In Apache Pulsar topic documentation it says can we set a topic time retention policy to -1 for infinite time based retention, What are the downsides of having infinite retention and can we use pulsar as message store where data lives forever in topics and build event sourcing application around them?
The downside is that your data will grow forever. However, due to the segment based architecture of the underlying storage (bookkeeper), more space can by added by adding storage nodes (i.e. all the data doesn't have to fit on one machine, as is the case in some other systems).
The segment based architecture also makes it fairly straightforward to move data to a bulk storage system (s3 or something) while still having it available from Pulsar. However, this is still in earlier stages of discussion right now.