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From Jason Iannone <>
Subject Re: MergeContent resulting in corrupted JSON
Date Fri, 12 Jun 2020 01:15:52 GMT
We currently have it encapsulated in code that allows proper isolation and
testing, as this is the same methodology applied for standard development.
What I wasn't sure is whether Nifi is opinionated and actually preferred
and/or performed better with callbacks. There's a lot of older Nifi
examples out there, including Nifi core processors and its hard to discern
what's recommended.

What's TWR?

Appreciate the replies, you both have been immensely helpful!


On Thu, Jun 11, 2020 at 8:40 PM Andy LoPresto <> wrote:

> To give another perspective on the “callback vs. non”, I’d say “heavy” or
> “messy” operations (like encryption, for example) should be contained in
> encapsulated code (other classes which provide a service) and then invoked
> from the callback or TWR. This allows for much more testable business
> logic, separation of concerns (a service which implements the behavior and
> then a component effectively calling the API), and
> composability/flexibility. If I want to build a processor which exposes a
> property allowing the user to select different encryption algorithms, I can
> either detect which one and delegate that to an implementation, or I could
> have a giant switch statement and the raw crypto primitive code all in a
> giant spaghetti method/callback definition. I know I would prefer the
> former.
> Andy LoPresto
> * <>*
> He/Him
> PGP Fingerprint: 70EC B3E5 98A6 5A3F D3C4  BACE 3C6E F65B 2F7D EF69
> On Jun 11, 2020, at 8:14 AM, Mark Payne <> wrote:
> Jason,
> Modify vs. clone vs. create new:
> You would clone a FlowFile if you want an exact copy of the FlowFile (with
> the exception that the clone will have a unique UUID, Entry Date, etc.).
> Very rare that a Processor will actually do this.
> Modify vs. create a “Child” FlowFiles (i.e., `session.create(original);` )
> - This is a judgment call really. Do you think it will be necessary to have
> a copy of the original FlowFile and a modified version of it? If so, you
> may want to create a child FlowFile and send the original FlowFile to
> original. In reality, you shouldn’t need this often. In most cases, if the
> user wants both the original and the modified version, they can just add
> two connections, one going to this processor and one going to wherever else
> they want the FlowFile. This will cause NiFi to implicitly clone the
> FlowFile. Where the “create a child and send out the original” matters is
> just when there’s a feasible use case in which the user would want to have
> a modified version of the FlowFile and the original version of the FlowFile
> and also not want to process the original version until after the modified
> version has been created. This is not common. However, over the years, it
> has become a common practice to create “original” relationships when they
> are not needed, likely because a few developers saw a pattern of creating
> an original relationship and duplicated this to many other processors
> without really understanding the difference.
> “Net New” - there are two ways to create a FlowFile: `session.create()`
> and `session.create(original);` - the first creates a FlowFile with no
> parent FlowFile. This should be done only if there is no inbound FlowFile
> to create it from. I.e., when this is a “source” processor. In 100% of all
> other cases, it should be done as `session.create(original);` Providing the
> original FlowFile does 2 important things. Firstly, it creates a linkage in
> provenance between them. Secondly, it causes the newly created FlowFile to
> inherit all attributes from the child.
> Call vs. non-callback: It doesn’t matter. The callback was originally the
> only way to read or write content of FlowFiles. It was done this way
> because it was a straight-forward way to ensure that the framework was able
> to properly manage InputStream, OutputStream, etc. But there were use cases
> that didn’t fit the callback mechanism well so we eventually added ability
> to get the InputStreams and OutputStreams directly and callers can just use
> try-with-resources. This is probably preferred now for most cases just
> because it results in cleaner code.
> Thanks
> -Mark
> On Jun 11, 2020, at 10:43 AM, Jason Iannone <> wrote:
> I confirmed what you mentioned as well.
> I also looked over many custom processor examples and looking for
> clarification on a few things which I didn't see explicitly called out in
> the developers guide.
>    - Are their guidelines on when one should modify the original flowfile
>    vs when you should clone vs when you should create net new?
>    - Should heavier lifting such as decryption, formatting, etc. be done
>    in a callback?
> Thanks,
> Jason
> On Wed, Jun 10, 2020 at 4:32 PM Mark Payne <> wrote:
>> I don’t think flushing should matter, if you’re writing directly to the
>> provided OutputStream. If you wrap it in a BufferedOutputStream or
>> something like that, then of course you’ll want to flush that. Assuming
>> that you are extending AbstractProcessor, it will call session.commit() for
>> you automatically when onTrigger() returns.
>> I did just notice that you said you’re merging 1,000+ FlowFiles. That
>> would make it kind of difficult to follow the provenance. Would recommend
>> for debugging purposes, at least, that you try making small batches, maybe
