Issue 139 - PartnerLink Semantics
Change the following text in section
From: A receive activity specifies the partner link it expects to
receive from, and the port type (optional) and operation that it expects the
partner to invoke.
To: A receive activity specifies the partner link that
contains the myRole used to receive messages, the
port type (optional) and operation that it expects the partner to invoke.
Note that the
value of the partnerRole on the partner link is not
used when processing a receive activity , that is, there is no
filtering of incoming messages based on the partnerRole’s value. Also note,
that the value of the partnerRole EPR MAY be initialized or updated when a
message is received if the underlying binding is able to transport EPR
information about the partner.
Change the following text in section 13.5.1:
From: The partnerLink attribute defines the partner link on which the
request is expected to arrive; the partnerLink must be defined in the partnerLinks section.
To: The partnerLink attribute defines the partner link that
contains the myRole EPR on which the request is
expected to arrive; the <partnerLink> must be defined in the <partnerLinks> section. As with <receive> the partnerRole EPR is ignored for purposes of
which messages to receive and may be
initialized/updated as a consequence of receiving a message using a binding
that is able to provide EPR information about the partner.
Rationale: An unanswered question in the BPEL
specification is what part, if any, do partnerLink roles play in message filtering/correlation?
<receive> is executed on a partnerLink with an initialized
partnerRole value is there a requirement that incoming messages accepted
by that receive MUST/SHOULD/MAY be sent by the endpoint identified by the
message arrives on a <receive> and the message (through an undefined
mechanism) contains an EPR for the partner MUST/SHOULD/MAY the partnerRole
on the partnerLink be set to that EPR?
sending a <reply> on a partnerLink with an initialized partnerRole
EPR MUST/SHOULD/MAY the reply message be required to be sent to the
endpoint identified by the partnerRole EPR?
using <invoke> on a partnerLink with an initialized myRole EPR
MUST/SHOULD/MAY the response to the request be required to be sent to the
Without a consistent answer to question #1 it will
be impossible to write portable BPEL code. After all, some implementations may
pay attention to the partnerRole as a filter for receiving incoming messages and
others may not and the result will be chaos. I believe the only reasonably portable
answer to question 1 is – partnerRole is ignored on
<receive>/<pick>/<onEvent>. Implementations are free to
override that with a mandatory extension where it makes sense but as a default,
portable behavior, I think ‘ignore’ is as good as it will get.
#2 however is a very different issue. Imagine you
are using WS-Addressing or some other protocol that provides EPR data. In that
case you would expect that when you receive a message on that partnerLink then
the partnerRole EPR will be initialized and you will be able to send messages
in return. In that case you wouldn’t bother manually setting EPRs. On the other
hand if your program is switched to something like XML over HTTP where there is
no WS-Addressing and thus no automatic EPR setting support then suddenly your
program’s key assumption falls apart and your code will fail as soon as it
tries to use what it thought was an initialized partnerLink.
If, your program was
originally written with XML over HTTP in mind then you would do your own EPR
management. But imagine that your BPEL is rebound to a transport using
WS-Addressing, in that case you would set the EPR on a partnerRole (from data
you got in an application message that is part of your protocol design and has
nothing to do with WS-Addressing), send a message and then receive a message
from the partner only to find that the EPR on the partnerRole may have been
changed as a consequence of receiving the message! On the other hand the EPR
would only be changed because the underlying transport said “I have a more up
to date EPR”. So shouldn’t the change have happened? The only place things can
really explode is if at some later point the BPEL process really needs to send
a message to the original (now altered) EPR, which the code thinks is still in
the partnerLink, and instead ends up sending a message to the new EPR. But this
sounds to me like a flaw in the underlying binding layer. If a binding allows
an existing EPR to be overwritten it better know what the heck it’s doing.
So the only problem case would
be if a BPEL that was designed to assume it was running on something like
WS-Addressing was switched to something like XML over HTTP. In that case the
program’s key assumption, that the EPR of a partner will be sent any time a
message from that partner is received, will prove to be false and the program
will fail when it tries to send a message on what will turn out to be an
best guess is that a process will have deployment information that declares if
the process requires a transport that can set EPRs. I suppose we could stick in
some kind of flag but I’m not 100% convinced it’s necessary. To me the key
difference between Q1 and Q2 is that in Q1 the chaos results from a
characteristic of how the BPEL engine works – does it accept messages by
filtering on the partnerRole? That is something under the control of the BPEL
engine and therefore something we need to specify in the standard. But in Q2
the behavior is out of control of the BPEL engine itself, it is a consequence of
the underlying binding. That’s something that I think we can safely stay away
my best guess is that we should just make it clear in the spec that receiving a
message can cause a partnerRole to be initialized/updated and leave it at that.
Questions 3 & 4 on the other hand I think we
can let slide. BPEL explicitly forbids users from setting myRole EPR values. So
when a myRole is initialized or how it is used is completely out of the hands
of the programmer, so there doesn’t appear to be a portability issue.