Remote Procedure Calls
Announcements
• Prelim II coming up in one week:
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Thursday, April 26th, 7:30—9:00pm, 1½ hour exam
101 Phillips
Closed book, no calculators/PDAs/…
Bring ID
• Topics:
– Since last Prelim, up to (and including) Monday, April 23rd
– Lectures 19-34, chapters 10-18 (7th ed)
• Project 5 due after Prelim II, Monday, April 30th
– Make sure to look at the lecture schedule to keep up with due dates!
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Review: Use of TCP: Sockets
• Socket: an abstraction of a network I/O queue
– Embodies one side of a communication channel
• Same interface regardless of location of other end
• Could be local machine (called “UNIX socket”) or remote machine (called
“network socket”)
– First introduced in 4.2 BSD UNIX: big innovation at time
• Now most operating systems provide some notion of socket
• Using Sockets for Client-Server (C/C++ interface):
– On server: set up “server-socket”
• Create socket, Bind to protocol (TCP), local address, port
• call listen(): tells server socket to accept incoming requests
• Perform multiple accept() calls on socket to accept incoming connection
request
• Each successful accept() returns a new socket for a new connection; can
pass this off to handler thread
– On client:
• Create socket, Bind to protocol (TCP), remote address, port
• Perform connect() on socket to make connection
• If connect() successful, have socket connected to server
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Socket Setup (Con’t)
Server
Socket
new
socket
socket
connection
Client
socket
Server
• Things to remember:
– Connection requires 5 values:
[ Src Addr, Src Port, Dst Addr, Dst Port, Protocol ]
– Often, Src Port “randomly” assigned
• Done by OS during client socket setup
– Dst Port often “well known”
• 80 (web), 443 (secure web), 25 (sendmail), etc
• Well-known ports from 0—1023
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Socket Example (Java)
server:
//Makes socket, binds addr/port, calls listen()
ServerSocket sock = new ServerSocket(6013);
while(true) {
Socket client = sock.accept();
PrintWriter pout = new
PrintWriter(client.getOutputStream(),true);
}
pout.println(“Here is data sent to client!”);
…
client.close();
client:
// Makes socket, binds addr/port, calls connect()
Socket sock = new Socket(“169.229.60.38”,6018);
BufferedReader bin =
new BufferedReader(
new InputStreamReader(sock.getInputStream));
String line;
while ((line = bin.readLine())!=null)
System.out.println(line);
sock.close();
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Goals for Today
• Implementing Distributed Applications
• Messages
– Send/receive
– One vs. two-way communication
• Remote Procedure Call
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Distributed Applications
• How do you actually program a distributed application?
– Need to synchronize multiple threads, running on different machines
• No shared memory, so cannot use test&set
Receive
Send
Network
– One Abstraction: send/receive messages
• Already atomic: no receiver gets portion of a message and two receivers
cannot get same message
• Interface:
– Mailbox (mbox): temporary holding area for messages
• Includes both destination location and queue
• For example, mbox is kernel buffer associated with a Socket
– Send(message,mbox)
• Send message to remote mailbox identified by mbox
– Receive(buffer,mbox)
• Wait until mbox has message, copy into buffer, and return
• If threads sleeping on this mbox, wake up one of them
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Using Messages: Send/Recv behavior
• When should send(message,mbox) return?
– When receiver gets message? (i.e. ack received)
– When message is safely buffered on destination?
– Right away, if message is buffered on source node?
• Actually two questions here:
– When can the sender be sure that receive actually received the
message?
– When can sender reuse the memory containing message?
• Mailbox provides 1-way communication from P1P2
– P1bufferP2
– Very similar to producer/consumer
• Send = V (i.e. signal)
• Receive = P (i.e. wait)
• However, can’t tell if sender/receiver is local or not!
