Andrzej Bialecki created SOLR-14683:
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Summary: Review the metrics API to ensure consistent placeholders
for missing values
Key: SOLR-14683
URL: https://issues.apache.org/jira/browse/SOLR-14683
Project: Solr
Issue Type: Improvement
Security Level: Public (Default Security Level. Issues are Public)
Components: metrics
Reporter: Andrzej Bialecki
Assignee: Andrzej Bialecki
Spin-off from SOLR-14657. Some gauges can legitimately be missing or in an
unknown state at some points in time, eg. during SolrCore startup or shutdown.
Currently the API returns placeholders with either impossible values for
numeric gauges (such as index size -1) or empty maps / strings for other
non-numeric gauges.
[~hossman] noticed that the values for these placeholders may be misleading,
depending on how the user treats them - if the client has no special logic to
treat them as "missing values" it may erroneously treat them as valid data.
E.g. numeric values of -1 or 0 may severely skew averages and produce
misleading peaks / valleys in metrics histories.
On the other hand returning a literal {{null}} value instead of the expected
number may also cause unexpected client issues - although in this case it's
clearer that there's actually no data available, so long-term this may be a
better strategy than returning impossible values, even if it means that the
client should learn to handle {{null}} values appropriately.
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