Posts

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Facebook Outbound Email

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.

Another interesting oddity from watching mail logs: Google’s Gmail servers seem to be the only mail servers properly implementing the BDAT command (binary data). I never see any other mail servers attempt to use it.

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Error Parsing YAML File: Mapping Values Are Not Allowed Here

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:

application: application-id
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Mail Service Error: Sender Is Not An Authorized Email Address

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";

For applications that need to send email originating from their custom domain, register a Google Apps account with the address you want to use, then register it as an administrator of the application.

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IOException: tmpFile.renameTo Failed

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.

If this error persists, close down and reopen Eclipse and the development app server – the file may have been left open from a previous run.

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Whitelisted PHP Extensions

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.)

If you need an extension not listed in the above link, you can request it via the App Engine issues tracker. For example, here’s a feature request for the ImageMagick extension: https://code.google.com/p/googleappengine/issues/detail?id=9424

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Downloading The Contents Of A MediaWiki Install

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 nohupinstruction 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/"

You can use this command to archive old wiki installs, or keep a local copy for quick reference.

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Accessing Google Cloud SQL From App Engine

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:

Click the Confirm button to finish setting up the database.

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App Engine Downtime Notices

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).

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Using Javascript To Forward A User (And Keeping Track Of The Forward)

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.

alert(location.hash);
location.hash = "forwarder";
alert(location.hash);
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);
}