At a glance
- Did you know
- Graph databases follow relationships directly, so multi-hop queries avoid large joins.
- Rule of thumb
- Traversals through super-nodes with millions of edges are the usual slow query.
- Watch out
- Deep traversals burn CPU; supernodes.
Its settings in ScaleMySystems
What you can change on it in the lab, and what each number does.
- Concurrency (slots)
- Calls it can work on at once (per instance when autoscaling). Needed slots ≈ calls per second × seconds each takes. Extra calls wait in the queue, then are rejected.
- Queue capacity
- Calls allowed to wait for a free slot. A bigger queue absorbs bursts but adds waiting time; 0 rejects straight away.
- vCPU
- CPU cores available. Needed ≈ calls per second × CPU per request ÷ 1000. Above about 90% busy it slows down and may crash.
- CPU per request (ms)
- CPU time each call burns. Part of what the component is; make the code cheaper to lower it.
- Latency (p50) (ms)
- Typical time one call takes here, before any calls it makes itself. Slots are held for this long.
- Latency (p99) (ms)
- The slowest 1% of calls. Callers must set their timeout above this or they cut off healthy calls.
- Timeout to dependencies (ms)
- How long it waits for each call it makes before giving up. Set it above the dependency’s p99 but low enough that a dead dependency does not hold every slot.
- Error rate
- Share of calls that fail on their own (bugs, declines, flaky tools). An agent that calls it on every step multiplies this.
- Quota (req/min)
- Calls allowed per minute (an API key or provider limit). Calls over it get 429s. Needed = calls per second × 60, with headroom.
Problems that start with it
- Friend recommendations on a graph database “People you may know” walks friends-of-friends on a graph database: 900 queries a second, 6 ms of CPU each, on 4 vCPUs.
- Facebook news feed: from launch build to real traffic The news feed as first built: no feed cache in front of the timeline store, and the feed API, ranking model, social graph and fan-out workers at an eighth of their real size. 5,000 requests a second arrive: 85% feed loads that are ranked on every load, 15% new posts fanned out to followers. Most feed loads fail.
Full lesson coming soon
Lessons are being added one component at a time: how it works, its settings, a live exercise to break and fix it, interview questions and a short check. See the Circuit breaker lesson for what one looks like.