Clarify use of date math in index names with custom date formats

[spinscale]

Elasticsearch version: 5.1

You can run this curl command to see that date math calculation with custom formats including more than year, month and day is crippled/bogus.

curl -g -v 'localhost:9200/<logstash-{now%2fd{YYYY.MM.dd.hh.mm.ss}}>'

{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index","resource.type":"index_or_alias","resource.id":"logstash-2016.12.22.12.00.00","index_uuid":"_na_","index":"logstash-2016.12.22.12.00.00"}],"type":"index_not_found_exception","reason":"no such index","resource.type":"index_or_alias","resource.id":"logstash-2016.12.22.12.00.00","index_uuid":"_na_","index":"logstash-2016.12.22.12.00.00"},"status":404}

The index name logstash-2016.12.22.12.00.00 does not seem to take minutes and seconds into account and the hour seems to be wrong as well.

The docs dont mention anything, that these formats are not supported - and I am not too sure they should (especially seconds), but this is unexpected behaviour so we should either fix it or document it.

[clintongormley]

I think we should just document it

[mayur-persistent]

@spinscale @clintongormley I want to take up this issue as a first contribution. Let me know what I need to do?

[clintongormley]

@mayur-persistent Thanks for showing interest in contributing to Elasticsearch.

Here's a guide to how to go about it: https://github.com/elastic/elasticsearch/blob/master/CONTRIBUTING.md

[mayur-persistent]

@clintongormley I have identified the file where I need to make those changes. https://github.com/elastic/elasticsearch/edit/5.1/docs/reference/api-conventions.asciidoc

Would you prefer that we show the message just below the table describing the valid regular expression as an important note?

[clintongormley]

@mayur-persistent sounds fine - open a PR and we can iterate there

[jpountz]

cc @elastic/es-search-aggs

[mayya-sharipova]

issue is still relevant

[matriv]

In my opinion this is the expected behavior.

• The hour turns to 12 because hh is used instead of HH. If it's changed to HH the hour becomes 00.
• Since now%2fd => now/d is used this means that the hours/minutes/seconds are all set to 0, so the logstash-2016.12.22.12.00.00 result as an index name is correct. (again if HH is used then it becomes: logstash-2016.12.22.00.00.00)

Imho, this behavior doesn't need any further explanation.