Lazy Type Changes in Object-oriented Database. Shan Ming Woo and Barbara Liskov MIT Lab. for Computer Science December 1999

Transcription

1 Lazy Type Chages i Object-orieted Database Sha Mig Woo ad Barbara Liskov MIT Lab. for Computer Sciece December 1999

2 Backgroud wbehavior of OODB apps compose of behavior of persistet obj wbehavior of objects govered their types wtype chageseeded to update OODB apps whow to execute type chage?

3 Requiremets w A type chage may affect er types oth w A upgradecosists of a set ed of re type chages w Upgrades are ordered w Executio of a upgrade have to b atomic w.r.t. app trasacti os to prevet type errors

4 Na ve Executio w Step 1: shut dow the database w Step 2: trasform objects w Step 3: restart the database w Drawback: database availability fers su w Not suitable for large-scal e databases ad missio-critical databases w Solutio: lazy type chages

5 Lazy Type Chages w Objects trasformed y, lazil i.e. just before use w Database availability t affected o w Workload of a upgrade: 1. Spread effectively ver time o 2. Distributed amog fairly apps w Suitable for all databases cludigi large-scale ad missio-cri tical oes

6 Theory w Type chage: C T =<T, T, f T > Pre-type Post-type w Upgrade: U=<, {C}> i Trasform fuctio Serial # Set of type chages

7 Theory (cot.) w Trasform fuctios: Act o oe object at a time Preserve object idetiti es, i.e. object refereces survive chages type Should ot modify ay data w Upgrades have to complete, be i.e. should ot affect her ot types

8 Implemetatio Desig (1) w Based o Thor [ECOOP99] Distributed cliet/server OODBMS Optimistic cocurrecy cotrol Servers store objects, validate trasactios Cliets cache objects, operate o cached objects o behalf of apps Object idetity partly locatio depedet Objects i cliet cache idexed by a residet object table (ROT) Each ROT etry stores a dispatch vector poiter ad a field poiter for object Poiters i empty a etry are ull Poiters i fulletry a are up-to-date

9 Implemetatio Desig (2) w Sigle Server Upgrades stored at the server ad pushed to cliets Objects trasformed by cliets before used by apps Objects trasformatios are regarded as modificatios Ivariat: All full ROT etries represet up-to date objects. At receipt of ew upgrade, cliet w aborts ruig trasactio if it used affected objects to guaratee atomicity w scas ROT ad emptie s affected etries to preserve ivariat

10 Implemetatio Desig (3) w Sigle server cotiued Special cliet cooperate with garbage collector to trasform rarely used objects Upgrades complete i order: whe all objects affected by the oldest upgrade has bee trasformed, it is discarded Problem: objects chage sizes across trasformatios Size decreases--- overwrite origial obje Size icreases---fid space o the same serve page: w If succeed, update page offset table w Otherwise, write to aother page ad replace origia l object with surrogate Referece through surrogate is shortcut whe the referrig obje is modified

11 Implemetatio Desig (4) w Multiple servers A master server stores the master copy of all upgrades Upgrades pushed from master server to other servers Each server seds upgrades to its cliets ad each cliet processes upgrades as i the sigle server implemetatio Problem: eed to maitai cosistecy whe a cliet talks to multiple servers---a upgrade may arrive at oe server before aother Cliet acts as relay to restore cosistecy: ea commit request is tagge with serial umber of ewest upgrade at clie

12 Coclusio w Lazy type chages preserve database availability ad ca be tly efficie implemeted--- essetial for large-scale missio-critical databases Future work w Project focused rsistet o pe objects: how avoid recompilig icatios? appl Uiversal applicatio framework? w How to exted o-database to eviromet