Tips, tricks, and thoughts about Google, AdWords, Google Cloud Platform, and all its subsidiaries. Not affiliated with or sponsored by Google.
While configuring a domain within Google Apps, you may see the error message “ The Term [Domain Name] Is Not Allowed ”:
This error can come up due to several problems. The most likely issue is that the domain has been configured as an independent domain, rather than as an alias domain. A quick fix to this issue is to delete this domain from Google Apps, and then to re-add it. When the prompt comes up to select independent domain versus alias domain, select the alias domain option.
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:
While using certain libraries on App Engine, you may encounter the following exception notice:
javax.servlet.ServletContext log:
Exception while dispatching incoming RPC call
threw an unexpected exception:
java.security.AccessControlException:
access denied (java.lang.RuntimePermission modifyThreadGroup)
If you see this exception message, your application or (more frequently the case) a library is attempting to create a new thread. App Engine doesn’t allow frontend instances to spawn threads, so any attempt to start up a thread will result in AccessControlExceptions.
However, App Engine does allow backend threads: threads which run within backend instances. If your application absolutely needs to run threads, run the threading component within a backend or a backend module.
On rare occasions, an application may spawn errors similar to the below:
com.google.appengine.api.datastore.DatastoreFailureException:
internal error.
at com.google.appengine.api.datastore.DatastoreApiHelper.translateError (DatastoreApiHelper.java:50)
at com.google.appengine.api.datastore.DatastoreApiHelper$1.convertException (DatastoreApiHelper.java:70)
As the exception message suggests, this log indicates that the datastore encountered an internal error while handling the datastore operation. Unfortunately, there’s nothing that a developer can do to fix this error since it’s an internal App Engine issue.
Generally this type of exception fixes itself sooner or later; if it persists, file an issue at the App Engine bug tracker: https://code.google.com/p/googleappengine/issues/entry?template=Production%20issue
While using JDO or another datastore helper library, applications may encounter a value of ExQ when requesting a query cursor. This value means that there are no more results (entities) to display for that query, and consequently that there are no more query pages to return.
All applications using query cursors should check for an invalid or end of query results marker, and handle this in an application-specific way. A query cursor string that is null or empty (zero length) generally denotes the end of query results. The cursor value ExQ can be handled in the same way.
A word of warning: some third-party datastore helper libraries spawn an exception or error condition when applications attempt to access invalid cursors, or attempt to request a cursor when end of results has already been reached. When using these libraries, it’s important to fully read the documentation and understand how these libraries interact with the App Engine datastore.
A few days ago, App Engine finished transitioning to the 1.8.8 runtime. Here’s a screenshot of an application in the middle of the transition (note that one instance is running 1.8.7, while the other is running 1.8.8):
My favorite part of this patch is the new ability to edit PHP strings in memcache:
A quick note: here’s how to write a line of text into logging on the PHP runtime:
syslog(LOG_INFO, "Log Text: " . $variable_to_log);
The first argument can be replaced with standard PHP logging levels, such as LOG_WARNING.
A reminder: App Engine ignores calls to openlog() and closelog() . While you can still call these functions (for instance if you have legacy code), they will not affect logging.
In SQL, the COUNT statement is available for counting the number of rows returned by a SELECT query. Here’s an example:
SELECT COUNT(*) FROM foods WHERE name="apple";
Unfortunately, the App Engine datastore offers no equivalent to the COUNT statement. To count the number of entities, an application has two options:
Here’s an example of how to configure servlets to accept XMPP requests. The XML below should be added to the web.xml file in the /war/WEB-INF directory.
This XML snippet configures three servlets: ChatServlet (handles inbound XMPP chat messages), PresenceServlet (handles requests for the application’s presence information, and presence info for other clients) and SubscribeServlet (when other XMPP users grant the application access to their presence information).
<!-- Handles the chat part of XMPP. -->
<servlet>
<servlet-name>Chat</servlet-name>
<servlet-class>com.example.xmpp.ChatServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Chat</servlet-name>
<url-pattern>/_ah/xmpp/message/chat/</url-pattern>
</servlet-mapping>
<!-- Handles the presence part of XMPP. -->
<servlet>
<servlet-name>Presence</servlet-name>
<servlet-class>com.example.xmpp.PresenceServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Presence</servlet-name>
<url-pattern>/_ah/xmpp/presence/*</url-pattern>
</servlet-mapping>
<!-- Handles the subscription part of XMPP. -->
<servlet>
<servlet-name>Subscribe</servlet-name>
<servlet-class>com.example.xmpp.SubscribeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Subscribe</servlet-name>
<url-pattern>/_ah/xmpp/subscription/*</url-pattern>
</servlet-mapping>Here is a short but incredibly useful piece of Javascript: it retrieves the value of a named URL parameter. For example, if the current page’s address is index.html?example=true&name=vinny this function would return vinny if passed the name name , and it would return true if passed the name example .
function getParameterByName(name) {
var match = RegExp('[?&]' + name + '=([^&]*)').exec(window.location.search);
return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
}
A note of warning: calling this method repeatedly is inefficient, since the regular expression needs to be repeated every time.