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General

CAP need to use storage media with persistence capabilities to store event messages, such as through databases or other NoSql facilities. CAP uses this approach to deal with the loss of messages in all environments or network anomalies. The reliability of messages is the cornerstone of distributed transactions, so messages cannot be lost under any circumstances.

Persistent

Before sent

Before the message enters the message queue, the 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 sent

After the message enters the message queue, the CAP will start the persistence function of the message queue. We need to explain how the 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.

!!! info “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.

Storage

After the CAP started, two tables are generated into the persistent, by default the name is Cap.Published and Cap.Received.

Storage Data Structure

Table structure of Published :

NAME	DESCRIPTION	TYPE
Id	Message Id	int
Version	Message Version	string
Name	Topic Name	string
Content	Json Content	string
Added	Added Time	DateTime
ExpiresAt	Expire time	DateTime
Retries	Retry times	int
StatusName	Status Name	string

Table structure of Received :

NAME	DESCRIPTION	TYPE
Id	Message Id	int
Version	Message Version	string
Name	Topic Name	string
Group	Group Name	string
Content	Json Content	string
Added	Added Time	DateTime
ExpiresAt	Expire time	DateTime
Retries	Retry times	int
StatusName	Status Name	string

Wapper Object

When the CAP sends a message, it will store the original message object in a second package in the Content field.

The following is the Wapper Object data structure of Content field.

NAME	DESCRIPTION	TYPE
Id	Message Id	string
Timestamp	Message created time	string
Content	Message content	string
CallbackName	Consumer callback topic name	string

The Id field is generate using the mongo objectid algorithm.

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