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Common Application Components

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  1. Common Application Components Some frequently needed elements in applications that involve communication between separate systems

  2. Common Elements of the Application Layer • Some things have to be done in all applications • Others are very common • Having a bag of tools handy can save recreating a solution every time the problem arises

  3. Base set of common services • Establish contact • Remote execution • Reliable transfer of data • Coordination of distributed processing elements

  4. Establishing contact • Locate the processing partner • Associate semantics with messages exchanged • Understand network service level • Match processing elements • Identify initiator of the contact

  5. ACSE • Association Control Service Element • Two types of services offered: • Association establishment • Association termination • release • abort by the user • abort by the lower layer network services

  6. A-ASSOCIATE • Association establishment • parameters describe the set of desired characteristics (the context) of the association • what application service elements will be active in the cooperating systems • identification of the destination process location for communication • OSI PSAP (Presentation Service Access Point) • TCP/IP port number • Presentation context • how should the receiver interpret data that arrives • {1{PersonnelData, Encrypted}, 2{Annual Report, Compressed}, 3{AnnualReport,Basic}}

  7. A-RELEASE, A-ABORT • A-RELEASE: Orderly termination of an association between cooperating processes • A-ABORT: Abrupt termination, with possible loss of data

  8. Closing an Association Confirmed service Unconfirmed service

  9. Communication between layers Peer to Peer Service Requestor / Service Provider Service Requestor / Service Provider Peer to Peer Service Requestor / Service Provider Service Requestor / Service Provider

  10. Within layers Peer to Peer Service Requestor / Service Provider Service Requestor / Service Provider Peer to Peer Service Requestor / Service Provider Service Requestor / Service Provider

  11. Parameters • Information shared between peers and between service requestors and providers • Each invocation of a service, or response to a service request, involves information passed both within the local stack and with peers in a cooperating stack. • The single list of parameters must be split into those that inform the service provider and those that inform the peer process.

  12. ASSOCIATION ESTABLISHMENT

  13. System state regarding protocols • A system is in one of a number of states at any given time. • The state depends on what conditions are in effect, what has happened so far. • The state determines what are possible responses to future events • Initial state (Idle) • limited set of events are acceptable • others are considered error conditions • For example, the arrival of data before an association is established would be an error condition

  14. Protocol machine Key: Event Response The event occurs, the protocol machine changes state and performs the indicated response. Here the response is something sent or issued. Two views: top shows the state transition in graphical form; bottom shows the layer interaction.

  15. Association Establishment Protocol Machine Idle A-ASSOCIATE.req AARQ AARQ A-ASSOCIATE.ind Outgoing association pending Incoming association pending AARE A-ASSOCIATE.conf A-ASSOCIATE.resp AARE Association Established

  16. Association Established • Once Association is Established • the context of the interaction is established • data can be exchanged • an orderly termination of the association can be done • ACSE is the OSI protocol for establishing an association. • Other stacks, like TCP/IP, do not have a named entity for this purpose, but must establish this sort of connection

  17. Remote Operation • Call for the execution of an operation on a remote system • Basic questions related to remote operation • What do we know about the data to be used? • How will the initiator know when the remote operation is complete? • Is it safe to repeat a request if the remote system crashes before completing? • Stop and wait or continue working?

  18. ROSE; remote procedure calls • What do we know about the data to be used? • ASN.1 and BER or alternatives such as XDR • How will the initiator know when the remote operation is complete? • Requesting some result from every operation allows the initiator to know when the operation is done • Is it safe to repeat a request if the remote system crashes before completing? • CCR or its equivalent • Stop and wait or continue working? • Synchronous or asynchronous invocation

  19. Remote operation services • In the OSI protocols, • ROSE - The Remote Operation Service Element • Other, • RPC - Remote Procedure Call • RMI - Java’s Remote Method Invocation

  20. ROSE service • RO-INVOKE • call a method located in another system • have it run in the other system • RO-RETURN-RESULT • send a result to the invoking method • RO-RETURN-ERROR • send an error notification to the invoking method • RO-REJECT-U/RO-REJECT-P • reject the invocation request

  21. ROSE characteristics • Minimal state information • All services unconfirmed • All responsibility for correctness is on the designer of the application

  22. Synchronous or Asynchronous • Synchronous • blocking • Asynchronous • non-blocking • report results and errors • report errors only • report results only • report nothing

  23. ASN.1 definition of a remote operation

  24. Using the definition • RO-INVOKE (…,0, “PS”…) • other required parameters identify remote system to be contacted, etc. • 0 is the identifier of getcount; PS is the name of the queue to examine. • RO-RETURN-RESULT (“PS”,250) • Queue PS has 250 entries • RO-RETURN-ERROR (1, “PS”) • Queue PS is not available

  25. Remote Operation Summary • Protocols cover only the communication between the caller and the called method • how method is identified • what information is provided • what is understood between them • Separate issue • write the method to carry out the desired action

  26. Reliable transfer of information • What is needed beyond a dependable transport layer? • Protect against errors that occur after the data is delivered to the machine • example: Jammed printer or a disk failure prevents completion of the desired action, but the transport layer is satisfied

  27. RTSE: Reliable Transport Service Element • Invokes services of the session layer, uses ACSE • Services provided by RTSE: • RT-OPEN • RT-TRANSFER • RT-TURN-PLEASE • RT-TURN-GIVE • RT-CLOSE • RT-U-ABORT/RT-P-ABORT

  28. RT-TRANSFER, RT-TURN • confirmed service • parameters specify data to be sent, maximum time allowed for completion • If two-way data transfer, RT-TURN-PLEASE, an unconfirmed service, requests the opportunity to send data; RT-TURN-GIVE, unconfirmed service to yield turn

  29. RTSE protocol • Use A-ASSOCIATE to establish connection with peer • Invoke turn-taking facilities of the Session layer • Invoke activity management facilities of the Session layer to establish checkpoints between data segments • used by X.400 mail system and available to ROSE when a lot of data must be moved

  30. Commitment, Concurrency, and Recovery • Commitment: • assurance that the server process will carry out a request regardless of difficulties that might arise • Concurrency: • protection from the intrusion of interleaved operations that interfere with correct completion of a requested task • Recovery: • establishment of procedures to overcome failures by one or more participating process during execution of a distributed application

  31. The Lost Transaction Problem Process 1 Credit Limit Process 2 5,000 Read credit 5000 Read credit 5000 Subtract new purchase (850) update credit 4,150 Subtract new purchase (54.50) update credit 4,945.50 Time

  32. CCR Services • Bracket atomic action • C-BEGIN, C-PREPARE • SERVER --> CLIENT ready • C-READY • SERVER --> CLIENT no • C-REFUSE • CLIENT, SERVER agree to go on • C-COMMIT • CLIENT --> SERVER no • C-ROLLBACK • CLIENT/SERVER go back • C-RESTART

  33. Multiple servers participate

  34. Server refuses request

  35. Some TCP/IP handy tools • Not exactly the same kind of thing, but useful nonetheless • ping • sends a message and looks for a response to determine that the other machine is alive and active • traceroute • displays the path taken by messages between two cooperating systems • on sun cluster: /usr/sbin/ping or traceroute • NOTE-- overuse of these is considered very unfriendly behavior. Be gentle, considerate.

  36. Overall summary • Net-centered computing has more elements than computing on a single machine. • Many characteristics are the same from one application to another • Understand what is the same and what varies between applications