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From "Hoss Man (JIRA)" <j...@apache.org>
Subject [jira] Commented: (SOLR-1729) Date Facet now override time parameter
Date Wed, 27 Jan 2010 01:48:34 GMT

    [ https://issues.apache.org/jira/browse/SOLR-1729?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12805313#action_12805313
] 

Hoss Man commented on SOLR-1729:
--------------------------------

bq. (e.g. they are in a different time-zone, not time-synced etc.).

time-zones should be irrelevant since all calculations are done in UTC ... lack of time-sync
is a legitimate concern, but the more serious problem is distributed requests and network
lag.  Even if all of the boxes have synchronized clocks, they might not all get queried at
the exact same time, and multiple requets might be made to a single server for different phrases
of the distributed request that expect to get the same answers.

It should be noted that while adding support to date faceting for this type of "when is now?"
is certainly _necessary_ to make distributed date faceting work sanely, it is not _sufficient_
... unless filter queries that use date math also respect it the counts returned from date
faceting will still potentially be non-sensical.

> Date Facet now override time parameter
> --------------------------------------
>
>                 Key: SOLR-1729
>                 URL: https://issues.apache.org/jira/browse/SOLR-1729
>             Project: Solr
>          Issue Type: Improvement
>          Components: search
>    Affects Versions: 1.4
>         Environment: Solr 1.4
>            Reporter: Peter Sturge
>            Priority: Minor
>         Attachments: FacetParams.java, SimpleFacets.java
>
>
> This PATCH introduces a new query parameter that tells a (typically, but not necessarily)
remote server what time to use as 'NOW' when calculating date facets for a query (and, for
the moment, date facets *only*) - overriding the default behaviour of using the local server's
current time.
> This gets 'round a problem whereby an explicit time range is specified in a query (e.g.
timestamp:[then0 TO then1]), and date facets are required for the given time range (in fact,
any explicit time range). 
> Because DateMathParser performs all its calculations from 'NOW', remote callers have
to work out how long ago 'then0' and 'then1' are from 'now', and use the relative-to-now values
in the facet.date.xxx parameters. If a remote server has a different opinion of NOW compared
to the caller, the results will be skewed (e.g. they are in a different time-zone, not time-synced
etc.).
> This becomes particularly salient when performing distributed date faceting (see SOLR-1709),
where multiple shards may all be running with different times, and the faceting needs to be
aligned.
> The new parameter is called 'facet.date.now', and takes as a parameter a (stringified)
long that is the number of milliseconds from the epoch (1 Jan 1970 00:00) - i.e. the returned
value from a System.currentTimeMillis() call. This was chosen over a formatted date to delineate
it from a 'searchable' time and to avoid superfluous date parsing. This makes the value generally
a programatically-set value, but as that is where the use-case is for this type of parameter,
this should be ok.
> NOTE: This parameter affects date facet timing only. If there are other areas of a query
that rely on 'NOW', these will not interpret this value. This is a broader issue about setting
a 'query-global' NOW that all parts of query analysis can share.
> Source files affected:
> FacetParams.java   (holds the new constant FACET_DATE_NOW)
> SimpleFacets.java  getFacetDateCounts() NOW parameter modified
> This PATCH is mildly related to SOLR-1709 (Distributed Date Faceting), but as it's a
general change for date faceting, it was deemed deserving of its own patch. I will be updating
SOLR-1709 in due course to include the use of this new parameter, after some rfc acceptance.
> A possible enhancement to this is to detect facet.date fields, look for and match these
fields in queries (if they exist), and potentially determine automatically the required time
skew, if any. There are a whole host of reasons why this could be problematic to implement,
so an explicit facet.date.now parameter is the safest route.

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