When c-tree receives the "CreatePersistenceTopic" operation, it does the following:
If the Persistence Topic already exists, the operation fails. c-tree logs an error to its CTSTATUS.FCS file.
c-tree ignores the operation and continues to collect data using the previously set up Persistence Topic.
c-tree creates a table to store data from future messages that will be published to the Persistence Topic.
If the specified table does not exist in the target database, c-tree creates it.
If the table exists, c-tree validates the fields in the table against the fields specified in the "CreatePersistenceTopic" operation.
If the operation specifies additional fields or different field data types, the operation fails and c-tree logs an error to its CTSTATUS.FCS file.
You can set properties in the operation to define table characteristics, such as:
Defining table fields
Setting a data retention policy
Adding an Automatic Timestamp Field with optional index
Making the table ready for Asynchronous Data Replication or optimizing the table for high-speed inserts (see "tableReplicationReady" below)
c-tree creates a message handler to process future messages published to the Persistence Topic.
When the message handler receives an incoming MQTT message that matches the Persistence Topic, it does the following.
It creates a record in the table assigned to the Persistence Topic when at least one non-null value can be placed in one field.
It extracts the JSON document from the payload of the message.
It extracts the value of each property in the JSON document.
It uses "mapOfPropertiesToFields" to look up how a property is mapped to a field in the table.
If necessary it converts the property value to match the target field’s data type.
It stores the value into the field.
It inserts the record.
The message handler sets a field value to NULL:
When a field does not have matching JSON properties; i.e. when a mapped JSON property is missing.
When an error occurs while converting a property value to the target field’s data type.
c-tree also logs an error to CTSTATUS.FCS that a data conversion failed.
The message handler does not insert a record:
When a JSON document is invalid.
When a JSON document in the message’s payload has no properties.
When no JSON properties are mapped to fields in the target table.
When no JSON property values can be converted to the field types in the target table.
In each case, c-tree logs an error to CTSTATUS.FCS that a data conversion failed.
Using CreatePersistenceTopic without a DeletePersistenceTopic
If a table is created via the MQTT "CreatePersistenceTopic", but never gets a "DeletePersistenceTopic" message, FairCom Edge will recreate those tables whenever it cannot find them. This usually happens on startup.
In that state, if you stop FairCom Edge, delete everything in the /data/ directory, and restart FairCom Edge, it will create those tables automatically. To get a "clean start" for FairCom Edge when it is in this state, you must delete all the files in the server/web/mqtt/active/ directory. The files in there are named <tableName>.json.
So to get out of that state, you need to delete the /data/ folder along with those JSON files from that directory.
