Contact tracing is a way to help prevent the spread of a disease, such as
COVID-19, by identifying an infected person’s contacts so that they
can be informed of the infection risk.
In the first part of this
series, we introduced open-source contact-tracing applications developed in
response to the current pandemic, and described how they work. In this
part, we look into the details of some of them, of both centralized and
decentralized design. These application projects have all released their
source code, but they differ in the implementation details, licenses used,
and whether they accept user requests or patches. We conclude with
the controversies around the tracing applications and the responses to
them.

Source: LWN.net – [$] Open-source contact tracing, part 2