CAP needs to use storage media with persistence capabilities to store event messages in databases or other NoSql facilities. CAP uses this approach to deal with loss of messages in all environments or network anomalies. Reliability of messages is the cornerstone of distributed transactions, so messages cannot be lost under any circumstances.
Before message enters the message queue, CAP uses the local database table to persist the message, which ensures that the message is not lost when the message queue is abnormal or a network error occurs.
To ensure the reliability of this mechanism, CAP uses the same database transactions as the business code to ensure that business operations and CAP messages are consistent in the persistence process. That is to say, in the process of message persistence, the database will be rolled back when any one of the exceptions occurs.
After the message enters the message queue, CAP will start the persistence function of the message queue. We need to explain how CAP message is persisted in RabbitMQ and Kafka.
For message persistence in RabbitMQ, CAP uses a consumer queue with message persistence, but there may be exceptions here.
Ready for production?
By default, queues registered by CAP in RabbitMQ are persistent. When used in a production environment, we recommend that you start all consumers once to create the queues with persistence, which ensures that all queues are created before the message is sent.
Since Kafka is born with message persistence using files, Kafka will ensure that messages are properly persisted without loss after the message enters Kafka.
CAP supports the following types of transaction-enabled databases for storage:
After CAP is started, two tables are generated in used storage, by default the name is
Storage Data Structure¶
Table structure of Published :
Table structure of Received :
Table structure of Lock (Optional):
|Instance||Acquired instance of lock||string|
|LastLockTime||Last acquired lock time||DateTime|
When CAP sends a message, it will store original message object in a second package in the
The following is the Wapper Object data structure of Content field.
|Timestamp||Message created time||string|
|CallbackName||Consumer callback topic name||string|
Id field is generate using the mongo objectid algorithm.
Thanks to the community for supporting CAP, the following is the implementation of community-supported storage