Durable Picos with celld

From Hello, Pico! to a persistent, addressable, hibernating actor

The architectural lesson

A Pico is not a container.
A Pico is not a process.
A Pico is not a Kubernetes Pod.
A Pico is not a Durable Object.
A Pico is not celld.

              Pico
                │
                ▼
        engineering entity
                │
                ▼
           Runtime SPI
          ╱      │      ╲
         ╱       │       ╲
     Native    celld     future
                │
                ▼
          Durable Actor

Pico is the Open Engineering abstraction. celld is one possible runtime implementation.

The central experiment

POST hello  →  event_count = 1
POST hello  →  event_count = 2
# restart runtime
POST hello  →  event_count = 3

State belongs to the Pico, not to the process that last executed it.

What you will build

Artifact	Role
HelloPico	Domain behaviour (runtime-independent)
PicoRuntime SPI	Abstract deploy / invoke / inspect / …
NativeRuntime	Volatile in-process baseline
DurableRuntime	SQLite-backed durable adapter (celld-shaped)
Contract tests	Same suite passes on both runtimes

Learning path

Hello, Pico!
     │
     ▼
State vs process
     │
     ▼
Actor model
     │
     ▼
Pico Runtime SPI
     │
     ▼
Durable state + restart recovery
     │
     ├─ multiple identities
     ├─ alarms / hibernation
     ├─ containers / Minikube
     └─ Crossplane / Manifold (reuse Academy material)

Runtime package

Working code lives in runtime/:

cd runtime
PYTHONPATH=src python -m pytest tests -q
# 18 passed

Principle

Prefer a semantic runtime request (class: durable) over hard-coding an implementation (implementation: celld).