A distributed edit-compile workflow

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1 Time Synhroniztion nd Logil Cloks Tody 1. The need for time synhroniztion 2. Wll lok time synhroniztion 3. Logil Time: Lmport Cloks COS 418: Distriuted Systems Leture 4 Kyle Jmieson 2 A distriuted edit-ompile workflow Physil time à 2143 < 2144 è mke doesn t ll ompiler Wht mkes time synhroniztion hrd? 1. Qurtz osilltor sensitive to temperture, ge, virtion, rdition Aury. one prt per million (one seond of lok drift over 12 dys) 2. The internet is: Asynhronous: ritrry messge delys Best-effort: messges don t lwys rrive Lk of time synhroniztion result possile ojet file mismth 3 4 1

2 Tody 1. The need for time synhroniztion 2. Wll lok time synhroniztion Cristin s lgorithm, Berkeley lgorithm, NTP 3. Logil Time: Lmport loks Just use Coordinted Universl Time? UTC is rodst from rdio sttions on lnd nd stellite (e.g., the Glol Positioning System) Computers with reeivers n synhronize their loks with these timing signls Signls from lnd-sed sttions re urte to out milliseonds Signls from GPS re urte to out one miroseond Why n t we put GPS reeivers on ll our omputers? 5 6 Synhroniztion to time server Suppose server with n urte lok (e.g., GPSdisiplined rystl osilltor) Could simply issue n RPC to otin the time: Client Server Time But this doesn t ount for network lteny Messge delys will hve outdted server s nswer 7 Cristin s lgorithm: Outline 1. Client sends request pket, timestmped with its lol lok T 1 2. Server timestmps its reeipt of the request T 2 with its lol lok 3. Server sends response pket with its lol lok T 3 nd T 2 4. Client lolly timestmps its reeipt of the server s response T 4 Client T 1 Server T 2 T 4 T 3 How the lient n use these timestmps to synhronize its lol lok to the server s lol lok? Time 8 2

3 Cristin s lgorithm: Offset smple lultion Tody Client Server 1. The need for time synhroniztion Gol: Client sets lok ß T 3 + δ resp Client smples round trip time δ = δ req + δ resp = (T 4 T 1 ) (T 3 T 2 ) T 1 δ req T 2 2. Wll lok time synhroniztion Cristin s lgorithm, Berkeley lgorithm, NTP But lient knows δ, not δ resp Assume: δ req δ resp T δ 3 resp T 4 3. Logil Time: Lmport loks Client sets lok ß T 3 + ½δ Time 9 10 Berkeley lgorithm A single time server n fil, loking timekeeping The Berkeley lgorithm is distriuted lgorithm for timekeeping Berkeley lgorithm Mster mhine: polls L other mhines using Cristin s lgorithm à { θ i } (i = 1 L) Mster Assumes ll mhines hve eqully-urte lol loks Otins verge from prtiipting omputers nd synhronizes loks to tht verge

4 Tody 1. The need for time synhroniztion 2. Wll lok time synhroniztion Cristin s lgorithm, Berkeley lgorithm, NTP 3. Logil Time: Lmport loks The Network Time Protool (NTP) Enles lients to e urtely synhronized to UTC despite messge delys Provides relile servie Survives lengthy losses of onnetivity Communites over redundnt network pths Provides n urte servie Unlike the Berkeley lgorithm, leverges heterogeneous ury in loks NTP: System struture Servers nd time soures re rrnged in lyers (strt) Strtum 0: High-preision time soures themselves e.g., tomi loks, shortwve rdio time reeivers Strtum 1: NTP servers diretly onneted to Strtum 0 Strtum 2: NTP servers tht synhronize with Strtum 1 Strtum 2 servers re lients of Strtum 1 servers Strtum 3: NTP servers tht synhronize with Strtum 2 Strtum 3 servers re lients of Strtum 2 servers NTP opertion: Server seletion Messges etween n NTP lient nd server re exhnged in pirs: request nd response Use Cristin s lgorithm For i th messge exhnge with prtiulr server, lulte: 1. Clok offset θ i from lient to server 2. Round trip time δ i etween lient nd server Over lst eight exhnges with server k, the lient omputes its dispersion σ k = mx i δ i min i δ i Client uses the server with minimum dispersion Users omputers synhronize with Strtum 3 servers

