What's the difference between snapshot and archive values in PI?
What's the difference between snapshot and archive values in PI?
Understanding the difference between snapshot and archive values is fundamental to working with PI Data Archive.
Snapshot Value
The snapshot is the most recent value received by PI Data Archive for a given tag. Key characteristics:
- Always represents the latest reading from the data source
- Updated every time a new value arrives from the interface
- Stored in memory for fast access
- There is exactly one snapshot per tag at any time
Archive Values
Archive values are the historically stored readings after compression. Key characteristics:
- Stored on disk in archive files
- Subject to compression — not every value is kept
- Form the historical time series you query with tools like PI Vision or PI DataLink
Compression: Why Archive Has Fewer Points
PI uses swinging door compression to decide which values to store:
- A new value arrives and is compared against the last archived value.
- If the new value falls within the compression deviation (a configurable tolerance), it may be discarded.
- Only values that represent significant changes are written to the archive.
This means:
- The interface may send 1,000 values per hour
- But the archive might only store 50-100 of those
- Data fidelity is preserved within the configured tolerance
When to Use Each
| Use Case | Value Type |
|---|---|
| Current display / real-time dashboards | Snapshot |
| Historical trends and reports | Archive (recorded) |
| Calculations over time ranges | Archive (interpolated or summary) |
| Alarming on current state | Snapshot |
Querying
- Snapshot:
PIPoint.Snapshot()or PI Web API/streams/{webId}/value - Archive:
PIPoint.RecordedValues()or PI Web API/streams/{webId}/recorded
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