Implementing Remote Procedure Calls Authored by Andrew D. Birrell and Bruce Jay Nelson Xerox Palo Alto Research Center Presented by Lars Larsson

Contents The RPC model and environment Aims of RPC facility and alternatives High level overview of the RPC facility RPC Binding Packet transport protocol Process Use Exception Handling Performance evaluation Conclusion

RPC Model Client server model for distributed computing Network I/O is ‘wrapped’ by stubs Programmer doesn’t need to know about message passing implementation

This RPC Environment Written for the Cedar project: a programming environment for building experimental programs and systems Coded mostly in Mesa but also some Smalltalk and InterLisp Communicate across the Xerox research inter-network

Aims of the RPC Facility Efficiency of communication Simplicity Security

Alternatives to Procedure Calls Some form of remote fork and shared address space This choice might introduce possible complications with the programming language Not likely to be as efficient Need more research of possible benefits

The 5 Pieces of a Procedure Call User process User Stub RPC Communications Package (RPC Runtime) Server Stub Server process

Generating the Pieces The RPC Runtime is part of Mesa - the major programming language of Cedar User and Server code are written as part of the distributed application User and Server Stubs are generated by Lupine Lupine uses specifications from the Mesa interface modules

Caller and Callee User process, user stub and an instance of the RPC Runtime exist on the caller machine Server process, server stub and an instance of the RPC Runtime exist on the callee machine

Overview of a simple call:

Binding Importers and Exporters Names bind importer with an exporter of an interface Each Name holds the type and possibly an instance of the interface Type describes which interface the caller expects the callee to implement Instance specifies which implementer of the interface is desired

Locating Exporters with Grapevine Each record is a Grapevine RName which have two varieties: individual or group Individual RNames hold the connect- site network addresses Group RNames hold a list of RNames identifying which instances belong to a particular group

Given type FileAccess.Alpine wih instance Ebbets.Alpine exported from a server at 3#22# and another instance of FileAccess.Alpine with an instance named Luther.Alpine exported from a server at 3#276# The members of the Grapevine group FileAccess.Alpine would include Ebbets.Alpine and Luther.Alpine. The Grapevine individual Ebbets.Alpine would hold 3#22# and the individual Luther.Alpine would hold 3#276# Example of Grapevine Group and Individual RName entries

Exporting and Importing Interfaces

Packet Level Transport Protocol Caller sends a CallPacket containing a call identifier, data specifying the desired procedure and arguments Result packet contains the same call identifier and the results Retransmitted packets request explicit acknowledgement While waiting for results caller sends periodic ‘probe’ packets Large results sent in multiple packets which request explicit acknowledgement (except for last packet)

Call Identifier Holds identifier of calling machine and machine relative identifier of calling process and a sequence number (with conversation identifier) (Machine, process identifier) pair is called an ‘activity’ Each activity has at most one outstanding remote call

Simple Call Example

Complicated Call Example

Exception Handling RPC returns an exception packet instead of a response packet Raises an exception in the appropriate process If a catch phrase executes a return, the return is passed to the callee and execution resumes otherwise the callee is notified of termination Only exceptions defined in the exported interface may be sent ‘Call failed’ exception may be raised incase of communication difficulty

Process Use Idle server processes wait for incoming packets If a process is waiting for a packet the Ethernet interrupt handler uses the ‘process identifier’ to pass the packet to waiting process Otherwise an idle process is passed the packet to decide what to do Initial destination process identifiers are ‘guessed’

Performance Evaluation

Conclusion RPC is fully implemented in Cedar Being used by Alpine – file server supporting multi-machine transactions Implementations made for BCPL, InterLisp, SmallTalk and C Still more work needs to be done