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67 lines
3.5 KiB
Markdown
67 lines
3.5 KiB
Markdown
# hive-jobq
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A job-DAG scheduler, extracted from hive-c0re's in-tree `job_queue`
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as a **domain-agnostic** library. It schedules a single in-memory graph of
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nodes over named resources; it knows nothing about containers, rebuilds, or any
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hyperhive type — the node payload `N` and resource name `R` are both generic, so
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the caller supplies its own domain.
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## When to use it
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Reach for this crate whenever you need to run a DAG of interdependent work items
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under bounded, named concurrency — the hive-c0re rebuild/lifecycle queue is the
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first consumer, but nothing here is specific to it. The caller defines the node
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kinds, wires deps, and supplies a runner; the scheduler decides what can start.
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## Model
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**Runtime-only: nothing writes this graph to disk.** The serde impls exist for
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the wire projection (`hive-jobq-wire`) and a possible future store; no caller
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loads one, so ids and timestamps are stable within a run, not across restarts.
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hive-c0re constructs an empty graph every boot and re-derives desired state with
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its reconcile sweep.
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One **shared graph** for the whole system, not a DAG per job. Enqueuing
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inserts a self-contained sub-DAG and returns the ids of the nodes the job
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_asked_ for, in the order it named them; the scheduler runs a continuous loop,
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starting every node whose deps are satisfied:
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- **Resource deps** are named counting semaphores over a caller-chosen type `R`
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— e.g. `build-slot` (capacity N), `agent/<name>` (capacity 1), or any
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unconfigured name (capacity 1, created on use). A node acquires _all_ its
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resource deps atomically at start (all-or-nothing) — no hold-and-wait, so no
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deadlock.
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- **Node deps** wait on another node per `DepWhen`: `AfterOk` needs success (a
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failed dep cancels the dependent), `AfterAny` only needs terminal.
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A node carries two independent axes: its `Dep`s (ordering + resource needs) and
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its `parent` (structural grouping). The **parent chain**, not the node edges, is
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what the scheduler consults for resource re-entrancy: a resource unit is held
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for the acquiring node _plus its whole parent subtree_, and a descendant needing
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a resource an ancestor already holds re-uses that grant (a re-entrant borrow,
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one branch at a time) rather than taking a fresh unit.
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A `NodeId` is opaque, stable and monotonic **within a run** — a fresh process
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mints ids from zero, so an id stored outside it is a historical record, not a
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handle that will resolve later. The scheduler is
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single-threaded — it owns the resource table and mutates it directly.
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## Shape
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- **`Graph<N, R>`** — the in-memory node store. `insert` mints ids and
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validates dep/parent references; `set_state` is the single state-transition
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choke point (and where each node's lifecycle timestamps —
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`started_at` / `finished_at`, `DateTime<Utc>` — are stamped).
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- **`Node<N, R>`** — `{ id, parent, payload, deps, state, started_at,
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finished_at, error }`. All fields public; derives serde for the wire
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projection (and so a store could be added — nothing calls one today).
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- **`Scheduler<N, R>`** — drives the graph: `settle()` starts every ready node
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(acquiring resources atomically), `complete(id, outcome)` reports a finished
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node's result and rolls terminality up the parent chain, releasing grants once
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a subtree is done. `Outcome::{Done, Failed(String)}` — the failure reason
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rides `Failed` onto the node's `error`.
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- **`ResourceTable<R>`** — per-name capacities; unconfigured names default to
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capacity 1.
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See the crate-root and `scheduler` module `//!` docs for the full borrow/release
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model.
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