.

---

SessionStore

// engine/port/store.go

type SessionStore interface {
    Save(ctx context.Context, s *session.Session) error
    Load(ctx context.Context, id session.SessionID) (*session.Session, error)
}

Save serializes the session's current state to stable storage and overwrites any prior snapshot for the same id. The call is idempotent over the session's state: calling Save twice on the same session object is safe — the second call replaces the first. Implementations must deep-copy the session at Save time so that mutations the caller makes afterward do not silently corrupt the stored snapshot.

Load deserializes the snapshot stored under id and returns a live *session.Session. The aggregate's invariants hold on the returned session — the state machine is driven through the correct lifecycle transitions during restore, not patched directly. If no session exists under id, Load returns an error that wraps the port-level sentinel port.ErrSessionNotFound, so callers that cannot import the adapter can distinguish "no such session" from an infrastructure failure via errors.Is:

sess, err := store.Load(ctx, id)
if errors.Is(err, port.ErrSessionNotFound) {
    // first run — no prior state
}

SessionCreator — atomic first publication

Stores used with caller ownership enforcement must also implement the additive port.SessionCreator capability:

type SessionCreator interface {
    Create(ctx context.Context, s *session.Session) error
}

Create publishes the first authoritative snapshot only when the session ID is absent. The check and publication must be backend-atomic across processes; a collision wraps port.ErrSessionAlreadyExists and leaves the existing snapshot, metadata, event log, and tool-call sidecar unchanged. Save remains the update operation after a successful create. Mecatl refuses ownership-enforced composition with a store that lacks this capability. The current gRPC session-store driver does not advertise atomic create, so --session-store-url cannot be combined with OIDC ownership enforcement; use an in-tree atomic backend until the driver protocol adds that operation.

PrunableStore — the optional retention seam

SessionStore is intentionally minimal. The retention seam is a separate, optional interface discovered by type assertion:

type PrunableStore interface {
    List(ctx context.Context) ([]StoredSession, error)
    Delete(ctx context.Context, id session.SessionID) error
}

List returns every stored session's id and last-save time in no guaranteed order. It applies no filtering — retention policy (which ids are prunable, age thresholds, per-family caps) is entirely the caller's concern.

Delete removes the snapshot for id plus any sidecars the adapter keeps (such as a tool-call log). It is idempotent: deleting an unknown id is success.

A store that does not implement PrunableStore is silently skipped by the composition-layer child-session GC — no error, no sweep. A store whose backend cannot enumerate or delete sessions should return an error wrapping port.ErrPruneUnsupported; the composition layer treats that sentinel as a permanent signal and disables further sweeps rather than retrying.

Bounded storage health

A backend may also implement port.SessionStorageHealthProvider. The management endpoint advertises this capability only when an authorizer is wired, and returns aggregate bytes plus format/kind/corruption counts from an existing metadata index and cheap object/file metadata—never by loading transcripts. Availability flags separate a real zero from unsupported or not-yet-measured data. Jsonlstore supports this view; memstore and remote backends that do not implement the seam report the feature as unsupported. The view is status-only: it does not run cleanup or migration.

Resumable v1-to-v2 migration

Jsonlstore and Redis implement the optional port.SessionMigrationStore management capability. For jsonlstore, an authenticated plan reports v1/v2/invalid/skipped families, current and estimated reclaimable bytes, and the maximum temporary space for one family. Apply processes a bounded batch and returns a durable job handle; use resume to process later batches or continue after a server restart. Each job's complete load-to-checkpoint drive is protected by a stable cross-process job exclusion, so overlapping resumes cannot replay a batch or regress counters. A stale concurrent resume returns conflict. Cancel waits for any committed in-flight batch, then becomes monotonic: later resume attempts conflict and cannot restore the running state. Already-migrated families remain committed.

Migration preserves the complete snapshot, owner, durable kind (including unknown), logical modification time, and tool/event sidecars. For jsonlstore it acquires the ordinary run-entry lease and the family's cross-process lock, rereads and verifies v2 before removing v1, and reports corrupt/torn records without discarding them. Redis uses the same authenticated, durable, bounded job to adopt the derivative metadata index on upgrade: inventory and cleanup remain unavailable while stale; stable inspection verifies that every valid snapshot has its exact global/owner index row and makes missing or bad coverage a repair candidate; each repair and final publication atomically verifies the job's exact lock token before any write. Concurrent Save/Delete advances the source generation, and ready is published only after clean per-snapshot coverage plus the final constant-work cardinality check. Invalid snapshots complete the job with failures while keeping paging unavailable; repair or remove them, then create a fresh plan/job. Job errors expose stable reason codes and sanitized text only. Memstore and remote stores advertise migration as unsupported rather than returning fabricated zero counts.