5 NTP opertion : Clok offset lultion Client trks minimum round trip time nd ssoited offset over the lst eight messge exhnges (δ 0, θ 0 ) θ 0 is the est estimte of offset: lient djusts its lok y θ 0 to synhronize to server Offset θ θ 0 Eh point represents one smple NTP opertion: How to hnge time Cn t just hnge time: Don t wnt time to run kwrds Rell the mke exmple Insted, hnge the updte rte for the lok Chnges time in more grdul fshion Prevents inonsistent lol timestmps δ 0 Round trip time δ Clok synhroniztion: Tke-wy points Cloks on different systems will lwys ehve differently Disgreement etween mhines n result in undesirle ehvior NTP, Berkeley lok synhroniztion Rely on timestmps to estimte network delys 100s μs ms ury Cloks never extly synhronized Tody 1. The need for time synhroniztion 2. Wll lok time synhroniztion Cristin s lgorithm, Berkeley lgorithm, NTP 3. Logil Time: Lmport loks Often indequte for distriuted systems Often need to reson out the order of events Might need preision on the order of ns

6 Motivtion: Multi-site dtse replition The onsequenes of onurrent updtes A New York-sed nk wnts to mke its trnstion ledger dtse resilient to whole-site filures Replite the dtse, keep one opy in sf, one in ny Client sends query to the nerest opy Client sends updte to oth opies Replite the dtse, keep one opy in sf, one in ny Deposit 100 Sn Frniso Inonsistent replis! Updtes should hve een performed 1,000 1,000 in the sme order t eh opy New York 1,010 1,100 1,110 1,111 Py 1 interest 21 Ide: Logil loks 22 Defining hppens-efore (à) Consider three proesses:,, nd Lndmrk 1978 pper y Leslie Lmport Nottion: Event hppens efore event ( à ) Insight: only the events themselves mtter Ide: Disregrd the preise lok time Insted, pture just hppens efore reltionship etween pir of events

7 Defining hppens-efore (à) Cn oserve event order t single proess Defining hppens-efore (à) 1. If sme proess nd ours efore, then à Defining hppens-efore (à) 1. If sme proess nd ours efore, then à 2. Cn oserve ordering when proesses ommunite Defining hppens-efore (à) 1. If sme proess nd ours efore, then à 2. If is messge reeipt of, then à

8 Defining hppens-efore (à) 1. If sme proess nd ours efore, then à 2. If is messge reeipt of, then à 3. Cn oserve ordering trnsitively Defining hppens-efore (à) 1. If sme proess nd ours efore, then à 2. If is messge reeipt of, then à 3. If à nd à, then à Conurrent events Not ll events re relted y à, d not relted y à so onurrent, written s d d Lmport loks: Ojetive We seek lok time C() for every event Pln: Tg events with lok times; use lok times to mke distriuted system orret Clok ondition: If à, then C() < C()

9 The Lmport Clok lgorithm Eh proess P i mintins lol lok C i 1. Before exeuting n event, C i ß C i + 1 The Lmport Clok lgorithm 1. Before exeuting n event, C i ß C i + 1: Set event time C() ß C i C 1 =0 C 2 =0 C 3 =0 C 1 =1 C() = 1 C 2 =1 C 3 = The Lmport Clok lgorithm The Lmport Clok lgorithm 1. Before exeuting n event, C i ß C i + 1: Set event time C() ß C i 1. Before exeuting n event, C i ß C i Send the lol lok in the messge m C 1 =2 C() = 1 C() = 2 C 2 =1 C 3 =1 C 1 =2 C() = 1 C() = 2 C(m) = 2 C 2 =1 C 3 =

