# When machine boots but reply disappears

> How Mainbrella saves machine starts without losing work.
- Title: When machine boots but reply disappears · Mainbrella
- Summary: How Mainbrella saves machine starts without losing work. Mainbrella's system saves receipts before booting machines so retries find previous attempts without…
- Keywords: mainbrella, linux, idempotency key, durable object, cloudflare, machine boot, receipt, retry, slot reservation, compute allowance, technology, startups
- Source: Mainbrella — https://mainbrella.com/blog/inside-our-brella
- Published: 2026-10-07T00:00:00+00:00
- Read time: 2 min
- Topics: mainbrella, linux, idempotency key, durable object, cloudflare, machine boot, receipt, retry, slot reservation, compute allowance, technology, startups
## The problem
When a machine boots but the reply never reaches you, do you retry and risk starting a second machine?
## We save a receipt
Mainbrella saves a receipt before booting so a retry can find the first attempt.
## The slot reservation
Each account gets a Cloudflare Durable Object to decide which requests get a slot and compute allowance.
## Reservation before boot
The coordinator saves the reservation before asking the runtime to boot so the next request sees the slot as taken.
## Idempotency key
A new key on every retry starts a new machine; keeping the key reuses the original attempt.
## Atomic write
The reservation and receipt land in one atomic write to prevent charging without a retry to find.
## Receipts last 24 hours
Receipts last twenty-four hours; if the machine stops or slot reuses, the key returns creation_no_longer_running.
## Running definition
The runtime boots the selected image with sleep infinity and gives uname -a sixty seconds to succeed.
## Old request meets new machine
An old request arrives with the right slot name but wrong lifetime after cancellation.
## Compute reservation
A free slot doesn’t mean unlimited runtime; we reserve compute allowance up front.
## Compute units
One hour reserved for a Medium machine costs ten units; five minutes used costs 0.833 units.
## Hard deadlines
The machine’s hard deadline is the earliest of its plan’s session limit, paid access expiration, next month boundary, or reserved compute.
## Managed execution
For longer work, create a managed execution with its own idempotency key and machine identity.
## File requests
The job writes /workspace/metrics.csv; we download it through a generation-bound file request.
## Save the disk
Saving a disk involves three parties: the account, runtime, and provider; none can atomically commit the other two.
## The consequence
Retries find previous attempts without starting new ones; the system prevents lost work.
## Key takeaway

Mainbrella's system saves receipts before booting machines so retries find previous attempts without starting new ones.