Introduction

• Network object is an object whose methods can be invoked over a network.
• Pure OO allows only message-passing, and no direct access to object state... good fit with distributed systems
  
• Mobile vs. stationary objects
• By copy or by reference object communication
• Activation
• Persistence
• Replication (fault-tolerance, performance)
• Distributed type-checking
• Distributed garbage collection
• Object granularity (software engineering: server, component)
• Features set: powerful marshaling, strong type-checking, garbage collection, stream support
• Missing: transactions, object migration, distributed shared memory
• System designed in Modula-3.
• Marshaling: compromise on inlining into stub and restricting marshallable types - Pickles.
• Stream vs. RPC: transferring large amounts of data, not restricted to duration of RPC.
• Performance

How network objects work

• Object: reference to data record, reference to method suite (procedures accepting object as first parameter), typecode, by reference
• subtyping: inheritance (single inheritance only), overriding
• Network object resides at owner; methods invoked by client
• Network interface type, T; pure object
• Client calls surrogate object, TSrg; object that subtypes interface and performs marshalling
• Communicates with implementation network object, TImpl: subtype of interface object, may contain state
• Example: File system

Implementation / features

• Fingerprint for every object type in system
• Communication over any type of transports: compression, encryption
• Garbage collection via counting/registration of surrogates. (Does not handle cycles across network)
• Crash detection to reclaim surrogates, false crash detections detected via (never reusing) object IDs
• Nodes are represented as Location objects: used to generate connections, request object interfaces
• Marshalling: inline code generated for simple types, more complex types are passed to a marshalling routine; generated code within a factor of two performance of hand-written code.
• NewSurrogate used to provide a connected surrogate object
• Marshalling streams, two possible implementations:
  • Open connection to known point and send unique identifier to pass connection to appropriate reader.
  • Marshall stream information in a voucher object back and forth; requires three messages instead of one
• Pickling: provides overrides for default marshalling behaviour, required for things like mutex and streams.
• Bootstrapping: id=0, well-known protocol/port
• Uses: packagetool/siphon

Discussion

• Synchronization
• State transfer, fault-tolerance
• Persistence, transactions, object databases
• Object transfer, connection transfer