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CS244B Final Review Session

CS244B Final Review Session. Anthony Ho. Administration. Distributed File Systems project: -Working on grading -Majority of students fail at least one test. Final exam. Tuesday 3:30 – 6:30 pm Gates B01 Cumulative Closed book Multiple choice and short answer. Agenda.

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CS244B Final Review Session

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  1. CS244B Final Review Session Anthony Ho

  2. Administration • Distributed File Systems project:-Working on grading-Majority of students fail at least one test.

  3. Final exam • Tuesday 3:30 – 6:30 pm Gates B01 • Cumulative • Closed book • Multiple choice and short answer

  4. Agenda • Directories and Naming • Accounts and Authentication • Transactions, Agreement, and Reconciliation

  5. Common, Proper Names, Description • Common names can refer to a set of objects • Proper names refer to a specific object • Common names have synonymy, ambiguity, substitutability with description

  6. Pure and Impure names • Pure names – No encoding or description of the entity being referred to • Impure names – Contains some information about the entity

  7. Advantages/Disadvantages of Impure Names • Can encode structure used for mappingEx: • Can include some information about the object. Ex: dev/cdrom • Can use structure to facilitate distributed allocation – Some controller assigned responsibility for some subset of names • Can lose flexibility – Entities cannot as easily change without changing their name • Longer names

  8. External/Internal names • External names are necessary for: -error reporting-referring to objects from user-visible interfaces-address-space and machine-independent identification of objects

  9. Internal Names • More efficient and simpler for software to implement • “External names are a necessity; internal names are an optimization” • Sometimes a necessary optimization • Internal names cannot capture intent • Examples: IP addresses

  10. UUID • Identifiers that can be unique for every object in the universe. An internal name • 256 bits? • May introduce impurities for structuring to facilitate mapping due to size of identifier • All problems of internal names. • Legacy issues. • Semantics of UUID naming unclear

  11. When to use internal names • Internal names are much shorter than external names • Only must be unique in the context that uses them. Transient • How large should they be, 32 usually. Up to 64. Or variable length

  12. Cheriton’s view of names • Usually, a server maps names to objects. • Cheriton argues that each object maps external names for objects that it implements • Fate-sharing violated if a name server used

  13. Decentralized/Centralized naming • Centralized. All naming lookups go to one place • Decentralized: Multiple name authorities • Security issue of decentralized: How to know a server is responsible for naming of a certain object

  14. Examples: • Lampson Directory Naming: Database changes updated in “sweeps” • Epidemic approach: Names spread like an epidemic • V-system: Uses internal naming

  15. URLs • Initially impurity limits the flexibility of this name. Cannot move object without renaming it • Benefit: Associated brand • Benefit: Subtree redirection • Benefit: Logical or virtual name

  16. Shared vs Public Key Encryption • Shared key – Symmetric key. Same key used for encryption and decryption • Public key – Asymmetric key. Different keys used for encryption and decryption. • Public key encryption much slower than Shared keys

  17. Encryption Terms • Confidentiality – Communication cannot be read by others • Integrity – Communication cannot be modified without detection • Authentication – Can determine who the message is from

  18. Three issues with key management • Lifetime – How long before regenerating key. • Selection – How to generate a key • Distribution – How to distribute keys among parties: Outside channel, chained key distribution, two-level key distribution, trusted third party

  19. How to Authenticate • Supply key or password • Encrypt a value, if decrypts to the right value, then the principal must have the key • Third party vouches for principal. Certificates

  20. Problems with Certificates • Revocation difficult • Difficult to determine liability • Verifying entities is expensive and might not be worth it

  21. Open Security vs Secrecy • Secrecy has a single point of failure, the key • Cannot tell when key is no longer secret • Open security goes for safety and availability • Open security puts everything in the open and clear to everyone • Unexpected actions will be contested and detected quickly

  22. Non-blocking commit • Disseminate result of prepare to commit and before to commit. • Two phase commit is blocking. Servers block while waiting for result of prepare to commit • Surviving server that knows state of prepare to commit can complete the protocol by committing

  23. Consistency vs Availability • The more servers you have the more possibility for inconsistency • Consistency is safer, but typically availability is more useful

  24. Any questions? • Good luck on the final!

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