Hi all, I've got an XML-RPC app that has to grab some data from a SQL database, then depending on the results, make some more SQL calls, make some more decisions, then finally return a result. So, I'm hip-deep in Deferreds. So, here's the idiom I've come up with, and I'm running into a limitation with it, namely that I can't seem to add callbacks to a Deferred after it has started running. Here's what I'm doing in abbreviated form, and it's an idiom I'd like to stick with, if possible: class MyXmlRpcClass(xmlrpc.XMLRPC): def__init__(self): self.db = adbapi.ConnectionPool(blah) def xmlrpc_foo(self): return FooClass(self.db).step1() # This returns a deferred, see below class FooClass(General): def __init__(self, db): self.db = db def step1(self): self.d = self.db.runQuery(blah) self.d.addCallback(self.step2) return self.d def step2(self, query): if query == 'yadda': return 'This bit works' else: self.d.chainDeferred(self.db.runOperation(blah)) self.d.addCallback(self.step3) def step3(self, data): return "Never gets here!" If I get to the part where it adds the callback for step3, it ends up giving an AlreadyCalled exception in Deferred. So, I expect there's a good way to add to a running Deferred, no? Thanks! Steve