darkmarmot
We have some code that creates unique hashes on documents to handle duplicate checks.
It wasn’t working on nodes that rebooted (letting dupes through) and as a last resort I changed from SHA to phash2 and it works perfectly now. This seems really strange to me. Is there some kind of timestamp seed or something in the crypto SHA I’m not aware of?
Here’s the code change that fixed our issues:
hash =
:erlang.phash2(doc, 1_000_000)
Here’s the original that is not working across restarts:
hash =
:crypto.hash(:sha, :erlang.term_to_binary(doc)) |> Base.encode64(case: :lower)
Maybe term_to_binary or encode64… but… it just surprised me a lot…
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cmkarlsson
Sorry for a few follow-up questions. I am not even sure I can help here.
doc? Is it a binary string or an erlang term? Something else?sha hashindividually to see which one is not working.My bet is that it is
term_to_binary. I am not sure it is meant to always produce the same result, only that it can be decoded withbinary_to_term.phash2on the other hand is designed to always produce consistent result when run on different version of erlang and machine architectures.benwilson512
The collision chance for phash2 is gonna be a lot higher than a sha though, so I’d definitely recommend finding a way to use a sha.