>> 25 FlowFiles or something like that. Would make it a lot easier to find the
>> culprit
>> On Jun 10, 2020, at 4:28 PM, Jason Iannone <> wrote:
>> Excellent advice, thank you! When writing via
>> ProcessSession.write(FlowFile, OutputStream) is it advised to flush and/or
>> session.commit()? I noticed we aren't doing either, but we are invoking
>> session.transfer.
>> Thanks,
>> Jason
>> On Wed, Jun 10, 2020 at 2:26 PM Mark Payne <> wrote:
>>> Jason,
>>> Control characters should not cause any problem with MergeContent.
>>> MergeContent just copies bytes from one stream to another. It’s also worth
>>> noting that attributes don’t really come into play here. MergeContent is
>>> combining the FlowFile content, so even if it has some weird attributes,
>>> those won’t cause a problem in the output content. NiFi stores attributes
>>> as a mapping of String to String key/value pairs (i.e., Map<String,
>>> String>). So the processor is assuming that if you want to convert a
>>> message header to an attribute, that header must be a string.
>>> Content in the repository is stored using “slabs” or “blocks.” One
>>> processor at a time has the opportunity to write to a file in the content
>>> repository. When the processor finishes writing and transfers the FlowFile
>>> to the next processor, NiFi keeps track of which file its content was
>>> written to, the byte offset where its content starts, and the length of the
>>> content. The next time that a processor needs to write to the content of a
>>> FlowFile, it may end up appending to that same file on disk, but the
>>> FlowFile that the content corresponds to will keep track of the byte offset
>>> into the file where its content begins and how many bytes in that file
>>> belong to that FlowFile.
>>> My recommendation to track this down would be to find a FlowFile that is
>>> corrupt, and then use the data provenance feature [1] to view its lineage.
>>> Look at the FlowFiles that were joined together by MergeContent and see if
>>> any of those is corrupt.
>>> Thanks
>>> -Mark
>>> [1]
>>> On Jun 10, 2020, at 2:07 PM, Jason Iannone <> wrote:
>>> Hey Mark,
>>> I was thinking over this more and despite no complaints from Jackson
>>> Objectmapper is it possible that hidden and/or control characters are
>>> present in the JSON values which would then cause MergeContent to behave
>>> this way? I looked over the code and nothing jumped out, but there is
>>> something we had to do because of how the publisher is setting kafka header
>>> attributes. Some attributes are bytes and not strings converted to bytes,
>>> and ConsumeKafka seems to assume that these can always be converted to a
>>> String. We had to change the encoding to be ISO8859 due to running into
>>> issues with the bytes getting corrupted.
>>> I'm also trying to better understand how the content is being stored in
>>> the content repository, and whether something is going wrong when writing
>>> it out.
>>> Thanks,
>>> Jason
>>> On Tue, Jun 9, 2020 at 8:02 PM Mark Payne <> wrote:
>>>> Hey Jason,
>>>> Thanks for reaching out. That is definitely odd and not something that
>>>> I’ve seen or heard about before.
>>>> Are you certain that the data is not being corrupted upstream of the
>>>> processor? I ask because the code for the processor that handles writing
>>>> out the content is pretty straight forward and hasn’t been modified in
>>>> 3 years, so I would expect to see it happen often if it were a bug in the
>>>> MergeContent processor itself. Any chance that you can create a flow
>>>> template/sample data that recreates the issue? Anything particularly unique
>>>> about your flow?
>>>> Thanks
>>>> -Mark
>>>> > On Jun 9, 2020, at 6:47 PM, Jason Iannone <>
>>>> >
>>>> > Hi all,
>>>> >
>>>> > Within Nifi 1.10.0 we're seeing unexpected behavior with
>>>> mergecontent. The processor is being fed in many flowfiles with individual
>>>> JSON records. The records have various field types including a hex-encoded
>>>> byte[]. We are not trying to merge JSON records themselves but rather
>>>> consolidate many flowfiles into fewer flowfiles.
>>>> >
>>>> > What we're seeing is that a random flowfile is split causing the
>>>> merge file to be invalid JSON. When running multiple bins we saw the
>>>> flowfile split across bins.
>>>> >
>>>> > Example
>>>> > Flowfile 1: {"name": "1", "hexbytes": A10F15B11D14", timestamp:
>>>> "123456789" }
>>>> > Flowfile 2:  {"name": "2", "hexbytes": A10F15D14B11", timestamp:
>>>> "123456790" }
>>>> > Flowfile 3:  {"name": "3", "hexbytes": A10F15D14B11", timestamp:
>>>> "123456790" }
>>>> >
>>>> > Merged Result:
>>>> > {"name": "1", "hexbyters": A10F15B11D14", timestamp: "123456789" }
>>>> > xbytes": A10F15D14B11", timestamp: "123456790" }
>>>> > {"name": "3", "hexbytes": A10F15D14B11", timestamp: "123456790" }
>>>> > {"name": "3", "h
>>>> >
>>>> > Mergecontent Configuration:
>>>> > Concurrent Tasks: 4
>>>> > Merge Strategy: Bin-Packing Algorithm
>>>> > Merge Format: Binary Concatenation
>>>> > Attribute Strategy: Keep Only Common Attributes
>>>> > Min. number of entries 1000
>>>> > Max number of entries: 20000
>>>> > Minimum group size: 10 KB
>>>> > Maximum number of bins: 5
>>>> > Header, Footer, and Demaractor are not set.
>>>> >
>>>> > We then backed off the below to reduce min and max entries, bin to 1,
>>>> and thread to 1 and still see the same issue.
>>>> >
>>>> > Any insights?
>>>> >
>>>> > Thanks,
>>>> > Jason

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