| An Overview of Cryptography |
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| Does increased security provide comfort to paranoid people? Or does security provide some very basic protections that we are naive to believe that we don't need? During this time when the Internet provides essential communication between tens of millions of people and is being increasingly used as a tool for commerce, security becomes a tremendously important issue to deal with. |
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| Conventional_Cryptography |
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| Asymmetric Cryptography,Message Digest,Digitally signed document |
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| Fault Tolerance |
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| The Byzantine generals problem for 3 loyal generals and 1 traitor. ... The same as in previous , except now with 2 loyal generals and one traitor. |
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| operating systems |
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| Issues in the design and functioning of operating systems. Emphasis on synchronization of concurrent activity in both centralized and distributed systems. Deadlock, scheduling, performance analysis, operating system design, and memory systems including distributed file systems. |
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| Types of architecture |
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| Architecture applies to various aspects of software systems development, understandably so given its generic and flexible terms of reference. The classification of an architecture into a particular type may be obvious from its context and is, arguably, irrelevant to its success. Nonetheless, it can be useful to have an understanding of the breadth of the discipline - it may also help to put the subject of these essays into perspective. |
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| Time, Clocks, and Global State |
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| The simplest case of clock
synchronization involves two processes
in a synchronous system.
Here, bounds are known for:
drift rate of clocks
maximum transmission delay
time for each step in the process |
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| Software Architecture |
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| Software architecture encompasses the set of significant decisions about the organization of a software system
Selection of the structural elements and their interfaces by which a system is composed
Behavior as specified in collaborations among those elements
Composition of these structural and behavioral elements into larger subsystems
Architectural style that guides this organization |
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| Scheduling and Migration |
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| A distributed scheduler is a resource management component of a distributed operating system that focuses on judiciously and transparently redistributing the load of the system among the computers to maximize the overall performance
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| Recovery |
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| Computer system recovery:
Restore the system to a normal operational state
Process recovery:
Reclaim resources allocated to process,
Undo modification made to databases, and
Restart the process
Or restart process from point of failure and resume execution
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| Protection and Security in Distributed Systems |
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| How to identify users to the system?
Passwords
Shared secret between two parties
Since only user knows password, someone types correct password ? must be user typing it
Very common technique
Smart Cards
Electronics embedded in card capable of providing long passwords or satisfying challenge ? response queries
May have display to allow reading of password
Or can be plugged in directly; several credit cards now in this category
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| Symmetric encryption |
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| Encryption involves applying an operation (an algorithm) to the data to be encrypted using the private key to make them unintelligible. The slightest algorithm (such as an exclusive OR) can make the system nearly tamper proof (there being so such thing as absolute security).
However, in the 1940s, Claude Shannon proved that to be completely secure, private-key systems need to use keys that are at least as long as the message to be encrypted. Moreover, symmetric encryption requires that a secure channel be used to exchange the key, which seriously diminishes the usefulness of this kind of encryption system.
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| Distributed Mutual Exclusion |
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| Mutual exclusion : makes sure that concurrent process access shared resources or data in a serialized way.
