1. Highly Available Database Systems Seminar im WS 2005/2006: Dependable Adaptive Information Systems (DAIS) Technische Universität Kaiserslautern Ou Yi

2. Slide 216 October 2015Highly Available Database Systems – Ou Yi Agenda Introduction of Basic Concepts Three Types of Coupling DDBS & PDBS Clustering HA in Practice

3. Slide 316 October 2015Highly Available Database Systems – Ou Yi Availability Concept Why? Availability “the fraction of the offered load that is processed with acceptable response times” A = MTTF / ( MTTF + MTTR ) The Number of Nines System Type Availability Unavailability (min/year) Well-managed 99.9 % 526 Fault-tolerant 99.99 % 53 High-availability 99.999 % 5 Very-high-availability 99.9999 % 0.5 Ultra-availability 99.99999 % 0.05

4. Slide 416 October 2015Highly Available Database Systems – Ou Yi HA and Multi-Computer System High Availability A → 1 downtime → 0 causes of downtime trouble-free components? redundancy! Multi-Computer System higher availability than single-computer system higher performance communication (network or shared HW)  tight coupling, close coupling, loose coupling

5. Slide 516 October 2015Highly Available Database Systems – Ou Yi Tight Coupling Characteristics shared main memory single copy of software Multiprocessor and SMP Pro & Contra + computing power + communication − availability − extensibility (max 16)

6. Slide 616 October 2015Highly Available Database Systems – Ou Yi Tight Coupling (2) Shared Everything (Shared Memory) a DBMS running on a multiprocessor multiprocessing with support of OS used as node architecture tight coupling

7. Slide 716 October 2015Highly Available Database Systems – Ou Yi Loose Coupling Characteristics interconnected through network independency Pro & Contra + error isolation + extensibility (unlimited?) − communication

8. Slide 816 October 2015Highly Available Database Systems – Ou Yi Loose Coupling (2) Shared Nothing independent computers (or nodes) database: physically partitioned, logically unified each node runs a copy of DBMS, which has direct access only to its own partition loose coupling

9. Slide 916 October 2015Highly Available Database Systems – Ou Yi Loose Coupling (3) Shared Disk external storage is shared, “all to all” each node runs a DBMS, independent a TA can be completed locally no need for  distributed query plan  distributed commit required  concurrency control  buffer coherency control

10. Slide 1016 October 2015Highly Available Database Systems – Ou Yi Close Coupling Characteristics coupling component private main memory and software Pro & Contra + communication + error isolation + extensibility (max 32) − proprietary design Shared Data multi-computer DBS hybrid of close coupling and shared disk

11. Slide 1116 October 2015Highly Available Database Systems – Ou Yi Close Coupling (2) Shared Data (cont.) Parallel Sysplex Cluster  shared disk & close coupling Coupling Facility  specialized SMP with large global main memory  useful for global tasks Sysplex Timer  global unique time  e.g. global log file

12. Slide 1216 October 2015Highly Available Database Systems – Ou Yi Shared Disk vs. Shared Nothing Node Failure SD: all the data still accessible to surviving nodes SN: data of the failed node can not be easily accessed Increased Workload SD: new nodes can be easily added and participate in query processing SN: new nodes have no direct access to data, reallocation is expensive!

13. Slide 1316 October 2015Highly Available Database Systems – Ou Yi DDBS & PDBS DDBS distributed database  a collection of multiple, logically unified databases distributed over a computer network distributed DBMS  manages the distributed DB  maintains distribution transparency  multiple DBMSs cooperating across sites (SN!) PDBS “locally distributed database systems of the types shared nothing, shared disk, or shared everything”

14. Slide 1416 October 2015Highly Available Database Systems – Ou Yi DDBS & PDBS (2) Replicated Data across nodes, improve data availability improve performance, consistency? ideally: one-copy equivalence ROWA (Read-Once/Write-All) algorithm  + one-copy equivalence  − response time strict consistency is expensive! snapshot consistency  snapshots, materialized views  consistent in some point of time in the past  acceptable for applications: analysis, reporting, etc.  periodical refreshing or triggered refreshing

