arkitekt-service

What a service of an Arkitekt hub is made with. Three parts, for three different things a service process does:

They share arkitekt_service.trust — no secrets between services: every instance signs with its own key and the hub vouches for the public halves — and nothing else. A process uses any of them without the others.

pip install arkitekt-service

The contract

Every service image answers the same entry point:

arkitekt-service describe    # what it needs from a hub and offers to it (JSON)
arkitekt-service render      # this release's config, from the hub's facts
arkitekt-service check       # does this release read a config as written?
arkitekt-service migrate     # its database migrations, as a step
arkitekt-service upgrade --from 5.2.0 --to 6.0.0

An installer (Konstruktor) then needs to know the hub, and nothing about a service that the service's image does not say. How a release spells its config, which keys it renamed, what it has to do to its data — all of that ships in the image, with the code it belongs to. Exit code 78 is a release's own refusal (facts it cannot be configured from, a setting it does not read), with the reason on stderr.

A container serves; a job prepares

A service's container does one thing: serve. Its start script starts the server and nothing else. Everything its database needs first is migrate: wait for the database, apply the release's migrations, then run the commands the service declared as its setup (an admin account, seeded rows), all in one process. Whoever starts the service runs it as a job of its own, once per build, before the first start and before an update's:

docker compose run --rm --no-deps mikro arkitekt-service migrate

or, in a compose file that has no installer, as a service the server waits for:

mikro-migrate:
  image: jhnnsrs/mikro:7
  command: arkitekt-service migrate
mikro:
  image: jhnnsrs/mikro:7
  depends_on:
    mikro-migrate:
      condition: service_completed_successfully

A container that merely restarts then does none of it. No run.sh migrates, seeds or waits for a database, and neither does a development one.

What describe prints also says how the image is run, so an installer assumes none of it:

• identifier: what the service is registered as, and what a client asks for.
• jobs: what can be run in the image beside its start, by name, each with the command that runs it. A service declares its own (Job(("ensureadmin",), "Create the operator account") on its contract); migrate is every service's, and lists the ones it runs as its setup (includes). An operator runs any of them again by name, e.g. konstruktor job run mikro ensureadmin, which is arkitekt-service job ensureadmin in a container of the image.
• prepare: which job brings the database to the release (migrate), or null for a service with nothing to prepare.
• sidecars: what the service does not run without and ships as an image of its own (rekuest's takt), named from the service's image.
• serve and debug: what a container of the image runs to serve, in production and in development (bash run.sh, bash run-debug.sh unless the service says otherwise). An installer writes the one that applies as the service's command.
• render: what writes the release's config from the hub's facts.

The image's own command is describe:

CMD ["arkitekt-service", "describe"]

So an installer asks by running the image with no command, and assumes nothing of what is inside: every other command it runs is one the answer named. A service written in another language takes part by printing the same description as its own command. Whoever starts the image by hand gets the description too, and beside it, on stderr, a note saying that nothing is being served and what to run instead.

A service declares itself in one module, named by ARKITEKT_SERVICE in its Dockerfile:

from arkitekt_service.contract import JSON, Contract, Description, Facts, Needs, Offers, blocks


def render(facts: Facts) -> dict[str, JSON]:
    document = blocks.server(facts)  # django, postgres, redis, authentikate
    document["datalayer"] = blocks.datalayer(facts, "media", "zarr")
    return document


contract = Contract(
    description=Description(name="mikro", identifier="live.arkitekt.mikro", needs=Needs(storage=["media", "zarr"])),
    settings=Settings,
    render=render,
)

The hub's facts (arkitekt_service.contract.facts) and a service's description (arkitekt_service.contract.description) are versioned documents; an image refuses facts it does not understand rather than dropping them.

A service, and a hook agent

from arkitekt_service.service import Descriptor, Service, organization_of
from arkitekt_service.hook import HookAgent

service = Service("mikro", description="Microscopy data")
dataset = service.structure(ArrayDataset, "@mikro/arraydataset", descriptors=[Descriptor("@mikro/n_channels", "INT")], describe=array_descriptors)
service.model_signal(dataset, organization=organization_of())

agent = HookAgent("mikro", description="mikro's housekeeping")


@agent.action
def reembed_stale(organization: str) -> dict:
    """Re-embed stale rows."""
    ...


urlpatterns = [..., *service.urls, *agent.urls]

A service says what exists; an agent says what can be done. Neither knows the other.

Development

uv sync
uv run pytest                                          # the contract and the service
uv run pytest tests/hook --ds=hook_project.settings    # the hook agent: a process that is no service
uv run basedpyright

Releases are cut from conventional commits on main (tag-only, like the other Arkitekt packages) and uploaded to PyPI.