Authenticated cleanup planning

The server exposes one retention planner to both automatic sweeps and authenticated manual cleanup. A manual dry-run is non-destructive and owner-scoped. On a shared store, it samples cross-process lease status at the planning instant with sequential bounded trial acquire/immediate-release calls; this does not promise that a candidate remains idle for apply, which always reacquires and revalidates. It reports only durable kind/state counts, age or cap reasons, modification times, and byte estimates; transcript, tool arguments, paths, credentials, and foreign-owner rows are never projected. Unknown, invalid, corrupt, running, awaiting, live, and leased sessions are protected and do not consume count-cap slots.

Apply requires the opaque confirmation token returned by the dry-run. The token binds the caller, exact kind scope, inventory generation, and effective policy version. A changed catalog or policy returns a stale-plan result without deleting anything. Cleanup-capable backends implement port.ConditionalPrunableStore: each candidate is revalidated under run-entry serialization and the maintenance lease, then the backend holds its family mutation exclusion across a final metadata comparison and sidecar-first/snapshot-last deletion. Remotely reachable and multi-writer composition must provide a working port.SessionLease; missing or backend-unsupported leasing suppresses migration/cleanup capability advertisement and fails apply closed. Only private embedded mecatui explicitly proves the local single-process posture that may substitute process-local IsLive plus family locking. Partial failures use stable, sanitized reason codes and can be retried by planning again. Unsupported stores report backend_unsupported; they never claim zero impact.

Snapshot mechanics via sessnap

The snapshot format is defined in engine/adapter/sessnap. The sessnap.Snapshot struct is a stable JSON DTO that the store adapters share:

• Conversation messages are serialized through messageDTO (explicit JSON tags, stable against field reordering in the domain type).
• The session lifecycle state is round-tripped through sessnap.RestoreState, which drives the session aggregate through the correct state-machine transitions on load — Complete, Stop, Cancel, Fail, or PauseForApproval — so the restored aggregate satisfies all invariants.
• Provider-private replay fields (Message.Reasoning, Message.ProviderPhase, ToolCall.ItemID) are captured in the snapshot and survive restart.
• Cumulative Usage (the budget brake's input), the Profile, ProviderID, ModelID, and ReasoningEffort labels are persisted so a restarted process can re-derive the exact same per-session engine.

---

EventLog

// engine/port/eventlog.go

type EventLog interface {
    Append(ctx context.Context, id session.SessionID, ev session.Event) error
    Read(ctx context.Context, id session.SessionID) iter.Seq2[session.Event, error]
}

Append durably records ev under the session id. The durability obligation is strict: Append must return nil only after the record is on stable storage or committed to the backing service. For local jsonlstore that means both the file and its directory have synced; without either capability append fails. An implementation that buffers without guaranteeing durability violates the contract. Callers must attempt each event at most once because an error may arrive after the write committed. The relay uses a cancel-detached context so a dead client's cancellation cannot abort it. Clients still receive every original chunk, while the run-scoped recorder coalesces message and reasoning into UTF-8-safe chunks capped at 1 MiB—small enough for the JSONL reader under worst-case escaping. Normal turns use one record per present kind; oversized turns use the minimum bounded count. Every chunk is cleared after its single attempt, append failure warns once per recorder, and later boundary/result events continue. A crash or failed append can leave a log gap; the completed session snapshot remains authoritative.

Read returns the session's events in append order as a lazy iter.Seq2[session.Event, error], following the same streaming idiom as port.LLMProvider.Stream. Streaming matters: a long session log can exceed a gRPC message limit if read as a single unary response; Read maps 1:1 to a server-streaming RPC and avoids the size cap. Key contract points:

• A miss (no log recorded for id) yields an empty sequence, not an error. Absence is data.
• On an infrastructure fault, Read yields (session.Event{}, err) and then stops — no further events after an error.
• An implementation must release any resource it opened (file handle, stream connection) when the consumer breaks out of the range early, exactly like a well-behaved iter.Seq2.

Concurrency contract. Implementations must be safe for concurrent Append and Read across session ids — the server shares one EventLog across all relay goroutines. Per-id append order is only well-defined for a single session's appends (the server serializes those through one relay loop per run).

EventLog vs EventSink

These are separate seams and must not be confused:

	port.EventSink	port.EventLog
Purpose	Live telemetry mirror	Durable chronological record
Direction	One-way write	Write + read-back
Durability	None guaranteed (fire-and-forget)	Strict (nil means on stable storage)
Consumer	Metrics, ACP, TUI relay	Restart rehydration, approval replay, Phase 3 consumers
Where it lives	engine/port/	engine/port/
Loop awareness	Loop emits; relay mirrors to sinks	Loop is storage-agnostic; relay calls Append

The loop never imports port.EventLog or calls Append. Persistence is a relay concern — it lives in internal/adapter/server.

