grpclb: support "pick_first" child policy #5438
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This is needed for GRPCLB pick_first support, which needs to attach tokens to headers, and the tokens are per server. In pick_first, all addresses are in a single Subchannel, thus the LoadBalancer needs to know which backend is used for a new stream. With this new transport attribute, the LoadBalancer can use one EAG per backend, put the token in the EAG attributes, and then access the token via the transport attribute available from ClientStreamTracer.StreamInfo.
ejona86
approved these changes
Mar 6, 2019
| PickResult picked(Metadata headers); | ||
| } | ||
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| @VisibleForTesting | ||
| static final class BackendEntry implements RoundRobinEntry { | ||
| final Subchannel subchannel; |
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Because it's always non-null in BackendEntry, and there are quite a few places that need to retrieve the subchannel from BackendEntry. entry.subchannel is easier to reason than entry.result.getSubchannel() that you won't get a null.
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The PICK_FIRST mode puts all backend addresses in a single Subchannel. There are a few points where it's different from the default ROUND_ROBIN mode:
TokenAttachingTracerFactory) than ROUND_ROBINGrpclbStateand starts a new one with the new mode. All connections will be closed during the transition. We don't expect this to happen in practice given the specific use case of PICK_FIRST.