You can completely, at all, avoid storing PII for the purpose of estimating the number of unique visitors. The answer is to use HyperLogLog (see e.g. https://en

You can completely, at all, avoid storing PII for the purpose of estimating the number of unique visitors. The answer is to use HyperLogLog (see e.g. https://en.wikipedia.org/wiki/HyperLogLog https://en.wikipedia.org/wiki/HyperLogLog or http://antirez.com/news/75 http://antirez.com/news/75) on a strong fingerprint of the user's device. In this case, only some small amount of aggregate data across all seen fingerprints is stored. NB: this is completely irrelevant except as a mathematical curiosity, because GDPR prohibits any processing of PII (in this case - the fingerprint), not just storing it anywhere, without a legal basis.