Glossary

Every Tape-specific term, defined. If a doc page assumes you know a word and it isn't on this page, file an issue — the docs need to be self-contained.

Core nouns

ADK

Google's Agent Development Kit. The host agent framework Tape is scoped to. Tape rides on ADK's plugin system, custom SessionService, LongRunningFunctionTool, and resumable invocation_id.

Agent

The thing that acts. In Tape, "agent" specifically means an ADK LlmAgent or composition of agents that calls tools. The agent process is separate from the runtime processes (server + reactors).

App

A named deployment unit. The name= you pass to durable_app(...). Scoping unit alongside user_id and session_id.

Budget

Run-level dollars and tokens, journalled. AdmitBudget before a costed boundary, ChargeBudget after. Spent counters survive crashes.

Compensation

A second operation that undoes a first, committed operation. The saga pattern (Garcia-Molina & Salem, 1987). Tape's compensation reactor walks obligations LIFO over a run.

Connector

A typed adapter to one upstream. Implements three methods: dispatch, observe, compensate. The outbox / reconciler / compensation reactors call into it.

Decision

A choice the agent makes — typically "call tool X with args Y" or "produce output Z." Decisions are journalled and replayed on resume, so the agent doesn't re-ask the model for choices it already made.

Effect

A tool call that touches the outside world. Has a declared semantics (IDEMPOTENT / NON_IDEMPOTENT / OBSERVE_ONLY) and a status (PENDING → CONFIRMED / FAILED / UNKNOWN).

Gate

A LongRunningFunctionTool (ADK term) that suspends the run until a named signal arrives or a timer fires. tape.gate_tool("name", timeout_ms=...).

Idempotency key

The deterministic identity of a tool call across replay. In Tape: r-<run>/d-<decision>/<tool>/<call_idx>. Derived from the journal, not from inputs.

Journal

The append-only history of decisions and effects per run. The source of truth. Replay reconstructs the agent's view from the journal.

Lease

A CAS-held claim by one reactor on one row (run / effect / obligation / timer). Time-bounded; the holder heartbeats. Other workers see the lease and skip.

Obligation

A pending compensation. Created when the reconciler detects a DUPLICATE or when compensate_run walks confirmed effects. The compensation reactor resolves obligations.

Outbox

The dispatch pattern for non-idempotent effects: tool body returns intent only; the outbox reactor performs the dispatch under a CAS lease. @tape.outbox_tool(...) is the decorator that opts in.

Policy version

A label ("cfo-2026.05") journalled on every decision. The agent can branch on tape.policy_is(tool_context, "..."). Tape doesn't automatically use the new code path for new runs — you do.

Reactor

A background loop that closes a specific gap between the journal and the world. Five ship: recovery, reconciler, outbox, timers, compensation. Each is idempotent.

Reactive KV

Tape's CAS-versioned, watchable key-value store. set_value / get_value / watch_value / delete_value. Not the journal. Used for fan-out by key, multi-subscriber.

Replay

The act of reconstructing an in-flight run from seq=0 to the resume point. Confirmed effects are short-circuited; recorded decisions are handed back to the agent loop.

Resume point

The first seq in the journal with no record. Where the agent actually starts running again on re-drive.

Run

One conversation / trajectory / session. Identified by run_id, typically derived from ADK's invocation_id. The unit of leasing, cancellation, and budget.

Saga

A sequence of operations where each step has a registered inverse (compensator). If a later step fails, earlier steps are compensated in LIFO order. Tape's compensation reactor implements this.

Signal

A named, point-to-point message to a specific run. SendSignal from outside; AwaitSignal from inside. Single-consumer.

Status check

A callable that asks the upstream — out of band — whether a specific effect committed. Used by the reconciler when an effect is UNKNOWN.

STUCK

A terminal-ish state for a run, effect, or obligation when Tape can't determine what happened. STUCK is good — the alternative is silently wrong.

Tape server

The Rust server that owns the wire protocol. Stateless; horizontally scalable; speaks one of SQLite / Postgres / AlloyDB / Bigtable / Spanner to the store.

Tool

ADK's primitive. Tape decorates tools with @tape.effect (idempotent upstream) or @tape.outbox_tool (non-idempotent upstream).

UNKNOWN

The third outcome. The request might have committed; we don't know. The reconciler reactor resolves it by calling observe().

WAL tail

SubscribeEvents — a cross-run stream of journal entries ordered by (ts, run_id, seq). Wire it through run_event_fanout for an in-process consumer or run_outbox_relay for an exactly-once-effective publisher.

Verbs

cancel_run

Mark a run CANCELLED. Cooperative — the agent bails at the next boundary. Does not automatically compensate.

compensate_run

Walk a run's confirmed effects LIFO and enqueue compensation obligations for each. The saga rollback.

dispatch

The forward path of an effect: the connector talks to the upstream.

heartbeat

Extend the lease on a run from within a long tool body, so the recovery reactor doesn't decide the run is stale.

observe

The out-of-band lookup the reconciler uses to resolve UNKNOWN. The connector implements it.

redrive

"Wake this run up." Operator's poke-with-a-stick. Schedules an immediate redrive-kind timer for the recovery reactor.

reconcile

What the reconciler reactor does — walk UNKNOWN effects, call observe, write the resolved status back.

sample

tape.sample(tool_context, fn) — call fn once per run, journal the result, return-from-history on replay. The escape hatch for nondeterminism (wall-clock, random, external reads).

set_value / watch_value

Reactive KV operations. CAS-versioned writes; watch streams that emit transitions (prev_value → value).

Three-letter words you'll see

ADC

Application Default Credentials. The Google Cloud auth source.

CAS

Compare-And-Swap. The lease primitive — write conditional on a previous version.

GSA / KSA

GCP Service Account / Kubernetes Service Account. Workload Identity binds the two.

KV

Key-Value (the reactive store).

OTel

OpenTelemetry. Tape opens spans on tape.begin_run, tape.begin_effect, etc.

SA

Service Account.

SDK

Software Development Kit. Python is the reference; Go / TypeScript / Java are wired clients with the protocol round-trip.

TTL

Time-To-Live. Leases have one; tokens have one.

WAL

Write-Ahead Log. The cross-run journal stream.

Two-letter words you'll see

H2

HTTP/2. tape-server runs with --use-http2 on Cloud Run.

IaC

Infrastructure as Code. Tape ships Terraform/OpenTofu + Helm.

See also

• Concepts — the same terms, in narrative form.
• Cheat sheet — every API, one page.