For output, you include the EXPORT TO TOPIC clause when you declare a stream or table. Once the schema object includes the EXPORT TO TOPIC clause and the topic is defined in the configuration file, any records written into the stream or table are made available to consumers through the topic port.

You can control what parts of the records are sent to the topic, using the WITH KEY/VALUE clauses. The WITH VALUE clause specifies which columns of the object are included in the body of the topic message and their order. The WITH KEY clause lets you specify one or more columns as a key for the message. Columns can appear in either the message body or the key, in both, or in neither, as needed. In all cases, the lists of columns are enclosed in parentheses and separated by commas.

So, for example, the following stream declaration associates the stream events with the topic eventLogs and selects two columns for the body of the topic message and one column as the key:

CREATE STREAM events 
  PARTITION ON COLUMN event_type 
  EXPORT TO TOPIC eventLogs
    WITH KEY (event_type) VALUE (when,what)
  ( event_type INTEGER NOT NULL,
    when TIMESTAMP NOT NULL,
    what  VARCHAR(256)
  );

Since VoltDB does not control what content producers send to the topic, it cannot dictate what columns or datatypes the stored procedure will receive. Instead, VoltDB interprets the content from its format. By default, text data is interpreted as comma-separated values. All other data is interpreted as a single value based on the data itself. On the other hand, if the topic is configured as using either JSON or AVRO formatted data in the configuration file, the incoming data from producers will be interpreted in the specified format.

Any errors during the decoding of the input fields is recorded in the log file. If the input can be decoded, the message fields are used, in order, as arguments to the store procedure call.

Only one key field is allowed for input. By default, the key is not passed to the specified stored procedure; only the message fields of the topic are passed as parameters to the stored procedure. If you want to include the key in the list of parameters to the stored procedure, you can set the property producer.parameters​.includeKey to true and the key will be included as the partitioning parameter for the procedure. For example:

<topics>
  <topic name="eventLogs" procedure="eventWatch">
     <property name="producer.parameters​.includeKey">true</property>
  </topic>
</topics>

When security is enabled for the database, the external clients must authenticate using a username and password when they initiate contact with the server. Access to the topic is handled separately for consumers and producers.

For producers, access to the topic is controlled by the security permissions of the associated stored procedure, as defined by the CREATE PROCEDURE... ALLOW clause or the generic permissions of the user account's role. (For example, a role with the ALLPROC or ADMIN permissions can write to any topic.)

For consumers, access to the topic is restricted by the allow attribute of the topic declaration in the configuration file. If allow is not specified, any authenticated user can read from the topic. If allow is included in the declaration, only users with the specified role(s) have access. You specify permissions by providing a comma-separated list of roles that can read from the topic. For example, the following declaration allows users with the kreader and operator roles to read from the topic eventLogs:

<topics>
  <topic name="eventLogs" allow="kreader,operator"/>
</topics>

Unlike export or import, where there is a single destination or source, topics can have multiple consumers and producers. So there is no specific event when the data transfer is complete and can be discarded. Instead, you must set a retention policy that defines when data is aged out of the topic queues. You specify the retention policy in terms of either the length of time the data is in queue or the volume of data in the queue.

For example, if you specify a retention policy of five days, after a record has been in the queue for five days, it will be deleted. If, instead, you set a retention policy of five gigabytes, as soon as the volume of data in the queue exceeds 5GB, data will deleted until the queue size is under the specified limit. In both cases, data aging is a first in, first out process.

You specify the retention policy in the retention attribute of the <topic> declaration. The retention value is a positive integer and a unit, such as "gb" for gigabytes or "dy" for days. The following is the list of valid retention units:

If you do not specify a retention value, the default policy is seven days (7 dy).

VoltDB topics are composed of three elements: a timestamp, a record with one or more fields, and an optional set of keys values. The timestamp is generated automatically when the record is inserted into the stream or table. The format of the record and the key depends on the data itself. Or you can specify a format for the record, for the key, or for both using properties of the topic declaration in the configuration file.

For single value records and keys, the data is sent in the native Kafka binary format for that datatype. For multi-value records or keys, VoltDB defaults to sending the content as comma-separated values (CSV) in a text string. Similarly, on input from producers, the topic record is interpreted as a single binary format value or a CSV string, depending on the datatype of the content.

You can control what format is used to send and receive the topic data using either the format attribute of the <topic> element, or separate <property> child elements to select the format of individual components. For example, to specify the format for the message and the keys for both input and output, you can use the attribute format="avro":

<topics>
  <topic name="eventLogs" format="avro"/>
</topics>

To specify individual formats for input versus output, or message versus keys, you can use <property> elements as children of the <topic> tag, where the property name is either consumer or producer followed by format and, optionally, the component type — all separated by periods. For example, the following declaration specifies Avro for both consumers and producers, and is equivalent to the preceding example using the format attribute:

<topics>
  <topic name="eventLogs">
    <property name="consumer.format">avro</property>
    <property name="producer.format">avro</property>
  </topic>
</topics>

The following are the valid formatting properties:

For input, note that you cannot specify the format of the key. This is because only a single key value is supported for producers and it is always assumed to be in native binary or string format.

Depending on what format you choose, you can also control specific aspects of how data is represented in that format. For example, you can specify special characters such as the separator, quote, and escape character in CSV format. Table 15.10, “Topic Formatting Properties” lists all of the supported formatting properties you can use when declaring topics in the configuration file.

When using AVRO format, you must also have access to an AVRO schema registry, which is where VoltDB stores the schema for AVRO-formatted topics. The URL for the registry is specified in the database configuration file, as described in the next section.