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Messages for Producer-Consumer
• Using send/receive for producer-consumer style:
Producer:
int msg1[1000];
Send
while(1) {
Message
prepare message;
send(msg1,mbox);
}
Consumer:
int buffer[1000];
while(1) {
Receive
receive(buffer,mbox);
process message;
Message
}
• No need for producer/consumer to keep track of space in
mailbox: handled by send/receive
– One of the roles of the window in TCP: window is size of buffer on far
end
– Restricts sender to forward only what will fit in buffer
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Messages for Req/Resp comm.
• What about two-way communication?
– Request/Response
• Read a file stored on a remote machine
• Request a web page from a remote web server
– Also called: client-server
• Client  requester, Server  responder
• Server provides “service” (file storage) to the client
• Example: File service
Request
File
Client: (requesting the file)
char response[1000];
send(“read rutabaga”, server_mbox);
receive(response, client_mbox);
Server: (responding with the file)
char command[1000], answer[1000];
receive(command, server_mbox);
decode command;
read file into answer;
send(answer, client_mbox);
Get
Response
Receive
Request
Send
Response
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Client/Server Paradigm
• Common model for structuring distributed computations
• A server is a program (or collection of programs) that
– provides some service, e.g., file service, name service, …
– may exist on one or more nodes.
• A client is a program that uses the service.
– It first binds to the server,
• i.e., locates it in the network and establishes a connection.
• The client then sends requests to perform actions;
– Using messages to indicate the desired service and params.
– The server returns a response.
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Why not use messages?
• Although messages are flexible, they have problems:
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Requires that programmer worry about message formats
Messages must be packed and unpacked
Server needs to decode messages to figure out the request
Messages are often asynchronous
They may require special error handling functions
• Basically using messages is not a convenient paradigm for
most programmers.
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Procedure Call
• More natural way is to communicate using procedure calls:
– every language supports it
– semantics are well defined and understood
– natural for programmers to use
• Basic idea: define server as a module that exports a set of
procedures callable by client programs.
• To use the server, the client just does a procedure call, as
if it were linked with the server
call
Client
Server
return
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(Remote) Procedure Call
• So, we would like to use procedure call as a model for
distributed communication.
• Lots of issues:
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how do we make this invisible to the programmer?
what are the semantics of parameter passing?
how is binding done (locating the server)?
how do we support heterogeneity (OS, arch., language)
etc.
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Remote Procedure Call
• The basic model for Remote Procedure Call (RPC) was
described by Birrell and Nelson in 1980, based on work done
at Xerox PARC.
• Goal to make RPC as much like local PC as possible.
• Used computer/language support.
• There are 3 components on each side:
– a user program (client or server)
– a set of stub procedures
– RPC runtime support
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RPC
• Basic process for building a server:
– Server program defines the server’s interface using an interface
definition language (IDL)
– The IDL specifies the names, parameters, and types for all clientcallable server procedures
– A stub compiler reads the IDL and produces two stub procedures for
each server procedure: a client-side stub and a server-side stub
– The server writer writes the server and links it with the server-side
stubs; the client writes her program and links it with the client-side
stubs.
– The stubs are responsible for managing all details of the remote
communication between client and server.
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RPC Stubs
• Client-side stub is a procedure that looks to the client as if it
were a callable server procedure.
• Server-side stub looks like a calling client to the server
• The client program thinks it is calling the server;
– in fact, it’s calling the client stub.
• The server program thinks it’s called by the client;
– in fact, it’s called by the server stub.
• The stubs send messages to each other to make RPC
happen.
