Graph User Manual

Graph ► Tools ► Fetch Data

Use the Fetch Data tool (available in Developer Mode only) to retrieve data from a web server. The Fetch Data tool is diagrammed here:

URL Types

The URL that can be used is defined here:

• Web: A web URL begins with "http://" and the rest of the URL is the path to the data file, such as: "http://www.vvi.com/data/mypoints.txt" (without the quotes). Notice that the path extension is "txt" in this case. Although the extension is arbitrary, txt is a safe extension as it informs a web server to use an ASCII MIME type. In practice, any extension, or no extension, also works. This is the same URL that you would type into a web browser to download the data. The data file must be a resource of the web server that the URL points to, either as a static file or a dynamic URL that retrieves the data file bytes from other places.

Notice that the Web URL gives access to a programmable system. For example, you can turn on your Web Sharing in the system preferences and then use PHP, perl or other scripting languages to write algorithms to retrieve computed data. The URL would be of this form: "http://localhost/cgi-bin/myalgorithm?data=lab1&section=2&param1=5" and you use the scripting engine's built-in form facilities to parse the URL parameters as input to an algorithm. Alternatively, the URL could point to an existing web service to retrieve SOA type information from queries.

Content Format Type

Generally the content is formatted as:

• A list of numbers for the Set Of Scalars task.

• A list of 2D points (two numbers) for the Set Of 2D Points task, such as: x1 y1 x2 y2 ... xN yN.

• A list of 3D points (3 numbers) for the 3D Points Task.

• A list of numbers for the Z Values and Density Values tasks.

On the task page, if the Fetch Data row does not have a text entry or "All" is entered then the content retrieved from a URL (the response bytes) can be XML. For an explanation of this content see the XML Fetch tutorial.

Note that the fetch content format is simple, it is just a list of numbers with a blank delimiter. No fancy XML formatting and no dimension data. It is intended be as simple as possible in order to facilitate ease of use. Data formats are also explained in each Task's Help tool and section in this manual.

Fetch Timing

Fetching data can occur at different time intervals as defined here:

• Fetch Data: Touching the Fetch Data button fetches the data immediately.

• Each Animation Step: Selecting the Each Animation Step switch causes the data to be fetched upon each animation, which is at an interval of one second. If you use this button then go to the Data tool and select Animate.

• At First Appearance: Selecting the At First Appearance switch causes the data to be fetched when the task is made visible. Tasks are only made visible when switching tasks so this operation does not occur too frequently.

Fetch Operation

In a nutshell, when a fetch occurs the data is retrieved, entered into the task and then displayed. Usually the fetch appears to be atomic, that is: it seems to be a single operation. However, fetching is asynchronous and non-atomic. You may notice this fact if the fetch takes a long time. Also, for tasks that have multiple columns the fetch is on a per-column basis and the fetch retrieves each column one at a time until all columns are fetched and then updates the task with fetched data. Multiple column fetches are done in parallel and asynchronously and each column fetch rendezvous to amalgamate the fetch into a single result.