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Posted to dev@nifi.apache.org by Russell Bateman <ru...@windofkeltia.com> on 2017/06/08 22:45:51 UTC

Clarification on "periodic task" processor

I'm doing something I've never done before in a custom processor. I'm 
writing a processor that will awaken every /n/ minutes, query a 
database, perform work as indicated by the result of the query, then go 
back to sleep for /n/ minutes.

This processor will never expect a flowfile because it doesn't work on 
flowfile attributes or content. It may, I think, emit some flowfiles 
downstream.

Will onTrigger()be called without a flowfile? That point seems unclear 
in the Developer Guide. Or is there some other lifecycle, like 
@OnScheduledthat I should expect to use for this?

Thanks,

Russ

Re: Clarification on "periodic task" processor

Posted by Russell Bateman <ru...@windofkeltia.com>.

Thanks, Andy. I guess point 2, "the processor has no in-coming 
connections", would answer this, but I'll annotate too--it's good 
"documentation" to have the annotation.


On 06/08/2017 05:03 PM, Andy LoPresto wrote:
> Russell,
>
> My understanding is that onTrigger() will be called based on the 
> scheduling of the processor, regardless of if there is an incoming 
> flowfile. A couple of examples support this line of thought — “source” 
> processors like GetFile, GenerateFlowFile, etc. run according to their 
> schedule even though they will never receive a flowfile. Similarly, in 
> ExecuteScript, the first task is always to attempt to retrieve a 
> flowfile from the session and then yield execution if none is returned.
>
> We may be able to add more explicit information to the Developer 
> Guide, but I would suggest looking at Performing the Work [1] and When 
> Processors Are Triggered [2] as they cover this topic. It seems in 
> your case, your processor will either have no incoming connections, or 
> you’ll need to add the @TriggerWhenEmpty annotation to your processor.
>
> [1] 
> https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#performing-the-work
> [2] 
> https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#when-processors-are-triggered
>
>
>
> Andy LoPresto
> alopresto@apache.org <ma...@apache.org>
> /alopresto.apache@gmail.com <ma...@gmail.com>/
> PGP Fingerprint: 70EC B3E5 98A6 5A3F D3C4  BACE 3C6E F65B 2F7D EF69
>
>> On Jun 8, 2017, at 3:45 PM, Russell Bateman <russ@windofkeltia.com 
>> <ma...@windofkeltia.com>> wrote:
>>
>> I'm doing something I've never done before in a custom processor. I'm 
>> writing a processor that will awaken every /n/ minutes, query a 
>> database, perform work as indicated by the result of the query, then 
>> go back to sleep for /n/ minutes.
>>
>> This processor will never expect a flowfile because it doesn't work 
>> on flowfile attributes or content. It may, I think, emit some 
>> flowfiles downstream.
>>
>> Will onTrigger()be called without a flowfile? That point seems 
>> unclear in the Developer Guide. Or is there some other lifecycle, 
>> like @OnScheduledthat I should expect to use for this?
>>
>> Thanks,
>>
>> Russ
>

Re: Clarification on "periodic task" processor

Posted by Andy LoPresto <al...@apache.org>.

Russell,

My understanding is that onTrigger() will be called based on the scheduling of the processor, regardless of if there is an incoming flowfile. A couple of examples support this line of thought — “source” processors like GetFile, GenerateFlowFile, etc. run according to their schedule even though they will never receive a flowfile. Similarly, in ExecuteScript, the first task is always to attempt to retrieve a flowfile from the session and then yield execution if none is returned.

We may be able to add more explicit information to the Developer Guide, but I would suggest looking at Performing the Work [1] and When Processors Are Triggered [2] as they cover this topic. It seems in your case, your processor will either have no incoming connections, or you’ll need to add the @TriggerWhenEmpty annotation to your processor.

[1] https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#performing-the-work <https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#performing-the-work>
[2] https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#when-processors-are-triggered <https://nifi.apache.org/docs/nifi-docs/html/developer-guide.html#when-processors-are-triggered>

Andy LoPresto
alopresto@apache.org
alopresto.apache@gmail.com
PGP Fingerprint: 70EC B3E5 98A6 5A3F D3C4  BACE 3C6E F65B 2F7D EF69

> On Jun 8, 2017, at 3:45 PM, Russell Bateman <ru...@windofkeltia.com> wrote:
> 
> I'm doing something I've never done before in a custom processor. I'm writing a processor that will awaken every /n/ minutes, query a database, perform work as indicated by the result of the query, then go back to sleep for /n/ minutes.
> 
> This processor will never expect a flowfile because it doesn't work on flowfile attributes or content. It may, I think, emit some flowfiles downstream.
> 
> Will onTrigger()be called without a flowfile? That point seems unclear in the Developer Guide. Or is there some other lifecycle, like @OnScheduledthat I should expect to use for this?
> 
> Thanks,
> 
> Russ