Service Oriented Architecture Lecture 9 Notes from Web

Service Oriented Architecture Lecture 9 Notes from: Web Services & Contemporary SOA Ch 6, Erl XML Transactions for Web Services, Faheem Khan Distributed Systems Text, Tanenbaum Microsoft Article at http: //msdn 2. microsoft. com/en-gb/library/ms 996526. aspx 95 -843 Service Oriented Architecture Master of Information System Management

Today’s Topics • • WS-Coordination WS-Atomic Transaction Two Phase Commit Protocol WS-Business. Activity 2

WS-Coordination • • • An OASIS standard Developed by IBM, Microsoft & BEA Implemented in Web. Spehere V 6 Implemented in JBOSS Part of MS Vista and the Windows Communication Foundation (Indigo project) • Apache Foundation Kandula Project 3

Coordination • WS-Coordination includes two lower level standards: • WS-AT Web Service Atomic Transaction • WS-BA Web Service Business Activity 4

From IBM 5

From Apache’s Kandula Project 6

activation Service registration service Three services defined by WS-Coordination participant coordinator Potential participants 7

Create. Coordination. Context activation Service participant registration service participant coordinator Potential participants 8

activation Service participant Potential participants registration register service participant coordinator participant 9

activation Service participant Potential participants registration service A set of coordination protocol services for each supported coordination type. participant coordinator participant 10

activation Service participant This participant wants to engage others in an atomic transaction. registration service Each coordinator has a type: currently either WS-Atomic. Transaction or WS-Business. Activity coordinator participant 11

Call create. Coordination. Context activation Service 1 registration service 2 coordinator participant 1. 2. Request coordination context. A Coordination Context is an XML document containing: an activity identifier, the type of coordination, a registration endpoint, expiration time and application specific extensibility elements. 12

Coordination Context <soapenv> <soapbody> <wscoor: Coordination. Context <Identifier>. . . </Identifier> <Expires>. . . </Expires> <wscoor: Coordination. Type>. . . </wscoor: Coordination. Type> <wscord: Registration. Service> <Address/> </wscoord: Registration. Service> <!--extensibility element -> </wscoor: Coordination. Context> </soapbody> </soapenv> 13

Call register registration service activation Service 3 4 coordinator participant 3. 4. Register to play a role in a coordinated activity. The role depends upon what type of activity is going to take place and how the participating application is involved in that activity. The registration service will register the role 14 of the participant application in the activity.

activation Service registration service coordinator participant Other players get a copy of the context. 15

Players Invited To Play <soapenv> <soapheader> <wscoor: Coordination. Context <wsme: Identifier> http: //my. Coordination. Service/ts/activity 1 </wsme: Identifier> <wsme: Expires> 2002 -06 -30 T 13: 20: 00. 000 -05: 00 </wsme: Expires> <wscoor: Coordination. Type> http: //xml-soap. org/2002/06/Atomic. Transaction </wscoor: Coordination. Type> <wscoor: Registration. Service> <Address> http: //my. Registration. Service </Address> </wscoor: Registration. Service> </soapheader> <soapbody> <debit. Account. . . /> </soapbody> </soapenv> 16

activation Service registration service coordinator participant With a copy of the context, other players register. 17

From Microsoft article 18

1. Sending a Create. Coordination. Context message to the Activation service creates an activity. The optional Current. Context parameter is absent, so a new activity is created and the returned Coordination. Context has a new activity identifier and Registration service A. 2. The Coordination. Context is propagated from application services A to B as a SOAP header in an application message. This acts as an invitation to application service B to participate in the activity using one of the coordination protocols for that coordination type. The service that receives this invitation can either register to participate or not. 3. Service B registers using the Registration service A from the propagated context. 4. The coordination protocol instance can then begin between the participants. This Coordination protocol enables coordination. For example, this may be either the WSAtomic. Transaction 2 PC or WS-Business. Activity protocol. 19

WS-Coordination • The participant application has gained possession of an instance of the coordination context. • The participant application then propagates the coordination context instance to other applications that it would like to take part in the same activity. • Those applications also register themselves with the coordinator for the same activity. • The different participating applications may use the same coordinator or they may want to use their own trusted coordinators. In case different participating applications use their respective coordinators, the coordinators will talk to each other in order to provide coordination services. 20