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The three reference backends

Backend	Package	Notes
In-memory	engine/adapter/memstore	Default; test/single-process; implements SessionStore + PrunableStore + EventLog as siblings
JSONL on disk	internal/adapter/store/jsonlstore	Default for mecated; triples as SessionStore + EventLog + ToolCallRecorder. The configured path and every ancestor must be physical non-symlink directories (use macOS /private/..., not a /var/... symlink path). Existing canonical snapshot Save may retain weaker capability; first legacy-family Save, EventLog append, Delete, retention, and migration fail closed when their required sync is unavailable. ToolCall audit is best-effort and may be unsynced or partially synced. Capability probes establish syscall support, not media persistence.
Redis	internal/adapter/redisstore	Used by mecak8s; validated by conformance suites over miniredis

engine/adapter/memstore — in-memory

memstore.Store implements port.SessionStore and port.PrunableStore. It stores sessions as sessnap.Snapshot values in a mutex-guarded map. Save round-trips through sessnap.Of / sessnap.Restore, so the stored copy is deep-isolated from both the caller's live session and from subsequently loaded copies — mutations to either side do not alias the store.

memstore.EventLog is the companion in-memory event log. It stores events by value (no serialization round-trip) under a mutex. Both are wired together in composition when no --store-dir flag is given.

Use this backend for unit tests (it is the reference the conformance suites validate first) and for single-process, non-persistent deployments.

import "github.com/stacklok/mecatl/engine/adapter/memstore"

store := memstore.New()
log   := memstore.NewEventLog()

internal/adapter/store/jsonlstore — JSONL on disk

The default backend for mecated. The same package triples as port.SessionStore, port.EventLog, and port.ToolCallRecorder.

Each session owns an authoritative v2 snapshot and two append-only sidecars under a sid-v1 subdirectory, sharing one stem:

<store-dir>/sid-v1/sid-v1-<token>.session.json      authoritative v2 current snapshot
<store-dir>/sid-v1/sid-v1-<token>.tools.jsonl       one record per tool call
<store-dir>/sid-v1/sid-v1-<token>.events.jsonl      one record per relayed event (eventlog-json/1)

Historical v1 .session.jsonl files remain readable and are lazily promoted on a later write. Do not infer the session id from a filename or edit these files; the store is plaintext and its files, including sidecars, are owner-only (0600).

Snapshot replacement uses a same-directory temporary file, full write, file sync where supported, atomic replacement, and directory sync where supported. SnapshotDurability reports the available primitives: only all three, on an underlying filesystem/storage stack that honors successful sync and atomic rename, make a successful save host-crash safe. The probe verifies syscall support; it does not make volatile storage such as tmpfs survive power loss. Event records are newline-committed and append fails closed when required directory sync is unavailable. Tool-call audit is best-effort and may drop. An interrupted final fragment is ignored or replaced on the next append, while a blank, whitespace-only, or otherwise malformed complete record fails loudly. A stable family flock coordinates cooperating jsonlstore processes, not arbitrary external writers. Quiesce every writer before copying the complete store directory for backup or restore.

Select it with --store-dir <path>. The directory is created if it does not exist, owner-only (0700). A store written by an older version keeps its files directly under <store-dir>; those are read as-is and moved into sid-v1/ the next time that session is written.

internal/adapter/redisstore — Redis-backed

The backend for mecak8s (the storage-free, Kubernetes-native composition root). Implements port.SessionStore, port.EventLog, port.PrunableStore, port.SessionMigrationStore, and port.ToolCallRecorder over a Redis connection. Existing snapshot databases without the derivative metadata index remain loadable but report paging and retention unsupported until an authenticated storage-migration job adopts every row and atomically publishes the index. Select it via --redis-url (mecak8s only). Validated by the same conformance suites as jsonlstore, running over miniredis.

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Event-sourced Load

Mecatl's own adapters use the snapshot model: Save writes a full snapshot; Load deserializes it. But a host whose system of record is an append-only event log can implement port.SessionStore.Load by folding the event stream into a *session.Session instead.

The reference implementation is engine/adapter/eventsource.Fold:

// engine/adapter/eventsource

func Fold(meta SessionMeta, events iter.Seq2[session.Event, error]) (*session.Session, error)

Fold walks the event stream and reconstructs the session's conversation, lifecycle state, counters, cumulative usage, and pending ask. SessionMeta carries the creation facts that no event records — id, mode, limits, workspace, profile, provider/model selector, reasoning-effort, and createdAt — which the host supplies out-of-band.

