Capture Fields for Generic Data Store Sample

This sample demonstrates how to use a capture field in the schema of a Query Table to make that table reusable in different copies of its containing module. Each instance of the table holds key-value data with different value data types for different instances.

This sample also includes a Java operator (defined in CaptureFieldsAwareOperator.java) that demonstrates the use of Operator API methods that affect how operators handle data from streams that include capture fields.

The schema for a Query Table that holds key-value pairs would normally have two fields, a key field and a data field of a particular data type. The schema for such a table might be, for example, {key long, data string}. Before capture fields, to store a key-value pair of with a different data type, you would need a separate Query Table with a different schema, such as {key long, data double}.

The Query Table in this sample instead uses a capture field for the data field. Capture fields are only active in the context of a module, so the Query Table, GenericDataStore, is placed in the module GenericDataStore.sbapp. Notice that its schema is {id long, data capture}. This capture field's field name is data, while its defined data type is dataFields. This schema means: expect the first field to be named id and to have data type long; then expect any number of fields with different types thereafter.

The inner module is completed with a Query operator to insert values, another to report the count of rows accumulated in the table so far, and one to read all rows so you can confirm its contents. It also contains two instances of the CaptureFieldsAwareOperator Java operator. See the Java comments in the source file for details on that operator.

The inner module, GenericDataStore.sbapp is referenced twice in the outer module, TopLevel.sbapp. In the first reference, an input stream, Points, with schema {id long, x int, y int} feeds into the inner module's DataIn stream. The first field, id, matches the Query Table's requirement for a first field of type long named id. The x and y fields are captured by the Query Table's capture field, which adapts the first instance of the GenericDataStore Query Table to have the same schema as the Points input stream.

In the second reference, the input stream, Names, with schema {id long, name string} feeds into another instance of the inner module's DataIn stream. In this case, the Query Table in the second instance of the inner module adapts its schema to match the Names input stream.

Reusing the inner module does not mean there is a single Query Table that changes its schema. It means there are two instances of the Query Table, one per module reference, with different schemas that match the two input streams. Notice that the same abstract Query Table schema definition is used without change in both module references, yet each instance of the Query Table ends up with different concrete schemas at runtime.

A simple feed simulation is provided that feeds generated values to all four input streams as follows:

The result of running the feed simulation is that the two instances of the inner module's Query Table are populated with generated values, each of the appropriate data type. Meanwhile, once per second, Studio's Output Streams view reports the number of rows accumulated so far in each of the two tables. You can send an empty tuple at any time to the ReadTable stream for each module reference, and see the contents of the table at that time on the respective TableContents output stream.

While the application is running, observe the console output from the Java operator (emitting on its logger at INFO level) to observe runtime output demonstrating the Operator API features for capture fields.

In StreamBase Studio, import this sample with the following steps:

StreamBase Studio creates a single project containing the sample files.

When you load the sample into StreamBase Studio, Studio copies the sample project's files to your Studio workspace, which is normally part of your home directory, with full access rights.

Using the workspace copy of the sample avoids permission problems. The default workspace location for this sample is:

studio-workspace/sample_CaptureGenericDataStore

See Default Installation Directories for the default location of studio-workspace on your system.