Re: [9fans] QTCTL?

[Latchesar Ionkov <lionkov@lanl.gov>]

The 5 seconds lease might work in the local network case, but not  
caching at all is going to work out pretty well too. What if you want  
to cache over Internet and you round-trip is 3-4 seconds :)

On Nov 1, 2007, at 11:03 AM, Russ Cox wrote:

>> The fact is we have loose consistency now, we just don't call it  
>> that.
>> Anytime you are running a file from a server, you have loose
>> consistency. It works ok in most cases.
>
> Because all reads and writes go through to the
> server, all file system operations on a particular
> server are globally ordered, *and* that global ordering
> matches the actual sequence of events in the
> physical world (because clients wait for R-messages).
> That's a pretty strong consistency statement!
>
> Any revocation-based system has to have the server
> wait for an acknowledgement from the client.
> If there is no wait, then between the time that the server
> sends the "oops, stop caching this" and the client
> processes it, the client might incorrectly use the
> now-invalid data.  That's why a change file doesn't
> provide the same consistency guarantees as pushing
> all reads/writes to the file server.  To get those, revocations
> fundamentally must wait for the client.
>
> It's also why this doesn't work:
>
>> Tcache	asks whether the server is prepared to cache
>> Rcache	makes lease available with parameters, Rerror says no.
>>
>> Tlease	says, ok start my lease now (almost immediately follows Rache)
>> Rlease	lease expired or lease needs to be given back early
>>
>> Tcache	done with old lease (may immediately ask for a new lease)
>> etc.
>
> because the Rlease/Tcache sequence is a s->c->s
> message.  If a client doesn't respond with the Tcache
> to formally give up the lease, the server has no choice
> but to wait.
>
> If you are willing to assume that each machine has
> a real-time clock that runs approximately at the
> same rate (so that different machines agree on
> what 5 seconds means, but not necessarily what
> time it is right now), then you can fix the above messages
> by saying that the client lease is only good for a fixed
> time period (say 5 seconds) from the time that the
> client sent the Tlease.  Then the server can overestimate
> the lease length as 5 seconds from when it sent the
> Rlease, and everything is safe.  And if the server
> sends a Rlease and the client doesn't respond with
> a Tcache to officially renounce the lease, the server
> can just wait until Tlease + 5 seconds and go on.
> But that means the client has to be renewing the
> lease every 5 seconds (more frequently, actually).
>
> Also, in the case where the lease is just expiring
> but not being revoked, then you have to have some
> mechanism for establishing the new lease before
> the old one runs out.  If there is a time between
> two leases when you don't hold any leases, then
> all your cached data becomes invalid.
>
> The following works:
>
> Tnewlease	asks for a new lease
> Rnewlease	grants the lease, for n seconds starting at time of  
> Tnewlease
>
> Trenewlease	asks to renew the lease
> Rrenewlease	grants the renewal for n seconds starting at time of  
> Trenewlease
>
> Now if the server needs to revoke the lease, it just
> refuses to renew and waits until the current lease expires.
>
> You can add a pseudo-callback to speed up revocation
> with a cooperative client:
>
> Tneeditback	offers to give lease back to server early
> Rneeditback	says I accept your offer, please do
>
> Tdroplease	gives lease back
> Rdroplease	says okay I got it (not really necessary)
>
> The lease ends when the client sends Tdroplease,
> *not* when the server sends Rneeditback.  It can't end
> at Rneeditback for the same reason change files don't work.
> And this can *only* be an optimization, because it
> depends on the client sending Tdroplease.  To get
> something that works in the presence of misbehaved
> clients you have to be able to fall back on the
> "wait it out" strategy.
>
> One could, of course, use a different protocol with
> a 9P front end.  That's okay for clients, but you'd still
> have to teach the back-end server (i.e. fossil) to speak
> the other protocol directly in order to get any guarantees.
> (If 9P doesn't cut it then anything that's just in front of
> (not in place of) a 9P server can't solve the problem.)
>
> Russ
>