10 The Lmport Clok lgorithm 3. On proess P j reeiving messge m: Set C j nd reeive event time C() ß1 + mx{ C j, C(m) } C 1 =2 C() = 1 C() = 2 C(m) = 2 C 2 =3 C() = 3 C 3 =1 Lmport Timestmps: Ordering ll events Brek ties y ppending the proess numer to eh event: 1. Proess P i timestmps event e with C i (e).i 2. C().i < C().j when: C() < C(), or C() = C() nd i < j Now, for ny two events nd, C() < C() or C() < C() This is lled totl ordering of events Mking onurrent updtes onsistent Totlly-Ordered Multist Rell multi-site dtse replition: Sn Frniso () deposited 100: New York () pid 1 interest: We rehed n inonsistent stte Gol: All sites pply updtes in (sme) Lmport lok order Client sends updte to one repli site j Repli ssigns it Lmport timestmp C j. j Key ide: Ple events into sorted lol queue Sorted y inresing Lmport timestmps Could we design system tht uses Lmport Clok totl order to mke multi-site updtes onsistent? Exmple: s lol queue: 1.2 ß Timestmps

11 Totlly-Ordered Multist (Almost orret) Totlly-Ordered Multist (Almost orret) 1. On reeiving n updte from lient, rodst to others (inluding yourself) 2. On reeiving n updte from repli: ) Add it to your lol queue ) Brodst n knowledgement messge to every repli (inluding yourself) queues, queues queues nd k s mrks fully k ed mrks fully k ed proesses k 3. On reeiving n knowledgement: Mrk orresponding updte knowledged in your queue 4. Remove nd proess updtes everyone hs k ed from hed of queue (Ak s to self not shown here) Totlly-Ordered Multist (Corret version) Totlly-Ordered Multist (Corret version) 1. On reeiving n updte from lient, rodst to others (inluding yourself) On reeiving or proessing n updte: ) Add it to your lol queue, if reeived updte ) Brodst n knowledgement messge to every repli (inluding yourself) only from hed of queue On reeiving n knowledgement: Mrk orresponding updte knowledged in your queue 4. Remove nd proess updtes everyone hs k ed from hed of queue k k (Ak s to self not shown here)

12 So, re we done? Does totlly-ordered multist solve the prolem of multi-site replition in generl? Not y long shot! 1. Our protool ssumed: No node filures No messge loss No messge orruption 2. All to ll ommunition does not sle 3. Wits forever for messge delys (performne?) Tke-wy points: Lmport loks Cn totlly-order events in distriuted system: tht s useful! We sw n pplition of Lmport loks for totllyordered multist But: while y onstrution, à implies C() < C(), The onverse is not neessrily true: C() < C() does not imply à (possily, ) Cn t use Lmport lok timestmps to infer usl reltionships etween events Wednesdy Topi: Vetor Cloks & Distriuted Snpshots Fridy Preept: RPCs in Go Why glol timing? Suppose there were n infinitely-preise nd glolly onsistent time stndrd Tht would e very hndy. For exmple: 1. Who got lst set on irplne? 2. Moile loud gming: Whih ws first, A shoots B or vie-vers? 3. Does this file need to e reompiled?

13 Totlly-Ordered Multist (Attempt #1) Totlly-Ordered Multist (Corret version) queues, queues queues nd k s mrks fully k ed mrks fully k ed proesses queues nd k s proesses mrks fully k ed proesses, then 1.2 k k Note: k s to self not shown here queues, queues queues queues nd k s mrks fully k ed proesses mrks fully k ed proesses k s mrks fully k ed proesses mrks fully k ed proesses 1.2 k 1.2 k 1.2 (Ak s to self not shown here) Time stndrds Universl Time (UT1) In onept, sed on stronomil oservtion of the sun t 0º longitude Known s Greenwih Men Time Interntionl Atomi Time (TAI) Beginning of TAI is midnight on Jnury 1, 1958 Eh seond is 9,192,631,770 yles of rdition emitted y Cesium tom Hs diverged from UT1 due to slowing of erth s rottion Coordinted Universl Time (UTC) TAI + lep seonds, to e within 0.9 seonds of UT1 Currently TAI UTC = 36 VC pplition: Order proessing Suppose we re running distriuted order proessing system Eh proess = different user Eh event = n order A user hs seen ll orders with V(order) < the user s urrent vetor