Adding / Removing appliances without restarting the application

[carllerche]

Hello,

I have an application that's already running and contains a web server. I would like to add a second web server and put it into operation without having to restart the whole application. However, I can't figure out how to build and start a single appliance without having to restart the entire application. Is this possible? Similarly, is it possible to remove an appliance without having to restart the application?

Also, as a bonus question, once I added a class to the catalog, is it possible to edit stuff like the volume size without having to branch the class?

Thanks,
-carl

[nobu]

Hi,

For the first question, we can't add an appliance without restarting the app.
However, you can edit the app while it's running, and then "restart" it.
So, the down time is limited to the time required for restarting the application (a few minutes for a small app).

Usually, I simply copy the app (to conserve running app), and then modify and test the new app with different IP address(es).
And when it's ready, I replace the IP (property) of the new up, stop the old app, and start the new one.


For the bonus question, I don't think we can edit the classes in catalog with the current version.
There might be a way to do thatwith the grid maintainer access, but I don't know yet.

Nobu

[PeterNic]

Carl,

While you can't add and remove appliances without restarting the app, you may leave some appliances on standby and start/stop them as needed. For example, we frequently put 4 web servers and start only one by default; the others are started/stopped manually (e.g., comp start myapp:main.srv2) as needed. Similarly, you can keep an admin gateway appliance stopped (or on standby) and enable it only during maintenance.

Once an appliance goes into a catalog, it cannot be modified without branching -- the singleton state is the only state in which classes can be modified. This allows you to make such changes, for example, while applications that use the old version of the class are running.

Regards,
-- Peter

[jasons]

You can't add an appliance without restarting the application? Maybe this is ok for small applications that don't take long to restart or apps that don't require to be up all the time. You should make this clear in your documentation and let people know upfront about a serious limitation like this. This is a deal breaker for many organization that can't afford downtime just to add another type of appliance to a running application. I'm considering canceling my grid for this very reason.

[PeterNic]

Jason,

Thanks for your candid comments.

We appreciate your concern and we do intend to address the issue of dynamic changes to applications. In the meantime, it may help you to understand why the limitation is there at the moment. AppLogic is the first and only system that deals with applications as entire entities. As you've already experienced, you can start/stop/backup or even migrate an entire distributed app to another data center with a single command. In order to achieve this level of simplicity in working with apps we needed bound the problem we were trying to solve in our early releases. One way in which we did this was to limit changes to applications while they're running. You can start/stop various applications without having them interfere with each other, you can log into virtual appliances or you can start/stop virtual appliances, but you can't add or remove virtual appliance from an app while it runs.

Again, this will be addressed as soon as we can work it into the engineering schedule. Users like Carl and you, as well as some who PM'd us, have shown us that this is an important feature -- and we are listening.

Regards,
-- Peter