Should%20Mirror%20Operations%20Be%20Dropped?

1
Should Mirror Operations Be Dropped?

• David BoothW3C Fellow / Hewlett-Packard

2
Current Status

• Four Message Exchange Patterns (MEPs)
• Input-Output (was "Request-Response")
• Input-Only (was "One-Way")
• Output-Input (was "Solicit-Response")
• Output-Only (was "Notification")
• "Mirror ops" Output-Input, Output-Only

3
Problems with Mirror Ops

• Multiple interpretations event? callback?
• Little evidence of use
• Where to get the Client's address?
• Inconsistent treatment of Faults?
• Input-Output Fault is an alternate Output
• Input-Only  (no Faults)
• Output-Input Fault is an alternate Input
• Output-Only (no Faults)

4
What I Did

• Abstract analysis
• Suppose we used WS descriptions in a larger
  context.  Would we want mirror ops?
• Example Markets

5
Potential Application Markets
Client A1
Service B1
Service A2
Service B2
Service A3
Service B3

• Multiple Clients, Multiple Services
• Any Client can talk to any Service (of the same
  type)

6
Markets
Client A1
Service B1
Service A2
Service B2
Service A3
Service B3

• Ways to match Client and Service
• Client and Service share same WSDL
• Client and Service have complementary WSDLs

7
Shared Service Descriptions
Client A1
Service B1
T
Service A2
Service B2
Service A3
Service B3

• Role must be separately indicated
• Client "I'm a T Client"
• Service "I'm a T Service"
• Binding information is lopsided (Service-centric)
• Binding has Service-specific info (address)
• Where is Client-specific info placed?

8
Shared One WSDL per Service
Client A1
Service B1
Service A2
Service B2
Service A3
Service B3

• T1, T2, T3 could be specific to B1, B2, B3
• T1 has B1's address, T2 has B2's address, etc.
• B1 "I'm a T1 Service"
• B2 "I'm a T2 Service", etc.
• Each Client could reference all T1, T2,
  T3"I'm a T1Client, a T2 Client and a T3 Client"

9
Shared Referencing a Common T
Client A1
Service B1
T
Service A2
Service B2
Service A3
Service B3

• T1, T2, T3 could reference generic T
• T1 has B1's address, T2 has B2's address, etc.
• B1 "I'm a T1"
• T is Service-centric, but not identity-centric
  (I.e., no address)
• Client could reference generic T
• "I'm a T Client"

10
Shared Client, Service Ref T
Client A1
Service B1
T
Service A2
Service B2
Service A3
Service B3

• TA1, TA2, TA3, TB1, TB2, TB3 are all
  identity-specific
• TA1 "A1 is a T Client"
• TB1 "B1 is a T Service"
• T does not contain address

11
Shared Role-Specific Descriptions
T

• TC and TS are role-specific, but not
  identity-specific
• TC "I am a T Client"
• TS "I am a T Service"
• T does not contain address or role info

12
Shared Conclusion
Client A1
Service B1
T
Service A2
Service B2
Service A3
Service B3

• Sharing requires mirror ops only if you think
  they're important
• Need Output-Input?
• Need Output-Only?

13
Complementary Service Descriptions
T
T
Service B1
Client A1
Service B2
Service A2
Service B3
Service A3

• Symmetric ("Peer-to-Peer")
• T describes Service T describes Client
• T, T indicate
• Generic info (T)
• Role-specific info (Client vs. Service)
• Identity-specific info (address)
• Requires (complementary) equivalence to match

14
Complementary Observation
T
T
Service B1
Client A1
Service B2
Service A2
Service B3
Service A3

• Complementary approach requires mirror ops
• Inputs of T are Outputs of T
• Outputs of T are Inputs of T

15
Complementary Identity-Specific Info
Client A1
Service B1
T
T
Service A2
Service B2
Service A3
Service B3

• TA1, TA2, TA3, TB1, TB2, TB3 are all
  identity-specific
• TA1 "A1 is a T"
• TB1 "B1 is a T"
• T, T do not contain addresses

16
Conclusions

• Mirror ops add flexibility
• Identity-specific info (address) should be
  separated from shared info
• Other binding info can be sharedtransport
  protocol, etc.

17
END
18
WSDL Information Sharing

• From most shared to least shared
• Message types
• Message direction (input/output)
• Transport protocol, Message encoding
• Address (Not shared!)
• The least shared info should be latest bound

19
Observations on MEPs
20
MEPs
B's View
A's View
1
2
3
4

• Sequence
• A sends W to B
• B sends X to A
• A sends Y to B
• B sends Z to A

21
MEP B's View
B's View
A's View
PWXYZ
1
2
3
4

• One big MEP
• PWXYZ Receive W, send X, receive Y, send Z

22
MEP B's View
B's View
A's View
PWX
1
2
PYZ
3
4

• Two "Request-Response" MEPs
• PWX Receive W, send X
• PYZ Receive Y, send Z

23
MEP B's View
B's View
A's View
PWX
1
2
PYZ
3
4

• Q Should B care how A models its interactions
  with B?
• A Of course not.

24
MEP A's View 1
B's View
A's View
PWX
1
2
PYZ
3
4

• Two "Solicit-Response" MEPs
• PWX Send W, receive X
• PYZ Send Y, receive Z

25
MEP A's View 2
B's View
A's View
PW
1
2
PXY
3
4
PZ

• Three MEPs
• PW Send W ("Notification")
• PXY Receive X, send Y ("Request-Response")
• PZ Receive Z ("One-way")

26
MEP A's View 3
B's View
A's View
PW
1
2
PX
PY
3
4
PZ

• Four MEPs
• PW Send W ("Notification")
• PX Receive X ("One-way")
• PY Send Y ("Notification")
• PZ Receive Z ("One-way")

27
MEP A's View 4
B's View
A's View
PWZ
1
2
PXY
3
4

• Two MEPs
• PWX Send W, receive X ("Solicit-Response")
• PYZ Send Y, receive Z ("Request-Response")

28
MEPs Are Relative
B's View
A's View
PWZ
PWX
1
2
PXY
PYZ
3
4

• Observation
• MEPs are role-specific

29
MEPs Are Relative
B's View
A's View
PWX
PWZ
1
2
PXY
PYZ
3
4

• A makes PWZ from half of PWX and half of PYZ

30
MEPs Are Relative
B's View
A's View
PWZ
PWX
1
2
PXY
PYZ
3
4

• A makes PXY from half of PWX and half of PYZ

31
Role-Specific Information
32
Role-Specific Information
X
ltingt
Service B
Service A
Receiver
Sender
T

• Suppose
• A must send B messages in format X
• X represents message format definition
• A, B reference X
• X contains role-specific information, e.g.,
  "ltingt" (from B's perspective)
• A, B use the same software T to generate Sender
  and Receiver from X

33
Role-Specific Information
X
ltingt
Service B
Service A
Receiver
Sender
T

• Observation
• Q How does T know whether to generate Sender or
  Receiver from X?
• A T must have other information
• Therefore "ltingt" is unnecessary in X

34
Role-Specific Information
X
ltingt
Service B
Service A
Receiver
Sender
T

• Conclusion Information that is shared between
  different roles should not contain role-specific
  information
• Examples of role-specific information
• "ltinputgt", "ltoutputgt", "one-way", address