34. Delay Queue Scheduler

The brief

Delay Queue Scheduler. Millions of future actions — reminders, retries, expiries — must fire near their due time. Scanning everything every minute does not scale, and an in-process timer dies with the process. Work from the scoping questions below. State assumptions for any unspecified load, guarantee or target, then trace your design end to end. Explain one difficult case and a credible alternative; the worked example is a reference, not a required implementation.

• Set the scope: Which actions run later and who chooses their due times? How late may a scheduled action run?
• Define the contract: Can an action be canceled or rescheduled after acceptance? What happens if a worker fails during execution?
• Test the boundaries: How many actions become due together, and how is recovery paced?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Delay Queue Scheduler. Separate stated behavior from assumptions, and identify what is outside your design.

Supported operating targets

Declare relevant volume, latency, freshness, quality or cost targets with units. Show calculations or an evaluation plan that can test them; unspecified targets are your assumptions, not hidden pass criteria.

Failure and boundary behavior

Explain how your guarantees hold in a difficult case relevant to this subject. Address: What happens if a worker fails during execution? How many actions become due together, and how is recovery paced?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: Which actions run later and who chooses their due times? How late may a scheduled action run?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: Can an action be canceled or rescheduled after acceptance? What happens if a worker fails during execution?

3. 03

   Capacity and operating targets

   Estimate the dominant workload and resource demand with units and explicit assumptions. For a learned system, also state how quality is measured and what data is available.

4. 04

   Architecture and central flow

   Draw or describe the responsibilities needed for Delay Queue Scheduler. Trace a representative request, event or job from its input to a visible result; identify durable state owners.

5. 05

   Failure and boundary walkthrough

   Walk through a difficult case step by step, including detection and recovery. Consider: How many actions become due together, and how is recovery paced?

6. 06

   Tradeoffs and operations

   Compare a credible alternative using your chosen workload and guarantees. Explain a remaining risk, a signal to watch and when you would change the design.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Delay Queue Scheduler

A table of things due later, a sweeper that claims the ones due soon, and a delay queue that holds each until its moment.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.