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RPC Call Structure
client
program
call foo(x,y)
client makes
local call to
stub proc.
server is
called by
its stub
proc foo(a,b)
begin foo...
server
program
end foo
call foo
client
stub
proc foo(a,b)
call foo
stub builds msg
packet, inserts
params
stub unpacks
params and
makes call
send msg
RPC
runtime
call foo(x,y)
server
stub
msg received
runtime sends
msg to remote
node
runtime
receives msg
and calls stub
RPC
runtime
Call
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RPC Return Structure
client
program
call foo(x,y)
client continues
server proc
returns
proc foo(a,b)
begin foo...
server
program
end foo
return
client
stub
proc foo(a,b)
return
stub unpacks
msg, returns
to caller
stub builds
result msg
with output
args
msg received
RPC
runtime
call foo(x,y)
server
stub
send msg
runtime
receives msg,
calls stub
runtime
responds
to original
msg
RPC
runtime
return
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RPC Information Flow
call
return
Machine B
Server
(callee)
return
call
marshal
return values
Server
Stub
unmarshal
args
send
receive
Network
Machine A
send
Client
RPC
Stub
Runtime
receive
unmarshal
mbox2
ret vals
Network
Client
(caller)
marshal
args
mbox1
RPC
Runtime
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RPC Binding
• Binding is the process of connecting the client and server
• The server, when it starts up, exports its interface,
– identifying itself to a network name server and
– telling the local runtime its dispatcher address.
• The client, before issuing any calls, imports the server,
– which causes the RPC runtime to lookup the server through the
name service and
– contact the requested server to setup a connection.
• The import and export are explicit calls in the code.
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RPC Marshalling
• Marshalling is packing of procedure params into message
packet.
• RPC stubs call type-specific procedures to marshall (or
unmarshall) all of the parameters to the call.
• On client side, client stub marshalls parameters into call
packet;
– On the server side the server stub unmarshalls the parameters to
call the server’s procedure.
• On return, server stub marshalls return parameters into
return packet;
– Client stub unmarshalls return params and returns to the client.
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Problems with RPC
• Non-Atomic failures
– Different failure modes in distributed system than on a single machine
– Consider many different types of failures
• User-level bug causes address space to crash
• Machine failure, kernel bug causes all processes on same machine to fail
• Some machine is compromised by malicious party
– Before RPC: whole system would crash/die
– After RPC: One machine crashes/compromised while others keep
working
– Can easily result in inconsistent view of the world
• Did my cached data get written back or not?
• Did server do what I requested or not?
– Answer? Distributed transactions/Byzantine Commit
• Performance
– Cost of Procedure call « same-machine RPC « network RPC
– Means programmers must be aware that RPC is not free
• Caching can help, but may make failure handling complex
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Cross-Domain Comm./Location
Transparency
• How do address spaces communicate with one another?
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Shared Memory with Semaphores, monitors, etc…
File System
Pipes (1-way communication)
“Remote” procedure call (2-way communication)
• RPC’s can be used to communicate between address
spaces on different machines or the same machine
– Services can be run wherever it’s most appropriate
– Access to local and remote services looks the same
• Examples of modern RPC systems:
– CORBA (Common Object Request Broker Architecture)
– DCOM (Distributed COM)
– RMI (Java Remote Method Invocation)
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Microkernel operating systems
• Example: split kernel into application-level servers.
– File system looks remote, even though on same machine
App
App
file system
VM
App
Windowing
Networking
Threads
Monolithic Structure
App
RPC
File
sys
windows
address
spaces
threads
Microkernel Structure
• Why split the OS into separate domains?
– Fault isolation: bugs are more isolated (build a firewall)
– Enforces modularity: allows incremental upgrades of pieces of software
(client or server)
– Location transparent: service can be local or remote
• For example in the X windowing system: Each X client can be on a
separate machine from X server; Neither has to run on the machine with
the frame buffer.
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Summary
• Remote Procedure Call (RPC): Call procedure on remote
machine
– Provides same interface as procedure
– Automatic packing and unpacking of arguments without user
programming (in stub)
• RPC is most common model now for communications in
distributed applications.
– It is language support for distributed programming.
• RPC relies on a stub compiler to automatically produce
client/server stubs from the IDL server description.
• RPC is commonly used, even on a single node, for
communication between applications running in different
address spaces. In fact, most RPCs are intra-node.
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