The v0.27 have support to negentropy but have some issues. I fixed all the issues in master branch (you can specify a git repo and commit in the Cargo.toml or wait for v0.28)
To use negentropy you need a persistent DB. Configured that with the ClientBuilder, just call "client.reconcile". All the reconciled event will be saved in the db: use client.database().query(filters) to query what you need.