> ## Documentation Index
> Fetch the complete documentation index at: https://docs.blobhub.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Execution Snapshot

> What an execution pins at creation, and how one without a snapshot reads its definition.

An execution with a snapshot runs and displays the definition as it was the moment it was created — or, for a
backfilled one, as it stood on the day of the backfill — regardless of any later edit, delete or re-branch of the
source. Every execution `create_execution` makes gets one; an older execution without one reads the live
definition. A new execution is never read from the live definition, not even as a fallback when its snapshot is
missing.

## The `snapshot` field

| Value | Meaning |
| :- | :- |
| `"created"` | taken at the moment this execution was created |
| `"backfilled"` | see below |
| *absent* | the execution predates snapshots; it reads the live definition, with that read's usual 403 if the live definition is later removed |

## The backfill

A small number of public example workflows on blobhub.io had their historical executions given a snapshot from
the definition as it stood on the day of the migration, marked `"backfilled"` rather than `"created"`.

## Where it's used

* [Download Execution Definition](/blob-types/workflow/operations/download-execution-definition) — the read
  path for a snapshot.
* [Create Execution](/blob-types/workflow/operations/create-execution#response) — when a snapshot is taken.