ops[tracing] charmlib api.py

class TracingEvents(Protocol):
    @property
    def request(self) -> EventSource: ...
    def broken(self) -> EventSource: ...

class CertEvents(Protocol):
    @property
    def something(self) -> EventSource: ...
    def other(self) -> EventSource: ...

class TracingRequirerProtocol(Protocol):
    @property
    def on(self) -> TracingEvents: ...
    def is_ready(self) -> bool: ...
    def get_endpoint(self, protocol: str) -> str|None:
    ...


class CertificatesRequirerProtocol(Protocol):
    @property
    def on(self) -> CertEvents: ...
    def is_ready(self) -> bool: ...
    def write(self, file_obj: xxx) -> None:
    ...


class Tracing(ops.Object):
    def __init__(
        self,
        charm: ops.CharmBase,
        tracing_requirer: TracingRequirerProtocol,
        certificates_requirer: CertificatesRequirerProtocol | None = None)
    ):
        self.tracing_requirer = tracing_requirer
        self.certificates_requirer = certificates_requirer
        for evt in (... the events for the above two ...):
            self.framework.observe(evt, self.reconcile)

    def reconcile(self):
        rv = None, None

        if not self.tracing_requirer.is_ready():
            return

        endpoint = self.tracing_requirer.get_endpoint("otlp_http")
        if not endpoint:
            return

        if not self.certificates_requirer.is_reaady():
            return

        ...

        # Finally
        self.charm.xxx.set_tracing_destination(*rv)

Charmers are left with this much boiler-plate:

class Charm(ops.CharmBase):
    def __init__(self, framework):
        self.tracing_endpoint = TracingEndpointRequirer(
            charm,
            relation_name="tracing-relation-name-must-match-metadata",
            protocols=["otlp_http"],
        )
        self.certificates = CertificatesRequirer(
            charm,
            relation_name=...,
        )
        self.tracing = Tracing(self, self.tracing_endpoint, self.certificates)

Could be a little better as:

class Charm(ops.CharmBase):
    def __init__(self, framework):
        self.tracing = Tracing(
            self,
            TracingEndpointRequirer,
            "tracing-relation-name",
            CertificatesRequirer,
            "certificates-relation-name",
        )

we're responsible to instantiate the requirers and must replicate their __init__() in the protocols.
we don't need to vendor the requirer source code though.

Decision from the stand-up: let's vendor the charm libs.

• easy to start
• relatively easy to upgrade
• the pydantic dep cost is manageable