Cesare Pautasso discusses composing RESTful web services using business process modeling languages. There are three main solutions proposed: 1) using an abstract workflow that is independent of invocation technology, 2) making REST interactions explicit constructs in workflow languages, and 3) modeling workflows as resources that are accessed via RESTful interfaces. Pautasso argues that workflow engines should support composing both RESTful and WS-* web services, and that representing workflows as resources accessed via standard REST operations is a promising approach.

1. Composing REST Services
Open Dagstuhl Session
Cesare Pautasso
Faculty of Informatics
University of Lugano (USI), Switzerland
http://www.pautasso.info
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 1

2. Web Service Composition Today
The WS-BPEL process
WS-BPEL 2.0
model is layered on top of
the service model defined WSDL 1.1
by WSDL 1.1. […]
Both the process and its
partners are exposed as
WSDL services
[BPEL 2.0 Standard, Section 3]
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 2

3. RESTful Web Services APIs…
WSDL 1.1
…do not use
WSDL 1.1
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 3

4. The Goal
Compose RESTful Web Services
Compose WSDL Web Services
Use Business Process Modeling Languages
One Solution: BPEL for REST
Extend BPEL to support RESTful Web Services
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 4

5. The Goal
Compose RESTful Web Services
Compose WSDL Web Services
Use Business Process Modeling Languages
Another Solution: JOpera
Visual Flow Language with Abstract Service Model
Extensible Autonomic Engine Architecture
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 5

6. REST in one slide PUT
Web Services expose their
data and functionality trough GET
R
resources identified by URI POST
DELETE
Uniform Interface Principle: Clients interact with
the state of resources through 4 verbs:
GET (read), POST (create), PUT (update), DELETE
Multiple representations for the same resource
Hyperlinks model resource relationships and
valid state transitions
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 6

7. Challenges for Composition Languages
Resource addressing through URI
How to interact with dynamic, variable set of URI?
Uniform Interface (GET, POST, PUT, DELETE)
Does it help to make the verbs explicit in the workflow?
Multiple resource representations
How to negotiate the most appropriate representation?
Hyperlinks
Can the workflow implement state transition logic of a
resource and generate new URIs dynamically as
processes run to guide the clients invoking them?
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 7

8. JOpera Example: Doodle Map Mashup
Setup a Doodle with Yahoo! Local search and
visualize the results of the poll on Google Maps
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 8

9. Doodle Map Mashup Architecture
Web Browser Workflow RESTful
Engine Web Services
RESTful API APIs
GET
POST
GET
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 9

10. 6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 10

11. BPM RESTful
Workflow Web Service
Languages Composition
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 11

12. Solution Space BPM
Workflow
RESTful
Web Service
Languages Composition
1. Abstract Workflow
Service invocation technology does not matter
2. Concrete Workflow
BPM
Expose service invocation
RESTful
Workflow Web Service
LanguagesComposition
technologies as explicit constructs
in the workflow language
3. RESTful Workflow
BPM
Workflow as one kind of resource
RESTful
Workflow
Web Service
Languages
Composition
exposed by a RESTful service
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 12

13. 1. Abstract Workflow Example: BPEL/WSDL
WSDL 2.0 HTTP Binding can wrap RESTful Web Services
(WS-BPEL 2.0 does not support WSDL 2.0)
BPEL WSDL 2.0 PUT
Operations
...
Op_1
<Invoke Op_1> Op_2
Op_3 GET
R
<Invoke Op_2> Op_4
... HTTP Binding POST
DELETE
<Invoke Op_3> Op URI Method
Op_1 R PUT
PUT
<Invoke Op_4> Op_2 R GET
GET
Op_3 R POST
POST
... Op_4 R DELETE
DELETE
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 13

14. 2. Concrete Workflow: BPEL for REST
Idea: Make REST interaction primitives first-class language constructs
BPEL for REST PUT
...
<Put R>
GET
R
<Get R>
... POST
DELETE
R>
<Post R>
<Delete R>
<Delete R>
...
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 14

