This does not address the loss of ACK messages in transit. (As mentioned
by Noel, earlier in this thread.) It is the ACK ACK that is missing from
SMTP, rather than the first ACK. (AKA 250 OK).
It also pre-supposes that a message can be uniquely identified in order to
check whether or not the message already exists in the store - an incorrect
assumption. It is possible to write an expect script (or similar) that
would connect to JAMES and send an SMTP message. That script could then be
run repeatedly, sending an absolutely identical message every time, and
remain completely within the SMTP spec.
Cheers
ADK
--------------------------------------------
There is no magic.
Maxime Levesque
<maxime.levesque@we To: James Users List
<[EMAIL PROTECTED]>
bhelp.com> cc:
Subject: Re: fault tolerance of smtp
server and mailets
10/12/2002 10:10
Please respond to
"James Users List"
A two phase commit would do the job indeed, but is not the only way
(at one condition se (*1)), a much
simpler approach, one that is better suited for James would be :
1) On the sender side (the SMTP source) :
boolean success = false;
while(!success) {
success = send(mail);// It's a prerequisite that send returns
"true" if and only if an acknowledgement is received,
// and that the later is sent if and only it the message was
persisted. (*1)
if(success)
deleteFromLocalStore(mail); // or "persist the fact that
'mail' was sent.
sleep(x);
}
2) On the receiver side (James) :
while(true) {
mail = receiveMail(); // a blocking call that unblocks when an
email gets in...
if(!alreadyExistsInPersistentStore(mail))
persist(mail);
}
The (1) part insures "at least once" guaranty of delivery, while (2)
by detecting duplicates takes care of the
"at most once" part guaranty.
The only prerequisite for this is that SMTP provides for some kind of
acknowledgement of receipt,
and that the SMTP source implementation uses this ack. to delete the
mail from its "to send" list...
Does SMTP have this ?
Note that this also takes care of DB failures, since the ACK. is
sent only when successfully stored.
Noel J. Bergman wrote:
>This is a transaction issue. But there is no mechanism within SMTP for a
>two-phase commit. There is always some chance that between storing the
>message and advising the client that the system could crash. For that
>matter, after sending an acknowledgment to the client, there is some
chance
>that the packet could be lost. This effects receipt and relaying by
James.
>
>Internally, mail is queued between the handler and the spool manager, and
>then within the spool manager for each transition between processors. Not
>for each mailet.
>
>What you want is a transaction, so that the process of your storing the
>mailet in the database, and the message being removed from the spool
appears
>to be atomic. But there is no two-phase commit within the spooling
>mechanism. If your mailet puts the message in a database, as
LocalDelivery
>can, then your mailet should set the state to GHOST, and the message will
be
>removed from the spool, so that the only copy will be the one you saved.
As
>I said, there is a small window between storing the message in the
database,
>and when it is cancelled from the spool. Very unlikely, but possible.
>
> --- Noel
>
>
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