Reliability and Security in Database Servers By Samuel Njoroge

Overview Role of Database Management Systems in creating reliable & secure systems. Sources of failure. Recovery from failure. Distribution, consistency, and managing databases. DBMS security and access.

Database failure and recovery Database must be able to recover from catastrophic failure. Database designers must always have a recovery plan Recovery plan ensures database is restored to a correct state ASAP. The plan must include what happens with new incoming data in the interim.

Sources of database Failures. Database server computer crashes  Information may be corrupted as it still resides in memory & not HDD Database server program crashes  Generally database program should never crash Database client computer crashes  Results in locks and inactive transactions. Network connection between client and server fails A transaction executes a rollback operation  A transaction has voluntarily cancelled itself therefore its updates must be removed from database.

Sources of database Failures A transaction executes an illegal operation  Systems detects that transaction has violated a protocol  Server must abort and recover through a rollback. Two or more transactions deadlock  System must abort one or more transactions to break dreadlock One or more transactions induce errors into the database  Application program runs incorrectly and updates database introducing inconsistent or nonfactual information Data on the disk is corrupted.  A hardware error causes some information on database to become unavailable.

Backup and checkpoints. Backup: Copy of the database at a certain time.  Contains enough info for restoration to same state prior to failure.  Updates that happened after the backup are not part of the state Checkpoint: Operation that forces the database on the disk to be in a state that is consistent with all committed transactions. If disk is not corrupted use the last checkpoint However if disk is corrupted, backup is used.

Transaction logs Records all activity within the server. Log contains:  Unique Transaction ID for every transaction upon execution.  Name of action preformed  Object that is referenced by the action  The Effect of the action on the object. Transaction log used to recreate state of database prior to failure Log should be in remote locations

Recovery: Roll Forward Steps: Recover database in a correct state Reapply all changes of the committed transactions in same order they were originally committed. Pre-requisite: System must have a redo log, which maintains all committed changes in order.

Recovery: Roll Back. Involves removing all the effect of transactions since failure. Begins from current state and undoes each update in reverse order Prerequisite: System must keep undo log Undo log contains original image of each updated value. Once in correct state, it may be possible to redo some transactions as long as there is no conflict.

Automatic recovery Rollback is supported by most commercial DBMSs Oracle has control file, containing redo logs, timestamps, names, locations of Databases. When failure occurs DB admin must first copy the damaged file from best backup. Recover command prompts for each redo log that must be run. Oracle has both “hot” and “cold” backups. Hot backup when in use, cold backup - when database is unavailable to users

Security in Relational Database System. Security always begins with the physical security of the actual systems. Database system must have support for 3 types of security:  Account security: validates users  Access security: protection of DB objects  Operating System security: Protects files

User Authorization: MySQL *. Create user Jane identified by ‘crockette’; alter user Dick account lock; alter user Dick identified by stop-please; alter user Dick password expire; create profile LimitedUser limit CONNECT_TIME 10; create user OPS$hannibal profile LimitedUser; alter user OPS$hannibal identified by use-this- password;

Protection of Database objects Involves privileges, to read, updated, append, create and drop.  grant insert on Customer to Jane;  grant select on Customer to public;  grant all on Employee to Jane;  revoke delete on Employee from Jane;  grant update on Customer(street, city, state, zip) to Jane;

Protection of Database objects create role Clerk not identified; grant all on Rental, PreviousRental to Clerk; grant role Clerk to Dick; create role FloorManager identified by ImInCharge; grant role Clerk to FloorManager;

Summary Reliability Backups and checkpoints Recovery via Roll Forward and Rollback Recovery from disk corruption Security: User authorization Security: protection of database objects

Works cited: Principles of Database Systems with Internet and Java Applications - by Greg Riccardi