duhengforever commented on issue #292: RocketMQ Feature Request: Transaction-Aware Message URL: https://github.com/apache/rocketmq/issues/292#issuecomment-394414833 Thanks for @lizhanhui's feature request，and as mentioned above, with the rise of micro service architecture，developers are eager to use a mechanism to ensure the eventual consistency of transaction, especially in distributed systems, and we also found transaction aware message can achieve this goal gracefully, so we intend to implement this feature in rocketMQ, and this process is as follows：  In our design, we will define a pair of message: named half message and commit/rollback message. As mentioned by @lizhanhui, half message means prepared message, which are not visible to consumers，and it will be sent before local transaction is executed. After the local transaction ends，producer will send a commit or rollback message to broker based on local transaction execution result. when producers are blocked or crashed after send a half message，the broker will check all of producers with the same role to obtain the transaction status, so producer must implement the check listener interface to response transaction check query request. In our implementation, we will make full use of the mechanism of rocketMQ itself to avoid external dependencies, so we will build two new system topic to store half message（RMQ_SYS_TRANS_HALF_TOPIC）and commit/rollback result message(RMQ_SYS_TRANS_OP_HALF_TOPIC)，and we will also add a checkback service in broker to periodically check every message in the half message queue, if its corresponding result message can be found in the result message queue, it will be consumed or discarded depends on its result status，and if this message cannot be found in the result queue，that indicates that it was in pending status， so the checkback service will send check request to all corresponding producers to obtain transaction status, at the same time，we will also support immunity time config for producer to avoid being checked prematurely, the pending status but still in immunity time message will be reload to the half message queue to wait for the next round of check.
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