The tool that makes this possible is gsutil, specifically its rsync option. This works very much like the usual rsync command in Unix but lets you perform bulk transfers/synchronisation of files between a Google Storage bucket and your local machine.
The gsutil rsync command makes the contents under dst_url the same as the contents under src_url, by copying any missing files/objects (or those whose data has changed), and (if the -d option is specified) deleting any extra files/objects. src_url must specify a directory, bucket, or bucket subdirectory.
So for my specific case where I wanted to have everything from Cloud Storage copied to my laptop, I would use a command like this...
Command
gsutil -m rsync -d -r gs://my-app.appspot.com/ /Backups/backup-files
This would make a mirror copy of my app's default bucket files on my laptop in the local /Backups/backup-files directory. The -m option makes transfers run in parallel, speeding up the entire backup. The -d option will delete any local files that are not in the Cloud Store bucket. The -r option performs a recursive copy of all the bucket data.
The output looks something like this (truncated)...
Terminal Output
Building synchronization state...
Starting synchronization...
Copying gs://my-app.appspot.com/xxxxx...
...
| [5.9k/5.9k files][325.9 MiB/325.9 MiB] 100% Done 76.6 KiB/s ETA 00:00:00
Operation completed over 5.9k objects/325.9 MiB.
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The operation was quite fast too. I had almost 6000 files to copy, which completed in a few minutes. Mind you it was around 300Mb of data, but transfers of many small files do not tend to make full use of the available bandwidth.
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This operation can also work in reverse, that is making the Cloud Store bucket a mirror image of the local file system; effectively restoring data that was backed up earlier. Doing that requires the source and destination arguments to be swapped, and possibly not using the -d option when copying back to the bucket.
Of course this will only back up the files and doesn't touch the Cloud Datastore, this is a topic for another post.
-i