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Refer to staleness in instant vector documentation

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The instant vector documentation does not explain which metric samples are selected - in particular, it makes no reference to staleness.

It's confusing when reading the docs to understand how exactly Prometheus selects the metrics to report: the most recent sample older than the search timestamp specified in the API request, so long as that metric is not "stale".

Signed-off-by: Craig Ringer <craig.ringer@enterprisedb.com>
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Craig Ringer 2023-10-18 14:11:51 +13:00
parent 5d8f0ef0c2
commit 15b68e989c
1 changed files with 11 additions and 2 deletions
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13
docs/querying/basics.md
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@ -109,8 +109,16 @@ single sample value for each at a given timestamp (point in time). In the simpl
form, only a metric name is specified, which results in an instant vector form, only a metric name is specified, which results in an instant vector
containing elements for all time series that have this metric name. containing elements for all time series that have this metric name.
The value returned will be that of the most recent sample at or before the
query's evaluation timestamp (in the case of an
[instant query](api.md#instant-queries))
or the current step within the query (in the case of a
[range query](api.md/#range-queries)).
The [`@` modifier](#modifier) allows overriding the timestamp relative to which
the selection takes place. Time series are only returned if their most recent sample is less than the [lookback period](#staleness) ago.
This example selects all time series that have the `http_requests_total` metric This example selects all time series that have the `http_requests_total` metric
name: name, returning the most recent sample for each:
http_requests_total http_requests_total
@ -359,7 +367,8 @@ cases like aggregation (`sum`, `avg`, and so on), where multiple aggregated
time series do not precisely align in time. Because of their independence, time series do not precisely align in time. Because of their independence,
Prometheus needs to assign a value at those timestamps for each relevant time Prometheus needs to assign a value at those timestamps for each relevant time
series. It does so by taking the newest sample that is less than the lookback period ago. series. It does so by taking the newest sample that is less than the lookback period ago.
The lookback period is 5 minutes by default. The lookback period is 5 minutes by default, but can be
[set with the `--query.lookback-delta` flag](../command-line/prometheus.md)
If a target scrape or rule evaluation no longer returns a sample for a time If a target scrape or rule evaluation no longer returns a sample for a time
series that was previously present, this time series will be marked as stale. series that was previously present, this time series will be marked as stale.