Add rack-aware load balancing policy #73
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The out was not helpful for understanding the failure of a test. Printing the query plans will help.
This argument was not used anywhere, it worked by chance since it had the same value everywhere.
The Datstax C++ driver has default ONE, the Scylla fork has LOCAL_ONE.
We need to prefer local rack as there are higher network costs when communicating with nodes in remote rack. This policy prefers nodes from the local rack, then local datacenter and then other nodes. The new RackAwarePolicy is similar to DCAwarePolicy, but does not have the deprecated options. TokenAwarePolicy and other code needed to be modified so that the local rack is propagated. The TokenAware policy was changed to prefer replicas in remote rack / remote DC before trying non-replica nodes. It does not make much sense to not try the replicas and trying the replicas simplifies the code as now we have three levels local/remote/remote2. This change might not be backwards-compatible, we don't know what exactly this project guarantees in terms of backwards compatibility. Co-Authored-By: Peter Navrátil <[email protected]>
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See also datastax#536 |
avelanarius
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Jan 4, 2023
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| if (local_dc_.empty()) { // Only override if no local DC was specified. | ||
| local_dc_ = local_dc; | ||
| } | ||
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| if (local_dc_.empty() && connected_host && !connected_host->dc().empty()) { | ||
| LOG_INFO("Using '%s' for the local data center " | ||
| "(if this is incorrect, please provide the correct data center)", | ||
| connected_host->dc().c_str()); | ||
| local_dc_ = connected_host->dc(); | ||
| } | ||
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| if (local_rack_.empty()) { // Only override if no local rack was specified. | ||
| local_rack_ = local_rack; | ||
| } | ||
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| if (local_rack_.empty() && connected_host && !connected_host->rack().empty()) { | ||
| LOG_INFO("Using '%s' for the local rack " | ||
| "(if this is incorrect, please provide the correct rack)", | ||
| connected_host->rack().c_str()); | ||
| local_rack_ = connected_host->rack(); | ||
| } |
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Minor issue: when only one of local_dc or local_rack is empty, the empty one is populated from connected_host (which corresponds to a control connection) while the non-empty one is populated from a user configuration, which could result in nonsensical DC+rack combination.
avelanarius
approved these changes
Jan 4, 2023
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We need to prefer local rack as there are higher network costs
when communicating with nodes in remote rack.
This policy prefers nodes from the local rack, then local datacenter
and then other nodes.
The new RackAwarePolicy is similar to DCAwarePolicy,
but does not have the deprecated options.
TokenAwarePolicy and other code needed to be modified
so that the local rack is propagated.
The TokenAware policy was changed to prefer replicas in remote
rack / remote DC before trying non-replica nodes.
It does not make much sense to not try the replicas and
trying the replicas simplifies the code as now we have three
levels local/remote/remote2.
This change might not be backwards-compatible,
we don't know what exactly this project guarantees in terms of
backwards compatibility.
Co-Authored-By: Peter Navrátil [email protected]