Reliable Distributed Systems

Reliable Distributed Systems

RPC and Client-Server Computing

Remote Procedure Call

Basic concepts Implementation issues, usual

optimizations Where are the costs? Reliability and consistency Multithreading debate

A brief history of RPC Introduced by Birrell and Nelson in 1985 Pre-RPC: Most applications were built

directly over the Internet primitives Their idea: mask distributed computing

system using a “transparent” abstraction Looks like normal procedure call Hides all aspects of distributed interaction Supports an easy programming model

Today, RPC is the core of many distributed systems

More history Early focus was on RPC “environments” Culminated in DCE (Distributed Computing

Environment), standardizes many aspects of RPC

Then emphasis shifted to performance, many systems improved by a factor of 10 to 20

Today, RPC often used from object-oriented systems employing CORBA or COM standards. Reliability issues are more evident than in the past.

The basic RPC protocol

client server“binds” to

serverregisters with name service



server

prepares, sends request

registers with name service

receives request



server



receives requestinvokes handler



server



receives requestinvokes handlersends reply



server


unpacks reply


receives requestinvokes handlersends reply

Compilation stage Server defines and “exports” a header file

giving interfaces it supports and arguments expected. Uses “interface definition language” (IDL)

Client includes this information Client invokes server procedures through

“stubs” provides interface identical to the server version responsible for building the messages and

interpreting the reply messages passes arguments by value, never by reference may limit total size of arguments, in bytes

Binding stage Occurs when client and server program

first start execution Server registers its network address

with name directory, perhaps with other information

Client scans directory to find appropriate server

Depending on how RPC protocol is implemented, may make a “connection” to the server, but this is not mandatory

Data in messages We say that data is “marshalled” into a

message and “demarshalled” from it Representation needs to deal with byte

ordering issues (big-endian versus little endian), strings (some CPUs require padding), alignment, etc

Goal is to be as fast as possible on the most common architectures, yet must also be very general

Request marshalling Client builds a message containing arguments,

indicates what procedure to invoke Due to the need for generality, data

representation a potentially costly issue! Performs a send I/O operation to send the

message Performs a receive I/O operation to accept the

reply Unpacks the reply from the reply message Returns result to the client program

Costs in basic protocol? Allocation and marshalling data into

message (can reduce costs if you are certain client, server have identical data representations)

Two system calls, one to send, one to receive, hence context switching

Much copying all through the O/S: application to UDP, UDP to IP, IP to ethernet interface, and back up to application

Schroeder and Burroughs Studied RPC performance in O/S

kernel Suggested a series of major

optimizations Resulted in performance

improvments of about 10-fold for Xerox firefly workstation (from 10ms to below 1ms)

Typical optimizations? Compile the stub “inline” to put arguments

directly into message Two versions of stub; if (at bind time) sender

and dest. found to have same data representations, use host-specific rep.

Use a special “send, then receive” system call (requires O/S extension)

Optimize the O/S kernel path itself to eliminate copying – treat RPC as the most important task the kernel will do

Fancy argument passing RPC is transparent for simple calls with a small

amount of data passed “Transparent” in the sense that the interface to the

procedure is unchanged But exceptions thrown will include new exceptions

associated with network What about complex structures, pointers, big

arrays? These will be very costly, and perhaps impractical to pass as arguments

Most implementations limit size, types of RPC arguments. Very general systems less limited but much more costly.

Overcoming lost packets

client serversends request



retransmit

ack for request duplicate request: ignored

Timeout!



retransmit

ack for request

reply

Timeout!



retransmit

ack for request

reply

ack for reply

Timeout!

Costs in fault-tolerant version? Acks are expensive. Try and avoid

them, e.g. if the reply will be sent quickly supress the initial ack

Retransmission is costly. Try and tune the delay to be “optimal”

For big messages, send packets in bursts and ack a burst at a time, not one by one

Big packets

client serversends request as a burst

ack entire burst

reply

ack for reply

RPC “semantics” At most once: request is processed 0 or

1 times Exactly once: request is always

processed 1 time At least once: request processed 1 or

more times... but exactly once is impossible because

we can’t distinguish packet loss from true failures! In both cases, RPC protocol simply times out.

