问题描述

Event Hub消费端消费事件延迟，并且有重新分配分区等行为，讨论这种现象的可能原因。

问题解答

Event Hub 如果有大量的 Rebalancing 请求，就会导致事件消费延迟。 而Rebalance的原因也与服务端和消费端的心跳探测有关，如果服务端失去了消费端的心跳请求，触发 Rebalance操作。 

整个过程大体的情况如下：

1）新消费者加入，导致Rebalance

2）大多数消费者加入，但是有些消费者没有及时加入或心跳探测，因此这些消费者被移除。

3）在等待加入完成时，被移除的消费者要么作为新成员重新加入，要么迟到。

4）因此，进一步导致再平衡重置。

上述行为可能是由于连接问题或消费者不稳定性引起的。 

Trace the initial cause of the rebalancing of the group and looks like it is related to consumer losing heartbeat. The group took a long time to become stable because there was a continuous pattern of missing heartbeats during the rebalancing. Kind of like the following sequence.

· New consumer join, causing rebalancing.

· Most of the consumers join, but some did not join or heartbeat in time, and so those consumers are removed.

· While waiting for join to complete, consumers that was removed either join as new member or return late.

· So it further caused rebalancing to reset.

The above behavior can happen due to connectivity, or consumer instability