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EEC-681/781
Distributed Computing
Systems
Lecture 4
Wenbing Zhao
Department of Electrical and Computer Engineering
Cleveland State University
[email protected]
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Outline
•
•
•
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Architecture Model
Server load balancing
End-to-end arguments in system design
Inter-process communications (part 1)
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Architectural Models
• Client-server model
• Peer-to-peer model
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Basic Client–Server Model
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Servers
• Servers: Generally provide services
related to a shared resource:
– Servers for file systems, databases,
implementation repositories, etc.
– Servers for shared, linked documents
– Servers for shared applications
– Servers for shared distributed objects
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Client
• Clients: Allow remote service access
– Programming interface transforming client’s local
service calls to request/reply messages
– Devices with (relatively simple) digital components
(barcode readers, teller machines, hand-held phones)
– Computers providing independent user interfaces for
specific services
– Computers providing an integrated user interface for
related services (compound documents)
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Application Layering
• Traditional three-layered view:
– User-interface layer contains units for an
application’s user interface
– Processing layer contains the functions of an
application, i.e. without specific data
– Data layer contains the data that a client
wants to manipulate through the application
components
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Multitiered Architecture
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Multitiered Architecture
1-30
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Horizontal Distribution
1-31
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Modern Architectures
• Vertical distribution
– According to application logic
• Horizontal distribution
– Scalability and availability
• In practice, a system is distributed in both
directions
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Peer-to-Peer Model
• A different way to construct client-server
systems where most, or all, of the server
functionality resides on the clients themselves
• Advantages
– Scalable to very large numbers (millions)
– Stable under very high load
– Self-repairing when disruptive failures occur
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Why Server Load Balancing?
• Scalable - Upgrade out instead of up
• Easily grow - Spread your load across your
existing low-cost servers as your needs grow
• HA - Can work closely with High Availability
solutions to add and remove servers as
necessary
Slides on server load balancing taken from Dustin Puryear’s talk on “LVS
– Load Balancing and High Availability for Free” at USENIX LISA 2003
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SLB Layout
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SLB Definitions
• Virtual IP (VIP) – The IP used by clients to
access a service
• Load Balancer (LB) or Server Load Balancer
(SLB) – the server that balances packets going
to or from the servers providing the service
• Real Server – The server providing the actual
service (i.e., the server running Apache)
• Real IP (RIP) – The IP of each real server
• Schedule – How the SLB determines which real
server gets the next connection
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SLB Transaction Walkthrough
1.
2.
3.
4.
5.
Client established TCP
connection with SLB.
SLB determines which real
server is ready for a
connection using configured
schedule, and creates a
connection to that server.
Client sends HTTP request.
SLB forwards HTTP request
to real server.
Real server responds back
to.. Who?
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So How Exactly Does the Real
Server Respond?
• There are two traditional methods to
solving this problem:
– Network Address Translation (NAT)
– Direct Routing/Direct Server Return
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NAT-Based SLB
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NAT-Based SLB
• SLB sits between the client and real servers for
all incoming and outgoing packets, just like a
router that does NAT
• The SLB can rewrite the IP packet in one of two
ways:
– Half-NAT: load-balancer rewrites the destination IP
– Full-NAT: load-balancer rewrites the source and
destination IP
• The VIP is only assigned on the load-balancer
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Half-NAT
1. SLB gets packet from client
2. SLB changes the destination IP in the header
of the packet
3. SLB forwards packet to real server
•
Major benefit of this method is that the real
server knows the real IP of the client
–
–
The real server can properly log client IP’s
The real server can do better security based on the
IP address of the client
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Full-NAT
1. SLB gets packet from client.
2. SLB changes the destination IP and source IP
in the header of the packet.
3. SLB forwards packet to real server.
•
Major drawback to this technique:
–
The real server has no idea who sent the original
request
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Direct Routing/Direct Server Return
Layer 2 LB
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Direct Routing/Direct Server Return
1.
2.
3.
4.
