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# Cold starts

Last updated 29 October 2026

Read this when your program starts on a new machine: when an open rebuilds the local copy from Tonbo, how long that takes, and how to avoid it.

A process rebuilds the database when it opens it on a new machine, or after its cache root, the directory that holds its local copy (the `dir` option of `open()`), was deleted: it downloads the latest snapshot and replays the commits since it. A process that restarts with the same cache root resumes from it instead, after a crash or a reboot too.

## How long it takes

In a test cluster of the same shape in the same region, a client inside the cluster rebuilt a 100 MB database in 0.3 to 1.1 seconds. From your host, add the download: the latest snapshot, up to 256 MiB compressed, plus the log written since it. A database near the size limit can take tens of seconds to rebuild over a slow link.

## Avoiding them

Keep the cache root on a disk that outlives the process, such as a volume of the same pod, so a restart resumes instead of rebuilding.

## Short-lived processes

A database works in any Node.js process with a writable local directory for its cache, `/tmp` included, as long as only one process opens it at a time. A process that starts without the last one’s directory, such as a fresh sandbox, a new container or a serverless function, rebuilds the local copy from Tonbo when it opens the database, a [cold start](https://tonbo.dev/docs/databases/cold-starts.md), and its open counts one write. Killed with its disk, a process can lose every commit since its last resolved `synced()`, so await `synced()` after each step that must survive, not only before it exits. Processes that open the same database at once, as concurrent invocations of a function do, take it over from each other, and the commits of all but the last are refused.

Such a process pays the rebuild above at every start, and one write for each open (see [Pricing](https://tonbo.dev/docs/databases/pricing.md)); before it exits, or is stopped, it has to have awaited `synced()` for everything it must keep (see [close()](https://tonbo.dev/docs/reference/nodejs/database.md#close)).