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Prerequisite: The cache meta of querycoord records which sealed segments and growing segments are loaded on each query node, and which dmchannels are watched by the query node. These metas are stored synchronously in etcd at the same time.
1.First of all, querycoord automatically generates handoff tasks and load balance tasks and writes them into etcd, and then clears them from etcd until the task is completed, ensuring that querycoord can accurately restore the task after restarting.
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