If a process , say Pi , is executing in its critical section, then no other processes can be executing in their critical sections
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| Multiprocessor OS |
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| A computer system in which two or more CPUs share full
access to the main memory
Each CPU might have its own cache and the coherence
among multiple cache is maintained
Write operation by a CPU is visible to all other CPUs
writes to the same location is seen in the same order by all
CPUs (also called write serialization) |
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| MultiProcesser-Concepts and Architectures |
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| Flynn, 1966+1972 classification of computer systems in terms of instruction and data stream organizations
Based on Von-Neumann model (separate processor and memory units
4 machine organizations
SISD - Single Instruction, Single Data
SIMD - Single Instruction, Multiple Data
MISD - Multiple Instruction, Single Data
MIMD - Multiple Instruction, Multiple Data
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| Multiple Processor Systems |
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| Goal of computer architects, engineers and scientists to get more and more computing power
Problem with the goal:
Previous approaches: make clock run faster to speed up
We are beginning to hit some fundamental limits on clock speed
Reason: Einstein’s special theory of relativity
No electrical signal can propagate faster than the speed of light: 30 cm/ns in a vacuum and 20 cm/ns over copper or optical fiber
This means:
with 10 GHz clock, signals cannot travel more than 2 cm total,
with 100 GHz clock, total signal path length at most 2 mm, and
at 1000 GHz (1 THz) computer would have to be smaller than 100 µm
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| The Design and Implementation of a Log-Structured File System |
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| This paper presents a new technique for disk storage
management called a log-structured file system. A logstructured
file system writes all modifications to disk
sequentially in a log-like structure, thereby speeding up
both file writing and crash recovery. |
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| The Design and Implementation of a Log-Structured File System |
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| Data needs to be flushed out to disk for safety reasons
Disk performance becomes dominated by write operations
Executing writes as soon as they occur reduces traffic, but less than 5% of the potential bandwidth is used for new data
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| Load Balancing and Stability Issues in Algorithms |
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| Abstract—Service composition enables flexible creation of new
services by assembling independent service components. We are
focused on the scenario where such composition takes place
across the wide-area Internet. We envision independent providers
deploying and managing service instances and portal providers
composing them to quickly enable new applications in nextgeneration
networks. |
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| Logical Clocks and Causal Ordering |
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| Transaction T transfers Rs 10,000 from S1 to S2
• Consider the situation when:
– State of S1 is recorded after the deduction and state of S2 is recorded before the addition
– State of S1 is recorded before the deduction and state of
S2 is recorded after the addition |
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| Inherent Limitations of a Distributed System |
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| A distributed system is a set of computers that communicate
over a network, and do not share a common memory or a
common clock
n Absence of a common (global) clock
u No concept of global time
u It’s difficult to reason about the temporal ordering of events
F Cooperation between processes (e.g.,
producer/consumer, client/server)
F Arrival of requests to the OS (e.g., for resources)
F Collecting up-to-date global state
u It’s difficult to design and debug algorithms in a distributed
system
F Mutua |
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| Recovery System |
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| Transaction failure :
Logical errors: transaction cannot complete due to some internal error condition
System errors: the database system must terminate an active transaction due to an error condition (e.g., deadlock)
System crash: a power failure or other hardware or software failure causes the system to crash.
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| Distributed Scheduling |
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| Resource management component of a system which moves jobs around the processors to balance load and maximize overall performance.
Typically makes sense in LAN level distributed systems due to latency concerns.
Needed because of uneven distribution of tasks on individual processors
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| Distributed Mutual Exclusion Algorithms |
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| Mutual exclusion: Concurrent access of processes to a shared resource or
data is executed in mutually exclusive manner.
Only one process is allowed to execute the critical section (CS) at any given
time.
In a distributed system, shared variables (semaphores) or a local kernel
cannot be used to implement mutual exclusion. |
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| Distributed Systems -Major Design Issues |
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| Integration of system services presenting a transparent view of a multiple computer system with distributed resources and control.Consisting of concurrent processes accessing distributed shared or replicated resources through message passing in a network environment.
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| Distributed Systems Architectures |
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| To explain the advantages and disadvantages of distributed systems architectures
To describe different approaches to the development of client-server systems |
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| Distributed Shared Memory |
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| Abstraction that supports the notion of shared memory in a physically non shared (distributed) architecture.
Local as well as remote memories can be accessed in a uniform manner, with the location of the shared region transparent to the application program. |
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| DISTRIBUTED MUTUAL EXCLUSION |
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| Mutual exclusion ensures that concurrent processes
make a serialized access to shared resources or data. |
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| Distributed DBMS |
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| Distributed transaction management.
Distributed concurrency control.
Distributed deadlock detection.
Distributed recovery control.
Distributed integrity control.
X/OPEN DTP standard.
Replication servers as an alternative.
