Forwarder's Operation : - After the forwarder sends a data block, it maintains a copy of the data in its wait queue until it receives an acknowledgment. - Meanwhile, it continues to send additional blocks. - If the forwarder doesn't get acknowledgment for a block within **300 seconds** (by default), it closes the connection. Possibility of Data Duplication : - It is possible for the indexer to index the same data block twice. - This can happen if there is a network problem that prevents an acknowledgment from reaching the forwarder. Network Issues : - When the network goes down, the forwarder never receives the acknowledgment. - When the network comes back up, the forwarder then resends the data block, which the indexer parses and writes as if it were new data. Duplicate Warning Example : 10-18-2010 17:32:36.941 WARN TcpOutputProc - Possible duplication of events with channel=source::/home/jkerai/splunk/current-install/etc/apps/sample_app /logs/maillog.1|host::MrT|sendmail|, streamId=5941229245963076846, offset=131072 subOffset=219 on host=10.1.42.2:9992 Note on useACK : - When `useACK` is enabled in the `outputs.conf` on forwarders, and there is either a network issue, indexer saturation (for example, pipeline blocks) or a replication problem, your Splunk platform deployment's indexers cannot respond to your deployment's forwarders acknowledgement. - Based on your deployment environment, data duplication can occur. https://docs.splunk.com/Documentation/Forwarder/8.2.5/Forwarder/Protectagainstthelossofin-flightdata
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