[Openais] [Fwd: Re: Several questions about Group Messaging Interface]

Steven Dake sdake at mvista.com
Fri Jun 25 10:52:52 PDT 2004


Folks
Forrest had asked some interesting questions about openais and others
may find answers of interest.

-----Forwarded Message-----
> From: Steven Dake <sdake at mvista.com>
> To: "Zhao, Forrest" <forrest.zhao at intel.com>
> Subject: Re: Several questions about Group Messaging Interface
> Date: Fri, 25 Jun 2004 10:47:30 -0700
> 

> On Fri, 2004-06-25 at 02:48, Zhao, Forrest wrote:
> > Hi, Steve
> > 
> > I'm not very clear about several points in Group Messaging Interface, so
> > your advices are highly appreciated.
> > 1 in todo list, I found an item " Add support for SAFE ordering". Dose 
> > this safe ordering mean totally ordering or extended virtual synchrony
> > delivery semantics or reliable multicast capability?
> > 
> 
> SAFE ordering is a type of ordering guarantee defined by VS model. 
> Whereas AGREED requires all processors to agree on a order before
> delivering the message, SAFE requires all processors to agree on a order
> AND all nodes have received the message before any processor can deliver
> the message.  By processor above, I mean node, but processor is a better
> term.
> 
> > 2 another item "Add support for low delivery-time delay FIFO messages".
> > Does it mean you'll make some optimization to group communication layer
> > to
> > reduce the delivery delay of FIFO messages?
> 
> It is possible to reduce latency on FIFO type messages by ignoring the
> agreed ordering rule.  The improvement would not be too dramatic, except
> for large rings.
> 
> > What types of ordering have been supported in current release?
> > 
> 
> AGREED ordering is supported in current release.  FIFO delivery can be
> fulfilled by AGREED ordering.
> 
> > 3 I'm not sure about the meaning of " Supports extended virtual
> > synchrony delivery semantics
> > 	with strong membership guarantees." in the develop map.
> > Does it mean it supports EVS, which is guaranteed by some strong
> > features of membership?
> > 
> 
> the strong membership guarantee means that a message is always delivered
> under a certain configuration.  How this works in the code:
> 
> the executive registers two callbacks deliver_fn and confchg_fn during
> startup with a call to gmi_join.
> 
> When a message is delivered, deliver_fn will be called.
> when a configuration change is delivered, confchg_fn will be called.
> 
> Messages that are delivered with deliver_fn are meant to be delivered
> under the configuration specified by the last call to confchg_fn.
> 
> At no time, may a message be delivered by deliver_fn for a configuration
> that was not previously specified by a call to confchg_fn.
> 
> If two or more nodes received the same message but with different
> configurations, they may make different decisions based upon the
> configuration information.  The "strong membership guarantee" prevents
> this from happening and allows the implementation of reliable
> distributed state machines in simple fashion. 





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