We have seen a strange problem on NiFi 1.4.0 where custom processors could 
suddenly not be started, because of incompatibility with custom services:
2018-03-02 13:40:35,490 ERROR [main] o.apache.nifi.controller.FlowController 
Unable to start …[id=5d57d39a-015c-1000-ffff-ffffd654d90b] due to 
java.lang.IllegalStateException: Processor … is not in a valid state due to …  
is invalid because … - 1.4-SNAPSHOT from …  is not compatible with … - 
It seems that the root cause was related to:
2018-03-02 13:39:55,086 WARN [main] org.apache.nifi.nar.NarClassLoaders While 
loading …:…:1.5-SNAPSHOT' unable to locate exact NAR dependency 
'…:…:1.4-20180302.111133-16'. Only found one possible match …:….:1.4-SNAPSHOT'. 
It turned out that our various custom NARs were  built on different built 
Maven has the (ugly) behaviour that when a snapshot version is present in the 
local repository after local build, the version number will be e.g. 
1.4-SNAPSHOT. However if it is downloaded from a remote repository, the version 
number includes a timestamp such as 1.4-20180302.111133-16.
So the manifest of the depending NARs contained:
Nar-Dependency-Version: 1.4-20180302.111133-16
while the other NAR file declared:
Nar-Version: 1.4-SNAPSHOT
Not sure if NiFi 1.5 would behave differently when loading such NAR files, but 
I would recommend to anyway fix the NiFi Maven Plugin:
Replace line 705:
narDependency = new NarDependency(artifact.getGroupId(), 
artifact.getArtifactId(), artifact.getVersion());
narDependency = new NarDependency(artifact.getGroupId(), 
artifact.getArtifactId(), artifact.getBaseVersion());
Explanation: artifact.getBaseVersion() will always consistenly return 
-SNAPSHOT, even in case the artifact was a timestamped snapshot downloaded from 
a remote repo.

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