36. Geospatial Index with TTL

The brief

Geospatial Index with TTL. Finding the nearby members of a set that is constantly moving. A table scan with a distance function is hopeless at any scale, and a stale position is worse than none — it sends a car to someone who left ten minutes ago. 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 moving objects are being located and how often do they report? What radius and result count does a nearby query request?
• Define the contract: When does an old location become unusable? How should results behave near cell boundaries and in dense areas?
• Test the boundaries: What location history may be retained or exposed?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Geospatial Index with TTL. 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: How should results behave near cell boundaries and in dense areas? What location history may be retained or exposed?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: Which moving objects are being located and how often do they report? What radius and result count does a nearby query request?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: When does an old location become unusable? How should results behave near cell boundaries and in dense areas?

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 Geospatial Index with TTL. 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: What location history may be retained or exposed?

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

Geospatial Index with TTL

Positions written to a cell-based index with a short expiry, so "who is near me" is a small read and anything that stops reporting disappears on its own.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.