grpclb: keep track of state updates for cached Subchannels. #5441
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The problem: GrpclbState tracks Subchannels' states as a mutable attribute in Subchannel.getAttributes(). However, GrpclbState only update this attribute for the Subchannels its managing. For those cached in SubchannelPool, their state attributes are stale. When they are given back to GrpclbState, IDLE state is assumed. As a result, if a Subchannel is READY when it's reclaimed from the pool, it will not be picked. To fix that, this change expands SubchannelPool interface to handle Subchannel state updates, which GrpclbState will call. SubchannelPool saves the latest state and delivers it when it's returned to GrpclbState by scheduling a call to handleSubchannelState() in the SynchronizationContext, so that GrpclbState will take the latest state as if it was just reported from the Channel.
ejona86
approved these changes
Mar 7, 2019
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The problem: GrpclbState tracks Subchannels' states as a mutable
attribute in Subchannel.getAttributes(). However, GrpclbState only
update this attribute for the Subchannels its managing. For those
cached in SubchannelPool, their state attributes are stale. When they
are given back to GrpclbState, IDLE state is assumed. As a result, if
a Subchannel is READY when it's reclaimed from the pool, it will not
be picked.
To fix that, this change expands SubchannelPool interface to handle
Subchannel state updates, which GrpclbState will call. SubchannelPool
saves the latest state and delivers it when it's returned to
GrpclbState by scheduling a call to handleSubchannelState() in the
SynchronizationContext, so that GrpclbState will take the latest state
as if it was just reported from the Channel.
Resolves #5435