Naming

1. Naming CSCI 4780/6780

2. Attribute-based Naming • Flat and structured names provide location transparency • Structured names are also human-friendly • Increasingly, users and application want to locate for resources based on features, contents, characteristics • Web search • P2P search

3. Attribute-based Naming • Describe entities as (attribute, value) pairs • Queries specify constraints on attributes • In P2P music search: • Title = “Four seasons” • Composer = “Vivaldi” • Orchestra = “Royal Philharmonic”

4. Directory Services • Naming systems for attribute-based names • Entities associated with a set of attributes that are searchable • Resource description framework (RDF) • Triplet consisting of (subject, predicate, object) • Subjects, predicates and objects can themselves be resources • Obtain information about subject using predicates and objects • RDFs can be implemented in a distributed fashion, but costs can be quite high

5. LDAP • Lightweight Directory Access Protocol • Hierarchical implementation • Combines structured naming and attribute-based naming • Consists of number of records • Collection of (attribute, value) pairs • Single-valued and multiple-valued attributes • Directory Information Base (DIB) – Collection of directory entries • Relative Distinguished Name • /C=NL/O=Vrije Universiteit/OU=Comp. sc.

6. LDAP Directory Entry

7. Directory Information Tree

8. LDAP • Two directory entries having Host_Name as RDN.

9. Lookups in LDAP • Read – Reads a particular entry • List – Lists all outgoing edges from a node • LDAP implementation is similar to DNS implementation • Supports richer sets of queries • Search(“&(C=NL)(O=Vrije Universiteit)(OU=*)(CN = Main Server) • Searching is expensive • Research on DHT-based implementation of attribute-based naming

10. Synchronization CSCI 4780/6780

11. Importance of Clocks & Synchronization • Avoiding simultaneous access of resources • Cooperate to grant exclusive access for small durations • Process may need to agree upon ordering of events • First person to complete assignment will receive bonus • How to infer who completed first? • Synchronization & ordering is difficult in distributed setting

12. Chapter Outline • Actual time-based synchronization • Physical clocks • Synchronizing multiple clocks • Relative ordering-based synchronization • Logical clocks, Lamport’s algorithm, Vector time stamps • Global state • Leader election algorithms (time permitting) • Mutual exclusion in distributed systems • Distributed transactions

13. Clocks in Systems • Notion of time in centralized system is unambiguous • Do a system call to get current time • Important property of time – Always moves forward • If B executes the system call after A, B will never get a lower value • Many system operations need some notion of time for correctness • Example – “Make” program in Unix • Re-compiles only files that were changed since last compilation • What happens when there is no global agreement on time?

14. Clock Synchronization • When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.

15. System Clocks & Clock Skew • Almost all computers have clocks (timers) • A precisely cut quartz crystal kept under tension • Oscillates at a well-defined frequency • Two registers – Counter & holding register • Each oscillation of crystal decrements counter by 1 • Interrupt when counter is zero & reload from holding reg. • Each interrupt is a tick • Clocks at multiple CPUs cannot be guaranteed to oscillate at exact same frequency • Difference between various clocks – Clock Skew

16. How is Time Actually Measured? • Solar time – Based on earth’s rotation • Transit of sun: Sun reaching the highest apparent in sky • Solar day: Time b/w two consecutive sun transits • Solar second: 1/86400th of a solar day • Mean solar second: 1/86400th of a mean solar day • Atomic time • Second: Time for cesium-133 atom to make 9,192,631,770 transitions • International atomic time (TAI) • Leap seconds to resolve difference b/w TAI & solar time (UTC) • NIST broadcasts UTC on radio station (WWV)

17. Physical Clocks (1) • Computation of the mean solar day.

18. Physical Clocks (2) • TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun.

19. Clock Synchronization Algorithms • Two related problems • If one machine has WWV receiver: synchronize all machines to machine with the WWV receiver • No WWV receiver: Keep all machines relatively synchronized • Many algorithms with some key assumptions • Each machine has timer that causes interrupt H times/sec • Increments software clock on each interrupt • Cp(t) indicates clock value when UTC time is t • In ideal world dC/dt = 1

20. Drift & Max. Drift Rate • In real world dC/dt is not one • Maximum drift rate: ρ such that 1- ρ ≤ dC/dt ≤ 1 + ρ • Specified by manufactures • The relation between clock time and UTC when clocks tick at different rates.

21. Clock Synchronization Algorithms • Two clocks drifting from UTC in opposite directions at rate of ρ need to be synchronized every δ/2ρ secs. • Christian’s Algorithm: • Suited when one machine has WWV receiver • Each machine sends a request to time server periodically (period < δ/2ρ ) seconds • Time server responds with its current time (CUTC) • Simple scheme • Set receivers time to CUTC • Two problems • Clock might run backward !!! • Doesn’t consider processing time

22. Cristian's Algorithm (T2 – T1) + (T4 - T3) θ = T3 + ---------------------------- - T4 2 • Introduce change gradually – Reduce time by a small amount