SLB gets packet from client
SLB changes destination IP in packet header
SLB forwards packet to real server, which also has VIP
Real server responds directly to the client with VIP as
the source IP
•
There are benefits and costs to this approach
– Without having to push data back through the SLB
you can decrease latency
– How do you handle ARP if multiple devices have the
same IP (the VIP)?
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End-to-End Arguments
in System Design
• Concerned with placement of functions
among modules of a distributed system
• Layered systems are very common
– Network protocols and Middleware
• Claim
– Functions at low levels of a system may be
redundant or of little use
– Moving a function upward in a layered system
closer to application that uses the function
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Careful File Transfer
• Objective: move a file from computer A’s
storage to computer B’s storage
• Application program: file transfer program
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Careful File Transfer
• Fault-free steps of file transfer
– A reads the file from its disk
– A sends the file to its networking stack and the file is
packetized at the networking protocol stack
– Communication network moves packets from A to B
– At B, B’s networking protocol stack delivers
transmitted file to the file transfer program running in
B
– B’s file transfer program asks its file system to write
the file to B’s disk
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Careful File Transfer
• What can go wrong in each step
– File corrupted due to disk error
– Software bugs in buffering and copying the file,
in A or B
– Processor or memory transient error during
buffering or copying
– Communication system error, e.g., drop or
change bits, or duplicate delivery
– Host might crash
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Careful File Transfer
• How to cope with the threats
– Reinforce each step along the way using
time-out and retry, error detection, crash
recovery, etc.
– Doing everything three times
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End-to-end Check and Retry
• Assuming low probability of the threats
• As a final additional step
– B reads back the file from disk into memory
– Recalculates checksum
– Sends this value back to A
– A compares the value with checksum of
original file
– If two checksums agree, file transfer
completes. Otherwise, retry from beginning
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Is Reliable Communication Useful?
• Communication system can be made reliable
with Packet checksums, sequence number,
internal retry, etc.
• Is it enough to ensure correct file transfer?
– No. Other threats can still corrupt the file
– The extra end-to-end check and retry still necessary
– Usefulness: Reducing frequency of retry
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Is Reliable Communication Useful?
• Conclusion
– Communication system to go out of its way to
be extraordinarily reliable does not reduce the
burden on the application program to ensure
reliability
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Performance Aspects
• Cannot conclude that lower levels should
play no part in obtaining reliability
– If file is long, retry the transfer of the file is too
expensive
– By providing reliable communication, can
significantly enhance the performance
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Performance Aspects
• Putting reliability measures in lower level
– An engineering trade-off based on performance
– Not a requirement for correctness
– Must be careful. Might not always improve
performance
• Other application might not need the extra
functionality
• Might not have enough information to operate
efficiently
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Inter-Process Communications
• Techniques:
– Shared memory
– Message passing
• Objectives:
– Data exchange
– Synchronization: processes at different
hosts, executing at different rates, need to
influence the overall execution pattern
=> Constraints on the order of events
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The OSI Network Architecture
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Low-level Layers
• Physical layer: contains the specification
and implementation of bits, and their
transmission between sender and receiver
• Data link layer: prescribes the
transmission of a series of bits into a
frame to allow for error and flow control
• Network layer: describes how packets in
a network of computers are to be routed
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Transport Layer
• The transport layer provides the actual
communication facilities for most
distributed systems.
• TCP: connection-oriented, reliable,
stream-oriented communication
• UDP: unreliable (best-effort) datagram
communication
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Application Layer
• Many application protocols are directly implemented on
top of transport protocols that do a lot of applicationindependent work
FTP
WWW
Transfer
FTP
HTTP
Encoding
7-bit text + 8-bit binary
8-bit + content type
Naming
Host + path
URL
Distribution
Pull
Pull
Replication
Caching + DNS tricks
Caching + DNS tricks
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Message Layout
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Middleware Layer
• Middleware is invented to provide common
services and protocols that can be used by
many different applications:
–
–
–
–
–
A rich set of communication protocols
Marshaling and unmarshaling of data
Naming protocols
Security protocols
Scaling mechanisms
• What remains are truly application-specific
protocols
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An Adapted Reference Model
with Middleware Layer
2-5
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