Writing Your First EventFlow Application

Double-click the MyFirstApp.sbapp file to open it in the EventFlow Editor. You can have multiple editors open in this area of your perspective, each with its own tab; your tab is labeled with the application file name, MyFirstApp.sbapp).

Also notice the Palette view in the lower left of StreamBase Studio: when you first open an EventFlow application, the Palette is loaded with application components (it is empty when no EventFlow is open).

In the Palette view, open the Streams category, then drag the Input Stream icon onto from the Palette into the EventFlow Editor (sometimes referred to as the canvas). The red arrow in the following example is added to indicate the drag and drop action:

The yellow background of the icon on the canvas indicates that this component does not typecheck yet, because there is no schema associated with it. We will explain typechecking soon, but for now let's go to the next step.

On the canvas, click the input stream. When you click any component in the EventFlow Editor, StreamBase Studio displays its Properties view. Initially, the Properties view and the editor look like this:

In the Properties view, on the General tab, change the name of the input stream to ItemsInputStream.

Then click the Edit Schema tab, and click the Add button twice to add rows where you will define two fields for this schema. Initially, the fields are named UnnamedField1 and UnnamedField2.

On the Edit Schema tab, click into the first row and enter these values for the first field:

Then click into the second row and enter these values for the second field:

(In our simple example, SKU is a stock keeping unit.)

At this point, the ItemsInputStream Properties view looks similar to the following:

This allows you to apply changes you make on the Properties tabs, without having to save the entire application. You can, however, enter File → Save to write the application's .sbapp file to disk.

Click the Outline view. Notice that, so far, the list of components in your StreamBase application contains the ItemsInputStream.

Back in the EventFlow Editor, notice how the color of your ItemsInputStream icon changed from yellow to white. That happened because the StreamBase Server running on behalf of StreamBase Studio (a client program) is now aware of the schema for this just-defined component, ItemsInputStream. (This assumes that automatic typechecking is enabled, as mentioned in the first step.)

Open the Operators section of the Palette tab. Drag and drop a Filter operator from the palette onto the canvas. Its default name is Filter1. At this point, the canvas will look similar to this:

Its background color is yellow, again meaning its typecheck status is unknown, because it has not been connected yet to a component with a known and valid schema.

Now click the Filter1 shape on the canvas. StreamBase Studio opens the Filter Properties view. In the General tab, change the name of the operator to skuGT5000Filter.

Click the Predicate Settings tag. Click the green plus sign (+) to enter the following expression for predicate #1:

SKU > 5000

A predicate is an expression that always evaluates to true or false.

Also select the Create output port for non-matching tuples option. (If we leave this option off, non-matching tuples will simply be omitted from the streaming data.) At this point the Filter Properties view should be similar to the following:

At runtime when a tuple comes into this Filter operator, if its SKU is greater than 5000 (predicate #1 is TRUE), it will be sent out on port 1. Because of the way this particular filter operator is defined, all other tuples will be sent out on port #2. Thus for each tuple, the filter evaluates the expressions in order. When a predicate returns TRUE, the tuple is sent out on that port immediately, and subsequent predicates are not checked.

On the canvas, StreamBase Studio shows the output ports as small black squares on the right side of the operator. The upper-most black square represents port 1; the next black square down represents port 2, and so on. In our example:

Now we need to connect the input stream to the filter operator. On the canvas, find the black square on the right side of the ItemsInputStream1 icon. Drag your cursor from that square to the black square on the left side of the filter operator. That square represents the operator's input port. StreamBase Studio draws an arc connecting the two components. Arcs represents internal streams within EventFlows. After the arc connects the two entities and you release your cursor, the input stream and the filter operator should turn white or transparent, indicating that both typechecked successfully. For example:

Conceptually, typechecking operates from the left side of the application's diagram — the starting points — and will proceed through the operators to the right. When the typecheck process finds an error condition, StreamBase changes the background color of the adjacent right-side operator to red. Also, operators to the right (or "downstream") of the offending operator are not typechecked yet. The typecheck can fail for a number of reasons, such as:

Now drag and drop an Output Stream from the Palette onto the canvas. Select the output stream on the canvas (click once).

In the Output Stream Properties view, change its name to ItemsOutputStream.

Click into a blank area of the canvas. Draw a connecting arrow:

Your EventFlow application should look similar to this screen shot:

Drag and drop a second Output Stream onto the canvas. Select the output stream on the canvas (click once).

In the Output Stream Properties view, change its name to AllTheRest.

Draw a connecting arrow from the lower output port (2) to the AllTheRest output stream.

To automatically arrange the components neatly on the canvas, you can click Diagram → Layout Application.

To improve the arrangement of arrows around components (for example, if an arc overlaps a component's name), try clicking Diagram → Reroute Connections.

Now the layout of your EventFlow application should look similar to the following:

Before we save the application, let's click the Outline tab again. If you expand the lists, you should see these components:

Now select File → Save from the menu, and save your StreamBase Application (for example, MyFirstApp.sbapp).