[kad] Provide a targeted store operation. #1988
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e.g. for write-back caching after a successful read. In that context, peers that were queried in a successful `Kademlia::get_record` operation but did not return a record are now returned in the `GetRecordOk::no_record` list of peer IDs. Closes libp2p#1577.
romanb
changed the title
protocols/kad/src/behaviour.rs
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| if source_key.distance(&key) < cache_key.distance(&key) { | ||
| } else { | ||
| no_record.push(source); | ||
| if quorum.get() == 1 { |
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I'm not sure if it would be worthwhile to be able to configure (i.e. enable/disable) the default behaviour of write-back caching upon lookups with a quorum of 1. Maybe some users would like full control over that behaviour by disabling it and doing any such writes themselves via Kademlia::put_record_to. Feedback appreciated.
I am fine with either solution. That said I do tend towards not treating quorum of 1 as a special case. Instead I suggest to always require the user to call put_record_to in order for write back caching to close nodes without the record to happen. Not having a special case for quorum of 1 seems more consistent to me, despite the fact that it is not perfectly inline with the paper.
I would deem an additional note on the GetRecordOk structure, informing the user of their responsibility, as well as an entry in the changelog enough. We could as well enforce this at the type level, requiring the collection of peers returned in GetRecordOk::no_record to be used, just like std::result::Result.
What do you think @romanb?
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I'm a bit reluctant to not do it by default for quorum 1 lookups, because I think libp2p-kad should offer "standard Kademlia" behaviour out of the box where you just put_record and get_record with the expected behaviour. It should of course be documented clearly that when using get_record with a quorum > 1 (or put_record with a quorum < ALL for that matter), you are leaving the territory of basic Kademlia and then you should need to care about put_record_to.
In the most recent commit I tried a different path: I made the caching more configurable, but retaining the default behaviour. Moreover, configuring the caching and enabling it gives control over how many "closest nodes to the key that did not return a record" are tracked and returned on a successful lookup. These are always returned in GetRecordOk::cache_candidates if caching is enabled, ordered by distance, but only for quorum 1 lookups do these also get automatically sent the record. I tried to document this clearly. What do you think? If you feel strongly that we should never do the caching automatically, let me know.
Co-authored-by: Max Inden <mail@max-inden.de>
Rather than returning the peers that are cache candidates in a `Vec` in an arbitrary order, use a `BTreeMap`. Furthermore, make the caching configurable, being enabled by default with `max_peers` of 1. By configuring it with `max_peers` > 1 it is now also possible to control at how many nodes the record is cached automatically after a lookup with quorum 1. When enabled, successful lookups will always return `cache_candidates`, which can be used explicitly with `Kademlia::put_record_to` after lookups with a quorum > 1.
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I'm a bit reluctant to not do it by default for quorum 1 lookups, because I think libp2p-kad should offer "standard Kademlia" behaviour out of the box where you just put_record and get_record with the expected behaviour.
Agree that this is something worth striving for.
In the most recent commit I tried a different path: I made the caching more configurable, but retaining the default behaviour. Moreover, configuring the caching and enabling it gives control over how many "closest nodes to the key that did not return a record" are tracked and returned on a successful lookup. These are always returned in GetRecordOk::cache_candidates if caching is enabled, ordered by distance, but only for quorum 1 lookups do these also get automatically sent the record. I tried to document this clearly. What do you think? If you feel strongly that we should never do the caching automatically, let me know.
I don't feel strongly about this and I am in favor of your recent changes, namely the introduction of KademliaCaching. In case people are in need for configuring the quorum == 1 caching case as well, I would hope for them to raise an issue in the future.
While always the case, thank you for the detailed documentation!
Co-authored-by: Max Inden <mail@max-inden.de>
This PR adds
Kademlia::put_record_tofor storing a record at specific nodes, e.g. for write-back caching after a successful read. In that context, peers that were queried in a successfulKademlia::get_recordoperation but did not return a record are now returned in theGetRecordOk::no_recordlist of peer IDs as additional information which may be used to choose which of these peers to send (one of) the found records to.The motivation for this PR originates in the discussion at #1577, i.e. the inability for libp2p-kad itself to do write-back caching of records after successful lookups when a quorum > 1 is used. It is deemed best and the most flexible to provide the means to do so on the API. There may be other use-cases for targeted store operations as well. That is to say,
Kademlia::put_record_tois intended as to augment the use ofKademlia::put_recordwith occasional targeted store operations, not as a replacement. Though if someone wishes to only useKademlia::put_record_to, that can obviously be done, the resulting DHT then just no longer has the properties attributed to Kademlia.I'm not sure if it would be worthwhile to be able to configure (i.e. enable/disable) the default behaviour of write-back caching upon lookups with a quorum of 1. Maybe some users would like full control over that behaviour by disabling it and doing any such writes themselves via
Kademlia::put_record_to. Feedback appreciated.