Term
| Show a diagram of the middleware layers for Remote Invocation |
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Definition
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Term
| What are the main remote invocation types? |
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Definition
- Request-reply protocols
- Low level support for remote operations
- Remote procedure calls (RPC)
- Remote method invocations (RMI)
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Term
| What are the characteristics of Request-Reply communcation |
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Definition
- Supports client-server interaction
- Client blocks until reply arrives
- Message structure: Message type, requestid, remoteReference, operationid, arguments
- Some implementations use TCP to ensure reliability
- Some use UDP for efficiency
- Timeouts used to detect server crashes or dropped messages, recovery is by resending request and filtering duplicate replies
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Term
| Show an example of Request-Reply communication |
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Definition
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Term
| What are the RPC fault-tolerant semantics? |
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Definition
- Underlyig request-reply protocol may
- Retransmit request messages until receive reply
- Filter out duplicate requests at server
- Retransmit results without re-excecuting operations
- Fault tolerant measures
- Maybe semantics
- At least once semantics
- At most once semantics
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Term
| What does fault tolerance maybe RPC semantics consist of? |
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Definition
- No fault tolerance built in
- RPC executed once or not at all
- Can suffer from communication omission failures or process crashes
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Term
| What does fault tolerance At least once RPC semantics consist of? |
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Definition
- Invoker receives either result (at least one execution) or exception)
- Achieved by retransmitting requests
- Can suffer from crash failures (of server) or arbitrary failres (whn procedure is excecuted more than once)
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Term
| What does fault tolerance At most once RPC semantics consist of? |
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Definition
- Invoker receives either result (exactly one execution) or exception (no excecution)
- Achieved by using all of the above fault-tolerant measures
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Term
| What are the roles of interfaces in Distributed Systems |
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Definition
- Used to control interactions between different components/objects
- Interfaces cannot specify direct access to variables
- Sevice interfaces(RPC)/remote interfaces(RMI) specify remote procedures/methods available for remote calls/invocations
- A different semantics for parameter passing is needed
- Interface definition languages (IDLs) allo procedures/objects implemented in different languages invoke each other
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Term
| What is the role of an Interface Definition Language (IDL)? |
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Definition
| Allow procedures/objects implemented in different languages to invoke one another |
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Term
| What is the role of Service Interfaces? |
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Definition
| Specifies remote procedures available for remote procedure calls (RPC) |
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Term
| What is the role of remote interfaces? |
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Definition
| Specify remote methods available for remote method invocations (RMI) |
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Term
| How is RMI similar to RPC? |
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Definition
- Supports programming with interfaces
- Uses request-reply protocols
- Range of possible semantics: at least/most once
- Similar levels of transparency
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Term
| What distinguishes RMI form RPC? |
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Definition
- Full expresive power of OOP in distributed systems
- Richer parameter-passing semantics: parameter include object references
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Term
| What does the object model consist of? |
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Definition
- Programs consist of collections of interacting objects (data + methods)
- Objects communicate via method invocation
- Objects can be accessed via object references
- Interfaces define sets of methods provided by objects
- Exceptions used to deal with error conditions
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Term
| What are the principles of the Distributed Object Model? |
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Definition
- Objects physically distributed into several processes
- Client-server architecture: remote objects are managed by servers and their methods can be invoked by clients using RMI
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Term
| How are remote objects managed by the client-server architecture? |
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Definition
- Client's request is sent in a message to server
- Method of the object excecuted at server
- Result is sent back to client in another message
- This enforces encapsulation; synchronisation primitives used to deal with concurrent addresses
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Term
| What are other distributed object models? |
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Definition
- Replicate objects to achieve fault tolerance and enhanced performance
- Migrate objects to enhance performance and availability
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Term
| Explain the Distributed Object Model and its components |
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Definition
- Several processes (on different machines), each containing a collection of objects
- Objects in the same process communicate via the local method invocation
- Objects in different processes communicate via remote method invocation
- Objects that can receive remote method invocations are called remote objects
- Remote object reference is needed to access a remote object
- Remote interface specifies which methods of a remote object can be invoked remotely
- RMIs should be able to raise exceptions due to either distribution or error conditions
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Term
| Show a diagram of the Distributed object model |
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Definition
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Term
| What are some principles of Remote Object References? |
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Definition
- Similar to local remote object references
- Can be passed as arguments to results of RMIs
- Serve as unique identifiers for remote objects, valid throughout a distributed system
- Format ensures uniqueness over space and time (first 4 fields), and describes the methods offered by the remote object (last field)
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Term
| What elements does remote object references contain? |
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Definition
- IP address
- Port number
- Time of creation
- Local object number
- Interface of remote object
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Term
| How are remote methods accessed? |
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Definition
- Through a remote interface
- Only methods of remote interface can be invoked by objects in other processes
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Term
| What are some issues that RMI must address? |
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Definition
- Location transparency: client should not know the location of the object it invokes
- Access transparency: remote and local objects accessed in the same way; can only be partially achieved
- Mechanism for acquisition of remote object references
- Ability to raise exceptions due to distribution
- Distributed garbage collection, if garbage collection is supported
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Term
| A diagram showing RMI implementation |
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Definition
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Term
| Explain the components of an RMI implementation |
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Definition
- Communication modules transmit requests and replies between client and server processes
- Remote reference modules create remote object references and translate between local and remote object references
- Remote object table records correspondence between local and remote references
- Proxy makes remote invokation transparent: client can make what looks like a local method invocation
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Term
| What role does the Proxy have in the RMI implementation? |
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Definition
- Makes remote invocation transparent
- Behaves like a local object to the invoker
- One proxy for each remote object for which a process holds a remote reference
- Its class (stub) implements the methods in the remote interface of the object it represents
- Instead of executing a method it forwards the invocation to the remote object (after marshaling the remote object reference, id and parameters)
- It returns the results to the invoking object (after un-marshaling them)
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Term
| What role does the dispatcher and skelleton have in the RMI implementation? |
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Definition
- One of each for each class, representing a remote object
- Dispatcher receives request messages and calls appropriate method of skeleton
- Skeleton class implements the methods in the remote interface by:
- Un-marshaling the request mesasge
- Invoking the corresponding method of the remote object
- Marshaling the results/exceptions into a reply message
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Term
| How is parameter and result passing operate? |
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Definition
- Client calls local method in stub
- Stub marshals target object, the method name and the parameters into a message which it sends to the skelleton on the server
- Server unpacks message and invokes actual method on server object
- Server then marshals result into a message which it sends back as a reply
- Client then unpacks reply message and returns result to client object
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Term
| What is object serialization? |
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Definition
- Flattening into serial form suitable for transmission in a message
- Carried out automatically by the middleware
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Term
| What is needed for the use of Remote Object References? |
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Definition
- Binders (Naming services) are used to maintain a list of remote objects held by server
- Servers register remote objects by textual names
- Clients lookup remote objects by names
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