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Re: Fast SCP
Rolf Andersson / Pantor Engineering
31 Jul 2008 9:05AM ETI was maybe a bit terse.
By using immutable template ids (in the short term) you can relax the requirements on synchrony between template definitions and messages using template ids. As a receiver, you still need a new template definition before you can decode a message with the new template id, but the existing definitions don't change (within a time window).
This allows for a lower overhead template distribution solution, which is what Dimitry was asking about, I think.
An example;
Let's assume you have a template named "foo/1" where 1 is the version of the template. Further let's assume the Template Name "foo/1" is associated with the Id 7. Now, if you want to modify this template (extend or change), you can create "foo/2" which differs in some aspect from "foo/1" and, depending on other Ids in use your first unused Id is 27, so you associate the name "foo/2" with the id 27.
Now receivers may see different representations of a message type. Template id 7 will still refer to the old layout and id 27 will be unknown until receivers see / retrieve the definition for id 27. The important difference is that you can relax strict ordering as id 7 is immutable for some period of time before it may be reused. Not seeing the new definition of id 27 can be detected.
You still need a level of synchronization between templates and messages in order to consume messages with newly created templates, but you don't need a strict ordering; new definitions can be sent out earlier as there is no (short-term) change to the meaning of an existing template.
There needs to be additional synchronization if several sources are allowed to allocate template ids, but that is a separate issue.
/Rolf
>> The remaining problem is then to make sure that a receiver
>> recognizes and acts on template id rollover. Implementors
>> need to make sure that senders receivers agree on the current
>> meaning of a template id (ie. the current association between
>> a template name and a template id). It is still an instance of
>> the reliable messaging problem, but with drastically lower
>> latency requirements.
>
> That's precisely the hard part: IMHO this is more than an instance of
> the Reliable messaging problem -- because it involves some ordering
> considerations. Given a sender and a possibly large number of
> receivers: no receiver should deliver a message prior to delivering its
> related template.
> e.g., ... [T_i] [M_i_1] ... [M_i_j] ... [T_(i+1)] [M_(i+1)_1] ....
>
> Not sure that's an instance of the distributed consensus, but rather
> instance of the virtual synchrony problem. That is, something (SCP)
> should guarantee that template changes within a group of receiver, are
> observed in the same order by all receivers. Furthermore, template
> changes in SCP must be totally ordered with respect to all regular
> FAST messages.
>
> BTW there is a typo in Fast SCP draft 1.1: page 15 section 7.3 third
> item one should read "...defined in 6.1" and not "defined in 5.1"
>
> I couldn't find mention of this in the SCP spec.
Fast SCP Dimitry London / Morgan Stanley 30 Jul 2008 10:16PM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 31 Jul 2008 3:28AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 31 Jul 2008 6:17AM ET Re: Fast SCP
Rolf Andersson / Pantor Engineering 31 Jul 2008 9:05AM ET Re: Fast SCP Hanno Klein / Deutsche Börse Systems 31 Jul 2008 10:06AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 31 Jul 2008 10:15AM ET Re: Fast SCP Greg Orsini / Cameron Systems 31 Jul 2008 11:44AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 2:06AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 2:39AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 3:13AM ET Re: Fast SCP Hanno Klein / Deutsche Börse Systems 1 Aug 2008 3:33AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 3:57AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 4:10AM ET Re: Fast SCP Rolf Andersson / Pantor Engineering 1 Aug 2008 3:06AM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 1 Aug 2008 6:28AM ET Re: Fast SCP Jacob Northey / The LaSalle Technology Group 1 Aug 2008 8:21AM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 1 Aug 2008 9:33AM ET Re: Fast SCP Greg Orsini / Cameron Systems 1 Aug 2008 10:01AM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 1 Aug 2008 10:16AM ET Re: Fast SCP Greg Orsini / Orc Software w/CameronFIX 1 Aug 2008 10:32AM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 1 Aug 2008 10:58AM ET Re: Fast SCP Jean-Marie Sulmont / RTS 31 Jul 2008 10:19AM ET Re: Fast SCP Jean-Marie Sulmont / RTS 31 Jul 2008 10:41AM ET Re: Fast SCP David Rosenborg / Pantor Engineering AB 31 Jul 2008 11:10AM ET Re: Fast SCP Dimitry London / Morgan Stanley 7 Aug 2008 10:53PM ET
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