Learn To Google It

Here's a short code snippet demonstrating how to retrieve and parse a XML file using Apps Script. First, here's how to download and parse the file:

var url = "URL to XML"; var xml = UrlFetchApp.fetch(url).getContentText(); var xml_document = XmlService.parse(xml); var xml_root = xml_document.getRootElement();

Once the file is downloaded, you can retrieve child elements by calling getChild/getChildren. GetChild returns the first instance of the named element, and getChildren returns an array listing every element instance:

var xml_items = xml_root.getChild("channel").getChildren("item");

And finally, here's how to retrieve the text content of an element:

Here's an example of how to use the Users API in Go. This example checks to see if the current user is logged in; if not, the user is given a link to log in. If the user is already logged in, then it creates and prints a link to log out.

You can search for specific users by using the filter labels function of App Engine logging. Here's a short example: the following filter searches for all users matching the substring inny :

The search finds the following request logs, where the user vinnyapp (my Gmail account) has logged in:

I'm in the middle of testing an email server on Compute Engine, and I noticed something unusual: apparently Facebook's outbound email servers insist on using extended SMTP to send email.

With extended SMTP, an email server sends email by opening up a connection and sending the EHLO command. The proper response is either 250 (to indicate success and that extended SMTP support is available) or 550 (the responding server did not understand the command, which is another way of saying that the responding server does not support ESMTP). In case of 550 errors, the usual practice is to fall back to the original SMTP command set and to send a HELO request.

But Facebook's outbound mail servers seem to only want to connect with ESMTP servers: FB mail servers send a quit command instead of falling back to sending a HELO command.

A quick reminder on how to configure Putty to access DigitalOcean Linux VMs:

Make sure that SSH version 2 is selected:

An improperly configured YAML file may show the error Error Parsing YAML File: Mapping Values Are Not Allowed Here . This error is demonstrated below:

Here is an example YAML file that causes this error:

application:application-id version:1 runtime:php api_version:1 threadsafe:true #Error happens at line 7 below, even though the incorrect lines are above. handlers: - url: /example

Even though the YAML parser reports the error at line 7, the actual incorrect lines are above that point: lines 1 - 5 are missing the space character between the colon and the value. If you encounter this error, make sure that the key: value pairs are separated by 1 colon and 1 space character, as shown below:

While using App Engine's Mail API, some applications may encounter the following error:

This error means that the application attempted to send email with a non-whitelisted from address.

To send email from App Engine, applications must declare a sending address matching one of the following: a registered administrator of the application, the Google user account of the currently-logged-in user, or an email address of the form:

[any string]@[Application ID].appspotmail.com

For most purposes, using the appspotmail string as a from address is perfectly fine. To generate this sending address, you can use App Engine's environment variables to collect the application ID. For example, here's how to do it in Java:

String application_id = SystemProperty.applicationId.get(); String sender = "donotreply@" + application_id + ".appspotmail.com";

On rare occasions, the Google App Engine dev server displays the following error:

The important part of the error is this text:

java.io.IOException: tmpFile.renameTo(classfile) failed

This exception crops up whenever an app file (in this case a JSP file) is currently being accessed by another program. If you see this exception, double check to ensure that the named file isn't being accessed by another program.

Google App Engine permits only specific whitelisted extensions to be used within PHP applications. If you use a non-whitelisted extension, you'll see the below error:

The [php_extension_name] extension is missing. Please check your PHP configuration.

If you need a certain extension for your PHP application, ensure that it's enabled in GAE: check the official list at https://developers.google.com/appengine/docs/php/#PHP_Enabled_extensions . If your preferred extension is not listed, you can also try searching for a pure-PHP implementation (Pure PHP extensions can always be uploaded as part of an application; C based extensions must be whitelisted.)

Here's an useful wget command: it downloads a static copy of a MediaWiki installation while skipping unimportant pages, such as the talk sections. Downloading large sites can take a long time, so the nohup instruction lets this command continue even when the user exits from the shell.

nohup wget --recursive --page-requisites --html-extension \ --convert-links --no-parent -R "*Special*" -R "*action=*" \ -R "*printable=*" -R "*title=Talk:*" \ "http://example.com/example/wiki/path/"

Accessing Google Cloud SQL from an App Engine application is relatively straightforward. To start, an application must first specify a host name for the Cloud SQL servers.

For example, here's the host name for PHP applications:

:/cloudsql/[Google-Cloud-Project-Name]:[Cloud-SQL-Instance-Name]

Java accesses Cloud SQL through a special JDBC driver. Here's the proper host name for it:

jdbc:google:mysql://[Google-Cloud-SQL-Project-Name]:[Cloud-SQL-Instance-Name]/[Database-Name]

Secondly, Cloud SQL must whitelist incoming connections from permitted App Engine applications. To do this, open up the Google Cloud console and select the project you're using. Then press the Cloud SQL option on the left hand navigation bar:

Click the New Instance button:

On the bottom of the form there's an option to whitelist named App Engine applications. Type in the application ID of the App Engine application using the database:

App Engine downtime and maintenance notices are posted to the Google Group google-appengine-downtime-notify, located at https://groups.google.com/forum/#!forum/google-appengine-downtime-notify . It's a good idea to monitor this list for any issues with the App Engine platform.

It's especially important to subscribe to these notices if your application is still on the M/S datastore. The M/S datastore is occasionally moved into a read-only state for maintenance, and these maintenance periods are announced over the downtime list.

Subscribing to the google-appengine-downtime-notify list is easy: go to the above linked address and click on the button marked Join Group. As you can see from the below pictures, this list is extremely low-traffic (less than 1 email a day).

URL hashes (the part after the # sign) are useful for keeping track of application state. Here's a short example of how they can be used.

The below code fragment forwards the user to a Google search for the value held in the variable q . The useful part is that this code also sets a hash to mark that the forward has taken place. This allows the page to keep track of the forwarding status, and to prevent reissuing the forward - for instance, if the user presses the back button after being forwarded.

if (location.hash != "#forwarder") { location.hash = "forwarder"; var search_url = "http://www.google.com/search?q=" + escape(q); //Set the timeout to 10 seconds. setTimeout(function(){location.assign(search_url);},10000); }

To see how this works, consider the below example. The first alert will show that there's no hash recorded in the URL (the alert will be blank). The second line sets a hash of forwarder and the third line shows the value of the current hash (which will be the value we set on the second line). This will cause the if statement to evaluate to false, preventing the forwarding from taking place. The same process (recognizing the hash and blocking the forward) happens when the user clicks the back button on the above forwarding code.

The App Engine system status console is located at https://code.google.com/status/appengine . It's a good idea to keep this page bookmarked to monitor GAE's performance.

Here's how the status page looks like when everything is running well:

Here's an example of how performance issues are reported:

While configuring a domain within Google Apps, you may see the error message " The Term [Domain Name] Is Not Allowed ":

I like to collect samples of downtime notices and error messages from popular websites; it's always fascinating to see how sites handle messaging on technical issues.

Here's an example downtime notice from Slashdot:

Here is the message in context: