dcapwell commented on code in PR #103:
URL: https://github.com/apache/cassandra-accord/pull/103#discussion_r1778967860
##########
accord-core/src/main/java/accord/topology/TopologyManager.java:
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@@ -98,36 +97,33 @@ static class EpochState
this.addedRanges =
global.ranges.subtract(prevRanges).mergeTouching();
this.removedRanges =
prevRanges.mergeTouching().subtract(global.ranges);
- this.prevSyncComplete = addedRanges.union(MERGE_ADJACENT,
prevSyncComplete.subtract(removedRanges));
- this.curSyncComplete = this.syncComplete = addedRanges;
+ this.syncComplete = addedRanges;
Review Comment:
`syncComplete` is based off `accord.local.Node#onTopologyUpdateInternal`
and `accord.api.ConfigurationService.EpochReady#coordination`
```
ready.coordination.addCallback(() -> {
synchronized (AbstractConfigurationService.this)
{
localSyncComplete(epochs.getOrCreate(ready.epoch).topology,
startSync);
}
});
```
This logic was changed (we talked about this a long time ago) so that each
epoch is now sequential and we don't allow epoch=N to happen-before epoch=N-1
(currently N can be syncComplete before N-1, simple case is that N-1 is a node
joining and must stream, and N is a table comment update).
If you look at `accord.local.Node#onTopologyUpdateInternal` coordination is
now sequenced so `N` happens-after `N-1`.
In Apache Cassandra this is notifies `localSyncComplete` which marks the
epoch as `epochState.setSyncStatus(SyncStatus.NOTIFYING);` and notifies the
*nodes in the topology* about the epoch. Those nodes will do the same which
cause `accord.topology.TopologyManager#onEpochSyncComplete` to get called,
which updates `syncComplete` (and `prevSyncComplete` and `curSyncComplete`)...
Now we have to turn to 2 cases that exist in Apache Cassandra: Expand
Cluster, and Host Replacement. In both of these cases a *new node* starts to
join and will know about past epochs (as it's needed for processing), but won't
be present in those past epochs. This leads us to a situation where those past
epochs have `syncComplete = []` and we include every single epoch for
coordination.
By sequencing `EpochReady.coordination`, once this new node *sees* epoch=N
be sync complete by its peers, it *knows* epoch=N-1 is also syncComplete.
Without this change, we would require every node in the cluster to notify
the joining node about every epoch they know and their sync status... if the
sync is also not complete then the node would be stuck with a partial update as
it wouldn't learn about new ones!
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