Lecture 5: RMI etc.
Java Remote Method Invocation
Invocation Semantics
Distributed Events
CDK: Chapter 5
TVS: Section 8.3
CDK Figure 5.7
The role of proxy and skeleton in remote method invocation
client
object A proxy for B
Request
server
remote
skeleton
object B
& dispatcher
for B s class
’
Reply
Communication
Remote
reference module
module
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servant
Communication Remote reference
module
module
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Servant
• An instance of a class which is referenced
remotely
• It exists within the server, which created it
and the remote reference to it
• It does the “real work” of the method
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1
The Dispatcher
• Server has a dispatcher & a skeleton for
each class of remote object
• The dispatcher receives the incoming
message, and uses its method info to pass it
to the right method in the skeleton.
(Dispatcher and proxy both work from the
remote interface to get same method info!)
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Skeleton
• Skeleton implements methods of the remote
interface to unmarshall arguments and
invoke the corresponding method in the
servant. It then marshalls the result (or any
exceptions thrown) into a reply message to
the proxy.
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Remote Reference Module
• Responsible for translating between remote
references and local ones
• Has a remote object table, recording:
– An entry for each remote object local to that
process (each process has its own table!)
– An entry for each local proxy
• Kept up-to-date appropriately
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2
Proxy
• There is one proxy per remote object a
process can reference
• The class of the proxy contains a method
for each method in the remote interface
which marshalls arguments and
unmarshalls results for that method.
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Garbage Collection of Remote
Objects
• Once references to objects can be remote,
Garbage Collection is more difficult
• Garbage Collection is done locally to the
process – but remote objects are not removed
• Each server keeps a set of clients with nonlocal references
• Each client tells server when removing proxy
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Generating classes for proxies,
dispatchers and skeletons
• Before JDK1.5, there was a separate tool to
use to generate these: rmic
• With JDK1.5 (and later) you don’t need to
do this
• It is done using “reflection”
• That can also simplify other things ….
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3
Reflection in Java
• Reflection allows a Java program to inspect
and manipulate itself
• It can get an object representing a class
whose name is a String
• It can then find out about this class’s
constructors, and methods, and their
arguments
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Reflection continued
• It can then invoke these – to construct objects
and call their methods
• E.g. Class c = Class.forname(“MyExClass”);
Constructors [] cc =
c.getDeclaredConstructors() ;
etc.
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rmiregistry
• This is how a server makes a remote object
available to clients
• A string is bound to the remote object, and
the clients interrogate the registry using the
string
• The client must know the server machine
name and the port the registry is on (there is
a default)
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4
Invocation Semantics
• This applies to both RPC and RMI
• For both normal method invocation and
normal procedure call, we have “exactly
once” semantics
• So what can we say in a distributed system
where machines can crash and messages
can be lost?
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Failures
•
•
•
•
Request could be lost
Reply could be lost
Server could fail
Client could fail
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Request Behaviour
• What does client do if it makes a request,
but gets no reply (after long enough)?
– It could just fail (not usual behaviour)
– It could resend the request
– If after several resends, still no reply -> failure
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5
Reply Behaviour
• What should the server do on getting a
request?
• If it doesn’t know it is a duplicate, it just
processes it (perhaps again)
• It could “filter” duplicates if it remembers
requests, and then:
– Discard duplicate if it hasn’t yet replied
– It could retransmit the reply (if it remembers it)
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CDK Figure 5.6
Invocation semantics
Fault tolerance measures
Retransmit request
message
Duplicate
filtering
Invocation
semantics
Re-execute procedure
or retransmit reply
No
Not applicable
Not applicable
Yes
No
Re-execute procedure
At-least-once
Yes
Yes
Retransmit reply
At-most-once
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Maybe
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Maybe semantics
• The invocation may or may not happen
• It is less effort – but not generally
satisfactory
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6
At-least-once semantics
• Unless the server crashes, this means that
the invocation definitely happens
• But it might happen more than once!
• OK for idempotent operations, ones which
– Can be performed >1 time with no different
effect to that when performed once
• Sun RPC does this
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At-most-once semantics
• Requires extra effort
• Operation won’t be done more than once!
• Java RMI provides this
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Distributed Event-based Systems
• You can build a distributed application
using RPC or RMI in a “normal” way
• Distributed event-based systems show
another possibility
• Events are notified to those objects which
subscribe to (i.e. register interest in) an
object
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7
Events are Published
• There is a way in which objects which are
prepared to notify events to subscribers can
make these known
• Notifications are sent asynchronously to all
subscribers
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Dealing room system
External
source
Dealer’s computer
Dealer
Dealer’s computer
Notification
Notification
Notification
Information
provider Notification
Notification
Dealer
Notification
Notification
Dealer’s computer
Dealer’s computer
Notification
Information
provider
Notification
Notification
Dealer
Dealer
External
source
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Next Lecture
I will talk about the 2nd lab exercise
This starts next week, and will occupy 3
sessions!
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