Hi Marcel,

Question: is this normal that a negative amount of unprocessed messages can 
> be shown or is this a bug?
>

That's a (display) bug as there is really no possibility to have a negative 
number of unprocessed messages in the journal without breaking the laws of 
space and time. ;-)
  

> Is this the expected behaviour? I so far understood that all messages 
> shall be written directly to the journal and at 'message_journal_flush_age' 
> intervals written to Elasticsearch. How could we lose messages here?
>

The message_journal_flush_age setting specifies how long to wait (at most) 
until the Kafka log is flushed to disk. The message_journal_flush_interval 
setting specifies how many messages should be buffered (at most) until the 
Kafka log is flushed to disk. The equivalent settings in the Kafka 
documentation (https://kafka.apache.org/08/configuration.html) are 
log.flush.interval.messages and log.flush.interval.ms. Flushing the Kafka 
log too often imposes a certain performance impact so it is a tradeoff 
between performance and how many messages you'll lose in case of an outage 
of the processing node (not Elasticsearch).

The settings for indexing batches of messages into Elasticsearch can be 
modified in the Graylog configuration with the output_batch_size and 
output_flush_interval settings (
https://github.com/Graylog2/graylog2-server/blob/1.0.1/misc/graylog2.conf#L193-203
).

 

> In order to verify that messages were lost on that node, can you tell me 
> what's the syntax for searching messages that were received over a certain 
> node? I only found the possibility to search by input - eg. 
> "gl_source_input:<input-here>", however since the input is a global input 
> it will show messages that were received over either node A or B for this 
> input. I would like to query only messages received from this input over 
> node A for example. How to do this?
>

You can use the (internal) message attributes gl2_source_node and 
gl2_source_input for this. An example query might look like 
"gl2_source_node:3c1749a2-b7ae-4e23-b761-f0b666e50e36 
gl2_source_input:52f51313e4b0fa055889cc5a".

You can also show messages received via a specific input in the web 
interface at System -> Inputs and select "Messages from this input" in the 
"Action" menu next to a specific input.


Cheers,
Jochen

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