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From "Johannes Peter (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (NIFI-3248) GetSolr can miss recently updated documents
Date Sun, 03 Sep 2017 12:15:00 GMT

    [ https://issues.apache.org/jira/browse/NIFI-3248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16151784#comment-16151784
] 

Johannes Peter commented on NIFI-3248:
--------------------------------------

[~ijokarumawak], [~bbende]
I examined the current GetSolr implementation and I found several issues, which I want to
discuss:
(1) Currently, a date field needs to be included into the index schema and the Solr documents
for indexing. Although this can be realized easily via Solrs' TimestampUpdateProcessor, it
should be better, simply to use Solrs' \_version\_ field for filtering subsequent retrieval.
This field is included in every well-configured Solr index as it is required for several functionalities.
By doing so, this processor could also be used for indexes, which were not created considering
NiFi interactions. 
(2) Iterating through a resultset will only be done if the processor runs the first time.
This will be problematic if the amount of newly indexed documents in a trigger interval exceeds
the configured batch size.
(3) Successively increasing the start parameter to retrieve Solr documents in batches is accompanied
by two problems in this context. First, this way shows a poor performance for large collections.
Second, updating the index during the iteration will probably lead to duplicates or a loss
of documents in the case that positions of documents change due to newly indexed documents
or deletions. Instead of increasing the start parameter, cursor marks should be used, and
the sorting should be fixed to an ascending order of the time when documents were indexed
(\_version\_ field). More details on this can be retrieved here https://lucene.apache.org/solr/guide/6_6/pagination-of-results.html
(4) Using the fq-parameter instead of the q-parameter should improve the performance in some
cases, as Solr is able to use caches for fq. The q-parameter should be fixed to "*:*". 

As a consequence, I suggest to redesign the GetSolr processor in a way that it mainly focuses
on retrieving documents reliably. This can be done better by using cursor marks and the \_version\_
field. Additionally, users should not be enabled to change the parameters sort and q. The
full query capabilities of Solr could be made available by integrating an additional processor,
e. g. "FetchSolr".

> GetSolr can miss recently updated documents
> -------------------------------------------
>
>                 Key: NIFI-3248
>                 URL: https://issues.apache.org/jira/browse/NIFI-3248
>             Project: Apache NiFi
>          Issue Type: Bug
>          Components: Extensions
>    Affects Versions: 1.0.0, 0.5.0, 0.6.0, 0.5.1, 0.7.0, 0.6.1, 1.1.0, 0.7.1, 1.0.1
>            Reporter: Koji Kawamura
>            Assignee: Johannes Peter
>         Attachments: nifi-flow.png, query-result-with-curly-bracket.png, query-result-with-square-bracket.png
>
>
> GetSolr holds the last query timestamp so that it only fetches documents those have been
added or updated since the last query.
> However, GetSolr misses some of those updated documents, and once the documents date
field value becomes older than last query timestamp, the document won't be able to be queried
by GetSolr any more.
> This JIRA is for tracking the process of investigating this behavior, and discussion
on them.
> Here are things that can be a cause of this behavior:
> |#|Short description|Should we address it?|
> |1|Timestamp range filter, curly or square bracket?|No|
> |2|Timezone difference between update and query|Additional docs might be helpful|
> |3|Lag comes from NearRealTIme nature of Solr|Should be documented at least, add 'commit
lag-time'?|
> h2. 1. Timestamp range filter, curly or square bracket?
> At the first glance, using curly and square bracket in mix looked strange ([source code|https://github.com/apache/nifi/blob/support/nifi-0.5.x/nifi-nar-bundles/nifi-solr-bundle/nifi-solr-processors/src/main/java/org/apache/nifi/processors/solr/GetSolr.java#L202]).
But these difference has a meaning.
> The square bracket on the range query is inclusive and the curly bracket is exclusive.
If we use inclusive on both sides and a document has a time stamp exactly on the boundary
then it could be returned in two consecutive executions, and we only want it in one.
> This is intentional, and it should be as it is.
> h2. 2. Timezone difference between update and query
> Solr treats date fields as [UTC representation|https://cwiki.apache.org/confluence/display/solr/Working+with+Dates|].
If date field String value of an updated document represents time without timezone, and NiFi
is running on an environment using timezone other than UTC, GetSolr can't perform date range
query as users expect.
> Let's say NiFi is running with JST(UTC+9). A process added a document to Solr at 15:00
JST. But the date field doesn't have timezone. So, Solr indexed it as 15:00 UTC. Then GetSolr
performs range query at 15:10 JST, targeting any documents updated from 15:00 to 15:10 JST.
GetSolr formatted dates using UTC, i.e. 6:00 to 6:10 UTC. The updated document won't be matched
with the date range filter.
> To avoid this, updated documents must have proper timezone in date field string representation.
> If one uses NiFi expression language to set current timestamp to that date field, following
NiFi expression can be used:
> {code}
> ${now():format("yyyy-MM-dd'T'HH:mm:ss.SSSZ")}
> {code}
> It will produce a result like:
> {code}
> 2016-12-27T15:30:04.895+0900
> {code}
> Then it will be indexed in Solr with UTC and will be queried by GetSolr as expected.
> h2. 3. Lag comes from NearRealTIme nature of Solr
> Solr provides Near Real Time search capability, that means, the recently updated documents
can be queried in Near Real Time, but it's not real time. This latency can be controlled by
either on client side which requests the update operation by specifying "commitWithin" parameter,
or on the Solr server side, "autoCommit" and "autoSoftCommit" in [solrconfig.xml|https://cwiki.apache.org/confluence/display/solr/UpdateHandlers+in+SolrConfig#UpdateHandlersinSolrConfig-Commits].
> Since commit and updating index can be costly, it's recommended to set this interval
long enough up to the maximum tolerable latency.
> However, this can be problematic with GetSolr. For instance, as shown in the simple NiFi
flow below, GetSolr can miss updated documents:
> {code}
> t1: GetSolr queried
> t2: GenerateFlowFile set date = t2
> t3: PutSolrContentStream stored new doc
> t4: GetSolr queried again, from t1 to t4, but the new doc hasn't been indexed
> t5: Solr completed index
> t6: GetSolr queried again, from t4 to t6, the doc didn't match query
> {code}
> This behavior should be at least documented.
> Plus, it would be helpful to add a new configuration property to GetSolr, to specify
commit lag-time so that GetSolr aims older timestamp range to query documents.
> {code}
> // with commit lag-time
> t1: GetSolr queried
> t2: GenerateFlowFile set date = t2
> t3: PutSolrContentStream stored new doc
> t4: GetSolr queried again, from (t1 - lag) to (t4 - lag), but the new doc hasn't been
indexed
> t5: Solr completed index
> t6: GetSolr queried again, from (t4 - lag) to (t6 - lag), the doc can match query
> {code}



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