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Update staleness documentation
Signed-off-by: Craig Ringer <craig.ringer@enterprisedb.com>
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@ -109,10 +109,14 @@ single sample value for each at a given timestamp (point in time). In the simpl
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form, only a metric name is specified, which results in an instant vector
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containing elements for all time series that have this metric name.
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The value returned will be the most recent sample within the the time frame
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specified by [the API call that submitted the query](api.md#instant-queries)
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or by the [`@` modifier](#modifier). Time series where the most recent sample
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is older than the [staleness threshold](#staleness) will not be returned.
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The value returned will be that of the most recent sample prior to the query's
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evaluation timestamp (in the case of an
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[instant query](api.md#instant-queries))
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or of the current resolution step within the query (in the case of a
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[range query](api.md/#range-queries)).
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The [`@` modifier](#modifier) allows overriding the timestamp relative to which
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the selection takes place. Time series where the most recent sample is older
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than the [staleness threshold](#staleness) will not be returned.
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This example selects all time series that have the `http_requests_total` metric
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name, returning the most recent sample for each:
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@ -364,7 +368,8 @@ cases like aggregation (`sum`, `avg`, and so on), where multiple aggregated
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time series do not precisely align in time. Because of their independence,
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Prometheus needs to assign a value at those timestamps for each relevant time
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series. It does so by taking the newest sample that is less than the lookback period ago.
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The lookback period is 5 minutes by default.
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The lookback period is 5 minutes by default, but can be
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[set with the `--query.lookback-delta` flag](../command-line/prometheus.md)
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If a target scrape or rule evaluation no longer returns a sample for a time
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series that was previously present, this time series will be marked as stale.
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