Implementing at most/least once Use a timer (clock) value and a unique id, plus

sender address Server remembers recent id’s and replies with

same data if a request is repeated Also uses id to identify duplicates and reject

them Very old requests detected and ignored by

checking time Assumes that the clocks are working In particular, requires “synchronized” clocks

RPC versus local procedure call Restrictions on argument sizes and

types New error cases:

Bind operation failed Request timed out Argument “too large” can occur if, e.g., a

table grows Costs may be very high ... so RPC is actually not very

transparent!

RPC costs in case of local destination process

Often, the destination is right on the caller’s machine!

Caller builds message Issues send system call, blocks, context switch Message copied into kernel, then out to dest. Dest is blocked... wake it up, context switch Dest computes result Entire sequence repeated in reverse direction If scheduler is a process, context switch 6 times!

RPC example

Source does

xyz(a, b, c)

Dest on same site

O/S

RPC in normal case

Source does

xyz(a, b, c)

Dest on same site

O/S

Destination and O/S are blocked

RPC in normal case

Source does

xyz(a, b, c)

Dest on same site

O/S

Source, dest both block. O/S runs its scheduler, copies message from source out-

queue to dest in-queue

RPC in normal case

Source does

xyz(a, b, c)

Dest on same site

O/S

Dest runs, copies in message

Same sequence needed to return results

Broad comments on RPC RPC is not very transparent Failure handling is not evident at all: if an RPC

times out, what should the developer do? Reissuing the request only makes sense if there is

another server available Anyhow, what if the request was finished but the

reply was lost? Do it twice? Try to duplicate the lost reply?

Performance work is producing enormous gains: from the old 75ms RPC to RPC over U/Net with a 75usec round-trip time: a factor of 1000!

Contents of an RPC environment

Standards for data representation Stub compilers, IDL databases Services to manage server directory,

clock synchronization Tools for visualizing system state

and managing servers and applications

Closely Related Topic Multithreading is a common

performance-enhancing technique Idea is that server is often idle while

doing I/O for one client, so use extra threads to allow concurrent request processing

In the limit, leads to database transactional concurrency model, but many non-transactional servers use threads for enhanced performance

Multithreading debate Three major options:

Single-threaded server: only does one thing at a time, uses send/recv system calls and blocks while waiting

Multi-threaded server: internally concurrent, each request spawns a new thread to handle it

Upcalls: event dispatch loop does a procedure call for each incoming event, like for X11 or PC’s running Windows.

Single threading: drawbacks Applications can deadlock if a request cycle

forms: I’m waiting for you and you send me a request, which I can’t handle

Much of system may be idle waiting for replies to pending requests

Harder to implement RPC protocol itself (need to use a timer interrupt to trigger acks, retransmission, which is awkward)

Multithreading Idea is to support internal concurrency as

if each process was really multiple processes that share one address space

Thread scheduler uses timer interrupts and context switching to mimic a physical multiprocessor using the smaller number of CPU’s actually available

Multithreaded RPC Each incoming request is handled by

spawning a new thread Designer must implement appropriate

mutual exclusion to guard against “race conditions” and other concurrency problems

Ideally, server is more active because it can process new requests while waiting for its own RPC’s to complete on other pending requests

Negatives to multithreading Users may have little experience with

concurrency and will then make mistakes Concurrency bugs are very hard to find due to

non-reproducible scheduling orders Reentrancy can come as an undesired surprise Threads need stacks hence consumption of

memory can be very high Deadlock remains a risk, now associated with

concurrency control Stacks for threads must be finite and can

overflow, corrupting the address space

Threads: can spawn too many

SCHED

event


SCHED

event

Thread spawned, but blocks


SCHED

eventEventually, application becomes bloated, begins to thrash. Performance drops and clients may think the server has failed

Upcall model Common in windowing systems Each incoming “event” is encoded as a

small descriptive data structure User registers event handling

procedures Dispatch loop calls the procedures as

new events arrive, waits for the call to finish, then dispatches a new event

Upcalls combined with threads

Perhaps the best model for RPC programming

Each handler can be tagged: needs thread, or can be executed “unthreaded”

Developer must still be very careful where threads are used

Recent RPC history RPC was once touted as the transparent

answer to distributed computing Today the protocol is very widely used ... but it isn’t very transparent, and

reliability issues can be a major problem Today the strongest interest is in Web

Services and CORBA, which use RPC as the mechanism to implement object invocation

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Reliable Distributed Systems