Make two servers talk to each other

Question

I have application written in GWT and hosted on Google AppEngine/Java. In this application user will have an option to upload video/audio/text file to the server. Those files could be big, up to 1gb or so and because GAE/J does not support large file I have to use another server to store those files. This would be easy to implement if there was no cross-domain security feature in browsers. So, what I'm thinking is to make GAE Server talk to my server (Glassfish or any other java servers if needed) to tell url to the file and if possible send status of uploaded file (how many percent was uploaded) so I can show status on clients screen. Here is what I'm thinking to do.

When user loads GWT page that is stored on GAE/J he/she will upload file to my server, then my server will send response back to GAE and GAE will send response to the client. If this scenario is possible what would be the best way to implement GAE to Glassfish conversation?

Answer 1

Actually before that maybe you can try using first approach via by-passing cross-domain security of browsers using iframe. There are some ready to use components for this but for your problem which of them can be usable I don't know. Just google for these components...

Answer 2

Doing it the original way you suggested use URL Fetch Service The down side to doing it the other way is that you introduce dependencies on multiple sites inside your web pages.
The downside of using the URL Fetch Service is that you have to pay by number of bytes transferred after you have reached the free quota.

Answer 3

One option would be to wait - the blobstore limit won't always be 50MB!

If you're in a hurry, though, I would suggest an approach like the following:

1. Have your App Engine app generate a signed token that signifies the user has permission to upload a file. The token should include the current date and time, the user's user ID, the maximum file size, and any other relevant information, and should be signed using HMAC-SHA1 with a secret key that your App Engine app and your server both know.
2. Return a form to the user that POSTs to a URL on your blob hosting server, and embeds the token you generated in step 1. If you want progress notifications, you can use a tool like plupload, and serve the form in an IFrame served by your upload server.
3. When the user uploads the file to your server, the server should return a redirect back to your App Engine app, with a new token embedded in the redirect URL. That token, again signed with a common secret, contains the ID of the newly uploaded file.
4. When your App Engine app receives a request for the redirect URL, it knows the upload was completed, and can record the new file's ID etc in the datastore.

Alternately, you can use Amazon's S3, which already supports all this with its HTML Form support.