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docs: design note for AppService start/stop ordering - #35
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Compares a global priority, phases, and provides/requires capabilities, and recommends phases with milestones inside a phase, so modules order against roles rather than each other's classes or numbers.
Code reviewNo issues found. Checked for bugs and CLAUDE.md compliance. |
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Design note only, nothing implemented: docs/service-ordering.adoc.
It came up while reviewing #34, where the Flyway and Exposed services set their order as offsets from
JdbcDataSourceService.PRIORITY. The note compares four approaches: the current priority with named bands, phases (layers), provides/requires capabilities, and a hybrid of the last two.Recommendation: use a closed set of phases (
INFRASTRUCTURE,INITIALIZATION,APPLICATION,SERVING) for the coarse order and to give every service a default position. Inside a phase, services order themselves with typed milestones:after: order after the providers of a milestone, if any exist.requires: the same, but the boot fails if no provider exists.There is no priority. Services that the phases and milestones leave unordered have no guaranteed order. A
TieOrderbreaks ties by name (in production) or shuffled with a logged seed (in development and tests, viaboot-env) to surface missing milestones.priority()is deprecated for one release and still honoured, then removed. With this,boot-db-exposedruns afterDbMilestones.SchemaMigratedwithout knowing about Flyway or Liquibase, and with no migration tool the constraint is simply met. Bootable computes the start plan once, before any service starts. Missing providers, cycles and constraints that point at a later phase fail the boot with a clear message. Shutdown runs the start plan in reverse. Wiring is the same in vanilla, Kodein and Metro, and existing apps keep their current behaviour during the deprecation release.Customizable:
PhaseandMilestoneare interfaces, so an enum is all it takes to define your own. The phases, the default phase, and how each service's ordering is read (orderingOf) come from aStartOrderpassed toBootable;DefaultStartOrderis used when none is passed. An app can add a phase of its own, replace the phases (mapping library services' default phases onto its own), or override a library service's ordering from the composition root without subclassing it.