REFERENCE / QUIZ

Quiz and flashcards

45 questions. Answer out loud first, then reveal. Every answer links to the Fly.io page it comes from.

Quiz

1. What does a Fly Machine run on, and what do you give Fly.io to start one?

Reveal answer

Machines are Firecracker microVMs. You hand Fly.io an OCI container image and it becomes a Machine.

Source: fly.io/docs/getting-started/essentials/

2. Does Fly.io charge a monthly platform fee?

Reveal answer

No. "Fly.io bills for the resources you use, with no monthly platform fee."

Source: fly.io/docs/about/pricing/

3. Is there a free tier for new organizations?

Reveal answer

No. New organizations do not have a free tier or a monthly free usage allowance. New accounts get a small trial credit instead.

Source: fly.io/docs/about/pricing/

4. What size servers do Fly.io workers run on?

Reveal answer

Dedicated physical servers with 8-32 physical CPU cores and 32-256GB of RAM.

Source: fly.io/docs/reference/architecture/

5. Do GPU Machines use Firecracker?

Reveal answer

No. GPU Machines use Intel Cloud Hypervisor because it supports PCI passthrough.

Source: fly.io/blog/wrong-about-gpu/

6. What is Corrosion?

Reveal answer

A Rust program that propagates a SQLite database with a gossip protocol. Fly.io uses it to spread routing and state between servers instead of a central database.

Source: fly.io/blog/corrosion/

7. What does flyd do?

Reveal answer

It is the source of truth for all the VMs running on a particular worker, with a local boltdb and state machines.

Source: fly.io/blog/carving-the-scheduler-out-of-our-orchestrator/

8. What is flaps, and what is the Machines API base URL?

Reveal answer

flaps is a stateful proxy for all the flyd instances; the Machines API talks to it. The public base URL is https://api.machines.dev.

Source: fly.io/docs/machines/api/working-with-machines-api/

9. How fast can the Machines API be called?

Reveal answer

The limit is 1 request per second per action (for example Create Machine or Start Machine), with a burst limit up to 3 req/s per action.

Source: fly.io/docs/machines/api/working-with-machines-api/

10. How are Machines billed while started versus stopped?

Reveal answer

Per second while started. Stopped or suspended Machines are billed only for root file system (rootfs) usage, $0.15 per GB per 30 days.

Source: fly.io/docs/about/billing/

11. What does a shared-cpu-1x Machine with 256MB cost per month in the cheapest regions?

Reveal answer

$2.19 per 30 days in Ashburn (iad) and Secaucus (ewr). Other regions and presets: check the pricing page or calculator [verify].

Source: fly.io/docs/about/pricing/

12. What is the baseline CPU quota of a shared vCPU?

Reveal answer

5ms of every 80ms period (6.25%), with a burst balance that starts at 5s and tops out at 500s. Performance vCPUs get the full 80ms.

Source: fly.io/docs/machines/cpu-performance/

13. What does a reservation block give you?

Reveal answer

A 40% discount on compute, reserved for performance or shared Machines in one region. You pay the yearly amount upfront and get a monthly credit.

Source: fly.io/docs/about/pricing/

14. What do volumes cost?

Reveal answer

$0.15 per GB per month of provisioned capacity.

Source: fly.io/docs/about/pricing/

15. How many days of automatic volume snapshots are kept by default?

Reveal answer

5 days by default, configurable up to 60 days. Snapshots should not be your primary backup method.

Source: fly.io/docs/volumes/overview/

16. How many volumes should an app have?

Reveal answer

"Always provision at least two volumes per app." Volumes have no built-in replication, so your app or database must replicate data.

Source: fly.io/docs/volumes/overview/

17. Where does a volume live, and how many Machines can use it?

Reveal answer

A volume exists on one server and is not network storage. It attaches to one Machine at a time.

Source: fly.io/docs/volumes/overview/

18. What happens to a single-Machine app when its host fails?

Reveal answer

The Machine goes down and does not automatically start on another host. Single-Machine apps are disproportionately affected; a second Machine on another host would keep serving.

Source: fly.io/docs/apps/trouble-host-unavailable/

19. How many Machines does Fly Launch create by default?

Reveal answer

Two for process groups with services, but only one when a volume is mounted, because there is no built-in replication between volumes.

Source: fly.io/docs/apps/app-availability/

20. Which suspend limit matters for memory?

Reveal answer

Suspend requires 2 GB of memory or less, because larger sizes make suspend slower. Resume takes a few hundred milliseconds.

Source: fly.io/docs/reference/suspend-resume/

21. Does autostop create or destroy Machines?

Reveal answer

No. Fly Proxy autostop and autostart only stop and start Machines that already exist.

Source: fly.io/docs/launch/autostop-autostart/

22. What does the fly-replay response header do?

Reveal answer

It tells Fly Proxy to replay the request to another region, Machine or app.

