1. Reliable Distributed Systems
Web Services

2. Today Web Services – Introduction “Remote Procedure Call” in WS
Binding, Marshalling…
Using TCP as the transport for RPCs
Connectivity Issues: NAT, Firewall

3. What are Web Services?
Today, we normally use Web browsers to talk to Web sites
Browser names document via URL (lots of fun and games can happen here)
Request and reply encoded in HTML, using HTTP to issue request to the site
Web Services generalize this model so that computers can talk to computers

4. What are Web Services? Web Service Client System SOAP Router
Backend Processes
Web Service

5. What are Web Services? Web Service
“Web Services are software components described via WSDL which are capable of being accessed via standard network protocols such as SOAP over HTTP.”
SOAP Router
Backend Processes
Web Service

6. What are Web Services? Web Service
“Web Services are software components described via WSDL which are capable of being accessed via standard network protocols such as SOAP over HTTP.”
SOAP Router
Backend Processes
Today, SOAP is the primary standard. SOAP provides rules for encoding the request and its arguments.
Web Service

7. What are Web Services? Web Service
“Web Services are software components described via WSDL which are capable of being accessed via standard network protocols such as SOAP over HTTP.”
SOAP Router
Backend Processes
Similarly, the architecture doesn’t assume that all access will employ HTTP over TCP. In fact, .NET uses Web Services “internally” even on a single machine. But in that case, communication is over COM
Web Service

8. What are Web Services? Web Service +
“Web Services are software components described via WSDL which are capable of being accessed via standard network protocols such as SOAP over HTTP.”
SOAP Router
WSDL documents are used to drive object assembly, code generation, and other development tools.
Backend Processes
WSDL document +
Web Service

9. Web Services are often Front Ends
Web Server
(e.g., IBM WebSphere, BEA WebLogic)
DB2 server
SAP
WSDL-described Web Service
Web App Server
Web Service invoker
COM App
CORBA App
C# App
Server Platform
Client Platform
SOAP messaging

10. The Web Services “stack”
BPEL4WS (IBM only, for now)
Business Processes
Reliable Messaging
Security
Transactions
Quality of Service
Coordination
WSDL, UDDI, Inspection
Description
SOAP
Other Protocols
Messaging
XML, Encoding
TCP/IP or other network transport protocols
Transport

11. What are Web Services? Web Service serverlet
Amazon would hand out “serverlets” for 3rd party developers to use
This connects their applications directly to Amazon’s system
SOAP Router
serverlet
Backend Processes
Web Service

12. Advantages of web services?*
Web services provide interoperability between various software applications running on various platforms.
“vendor, platform, and language agnostic”
Web services leverage open standards and protocols. Protocols and data formats are text based where possible
Easy for developers to understand what is going on.
By piggybacking on HTTP, web services can work through many common firewall security measures without requiring changes to their filtering rules.
also be a disadvantage – security issues.
*: From Wikipedia

13. How Web Services work First the client discovers the service.
More in next lecture!
Typically, client then binds to the server.
By setting up TCP connection to the discovered address .
But binding not always needed.
OK to say client/server here?

14. How it works… Next build the SOAP request: (Marshaling)
Fill in what service is needed, and the arguments. Send it to server side.
XML is the standard for encoding the data (but is very verbose and results in HUGE overheads)
SOAP router routes the request to the appropriate server(assuming more than one available server)
Can do load balancing here.
Are these all request-response? OR is it possible to say something like setVal(X, 100) also?

15. How it works…
Server unpacks the request, (Demarshaling) handles it, computes result.
Result sent back in the reverse direction: from the server to the SOAP router back to the client.

16. Marshalling Issues
Data exchanged between client and server needs to be in a platform independent format.
“Endian”ness differ between machines.
Data alignment issue (16/32/64 bits)
Multiple floating point representations.
Pointers
(Have to support legacy systems too)

17. Discovery This is the problem of finding the “right” service
In our example, we saw one way to do it – with a URL
Web Services community favors what they call a URN: Uniform Resource Name
But the more general approach is to use an intermediary: a discovery service

18. Example of a repository
Name
Type
Publisher
Toolkit
Language OS
Web Services Performance and Load Tester
Application
LisaWu
N/A
Cross-Platform
Temperature Service Client
vinuk
Glue
Java
Weather Buddy
rdmgh724890
MS .NET C#
Windows
DreamFactory Client
billappleton
DreamFactory
Javascript
Temperature Perl Client
Example Source
gfinke13
Perl
Apache SOAP sample source
xmethods.net
Apache SOAP
ASS 4
TVG
SOAPLite
PocketSOAP demo
simonfell
PocketSOAP
C++
easysoap temperature
a00
EasySoap++
Weather Service Client with MS- Visual Basic
oglimmer
MS SOAP
Visual Basic
TemperatureClient
jgalyan

19. Repository summary A database listing servers
Each is described using the UDDI language, which is defined over XML
Hence can be searched with XML queries
An extensible standard
Defines some required information about interfaces available and argument types, etc
But services can provide extra information too.

20. Roles?
UDDI is used to write down the information that became a “row” in the repository (“I have a temperature service…”)
WSDL documents the interfaces and data types used by the service
But this isn’t the whole story…

21. Discovery and naming The topic raises some tough questions
Many settings, like the big data centers run by large corporations, have rather standard structure. Can we automate discovery?
How to debug if applications might sometimes bind to the wrong service?
Delegation and migration are very tricky
Should a system automatically launch services on demand?

