[Backport] Tweaks around libp2p pubsub seen messages cache #3783
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By default, the libp2p seen messages cache uses the `FirstSeen` strategy which expires an entry once TTL elapses from the time it was added. This means that if a single message is being received frequently and consistently, pubsub will re-broadcast it every TTL, rather than never re-broadcasting it. In the context of the Keep client which additionally uses app-level retransmissions, that often leads to a strong message amplification in the broadcast channel which cause a significant increase in the network load. As the problem is quite common (see libp2p/go-libp2p-pubsub#502), the libp2p team added a new `LastSeen` strategy which behaves differently. This strategy expires an entry once TTL elapses from the last time the message was touched by a cache write (`Add`) or read (`Has`) operation. That gives the desired behavior of never re-broadcasting a message that was already seen within the last TTL period. This reduces the risk of unintended over-amplification. (cherry picked from commit 89a7b77)
Once a message is received and validated, pubsub re-broadcasts it to other peers and puts it into the seen messages cache. This way, subsequent arrivals of the same message are not re-broadcasted unnecessarily. This mechanism is important for the network to avoid excessive message flooding. The default value used by libp2p is 2 minutes. However, Keep client messaging sessions are quite time-consuming so, we use a longer TTL of 5 minutes to reduce flooding risk even further. Worth noting that this time cannot be too long as the cache may grow excessively and impact memory consumption. (cherry picked from commit a712112)
tomaszslabon
approved these changes
Feb 12, 2024
efbb583
into
releases/mainnet/v2.0.0-m7
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This pull request backports #3773 to the
releases/mainnet/v2.0.0-m7branch.