Source: fly.io/docs/reference/fly-proxy/

23. What happens to a Machine above its hard_limit?

Reveal answer

Fly Proxy sends it no new traffic. At or above soft_limit it is used only if all other Machines are also above soft_limit.

Source: fly.io/docs/reference/load-balancing/

24. What is 6PN?

Reveal answer

The private network of IPv6 WireGuard tunnels shared by the apps in an organization. It is always available to apps by default.

Source: fly.io/docs/networking/private-networking/

25. What does an app need to bind to be reachable on 6PN?

Reveal answer

The fly-local-6pn address (found in the Machine /etc/hosts), not just 127.0.0.1.

Source: fly.io/docs/networking/private-networking/

26. Which IP addresses are free and which cost money?

Reveal answer

Shared IPv4 and IPv6 are free. A dedicated IPv4 address is $2 per month.

Source: fly.io/docs/about/pricing/

27. What does outbound data to the internet cost in North America and Europe?

Reveal answer

$0.02 per GB. Inbound data and traffic inside one region are free.

Source: fly.io/docs/about/pricing/

28. What does a static egress IP cost?

Reveal answer

$0.005 per hour, about $3.60 per month.

Source: fly.io/docs/about/pricing/

29. How much do the volume snapshots cost?

Reveal answer

$0.08 per GB per month, with the first 10GB free each month.

Source: fly.io/docs/about/pricing/

30. What is the cheapest Managed Postgres plan?

Reveal answer

Basic, Shared-2x with 1GB, at $38 per month. Storage is $0.28 per GB for a 30-day month.

Source: fly.io/docs/postgres/

31. What is a Sprite?

Reveal answer

"Sprites are persistent, hardware-isolated Linux VMs on Fly.io for AI agents, dev environments and untrusted code, billed by the second and free when idle."

Source: fly.io/docs/sprites/

32. How fast does a Sprite create, and how big is its disk?

Reveal answer

In "a second or two". Every Sprite has a 100GB durable root filesystem.

Source: fly.io/blog/design-and-implementation/

33. How many vCPUs and how much storage does a Sprite have?

Reveal answer

8 vCPUs and 100 GB of storage. Storage does not autoscale yet.

Source: fly.io/docs/sprites/concepts/lifecycle/

34. After how long idle does a Sprite pause, and what is warm versus cold?

Reveal answer

About 30 seconds today. Warm: memory frozen, billing stops, resume in 100-500ms with processes intact. Cold: stopped, memory dropped, wake in 1-2s with fresh processes.

Source: fly.io/docs/sprites/concepts/lifecycle/

35. What does a Sprite checkpoint capture?

Reveal answer

The writable filesystem overlay: files, packages, config and on-disk databases. Not running processes, in-memory state or open connections.

Source: fly.io/docs/sprites/concepts/checkpoints/

36. What does the Sprites CPU meter cost?

Reveal answer

$0.0385 per CPU-hour. Memory is $0.021875 per GB-hour; the page says RAM is almost always the line that dominates.

Source: fly.io/sprites

37. Is a Sprite billed while it sits warm or cold?

Reveal answer

No. A Sprite is billed while running; one that does nothing costs nothing beyond its storage.

Source: fly.io/sprites

38. What does Fly.io Standard support cost and promise?

Reveal answer

$29 per month with a 36-hour first response time.

Source: fly.io/support

39. What does Premium support cost and promise?

Reveal answer

$199 per month with a 24-hour first response time (1 hour for urgent issues), a dedicated Slack channel and quarterly Solutions Architecture sessions.

Source: fly.io/support

40. What does Enterprise support include?

Reveal answer

From $2,500 per month: 4-hour 24x7 first response, a dedicated Slack channel and a 99.9% uptime SLA.

Source: fly.io/support

41. Where does Fly.io publish write-ups of its incidents?

Reveal answer

The Infra Log, which says it aims for 100% fidelity to internal incidents, in addition to the status page.

Source: fly.io/infra-log

42. What went wrong in the 1 September 2024 global outage?

Reveal answer

A new Machine with a new virtual service option made every proxy lock up, and no end-user requests could reach customer apps for a period.

Source: fly.io/blog/corrosion/

43. How does Fly.io contrast with Kubernetes-style central state?

Reveal answer

Individual servers are the source of truth for their workloads, with gossip between them: no locking, no central servers, no distributed consensus.

Source: fly.io/blog/corrosion/

44. Why did Fly.io pick Firecracker over plain containers?

Reveal answer

It surveyed namespaces, gVisor and Kata and called Firecracker a VMM optimized for security, giving hardware-level isolation.

Source: fly.io/blog/sandboxing-and-workload-isolation/

45. When should you use a Sprite instead of a Machine?

Reveal answer

Sprites suit agents, dev boxes, untrusted code and prototypes. Machines remain the runtime for production apps with CI/CD and availability needs.

Source: fly.io/blog/design-and-implementation/

Flashcards

20 cards. Click or tap a card to flip it.