22. Example: Why discovery is tricky
Client has opinions
“I want current map data for Disneyland showing line-lengths for the rides right now”
Service has opinions
Amazon.com would like requests from Ithaca to go to the NJ-3 datacenter, and if possible, to the same server instance within each clustered service
DNS has opinions
Many systems play with name -> IP bindings
Internet has opinions (routing)

23. So, what’s tricky?
Web Services doesn’t standardize these four steps, it just assumes that people will hack solutions
Hence some are hard to implement, we lack standards, and in some cases, solutions are poor ones
UDDI and WSDL are just a corner of the overall picture!

24. Network address translation…
Another issue: Often, the internal address is not addressable from outside!
A tiny bit of security.
But if RPC server is behind a NAT, trouble!
NAT needs the host behind it to start the connection process.
Need to configure NAT to let specified traffic through.
Generally: (WS traffic)HTTP is let through.
Tough to have a connection in between two hosts behind NATs.
There are some tricks to bypass this though.

25. Firewalls
These allow/disallow traffic, depending on source, destination, protocol used, etc.
Often only allow connection from the inside to the outside!
Stateful: remember active flows, and disallow unexpected packets (NAT)
Again, need to configure to ensure server traffic gets through. (General RPC)
Again, (WS)HTTP does not face as much of a restriction.
Get traffic statistics.
Spam/virus checking, etc.
NAT and firewall typically in the same box.

26. Demilitarized Zone (DMZ)
DMZ: used to host publicly accessible services like company webpages, ftp, dns.
Good place to host the Web Service!
DMZ situated outside the private network.
No outgoing connections from DMZ.
If DMZ attacked, damage limited to DMZ.

27. Client talks to eStuff.com
Moving on… let’s oversimplify and just assume the client manages to find the data center
We think of remote method invocation and Web Services as a simple chain:
Client system
SOAP router
Web Service
Web Service
Web Services
Soap RPC

28. So… suppose we get in
Assuming we can connect to the data center (to its Web Services router), then what?
If you just use Visual Studio out of the box, you end up with a single-machine Web Server
But massive datacenters are common!

29. A glimpse inside eStuff.com
“front-end applications”
Pub-sub combined with point-to-point communication technologies like TCP LB
service LB
service LB
service LB
service LB
service LB
service

30. Clusters and load balancing
Idea here is that some form of load balancer spreads work over a cluster
And cluster replicates data for availability and load management
How it does this is a topic we need to discuss in more detail (not today)

31. What about “legacy” applications?
Some of these Web services are really just front-ends to older legacy applications
So to talk to an old IBM database, we might
Run the database on some sort of machine, or virtual machine
Build one of these translator front-ends
And then register it with the Web Services router
This may sound expensive (it is) but it works!
Obviously, our fancy clustering and load-balancing won’t apply to a legacy application, so those fancy tricks are only for “new” code

32. Discovery in eStuff.com
Data centers are increasingly common
And they raise hard questions!
How can a data center in California control decisions a client is making in Ithaca?
Services are clustered. How should client request be “routed” to the right member
Once you start talking to a server it may cache data for you. How can you be sure to get the right one next time?

33. These are modern challenges
Web Services can be seen as evolving from prior work
Most often cited: CORBA, which also was used in many big data centers
But CORBA didn’t assume that clients came in over the public Internet
More often, CORBA was used between a hand-built client and the service it talks to

34. CORBA approach CORBA had what are called
Ways to export specialized client stubs
The client stub could include server provided decision logic, like “which data center to connect with”
Gives data center a form of remote control
Factory services: manufacture certain kinds of objects as needed
Effect was that “discovery” can also be a “service creation” activity

35. CORBA is object oriented
Seems obvious… and it is. CORBA is centered around the notion of an object
Objects can be passive (data)
… active (programs)
… persistent (data that gets saved)
… volatile (state only while running)
In CORBA the application that manages the object is inseparable from the object
And the stub on the client side is part of the application
The request per-se is an action by the object on itself and could even exploit various special protocols
We can’t do this in Web Services

36. Web Services are document-centric
That is, communication is by sending documents (like pages) from client to server and back
And most guarantees or properties are associated with the document itself, not the service
For example, WS_RELIABILITY isn’t about making services reliable, it defines rules for writing reliability requests down and attaching them to documents
In contrast, CORBA fault-tolerance standard tells how to make a CORBA service into a highly available clustered service

37. Will Web Services “help” with naming and discovery?
Web Services tells us how
One client can…
… find one server and
… bind to that server and
… send a request that will make sense
… and make sense of the response
So sure, WS will help

38. But Web Services won’t…
Allow the data center to control decisions the client makes
Assist us in implementing naming and discovery in scalable cluster-style services
How to load balance? How to replicate data? What precisely happens if a node crashes or one is launched while the service is up?
Help with dynamics. For example, best server for a given client can be a function of load but also affinity, recent tasks, etc

39. How we do it now Client queries directory to find the service
Server has several options:
Web pages with dynamically created URLs
Server can point to different places, by changing host names
Content hosting companies remap URLs on the fly. E.g. (reroutes requests for to Akamai)
Server can control mapping from host to IP addr.
Must use short-lived DNS records; overheads are very high!
Can also intercept incoming requests and redirect on the fly

40. Why this isn’t good enough
The mechanisms aren’t standard and are hard to implement
Akamai, for example, does content hosting using all sorts of proprietary tricks
And they are costly
The DNS control mechanisms force DNS cache misses and hence many requests do RPC to the data center
We lack a standard, well supported, solution!

41. Coming up?
How content is managed in even larger systems, that have multiple data centers
The main example is Akamai…