15. Slide 1516 October 2015Highly Available Database Systems – Ou Yi DDBS & PDBS (3) Disaster Recovery destroys complete computer center (site failure) keeping backup medium off-site  time consuming  server to be set up  whole database to be retrieved  loss of TAs online replication  identical configuration  highly up-to-date backup database  quick takeover possible  variations

16. Slide 1616 October 2015Highly Available Database Systems – Ou Yi DDBS & PDBS (4) Online Replication (cont.) 1-safe  asynchronous log transfer  + response time  − loss of TAs very-safe  distributed two-phase commit  + no loss of TAs  − availability 2-safe  backup involved in commit process in normal case  primary independent from backup, unilaterally commit

17. Slide 1716 October 2015Highly Available Database Systems – Ou Yi Clustering Definition interconnecting of a group of computers so that they work together closely and it can be viewed as if it were a single computer special components for  load distribution and failure detection architecture  shared nothing, shared disk, shared data Cluster At Different Levels OS: MS Windows NT/2000/2003, IBM AIX middleware: C-JDBC application: Oracle RAC (Real Application Clusters)

18. Slide 1816 October 2015Highly Available Database Systems – Ou Yi Clustering Purpose high-availability cluster  fail-over & switch-over  backup is idle load balancing cluster  all nodes are active Private Network used to detect node failure  heartbeat: status info. sent by the nodes to each other at regular intervals network partition problem  recommended to be redundant, fast and reliable

19. Slide 1916 October 2015Highly Available Database Systems – Ou Yi HA in Practice Mainframe (“big iron”) used by large companies mission critical applications and bulk data processing  financial TA processing  airline booking  railway systems RAS, years without interruption IBM zSeries family z9-109 the most powerful with up to 54 configurable PUs (processor) and many HA features z9-109 external view

20. Slide 2016 October 2015Highly Available Database Systems – Ou Yi HA in Practice - zSeries Memory-coherent SMP  typical problem of SMP: NUMA  single large L2, uniform memory access, cache coherency

21. Slide 2116 October 2015Highly Available Database Systems – Ou Yi HA in Practice - zSeries PU Sparing & Instruction Retry  two spare PUs per server, protection against PU failure  roles can be reassigned dynamically and transparently  each instruction is executed two times  detect soft error Modular Multi-book Design  multiple “books” per server  a book hosts PUs (12 / 16), memory and I/O connectors  Concurrent Book Add (CBA)  Enhanced Book Availability (EBA)

22. Slide 2216 October 2015Highly Available Database Systems – Ou Yi HA in Practice - zSeries Virtual Machine  protected and isolated copy of the physical machine  private address space, independency  user illusion: having a dedicated physical machine  Hypervisor  software which emulates the underlying physical machine’s architecture very efficiently (machine code)  advantages  error isolation  overcoming hardware boundaries  PR/SM of zSeries  a pool of resources  support up to 60 LPARs

23. Slide 2316 October 2015Highly Available Database Systems – Ou Yi HA in Practice - zSeries  LPAR (Logical Partition)  CPU resources  zone (part of the physical main memory)  I/O resources (statically assigned)

24. Slide 2416 October 2015Highly Available Database Systems – Ou Yi HA in Practice - DBMS System Failure database crash recovery & clustering Data Failure storage failure  RAID and (online) backups DB2 backup db sample online to /dev3/backup human error  Flashback technology FLASHBACK TABLE account TO BEFORE DROP FLASHBACK DATABASE TO TIMESTAMP (...) Site Failure cross-sites online replication: Oracle Data Guard ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE {PROTECTION | AVAILABILITY | PERFORMANCE}

25. Slide 2516 October 2015Highly Available Database Systems – Ou Yi HA in Practice - MAA Designing HA Architecture Oracle MAA (Maximum Availability Architecture)

26. Slide 2616 October 2015Highly Available Database Systems – Ou Yi Summary HA DBSs are multi-computer DBSs Basic architectures are the three types of coupling Error isolation and redundancy are effective and widely adopted approaches A promising market

27. Thank You! Technische Universität Kaiserslautern Ou Yi o_yi@informatik.uni-kl.de