What is the best way make a copy of a blobstore entity on app engine in Java?

Question

Here is our simple usecase: user2 wants to copy user1's document into his or her own repository within our application. Should be simple, right? All we need to do is create a second identical blob in the blobstore with the key returned that we can associate with user2. We must be missing something here. It appears that app engine blob store's primary function is to handle blobs uploaded from and downloaded to a browser, and a simple copy operation initiated server-side is not so simple.

The obvious solution seemed to be using the the experimental file api in java, but no love. It works, until you get up in file size beyond a MB or so, then it fails, somewhat unpredictably. Reading it all into the server layer also seems silly, when we just need to make a copy in the storage layer. In addition, the odds of us getting an experimental feature through into our production environment is slim, although non-zero.

Some information about our environment: the app is written in Java and we're using the blobstore, not cloud storage and are committed to it for now. We're a small departmental team and would like to make the case that app engine is a great platform to use, but this one has us stumped. S3 makes this blindingly simple, are we missing something really stupid here?

Answer 1

We ended up scrapping the idea of making a programatic copy of the blob with the file api and going with a reference as Kalle suggested in his comment, and created a new xref entity that stores information about the copy and the original. When an image or file is deleted, we check the xfef entity for references and delete the ones that point to that image/file (ie created if the deleted image/file was copied from another one). If we don't find any xrefs at all, we delete the blob itself. We didn't like the privacy/compliance implications of leaving orphaned blobs laying around, and even though storage is cheap every $$$ helps. We also liked the idea of keeping a clean house so to speak.