Github user queeniema commented on a diff in the pull request:
https://github.com/apache/incubator-quarks-website/pull/13#discussion_r56919226
--- Diff: site/docs/recipe_source_function.md ---
@@ -0,0 +1,86 @@
+---
+title: Recipe 2. Writing a Source Function
+---
+In the previous [Hello Quarks!](recipe_hello_quarks) example, we create a
data source which only generates a single Java String and prints it to output.
Yet Quarks sources support the ability generate any data type as a source, not
just primitive Java types such as Strings. Moreover, because the user supplies
the code which generates the data, the user has complete flexibility for *how*
the data is generated. This recipe demonstrates how a user could write such a
custom data source.
+
+## Custom Source: Reading the Lines of a Web Page
+{{site.data.alerts.note}} Quarks' API provides convenience methods for
performing HTTP requests. For the sake of example we are writing a HTTP data
source manually, but in principle there are easier methods.
{{site.data.alerts.end}}
+
+One example of a custom data source could be retrieving the contents of a
web page and printing each line to output. For example, the user could be
querying the Yahoo Finance website for the most recent stock price data of Bank
of America, Cabot Oil & Gas, and Freeport-McMoRan Inc:
+
+``` java
+ public static void main(String[] args) throws Exception {
+ DirectProvider dp = new DirectProvider();
+ Topology top = dp.newTopology();
+
+ final URL url = new
URL("http://finance.yahoo.com/d/quotes.csv?s=BAC+COG+FCX&f=snabl";);
+ }
+```
+
+Given the correctly formatted URL to request the data, we can use the
*Topology.source* method to generate each line of the page as a data item on
the stream. *Topology.source* takes a Java Supplier that returns an Iterable.
The supplier is invoked once, and the items returned from the Iterable are used
as the stream's data items. For example, the following *queryWebsite* method
returns a supplier which queries a URL and returns an Iterable of its contents:
+
+``` java
+ private static Supplier<Iterable<String> > queryWebsite(URL url)
throws Exception{
+ return () -> {
+ List<String> lines = new LinkedList<>();
+ try {
+ InputStream is = url.openStream();
+ BufferedReader br = new BufferedReader(
+ new InputStreamReader(is));
+
+ for(String s = br.readLine(); s != null; s = br.readLine())
+ lines.add(s);
+
+ } catch (Exception e) {
+ e.printStackTrace();
+ }
+ return lines;
+ };
+ }
+```
+
+ When invoking *Topology.source*, we can use *queryWebsite* to return the
required supplier, passing in the URL.
+
+ ``` java
+ public static void main(String[] args) throws Exception {
+ DirectProvider dp = new DirectProvider();
+ Topology top = dp.newTopology();
+
+ final URL url = new
URL("http://finance.yahoo.com/d/quotes.csv?s=BAC+COG+FCX&f=snabl";);
+
+ TStream<String> linesOfWebsite = top.source(queryWebsite(url));
+}
+ ```
+
+ Source methods such as *Topology.source* and *Topology.strings* return a
*TStream*. If we print the *linesOfWebsite* stream to standard output and run
the application, we can see that it correctly generates the data and feeds it
into the Quarks runtime:
+
+Output:
+
+```java
+"BAC","Bank of America Corporation Com",13.150,13.140,"12:00pm -
<b>13.145</b>"
+"COG","Cabot Oil & Gas Corporation Com",21.6800,21.6700,"12:00pm -
<b>21.6775</b>"
+"FCX","Freeport-McMoRan, Inc. Common S",8.8200,8.8100,"12:00pm -
<b>8.8035</b>"
+```
+
+## Polling source: reading data periodically
+A much more common scenario for a developer is the periodic generation of
data from a source operator -- a data source may need to be polled every 5
seconds, 3 hours, or any time frame. To this end, *Topology* exposes the *poll*
method which can be used to call a function at the frequency of the user's
choosing. For example, a user might want to query Yahoo Finance every two
seconds to retrieve the most up to date ticker price for a stock:
+
+```java
+ public static void main(String[] args) throws Exception {
+ DirectProvider dp = new DirectProvider();
+ Topology top = dp.newTopology();
+
+ final URL url = new
URL("http://finance.yahoo.com/d/quotes.csv?s=BAC+COG+FCX&f=snabl";);
+
+ TStream<Iterable<String>> source = top.poll(queryWebsite(url), 2,
TimeUnit.SECONDS);
+ source.print();
+
+ dp.submit(top);
+ }
+```
+
+**Output:**
+<br>
+<img src="images/pollingSource.gif">
+
+It's important to note that calls to *DirectProvider.submit* are
non-blocking; the main thread will exit, and the threads executing the topology
will continue to run. (Also, to see changing stock prices, the above example
needs to be run during open trading hours. Otherwise, it will simple return the
same results every time the website is polled).
--- End diff --
Typo: "simple" should be "simply"
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