Depending on the specifics of the inputs (source, format, correlating attributes, etc.) it sounds like you have a few good options to achieve this.
1. MergeContent  — This processor is very old (read: stable & widely-used, not legacy) and is used to merge content from multiple incoming flowfiles into batches according to a variety of strategies. There are many threads on these lists and articles/Stack Overflow/HCC posts about it, as it is very versatile.
2. Wait/Notify  — New in NiFi 1.2.0 (not yet released but available currently on the master branch), Joe Gresock and Koji Kawamura have contributed some very useful processors which allow you to wait for an arbitrary number of “signals” to invoke further processors. See the PRs  and excellent explanation .
Hope that helps. Please let us know if you have any other questions.
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I’ve just started taking a look at Nifi as it seems perfect for a project I’m working on right now.
The one thing I’m trying to solve is this – I have a process (say a batch job or similar) that I want to invoke,
only once all the inputs have arrived ( a simple dependency graph ).
i.e. I have 3 files and I don’t want to run a process after each file arrival, but once all 3 files have arrived.
Is this easily achievable in Nifi ? Or would I have to think about custom processor?
Thanks in advance,
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