checkpoint_and_rehydrate

What this lesson demonstrates

A cyclic guess-the-number workflow proves that a checkpoint is real by discarding the running instance and rehydrating a brand-new workflow from a mid-run snapshot — which then finishes the search from exactly where the snapshot left off. Two executors form a loop: Guess binary-searches for a hidden target inside its [lower, upper] bounds; Judge compares each guess to 42 and sends back Above / Below until the guess is exact, then yields the final string.

Crucially, this lesson has no model, no credential, no network — it teaches a pure in-process workflow, so it runs identically everywhere.

First run, then rehydrate

The runtime is decorated with checkpointing; every super-step boundary produces a CheckpointInfo you collect. Then a fresh graph is built and resumed from one of them:

newWorkflow := buildWorkflow()   // fresh executors, initial bounds
savedCheckpoint := checkpoints[checkpointIndex]

newCheckpointedRun, err := inproc.Default.
    WithCheckpointing(checkpointManager).
    ResumeStreaming(ctx, newWorkflow, savedCheckpoint)

buildWorkflow() is deliberately called again — the resume path starts from a clean slate and lets the snapshot overwrite it.

What to notice

• State lives in the executor; checkpointing lives in the runtime. Guess and Judge hold plain fields (lowerBound, tries). They opt in by implementing OnCheckpoint (queue state via ctx.QueueStateUpdate) and OnCheckpointRestored (read it back via ctx.ReadState). Attach them with Extend(&workflow.Executor{OnCheckpointFunc: ..., OnCheckpointRestoredFunc: ..., ResetFunc: ...}).
• Rehydration is distinct from resuming the same run. Here you build a new *workflow.Workflow; Reset gives it a clean slate and OnCheckpointRestored overwrites it with the snapshot. A missing key falls back to the reset state. (The next lesson rewinds the same live run instead.)
• The graph is cyclic and terminates by yielding. AddEdge(Guess→Judge) and AddEdge(Judge→Guess) form the loop; WithOutputFrom(Judge) marks the output node. The loop ends because Judge yields (instead of sending) once the guess is correct.
• Typed message contracts. The zero-size _ workflow.AttrSendsMessage[int] / AttrYieldsOutput[string] marker fields declare each executor’s message and output types to the builder.

How it maps to the Agent Framework Go SDK

inproc.Default.WithCheckpointing(manager) plus checkpoint.NewInMemoryManager() and the executor’s OnCheckpoint / OnCheckpointRestored hooks are the SDK’s durability story for workflows. ResumeStreaming rebuilds a run from a CheckpointInfo handle — the foundation for crash recovery and long-running, resumable agent workflows.

Run it

go run  ./tutorial/03-workflows/checkpoint/checkpoint_and_rehydrate
go test ./tutorial/03-workflows/checkpoint/checkpoint_and_rehydrate

There is no live path — AF_LIVE=1 changes nothing here, since this lesson is a pure in-process workflow. The offline test drives a full checkpoint/resume round-trip.

Next: checkpoint · Checkpoint and Resume