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https://issues.apache.org/jira/browse/NIFI-5406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16541395#comment-16541395
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Koji Kawamura commented on NIFI-5406:
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[~joewitt] Thanks for your comments. I've updated the PR to let user to choose
listing strategy. It behaves as is by default setting.
> Add new listing strategy by tracking listed entities to ListXXXX processors
> ---------------------------------------------------------------------------
>
> Key: NIFI-5406
> URL: https://issues.apache.org/jira/browse/NIFI-5406
> Project: Apache NiFi
> Issue Type: Improvement
> Components: Extensions
> Reporter: Koji Kawamura
> Assignee: Koji Kawamura
> Priority: Major
>
> Current List processors (ListFile, ListFTP, ListSFTP ... etc) implementation
> relies on file last modified timestamp to pick new or updated files. This
> approach is efficient and lightweight in terms of state management, because
> it only tracks latest modified timestamp and last executed timestamp.
> However, timestamps do not work as expected in some file systems, causing
> List processors missing files periodically. See NIFI-3332 comments for
> details.
> In order to pick every entity that has not seen before or has been updated
> since it had seen last time, we need another set of processors using
> different approach, that is by tracking listed entities:
> * Add new abstract processor AbstractWatchEntries similar to
> AbstractListProcessor but uses different approach
> * Target entities have: name (path), size and last-modified-timestamp
> * Implementation Processors have following properties:
> ** 'Watch Time Window' to limit the maximum time period to hold the already
> listed entries. E.g. if set as '30min', the processor keeps entities listed
> in the last 30 mins.
> ** 'Minimum File Age' to defer listing entities potentially being written
> * Any entity added but not listed ever having last-modified-timestamp older
> than configured 'Watch Time Window' will not be listed. If user needs to pick
> these items, they have to make 'Watch Time Window' longer. It also increases
> the size of data the processor has to persist in the K/V store. Efficiency vs
> reliability trade-off.
> * The already-listed entities are persisted into one of supported K/V store
> through DistributedMapCacheClient service. User can chose what KVS to use
> from HBase, Redis, Couchbase and File (DistributedMapCacheServer with
> persistence file).
> * The reason to use KVS instead of ManagedState is, to avoid hammering
> Zookeeper too much with frequently updating Zk node with large amount of
> data. The number of already-listed entries can be huge depending on
> use-cases. Also, we can compress entities with DistributedMapCacheClient as
> it supports putting byte array, while ManagedState only supports Map<String,
> String>.
> * On each onTrigger:
> ** Processor performs listing. Listed entries meeting any of the following
> condition will be written to the 'success' output FlowFile:
> *** Not exists in the already-listed entities
> *** Having newer last-modified-timestamp
> *** Having different size
> ** Already listed entries those are old enough compared to 'Watch Time
> Window' are discarded from the already-listed entries.
> * Initial supporting target is Local file system, FTP and SFTP
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