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https://issues.apache.org/jira/browse/SOLR-14683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17228512#comment-17228512
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Andrzej Bialecki edited comment on SOLR-14683 at 11/9/20, 10:57 AM:
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{quote}Solr's JSON Response writer already has long standing support to output
{{Float.NaN}} as a quoted string {{"NaN"}}
{quote}
Therein lies the problem ;) since there is no standard way to do it Solr
decided to use a STRING (quoted) value of {{"NaN"}}... but some other libraries
(and the popular extended spec [http://json5.org|http://json5.org%29/]) use
unquoted values for {{NaN, Infinity, -Infinity}}. Some other parsers use {{nan,
inf, -inf}} - that's the beauty of standards, there are so many of them to
choose from... /s
Taking all this into account serializing NaN as {{null}} seems like the safest
option, unless we add this configurability to our JSONWriter.
Also, since from the point of view of metrics it seems it conveys the same
message when it returns NaN or null when the value is unknown - so for
simplicity and easier compatibility we could always return {{null}} as a metric
value, regardless of how it's serialized.
was (Author: ab):
{quote}Solr's JSON Response writer already has long standing support to output
{{Float.NaN}} as a quoted string {{"NaN"}}
{quote}
Therein lies the problem ;) since there is no standard way to do it Solr
decided to use a STRING (quoted) value of {{"NaN"}}... but some other libraries
(and the popular extended spec [http://json5.org|http://json5.org%29/]) use
unquoted values for {{NaN, Infinity, -Infinity}}. Some other parsers use {{nan,
inf, -inf}} - that's the beauty of standards, there are so many of them to
choose from... /s
Taking all this into account returning {{null}} for NaN or undefined seems like
the safest option.
> 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
> Components: metrics
> Reporter: Andrzej Bialecki
> Assignee: Andrzej Bialecki
> Priority: Major
>
> 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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