Field-by-field reconstruction contract

The table below is derived from engine/COMPATIBILITY.md ("Session reconstruction contract"):

Field	Round-trip obligation	Event source
Conversation (user prompts, assistant text, tool calls, tool results — pairing-valid)	MUST	EvUserPrompt (genuine prompt + harness continuations), EvMessageDelta, EvToolCall, EvToolResult; pre-compaction head from EvCompactionArchive
State (idle / running / awaiting / completed / failed / cancelled)	MUST	derived from terminal EvResult.Stop; trailing unanswered EvPermissionAsk → awaiting; no terminal → idle
Recorded stop reason (RecordedStopReason)	MUST	EvResult.Stop
Pending ask (PendingAsk, when awaiting)	MUST	trailing EvPermissionAsk with no following EvApproval or EvResult
Cumulative Usage	MUST	SUM of every per-run EvResult.Usage (each is per-run; the budget brake reads the cumulative aggregate)
Creation metadata (id, mode, limits, workspace, profile, provider/model selector, reasoning-effort, createdAt)	MUST — supplied out-of-band	Not in any event — provided via eventsource.SessionMeta
Counters (turns / tool calls / consecutive failures)	run-scoped — latest run segment only	EvTurnStart (turns), EvToolResult (tool calls / consecutive failures)
Run plumbing (diagnostics binding, askID serials)	safe to lose — rebuilt fresh	n/a

Replay-fidelity limitation

A fold reconstructs the structural conversation faithfully and is byte-identical-replay-faithful only for providers that do not use the provider-private opaque replay fields. Three fields reach the conversation only via Session.RecordAssistant in the loop and are never emitted on the event stream:

• Message.Reasoning — the provider reasoning replay blob (OpenAI encrypted reasoning content; Anthropic (thinking, signature) pair)
• Message.ProviderPhase — the OpenAI Responses phase marker
• ToolCall.ItemID — the provider-assigned item id

EvReasoningDelta carries a human-readable reasoning summary, which the loop deliberately never places on Message.Reasoning — a fold must not do so either.

For plain-chat providers (including mockllm) those fields are empty and the fold is byte-identical. For reasoning providers (OpenAI, Anthropic) the fold produces a structurally correct but not byte-identical conversation. This is why Mecatl's own resume uses the snapshot; the fold is for event-log-SoR hosts that accept this boundary or carry those fields in their own richer event schema.

This is a documented contract limitation, not a bug. See engine/COMPATIBILITY.md and ADR 0038.

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Conformance suites

Any SessionStore implementation — including your own — can be validated against the shared conformance table in engine/adapter/storeconformance:

import "github.com/stacklok/mecatl/engine/adapter/storeconformance"

func TestMyStore(t *testing.T) {
    storeconformance.Run(t, func(t *testing.T) port.SessionStore {
        return mystore.New(t.TempDir())
    })
}

If your store also implements port.PrunableStore, run the retention suite too:

    storeconformance.RunPrunable(t, func(t *testing.T) port.SessionStore {
        return mystore.New(t.TempDir())
    })

The store suite pins the contract — snapshot encoding, isolation (Save and Load must not alias the caller's pointer), lifecycle-state fidelity for all six session states, multi-session keying, not-found sentinel wrapping, and overwrite semantics. It does not assert wire format or file layout.

Similarly, any EventLog implementation can be validated against engine/adapter/eventlogconformance:

import "github.com/stacklok/mecatl/engine/adapter/eventlogconformance"

func TestMyEventLog(t *testing.T) {
    eventlogconformance.Run(t, func(t *testing.T) port.EventLog {
        return mylog.New(t.TempDir())
    })
}

The event log suite covers: append-then-read order, miss as empty sequence (not error), distinct-session isolation, no Seq-based reordering (contract is raw append order), large event round-trip, cumulative log crossing the framing boundary (the streaming-vs-unary correctness test), early-break resource release, and concurrent append/read safety under -race.

Both suites are the same ones jsonlstore and redisstore pass — passing them is the bar for a production-grade implementation.

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Wiring

Direct embed (engine consumer)

Pass your store and event log through agent.Deps:

deps := agent.Deps{
    Store:    mystore.New(),
    // EventLog is wired at the relay layer (internal/adapter/server), not in Deps
    // ...other fields
}
eng := agent.NewEngine(deps)

The EventLog is not a Deps field — the loop is storage-agnostic by design and never imports port.EventLog. Wire it in your relay layer, calling log.Append for every event your relay observes before forwarding it to clients.

mecated (standalone server)

• --store-dir <path> — selects the jsonlstore backend (JSONL on disk). Both SessionStore and EventLog are wired from the same store directory.
• No flag — defaults to memstore (in-memory, no persistence across restarts).

mecak8s

• --redis-url <url> — selects the Redis backend. Both SessionStore and EventLog are wired through the Redis adapter.

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What's next

• LLM provider — implement the port.LLMProvider interface to plug in a custom model backend.
• Permission policy — implement port.PermissionPolicy to replace the built-in rule engine.
• Hook system — the port.HookRunner lifecycle hooks that fire around tool calls.
• The agent loop — how the loop drives turns, emits events, and reaches terminal states.