Now that the `torrent-repository is emitting events, we can simplify the announce handler in the tracker-core package.

The bittorrent_tracker_core::announce_handler::AnnounceHandler has a method to handle the announce request:

    pub async fn announce(
        &self,
        info_hash: &InfoHash,
        peer: &mut peer::Peer,
        remote_client_ip: &IpAddr,
        peers_wanted: &PeersWanted,
    ) -> Result<AnnounceData, AnnounceError> {
        self.whitelist_authorization.authorize(info_hash).await?;

        let opt_persistent_torrent = if self.config.tracker_policy.persistent_torrent_completed_stat {
            self.db_torrent_repository.load(info_hash)?
        } else {
            None
        };

        peer.change_ip(&assign_ip_address_to_peer(remote_client_ip, self.config.net.external_ip));

        let number_of_downloads_increased = self
            .in_memory_torrent_repository
            .upsert_peer(info_hash, peer, opt_persistent_torrent)
            .await;

        if self.config.tracker_policy.persistent_torrent_completed_stat && number_of_downloads_increased {
            self.db_torrent_repository.increase_number_of_downloads(info_hash)?;
        }

        Ok(self.build_announce_data(info_hash, peer, peers_wanted).await)
    }

When persistent_torrent_completed_stat is enabled, it updates a DB counter with the number of downloads for that torrent.

I proposed a change here.

We can now:

• Listen for events from the torrent-repository package.
• Filter the event PeerDownloadCompleted.
• When we received that event, we call the db_torrent_repository.increase_number_of_downloads(info_hash) function.

The downloads will be incremented asynchronously.

Pros:

• Response should be faster; the peer does not have to wait until we increase the counter in the DB.
• The announce method will be simpler. It does not have to handle metrics.
• We can simplify the upsert_peer method. We don't need to return the boolean to know if downloads were increased.
• The EventBus acts as a buffer to write to the database (to increase counters). We could use batches to increase many counters with one SQL sentence.

Cons:

• It will be harder to follow what's happening by only reading the announce method. Anyway, we are intensively using events everywhere.
• It can be inefficient to listen to all torrent-repository events. Ideally we should be able to listen only to the PeerDownloadCompleted event. But I think that would not be possible if we don't use another EventBus.