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Merge pull request #2737 from prometheus/beorn7/histogram

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native histogram spec: More clarifications about schemas
This commit is contained in:
Björn Rabenstein
2025-09-24 18:07:38 +02:00
committed by GitHub
parent 3d2339c49e b27dead365
commit aea54d3d9d
1 changed files with 22 additions and 4 deletions
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docs/specs/native_histograms.md
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@@ -248,10 +248,12 @@ observations are a very rare use case for Prometheus histograms and summaries.)
The _schema_ is a signed integer value with a size of 8 bits (short: int8). It
defines the way bucket boundaries are calculated. The currently valid values
are -53 and the range between and including -4 and +8. More schemas may be
added in the future. -53 is a schema for so-called _custom bucket boundaries_
or short _custom buckets_, while the other schema numbers represent the
different standard exponential schemas (short: _standard schemas_).
are -53 and the range between and including -4 and +8 (with a larger range
between and including -9 and +52 being reserved, see below for details). More
schemas may be added in the future. -53 is a schema for so-called _custom
bucket boundaries_ or short _custom buckets_, while the other schema numbers
represent the different standard exponential schemas (short: _standard
schemas_).
The standard schemas are mergeable with each other and are RECOMMENDED for
general use cases. Larger schema numbers correspond to higher resolutions.
@@ -313,6 +315,22 @@ Receivers MAY accept schemas between 9 and 52 if they reduce the schema upon
ingestion to a valid number (i.e. between -4 and 8), following the formulas for
bucket boundaries above.
If, after this optional schema conversion, the schema is still unknown to the
receiver, there are the following options:
- If a scrape (including federation) contains one or more histograms with an
unknown schema, the entiry scrape MUST fail, following the Prometheus
practice of avoiding incomplete scrapes.
- For any other ingestion paths (including replaying the WAL/WBL), the receiver
MAY ignore histograms with unknown schemas and SHOULD notify the user about
this omission in a suitable way.
When a TSDB implementation reads histograms from its permanent storage
(excluding replaying the WAL/WBL), similar guidelines apply: Schemas between 9
and 52 MAY be converted to valid schemas. Otherwise, unknown schemas MUST
return an error on retrieval, and the PromQL query that triggered the retrieval
MUST fail.
For schema -53, the bucket boundaries are set explicitly via _custom values_,
described in detail in the [custom values section](#custom-values) below. This
results in a native histogram with custom bucket boundaries (or short _custom