How DBMSs can support the mobile worker. |
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| Distributed File Systems |
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| The purpose of a distributed file system (DFS) is to allow users of physically distributed
computers to share data and storage resources by using a common file system. A typical
configuration for a DFS is a collection of workstations and mainframes connected by a
local area network (LAN). |
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| Distributed Deadlock Detection |
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| Distributed deadlock models are presented for resource and communication deadlocks. Simple
distributed algorithms for detection of these deadlocks are given. We show that all true deadlocks are
detected and that no false deadlocks are reported. |
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| Distributed Computation |
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| Abstract. We describe a new circulation-based method to determine cuts in an undirected graph. A
circulation is an oriented labeling of edges with integers so that at each vertex, the sum of the in-labels
equals the sum of out-labels. For an integer k, our approach is based on simple algorithms for sampling a
circulation (mod k) uniformly at random. |
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| Distributed Deadlock |
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| Prevention
Too expensive in time and network traffic in a distributed system
Avoidance
Determining safe and unsafe states would require a huge number of messages in a DS
Detection
May be practical, and is primary chapter focus
Resolution
More complex than in non-distributed systems |
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| Distributed Deadlock Detection Algorithm |
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| We propose an algorithm for detecting deadlocks among transactions running concurrently in a
distributed processing network (i.e., a distributed database system). The proposed algorithm is a
distributed deadlock detection algorithm. A proof of the correctness of the distributed portion of the
algorithm is given, followed by an example of the algorithm in operation. The performance characteristics
of the algorithm are also presented. |
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| Database Concurrency Control and Recovery |
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| Pessimistic concurrency control
Two-phase locking (2PL) and Strict 2PL
Timestamp ordering (TSO) and Strict TSO
Optimistic concurrency control (OCC)
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| Database System Architectures |
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| Database functionality can be divided into:
Back-end: manages access structures, query evaluation and optimization, concurrency control and recovery.
Front-end: consists of tools such as forms, report-writers, and graphical user interface facilities.
The interface between the front-end and the back-end is through SQL or through an application program interface.
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| Database Recovery Techniques |
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| Purpose of Database Recovery
To bring the database into the last consistent state, which existed prior to the failure.
To preserve transaction properties (Atomicity, Consistency, Isolation and Durability).
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| Cryptography |
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| Cryptography – Greek for hidden and writing is a means of transforming data in a way that renders it unreadable by anyone except the intended recipient.
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| Conventional Encryption |
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| An encryption scheme has five ingredients:
– Plaintext
– Encryption algorithm
– Secret Key
– Ciphertext
– Decryption algorithm
• Security depends on the secrecy of the key, not the secrecy
of the algorithm |
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| Concurrency Control |
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| A lock is a mechanism to control concurrent access to a data item
Data items can be locked in two modes :
1. exclusive (X) mode. Data item can be both read as well as
written. X-lock is requested using lock-X instruction.
2. shared (S) mode. Data item can only be read. S-lock is
requested using lock-S instruction |
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| Causal Order of Messages |
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| The purpose of causal ordering of messages is to insure that the same causal relationship for the "message send" events correspond with "message receive" events. (i.e. All the messages are processed in order that they were created.)
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| Backward Error Recovery in Redundant Disk Arrays |
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| Redundant disk arrays are single fault tolerant, incorporating a layer of error handling
not found in nonredundant disk systems. Recovery from these errors is complex,
due in part to the large number of erroneous states the system may reach.
The established approach to error recovery in disk systems is to transition directly
from an erroneous state to completion. |
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| Authentication for Distributed Systems |
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| A fundamental concern in building a secure distributed system is authentication of local and remote entities
in the system. We survey authentication issues in distributed system design. Two basic paradigms
underlying the design of authentication protocols are presented. |
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| AgreementProtocols |
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| Classification of Tolerance
• Types of tolerance:
– Masking – system always behaves as per specifications
even in presence of faults
– Non Non-masking – system may violate specifications in
presence of faults. Should at least behave in a well well-defined
manner |
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| A Cryptographic Model for Access-Control |
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| Trying to reconcile “trust models” in cryptography, access-control,Cryptographic models,Access-control models
Case study: object storage
The issue: Distributed storage servers
The (obvious?) protocol: Capabilities
Delegation: problem and solution
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