[Zeppelin 329] Notebook copied in file system level should be reflected in notebook list

[minahlee]

This PR enables Zeppelin to reflect addition/changes/removal of {ZEPPELIN_NOTEBOOK_DIR}/{notebookId}/note.json on the file system without restarting ZeppelinServer.

To activate this functionality, user needs to set ZEPPELIN_NOTEBOOK_GET_FROM_REPO to be true in conf/zeppelin-env.sh.

[Leemoonsoo]

Nice improvement. Could you choose little more intuitive name than ZEPPELIN_NOTEBOOK_GET_FROM_REPO ?

[minahlee]

@Leemoonsoo @khalidhuseynov ready for review

[anthonycorbacho]

why do you need to activate this functionality? shouldnt be a expected behavior?

[minahlee]

Beacause this PR will load notebooks from storage everytime client requests notebook list, which can be burden to Zeppelin with a bunch of notebooks. So I wanted to make as less effect as possible on current behavior. Would it be better to activate it by default?

[anthonycorbacho]

so why not using a watcher? like if someone drop a notebook in the notebook dir, you update the list of notebook.

[Leemoonsoo]

I think activating this function by default will brings some IO overhead in some cases. (large number of big notebooks). As @minahlee mentioned, it's better to keep it de-activated by default. When the implementation becomes more efficient it can be activated by default. That could be future work.
So, Looks good to me.

[Leemoonsoo]

Merging if there're no more discussions