nifi-signal-messenger is a NiFi extension that makes it possible to send and receive Signal messages in NiFi. This extension uses the daemon mode of signal-cli is therefore highly dependent on signal-cli. You can use either linked or registered numbers with this extension.
This NiFi extension needs an existing signal-cli daemon running with the flag --http to start and work properly. So the first thing you'll need to do (if you don't already have a signal-cli daemon running) is to download and install signal-cli v0.11.5 which is really well describes in this how-to. Then either register a number (as described in the how-to) or make a link to an existing number.
/path/to/signal-cli/bin/signal-cli --config /path/to/configuration/Signal daemon --receive-mode manual --send-read-receipts --http 0.0.0.0:8090
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mvn package
and grab the following two nars and drop them in either ''lib'' or ''extensions'' directory in your NiFi installation.
./nifi-signal-messenger-nar/target/nifi-signal-messenger-nar-<version>.nar
./nifi-signal-messenger-api-nar/target/nifi-signal-messenger-api-nar-<version>.nar