From Microsoft article 21

In the above, after the import of the activity or the interposition of the trusted coordinator service B, Application B can deal with its own coordination services, which in turn deals with A's coordination services. 1. Create the activity and receive a Coordination. Context. 2. Propagate A's Coordination. Context to B in an application message. 3. B has a choice of whether to deal directly with A's coordination services, as in our first example, or use another set of coordination services as its representative. It decides to import the activity to B's coordination services by sending its Activation service the Create. Coordination. Context message with the context from A as the optional Current. Context parameter. The returned Coordination. Context has the same activity identifier, but has B's Registration service. 4. Register B with its own Registration service obtained from its Coordination. Context identifier. 5. B's coordination services delegate the registration to A's Registration service, which it obtained from the Current. Context parameter during import. This creates a new coordination protocol instance between A and B. 6. The coordination protocol instance can then begin between the participants A and B. 22

WS-Coordination • But what is the coordinated activity (the actual sequence of operations) that will take place? • WS-Coordination says nothing about the actual activity. It leaves it up to the participating applications to decide what they want to do with the coordination context. 23

activation Service registration service coordinator participant Suppose everyone is registered for an atomic transaction using 2 PC 24

Hypothetical Web Service Transaction Begin transaction Book. Trip book plane book hotel book rental car End transaction Book. Trip Notes adapted from Tanenbaum’s “Distributed Systems Principles and Paradigms” 25

activation Service registration service coordinator Book Trip WS Book Plane WS Book Car WS Book Hotel WS Suppose everyone is registered for an atomic transaction using 2 PC 26

Transactions (ACID) • Atomic: All or nothing. No intermediate states are visible. • Consistent: system invariants preserved, e. g. , if there were n dollars in a bank before a transfer transaction there will be n dollars in the bank after the transfer. • Isolated: Two transactions do not interfere with each other. They appear as serial executions. • Durable: The commit causes a permanent change. 27

Participant Talks to Coordinator Different servers Any server Coordinator Book. Plane Participant Recoverable objects needed to book a plane Book. Hotel Participant open. Trans Unique Transaction ID TID Book. Trip Client Recoverable objects needed to book a hotel. Book. Rental. Car Participant Recoverable objects needed to rent a car. TID = open. Transaction() 28

Client Calls Methods Different servers Any server Book. Trip Coordinator Book. Plane Participant Recoverable objects needed to book a plane Book. Hotel Participant Recoverable objects needed to book a hotel. Call + TID Book. Rental. Car Participant Book. Trip Client Recoverable objects needed to rent a car. plane. book. Flight(111, ”Seat 32 A”, TID) 29

The participant only calls join if it has not already done so. Plane Joins the Transaction Different servers Book. Plane Participant Coordinator join(TID, ref to participant) Recoverable objects needed to book a plane Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Rental. Car Participant Book. Trip Client The participant knows where the coordinator is because that information can be included in the TID (eg. an IP address. ) The coordinator now has a pointer to the 30 participant.

Suppose All Goes Well (1) Different servers Book. Plane Participant Coordinator Recoverable objects needed to book a plane Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Trip Client Book. Rental. Car Participant Recoverable objects needed to rent a car. OK returned 31

Suppose All Goes Well (2) Different servers Book. Plane Participant Recoverable objects needed to book a plane Coordinator begins 2 PC and this results in a GLOBAL COMMIT sent to each participant. Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Rental. Car Participant Book. Trip Client Recoverable objects needed to rent a car. OK returned Close. Transaction(TID) Called 32

This Time No Cars Available (1) Different servers Book. Plane Participant Recoverable objects needed to book a plane Coordinator Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Rental. Car Participant Book. Trip Client Recoverable objects needed to rent a car. OK returned NO CARS AVAIL abort. Transaction(TID) called 33

This Time No Cars Available (2) Different servers Book. Plane Participant Recoverable objects needed to book a plane Coordinator sends a GLOBAL_ABORT to all particpants Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Rental. Car Participant Book. Trip Client Recoverable objects needed to rent a car. OK returned NO CARS AVAIL abort. Transaction(TID) called 34

This Time No Cars Available (3) Different servers Book. Plane Participant ROLLBACK CHANGES Coordinator abort. Transaction Each participant Gets a GLOBAL_ABORT Book. Trip Client Book. Hotel Participant ROLLBACK CHANGES Book. Rental. Car Participant ROLLBACK CHANGES OK returned NO CARS AVAIL abort. Transaction(TID) 35

Book. Plane Server Crashes After Returning ‘OK’ (1) Different servers Book. Plane Participant Coordinator Recoverable objects needed to book a plane Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Trip Client Book. Rental. Car Participant Recoverable objects needed to rent a car. OK returned 36

