Readiness Scan¶
Detection mechanism — instantiates Head-of-Line Blocking Relief
Sweeps the items waiting behind a stalled head and flags which ones are independent and ready to move, turning a blocked queue into a list of what can safely proceed.
Every relief action in this archetype needs the same fact first: given that the head is stuck, which items behind it can actually move? Readiness Scan is the sensing layer that supplies it. It does two things and stops there — it confirms the head is genuinely blocked (not merely slow), and it walks the items waiting behind it, marking each one ready, not-yet-ready, or depends-on-the-head. Its output is an eligibility list plus a legible picture of the queue's state; it never routes, reorders, or escalates anything. That restraint is the point: by separating "what can move" from "move it," the scan lets every downstream mechanism act on evidence instead of a hunch, and keeps the correctness question (independence) from being tangled up with the throughput question (flow).
Example¶
A batch-processing cluster pulls jobs from a single ordered queue. The job at the head is stuck — it is waiting on a database lock another process holds — and the workers behind it sit idle even though the queue is full. A Readiness Scan runs across the head and the next span of jobs: it marks the head blocked (lock not acquired), then inspects each waiting job's declared inputs. Jobs that read only their own partitions are ready; two that also touch the locked row are depends-on-head; one whose upstream feed hasn't landed is not-yet-ready. The scan publishes this as a small table — head blocked, five ready, two dependent, one pending — visible to the scheduler and to an on-call engineer. Nothing has moved. But now the scheduler has exactly what it needs to safely release the five ready jobs, and the engineer can see at a glance that the blockage is a lock, not a capacity shortage.
How it works¶
What distinguishes a scan from a generic queue monitor is that it evaluates independence, not just presence:
- Confirm true blockage. Separate a head that cannot proceed (missing input, held lock, unmet dependency) from one that is merely slow but progressing — only the former should trigger relief.
- Test each follower for independence. For every item in the scan window, check whether it can be served without the blocked head — no shared lock, no data dependency, no ordering commitment tying it to the head.
- Publish state, don't act. Emit the result as a legible list (ready / dependent / pending) that downstream mechanisms and humans can read; the scan's contract is to inform, never to move work.
Tuning parameters¶
- Scan window — how far behind the head the scan looks. A deeper window finds more ready work per pass but costs more and risks acting on stale readiness.
- Blockage sensitivity — how quickly a slow head is classified as blocked. Too eager and you re-scan constantly on transient slowness; too lax and real blockages sit undetected.
- Dependency granularity — how finely independence is tested (whole-queue vs. per-record vs. per-lock). Finer tests catch more true dependencies but cost more to compute and maintain.
- Refresh cadence — one-shot vs. continuous re-scan. Continuous scanning keeps the eligibility list fresh as items complete, at ongoing overhead.
- Visibility surface — how much state is exposed and to whom. Richer surfaces help humans govern bypass but can leak queue internals.
When it helps, and when it misleads¶
Its strength is that it turns a blocked queue into an explicit, checkable list of what can safely move, and it prevents the single most dangerous relief error — advancing an item that only looked independent. Because it publishes rather than acts, one scan can front several different relief mechanisms without committing to any.
Its failure mode is a false readiness verdict: an item marked ready that actually shares a hidden dependency with the head, so serving it violates correctness downstream. The classic misuse is trusting a shallow scan — presence-only, no true independence test — which effectively rubber-stamps cherry-picking. The discipline that guards against this is to make the independence test as strong as the domain's real coupling (locks, data, commitments) and to re-scan rather than cache a stale verdict. The scan's stance mirrors readiness-notification I/O, which reports which connections can make progress rather than acting for them.[1]
How it implements the components¶
Readiness Scan fills the sensing components of the archetype — the facts every relief action consumes:
blockage_detection— its first job: confirm the head is genuinely stalled rather than merely slow.readiness_or_dependency_check— its core: per-item independence testing that marks which followers can be served without the head.backlog_visibility— its output surface: a legible ready / dependent / pending picture of the waiting set.
It does not route ready work around the head (bypass_rule — that is Bypass Queue), relax the service order in place (resequencing_policy — Out-of-Order Processing), open a second lane (parallel_service_path — Parallel Lane Activation), or put a clock on the stall (stall_threshold — Timeout and Escalation); the scan only reports what can move.
Related¶
- Instantiates: Head-of-Line Blocking Relief — Readiness Scan is the sensor the whole archetype depends on; every governed relief action reads its verdict first.
- Sibling mechanisms: Bypass Queue · Out-of-Order Processing · Parallel Lane Activation · Resequencing Buffer · Blocked Item Escalation · Timeout and Escalation · Exception Queue
Editorial Notes¶
Form Classification¶
Form family: Decision, Gate & Allocation
Rationale: Readiness Scan operates by classifies a blocked head and routes an independent follower into the eligible relief path. That concrete deployed or enacted form is Decision, Gate & Allocation under the frozen taxonomy.
Nearest alternative: Assessment, Review & Assurance — Although Assessment, Review & Assurance can support this mechanism, the frozen evidence makes its operative form the act that classifies a blocked head and routes an independent follower into the eligible relief path; the alternative is therefore secondary rather than defining.
Review outcome: Adjudicated after independent review; medium confidence.
Origin Attribution¶
Primary origin: Computer Science & Software Engineering
Origin pattern: Cross-disciplinary synthesis
Present-day reach: Multi-domain
Rationale: Scanning past a stalled queue head to find independently executable work responds directly to head-of-line blocking in computer and network scheduling, using operations-research queueing concepts.
Related originating lineages:
- Operations Research — Queueing and scheduling theory independently formalized selecting ready work under constraints.
Review resolution: The blind reviewers disagreed on primary lineage. Light authoritative research resolves the defining form in favor of computer_science: Scanning past a stalled queue head to find independently executable work responds directly to head-of-line blocking in computer and network scheduling, using operations-research queueing concepts. The rejected primary is retained only when it materially shaped the mechanism, and present-day breadth is recorded separately as domain_reach=multi_domain.
Review outcome: Researched adjudication after independent review; high confidence.
Sources consulted:
References¶
[1] Stevens, W. R. UNIX Network Programming, Volume 1: Networking APIs—Sockets and XTI, 2nd ed. Prentice Hall PTR (1998). Stevens describes readiness-notification I/O as reporting which descriptors are ready for progress without performing the connection I/O itself. The source does not establish that this mechanism's scan mirrors that API. registry ↩