Introduction

There seems to be a trend in IVOA standards for web services to be specified as asynchronous. While this is probably a good thing, it is undoubtedly more complex and, for a query interface, does present problems with how long the calling service waits for a response.

Issues

• CallbackOrPoll
• if SACs are asynch, should they provide callback interfaces or expect callers to poll for new results
• QueryForwardDepth
• if SACs are synchronous, might there be a problem when we have numerous such services available with queries being forwarded, perhaps endlessly if we get into a loop