Book. Plane Server Crashes After Returning ‘OK’ (2) Different servers Book. Plane Participant Recoverable objects needed to book a plane Coordinator excutes 2 PC: Ask everyone to vote. No news from the Book. Plane Participant so multicast a GLOBAL ABORT Book. Hotel Participant Recoverable objects needed to book a hotel. Book. Rental. Car Participant Book. Trip Client Recoverable objects needed to rent a car. OK returned Close. Transaction(TID) Called 37

Book. Plane Server Crashes After Returning ‘OK’ (3) Different servers Book. Plane Participant Recoverable objects needed to book a plane Coordinator GLOBAl ABORT Book. Hotel Participant ROLLBACK Book. Rental. Car Participant Book. Trip Client ROLLBACK OK returned ROLLBACK Close. Transaction(TID) Called 38

Two-Phase Commit Protocol Phase 1 Book. Plane Vote_Request Vote_Commit Coordinator Vote Request Vote Commit Vote Request Phase 1 Coordinator sends a Vote_Request to each Vote Commit process. Each process returns a Vote_Commit or Vote_Abort. Book. Hotel Book. Rental. Car 39

Two-Phase Commit Protocol Phase 2 Book. Plane Global Commit Coordinator ACK Book. Hotel Global Commit ACK Global Commit Book. Rental. Car Phase 2 Coordinator ACK checks the votes. If every process votes to commit then so will the coordinator. In that case, it will send a Global_Commit to each process. If any process votes to abort the coordinator sends a GLOBAL_ABORT. Each process waits for a Global_Commit message before committing its part 40 of the transaction.

2 PC Finite State Machine From Tanenbaum Coordinator Participant State has already been saved to permanent storage. Init Vote-request ----------------Vote-commit Vote-abort Init Commit -----Vote-request Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 41

2 PC Blocks in. If waiting Three Places too long for a Vote-Request send a Vote-Abort Init Vote-request --------Vote-commit Vote-request --------Vote-abort Commit -----Vote-request Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 42

2 PC Blocks in Three Places Init Vote-request If waiting too long --------After Vote-request Vote-commit Send a Global-Abort Vote-request Ready --------wait Vote-abort Vote-commit Vote-abort Global-commit -----------------------Global-abort Global-commit ACK Global-abort --------Commit Abort ACK Commit Abort Commit -----Vote-request 43

2 PC Blocks inwe. Three Places If waiting too long can’t simply abort! We must wait until the coordinator recovers. We might also make queries on other participants. Init Commit -----Vote-request --------Vote-commit Vote-request --------Vote-abort Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 44

2 PC Blocks in Three If this process learns that another has. Places committed then this process is free to commit. The coordinator must have sent out a Global-commit that did not get to this process. Init Commit -----Vote-request --------Vote-commit Vote-request --------Vote-abort Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 45

2 PC Blocks in Three If this process learns that another has. Places aborted then it too is free to abort. Init Commit -----Vote-request --------Vote-commit Vote-request --------Vote-abort Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 46

2 PC Blocks in Three Places Suppose this process learns that another Init Commit -----Vote-request Vote-abort -------Global-abort Abort wait process is still in its init state. The coordinator must have crashed while multicasting the Vote-request. It’s safe for this process (and the queried process) to abort. Init Vote-request ----------------Vote-commit Vote-abort Ready Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 47

2 PC Blocks inqueried Three Tricky case: If the processes Places are all still in their ready state what do we know? We have to block and wait until the Coordinator recovers. Init Commit -----Vote-request --------Vote-commit Vote-request --------Vote-abort Ready Vote-abort -------Global-abort Abort wait Vote-commit --------Global-commit Global-abort --------ACK Commit Abort Global-commit ---------ACK Commit 48

Strong Division of Function • With atomic transactions there is a strong division of function between the business activity and coordination of the transaction. • The applications decides who to involve in the transaction and whether to commit or abort. • After this, coordination takes over and decides the outcome. 49

Mutual Trust Is Required • Any system can abort the entire transaction. Systems must be trusted to have cooperative intentions. • Systems must trust each other to be responsive. 50

Business Activity Differs from Atomic Transactions • Atomic transactions not always appropriate. • May have longer duration (minutes, days, weeks) • Locks should not be held for long periods. • Changes become visible. Say, a process sends an email. We can’t rollback. • Application logic is involved in the coordination. • Compensation used if parts of a process can’t 51 complete.