async_storm--benja: Supporting aynchronicity in Storm

Author: Benja Fallenstein
Date: 2002-11-24
Revision: 1.3
Status: Accepted

The vision of Storm has always been to operate on top of a peer-to-peer network, providing network transparency to all user data. However, the current Storm interfaces are woefully inadequate for a network-based implementation, because they do not support asynchronicity: All operations block until they are successfully completed.

This PEG defines a pattern to support asynchronicity. A JobListener is introduced that can be used to inform clients when new data arrives. All Storm lookups take an optional JobListener parameter. If the requested data is available locally, they simply return it. Otherwise, they return null and start a new thread to retrieve the data. Once that job is done, the JobListener is informed; it can at that time re-request the data and expect it to be available.




In a network environment, blocking IO is simply unbearable-- firstly because blocking user input during IO operations is not acceptable, and secondly because it is vital for performance to be able to perform several network lookups in parallel.

The classic pattern to support network asynchronicity is to use GUI events to get new data to a program that has requested it (it works like this both in GNOME and in KDE).

In this pattern, the callbacks include information about the newly arrived data, like this:

dataArrived(Job job, byte[] bytes);

A browser application, for example, could support progressive image rendering by updating an image on the screen whenever dataArrived() is called.

However, this doesn't fit very well with the Gzz view architecture, where the whole view is re-generated each time there are changes (possibly re-using some cached parts). For example, when an image block has been loaded from the network (we can currently only check the data when it has completely arrived, so we don't need to worry about progressive rendering at this time), we would re-generate the view, putting in the real image instead of the placeholder we've had before. When new xanalogical links to a page we're viewing arrive from the network, we regenerate the view to incorporate them.

This architecture is better served with something like the Obs interface, which only provides a single, parameterless chg() method; the callback then re-requests the data when chg() is called.


I propose a similar interface for Storm, JobListener:

/** Called when new data has arrived. */
void newDataAvailable();

/** Called when the lookup finishes.
 *  @param timeout Whether the lookup was finished
 *                 because of a time-out. If this
 *                 is false, the lookup was finished
 *                 because the pool implementation
 *                 believes that all relevant data
 *                 has been retrieved now.
void finished(boolean timeout);

The two separate callbacks make it easy to use the same interface for lookups like for xu links, where we update the view each time new information arrives, and lookups like for pointer blocks, where we can only continue when all relevant data is known.

The lookup methods in StormPool would simply get an additional JobListener parameter:

Block get(BlockId id, JobListener listener);
Set getIds(JobListener listener);

(IndexedPool would be similar, but since that class isn't fully thought-out yet, I'm not giving its methods here.)

- Benja