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Extension Patterns
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The following sequence diagram shows how a client uses an INVOCATION INTERCEPTOR to add a security token to each invocation. The security token will be used within the server application to authenticate the client.
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create
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registerInterceptor(sci)
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Client
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sci:Security Client Interceptor
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Requestor
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i:Invocation Data
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c: Invocation Context
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create
beforeInvocation(i,c)
add("securityToken", aToken)
send(i,c)
As the diagram shows, the client registers a security interceptor with the framework facade, a Facade [GHJV95] object that provides access to the distributed object middleware infrastructure on start-up. When the client invokes an operation, the REQUESTOR uses the registered INVOCATION INTERCEPTORS to add the security token to the INVOCATION CONTEXT transparently. The invocation itself is passed to the interceptors as an invocation data object. Potentially, the interceptors can change the information in this object and thereby manipulate the invocation. The same happens on the server side. The respective parts in the server application are shown next. An INVOCATION INTERCEPTOR is used to access the security token in the INVOCATION CONTEXT. The token is used to authenticate the client, and the operation is invoked only if access can be granted. If the token is invalid, a security exception is raised and a REMOTING ERROR is sent back to the client. In the following sequence diagram, we assume that the interceptor has been registered by the server application upon start-up, using the server-side framework facade. Here, the interceptor decides
24.09.2004 21:08 c06.fm 0.3
Interactions among the patterns
whether the invocation can be performed or, if the security token is not valid, the invocation is rejected using a security exception.
Server Message Handler
i:Invocation Data
c: Invocation Context
Invoker
Interceptor Registry
ssi:Security Server Interceptor
Remote Object
create
create
invoke(i,c)
getInterceptors()
beforeInvocation(i,c)
get("securityToken") opt [token is ok]
anOperation()
opt [token is not ok]
throw ( SecurityException ) report Exception back to client
Session handling is another interesting feature that can be implemented easily using the patterns introduced in this chapter. As explained more deeply in 13, Related Concepts, Technologies, and Patterns, sessions can be used to provide state to otherwise stateless remote objects. Using a client-specific session object, instead of making all the remote objects stateful, CLIENT-DEPENDENT INSTANCES makes scalability, failover, and load-balancing easier to achieve in most cases. The example in the next diagram uses non-persistent sessions. These are only stored in memory by the SessionRegistry. If persistent sessions must be supported, the respective INVOCATION INTERCEPTOR would have to make sure the session data is loaded in beforeInvocation and also save potential changes in afterInvocation. In the example below, beforeInvocation handles the session using the session registry and afterInvocation does nothing. The sequence diagram shows two session-handling scenarios: The session does not yet exist. A session object is created, added to the session registry, then used for the invocation.
24.09.2004 21:08 c06.fm 0.3
Extension Patterns A session object is found. It can therefore be obtained and used for the invocation. In the sequence diagram, we see that the remote object instance can optionally interact with the session object. Here, we store and retrieve values from the session object. This requires the remote object to know the current session object. The INVOCATION INTERCEPTOR can be used to add this information to the invocation.
c: Invocation Context
i:Invocation Data
Invoker
invoke(i,c)
si:Session Server Interceptor
Session Registry
Remote Object
s: Session
beforeInvocation(i,c) clientID := get("ClientIdentifier")
opt [ no session found for client ]
{} := getSession(clientID)
create
addSession(clientID, s)
opt [ session found ]
s := getSession(clientID)
setSession(s)
someMethod(x) x := getValue("x") incr(x) storeValue("x", x) afterInvocation(i,c)
24.09.2004 21:08
c06.fm
Extended Infrastructure Patterns
Extended Infrastructure Patterns This chapter presents patterns that are used to implement advanced features of distributed object middleware. While they are part of the infrastructure, they are still visible and relevant to the developer who is using distributed object middleware.
QOS OBSERVER
monitors optimizes resource consumption
nito
INVOKER CLIENT REQUEST HANDLER SERVER REQUEST HANDLER
Remote Object
manages lifecycle for
ABSOLUTE OBJECT REFERENCE
updates client's
on ati ap loc y for s pe ide enc d of ar rov spar p n LOCATION s an ts lik tra ps s se FORWARDER e u e ro iz g n a LOCAL OBJECT g or LIFECYCLE MANAGER CONFIGURATION GROUP