Stream Topics

Topics are created in streams and contain logical collections of messages. These collections of messages are published to partitions in the topic.

Using HPE Ezmeral Data Fabric Event Store with filesystem and HPE Ezmeral Data Fabric Database in the Data Fabric Converged Data Platform, enables organizations to create a centralized, secure data lake that unifies files, database tables, and message topics.

Note: Topics inherit security permissions, time-to-live data retention, and data compression policies at the stream-level.

You can design topic usage in a variety of ways. For example, you might have an application that monitors the logs for mission-critical software. Your monitoring application could send informational messages to a topic named info, warning messages to a topic named warnings, and error messages to a topic named errors. Different downstream applications might monitor each topic.

You can manage topics for different scenarios:

Set security policies that apply all of the topics in that stream. Security policies are set at the stream-level. See Stream Security for more information.
Set a default number of partitions for each new topic that is created in the stream. The default number partitions is set at the stream-level, however, individual topics can override the default. See Topic Partitions for more information.
Set a time-to-live for messages in every topic in the stream. Every message in every topic in a stream expires after a duration of time, unless you set the time-to-live to 0, meaning messages never expire. Time-to-live is set at the stream-level. See Time-to-Live for Messages for more information.

Restrictions

After a topic is created in a stream, it is not possible to move that topic to a different stream.
For example, suppose you create the topic structural_integrity_sensors_us_western_region, one of a number of topics that collect data from sensors that keep watch over various measurements for bridges, buildings, and other structures. However, you have mistakenly created the topic in the stream ventilation_systems instead of the stream structural_integrity_sensors.

Note: There is no command that will move a topic from its current stream to a different stream.
To rectify this mistake:
- You must delete the topic and recreate it in the other stream.
- Any producers that published messages to the topic and any consumers that read messages from the topic must be modified to point to the new location of the topic. This is because producers and consumers refer to topics with a combination of stream name and topic name.
Only the following characters are allowed for stream topic names:
- Alphanumeric characters
- Period, underscore, and dash
When producing or consuming stream topic messages, you must specify the stream's path and name along with the topic name in the following manner:
```
/<stream name>:<topic name>
```
Note: If a topic is specified but the stream's path and name is not, depending on the application's programming language, you might get an error or nothing. If nothing happens and you are using Java, the assumption is that you are publishing to Apache Kafka.