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better convergence when peers restart #1641
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I have run the test from #1554 (comment) against this. No connection drops at all, even after resuming all the gossiping. And all peers end up with the correct info about the restarted peer (new UID, etc). |
| // received by other peers. | ||
| pending.localPeerModified = peers.ourself.setVersionBeyond(newPeer.Version) | ||
| } | ||
| default: |
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Apart from my comment on the comment, LGTM |
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Previously when a peer restarted, information about the new incarnation (i.e. with a different UID) was not accepted by other peers (and connections would be dropped) unless all knowledge of the previous incarnation had been purged. This could result in a lot of connection churn and hence connectivity disruption, and, in some pathological cases, very slow convergence and hence acceptance of the new incarnation into the network. We now no longer drop connections when encountering different incarnations of a peer. There are two situations when that can happen: 1) on connection establishment we simply proceed 2) on receipt of gossip to ensure convergence we a) treat the UID as an additional discriminator when deciding whether we should update our information about a peer with that which was gossiped. Specifically, we update the information we hold when a) the gossiped version is greater, or b) is the same and the UID is greater. b) include the UID in the information we update c) move our own version number beyond any we receive for ourselves, if the received UID differs from ours. With (a) we establishes a total order of peer information across several incarnations of the same peer. i.e. we consider information to be fresher if it has a higher version, or the same version and higher UID. This may seem somewhat counter intutive, since it will generally treat information about new incarnations as older than old incarnations, since incarnations always start life with version 1. But to do better we'd need to establish a total order of incarnations that matches their temporal occurrence. Which requires some sort of durable state. So instead we have (c). Through that we learn the highest version number of any old incarnation of ourselves that other peers still hold, and then make sure that our version is greater than that. Essentially we continue where the old incarnations left off. It's as if instead of restarting we had simply changed UIDs. And due to (a) and (b) the information about the new incarnation of ourselves, now with a higher version, will supersede that of the old incarnations. Fixes #1554.
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bboreham
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Nov 9, 2015
…ange better convergence when peers restart; fixes #1554.
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Previously when a peer restarted, information about the new incarnation (i.e. with a different UID) was not accepted by other peers (and connections would be dropped) unless all knowledge of the
previous incarnation had been purged. This could result in a lot of connection churn and hence connectivity disruption, and, in some pathological cases, very slow convergence and hence acceptance of the new incarnation into the network.
We now no longer drop connections when encountering different incarnations of a peer. There are two situations when that can happen:
we simply proceed
to ensure convergence we
a) treat the UID as an additional discriminator when deciding whether we should update our information about a peer with that which was gossiped. Specifically, we update the information we hold when a) the gossiped version is greater, or b) is the same and the UID is greater.
b) include the UID in the information we update
c) move our own version number beyond any we receive for ourselves, if the received UID differs from ours.
With (a) we establishes a total order of peer information across several incarnations of the same peer. i.e. we consider information to be fresher if it has a higher version, or the same version and higher UID. This may seem somewhat counter intutive, since it will generally treat information about new incarnations as older than old incarnations, since incarnations always start life with version 1. But to do better we'd need to establish a total order of incarnations that matches their temporal occurrence. Which requires some sort of durable state.
So instead we have (c). Through that we learn the highest version number of any old incarnation of ourselves that other peers still hold, and then make sure that our version is greater than that. Essentially we continue where the old incarnations left off. It's as if instead of restarting we had simply changed UIDs. And due to (a) and (b) the information about the new incarnation of ourselves, now with a higher version, will supersede that of the old incarnations.
Fixes #1554.