Parking permit issuer
A small system that decides how many permits to issue against a fixed number of spaces, applies an allocation rule that people can live with, reclaims permits nobody uses, and handles the part that damages relationships. Parking is the smallest system in this series and produces more complaints than all the others combined. Seven posts on the same system, one diagram at a time, with a cost breakdown and an engineering reference at the end.
- 01
A parking permit issuer on AWS for a few dollars a month
The whole system on one page — ratio, allocation, reclaim — and why the published rule matters more than the rule itself.
- 02
How many permits for how many spaces
How occupancy is measured cheaply, how a ratio is derived, why the cost is asymmetric, and how the ratio is revised.
- 03
How allocation gets decided without a riot
Four allocation rules and what each rewards, why the published rule matters more than the choice, how a waiting list is run, and the exceptions that are not exceptions.
- 04
What happens to a permit nobody uses
How usage is measured proportionately, the warning and explanation process, what counts as a good reason, and why this beats any other capacity measure.
- 05
Why enforcement is the hard part
Why enforcement damages more than it recovers, how a graduated response works, what number plate recognition actually creates, and why a person is always in the loop.
- 06
What the parking permit issuer costs
About $2 a month. One Bedrock read per permit is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the parking permit issuer architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is a parking permit issuer?
- A small serverless system that manages permits against a fixed number of spaces, applies a stated allocation rule, tracks usage, and reclaims permits that are not being used.
- Why issue more permits than spaces?
- Because on any given day a substantial share of permit holders are not driving in. Issuing one permit per space wastes a good deal of the car park.
- How is allocation decided?
- By a rule that is published before anybody applies. The post on this compares four rules and argues that stating the rule matters more than which one you pick.
- Are accessible spaces part of this?
- No. Accessible bays are allocated on need and are outside the ratio and the ballot entirely.
- What does it cost to run?
- A couple of dollars a month. See part six.