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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
If you're looking for a really good alternative that's decentralised, check out Matrix. https://matrix.org
There's several Messenger Apps, the most developed (but with many very promising looking alternatives) would be Element. https://element.io
A short search offers me this build-instruction. https://github.com/aqnouch/Signal-Setup-Guide/tree/master/signal-android I have no experience but looks like you can do this by yourself.
I think thats the exactly the point. It depends on how you use it. Matrix is a protocol at first. Your own Community-Matrix-Server without federation is perfect, with federation it could leak some data, depending on what you are doing (but yes, you should be aware of). For the profile data there are options to limit the „leakage“ a bit: https://github.com/matrix-org/synapse/blob/develop/docs/sample_config.yaml
But for a phone numberless (open source) app there's https://getsession.org/. I haven't use this and don't know enough to vouch for it. But it purports to tick all the boxes.