15. BPEL for REST – New Activities/Handlers
PUT
<put> <onPut>
<get>
GET
R esource <onGet>
<post> <onPost>
POST
<delete> DELETE <onDelete>
Web <receive>
<invoke>
Service <reply>
<invoke> Web
<receive>
Service
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 15

16. BPEL for REST – Resource Block
Dynamically publish resources from BPEL
processes and handle client requests
BPEL for REST PUT
<Resource P>
PUT
<onGet>
<Put R> GET
R
<Get S> POST
GET
P </onGet>
DELETE
<onDelete> PUT
POST
DELETE <Post R>
<Delete S>
GET
S
</onDelete>
</Resource> POST
DELETE
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 16

17. 3. RESTful Workflows
PUT P
GET
R
POST
DELETE
Use the resource interface
abstraction to publish
the state of the workflow
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 17

18. Workflows as Resources – URI
Publish workflows as resources identified by the URIs:
/package/process
/package/process/version
/package/process/version/instance
/package/process/version/instance/task
/package/process/version/instance/task/parameter
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 18

19. Reading the state of the workflow resources
GET /package/process
Enumerate deployed process versions
GET /package/process/version
Enumerate active instances of a given process version
GET /package/process/version/instance
Read the current state of a workflow instance
GET /package/process/version/instance/task
Read the current state of a workflow instance task
GET /package/process/version/instance/task/param
Read the current value of a workflow instance parameter
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 19

20. Creating new workflow resources
POST /package
Deploy new process template into package
POST /package/process
Deploy new version of a process
POST /package/process/version
Create new process instance
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 20

21. Updating the workflow resource
PUT /package/process/version/instance/task
Update the state of a workflow task (e.g., finished, failed)
PUT /package/process/version/instance/task/param
Write into task parameters some values
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 21

22. Deleting workflow resources
DELETE /package/process
Undeploy all versions of a process
(and all the corresponding process instances)
DELETE /package/process/version
Undeploy a version of a process (and all of its instances)
DELETE /package/process/version/instance
Remove the state of a specific process instance only
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 22

23. Discussion
Why should a workflow engine care about REST?
Use workflows to compose RESTful Web services
Implement RESTful services with a workflow
Should a process explicitly include RESTful activities?
Or it is better/enough to model REST implicitly?
How much of the state of a process instance should
be exposed as a resource?
How to control which “parts” are visible?
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 23

24. Conclusion
Business Process Modeling Languages have been
applied with success to compose “traditional” WS-*
Web Services (BPM = SOA + BPEL)
Business Process Modeling Languages should also be
applied to compose RESTful Web Services
BPEL for REST is a lightweight BPEL extension for
REST and WS-* service composition
JOpera for Eclipse is a visual process modeling tool
with an extensible engine for composing both kinds of
services (and many more)
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 24

25. Some References
R. Fielding, Architectural Styles and the Design of Network-
based Software Architectures, PhD Thesis,
University of California, Irvine, 2000
C. Pautasso, O. Zimmermann, F. Leymann, RESTful Web
Services vs. Big Web Services: Making the Right
Architectural Decision, Proc. of the 17th International World
Wide Web Conference (WWW2008), Bejing, China, April 2008
C. Pautasso, BPEL for REST, Proc. of the 7th International
Conference on Business Process Management
(BPM 2008), Milano, Italy, September 2008
Xiwei (Sherry) Xu, Liming Zhu, Yan Liu, Mark Staples, Resource-
Oriented Architecture for Business Processes, APSEC’08
6 January 2009 ©2009 Cesare Pautasso | www.pautasso.info 25

26. PhD positions available!
Prof. Cesare Pautasso
University of Lugano, Switzerland
c.pautasso@ieee.org
http://www.pautasso.info
26
©2008 Cesare Pautasso