Introduction to Distributed Systems

1. Introduction to Distributed Systems

2. Outline • about the course • relationship to other courses • the challenges of distributed systems • distributed services • *ility for distributed services • some basic problems and techniques

3. What is CPS 212 about? • What do I mean by “distributed information systems”? • distributed: a bunch of “computers” connected by “wires” • Nodes are (at least) semi-autonomous... • but run software to coordinate and share resources. • Information systems: focus on systems to store/access/share data and operations on data, rather than on computing. • Move {data, computation} around and deliver it to the right places at the right times, safely and securely. • Information systems is more general than “relational databases”. • In this course, we view database systems as local components of larger distributed systems. (The topics also apply to building very large database systems.) We study database concurrency control and recovery, but not the relational model.

4. Why are you here? • You are a second-year (or later) CPS graduate student. • You have taken CPS 210 and CPS 214 and you want more. • familiarity with TCP/IP networking, threads, and file systems • Or: we have talked and we agreed that you should take the class. • You are comfortable with concurrent programming in Java. • (You want to do some Java programming labs.) • You want to prepare for R/D in this exciting and important area. • (You want to read about 15 papers and take some exams.) • You want to get started... • (You want to spend time tinkering with a related software artifact of your choice, and doing a semester group project.)

5. Continuum of Distributed Systems Issues: naming and sharing performance and scale resource management Parallel Architectures CPS 221 Networks CPS 214 small fast big slow ? ? Global Internet Multiprocessors clusters LAN fast network trusting hosts coordinated slow network untrusting hosts autonomy high latency low bandwidth autonomous nodes unreliable network fear and distrust independent failures decentralized administration low latency high bandwidth secure, reliable interconnect no independent failures coordinated resources

6. Assumptions About the Network • Most of what we study in this class is at the session or presentation levels of the OSI “layer cake”. • We assume properties of the transport and network layers: • uniform network address space (IP address, port) • best-effort delivery of messages of arbitrary size • reliable ordered stream communication (TCP) • flow control • The key issue is: how to use the network to build networked applications and services with the properties we want? • In practice, many critical structuring and performance issues do not permit us to draw so clean a line...but we’ll try.

7. The Challenges of Distributed Systems • private communication over public networks • who sent it (authentication), did anyone change it, did anyone see it • building reliable systems from unreliable components • reliable communication over unreliable networks • autonomous nodes can fail independently; a distributed system can “partly fail” • Lamport’s characterization: “A distributed system is one in which the failure of a machine I’ve never heard of can prevent me from doing my work.” • location, location, location • Placing data and computation for effective resource sharing, and finding it again once you put it somewhere. • coordination and shared state • What should we (the system components) do and when should we do it? • Once we’ve all done it, can we all agree on what we did and when?

8. The Importance of Authentication This is a picture of a $2.5B move in the value of Emulex Corporation, in response to a fraudulent press release by short-sellers through InternetWire last Friday. The release was widely disseminated by news media as a statement from Emulex management, but media failed to authenticate it. EMLX [reproduced from clearstation.com]

9. Broader Importance of Distributed Software Technology • Today, the global community depends increasingly on distributed information systems technologies. • There are many other recent examples of high-profile meltdowns of systems for distributed information exchange. • denial-of-service attacks against Yahoo etc. (spring 00) • the Starr report melts the ‘net (fall 98) • stored credit card numbers stolen from CDNow.com (spring 00) • People were afraid to buy over the net at all just a few years ago! • Network Solutions DNS root server failure (fall 00) • MCI trunk drop interrupts Chicago Board of Exchange (summer 99) • These reflect the reshaping of business, government, and society brought by the global Internet and related software. • We have to “get it right”!

10. Services “Do A for me.” “OK, here’s your answer.” “Now do B.” “OK, here.” Client Server(s) • request/response paradigm ==> client/server model • examples: • Remote Procedure Call (RPC) • object invocation, e.g., Remote Method Invocation (RMI) • HTTP • device protocols (e.g., SCSI) • Is Napster a “service”? client/server vs. peer/peer

11. Challenges for Services: *ility • We want our distributed applications to be useful, correct, and secure. We also want reliability. Broadly, that means: • recoverability • Don’t lose data if a failure occurs. • availability • Don’t interrupt service if a failure occurs. (also survivability) • scalability • The system can grow effectively with the workload. See also: manageability, adaptibility, agility, performability. • These affect how/where we place functions and data in the network. • It turns out that there are many common problems and techniques that can be (mostly) “factored out” of applications and services. That is (mostly) what this course is about.

12. Failure • Before we talk about recoverability and availability, we need to know what we mean by failure. • packet drop or packet delay • Is delay bounded or unbounded? How long must I wait? • synchronous vs. asynchronous distributed systems • network partition • “split brain” syndrome • Byzantine failure • component behaves incorrectly or unexpectedly • could be an attack that corrupts or replays messages • component fail-stop or halt • discard state, or recover with stale state (e.g., pause)? • For now, assume fail-stop and use the term “node” as shorthand for “component”.

13. Recoverability • Some basic assumptions: • Nodes have volatile and (optional) nonvolatile storage. • Volatile storage is fast, but its contents are discarded in a failure. • OS crash/restart, power failure, untimely process death • Nonvolatile (stable) storage is slow, but its contents survive failures of components other than the storage device itself. • E.g., disk: high latency but also high bandwidth (if sequential) • Low-latency nonvolatile storage exists. It is expensive but getting cheaper: NVRAM, Uninterruptible Power Supply (UPS), flash memory, etc...these help keep things interesting. • Stability is never absolute: it is determined by probability of device failure, often measured by “mean time between failure” (MTBF). • How about backing up data in remote memory?

14. Memory and Stable Storage volatile memory • Servers typically manage volatile memory as a cache over stable storage: • database • file system • object store • tuple store • Stable storage holds the authoritative copy of the data. • Volatile memory acts as a write-back “scratch pad” for updates and cached data in active use. • How does low-latency stable storage change this picture? stable storage (home)

15. The Key to Recoverability • Software must manage the flow of data from volatile to nonvolatile storage so that: • stable storage updates are efficient; • recovered state preserves atomicity of groups of updates made together • state is internally consistent or self-consistent • e.g., atomictransactions (later) • each node recovers necessary state after a failure • The right stuff has to get to the “disk” at the right time; a failure can occur at any time (even while recovering). • How do you debug something like this? • We need some basic techniques for preserving state...

16. log Logging volatile memory • Key idea: supplement the home data image with a log of recent updates and/or events. • append-only • sequential access (faster) • preserves order of log entries • enables atomic commit with a single write • Recover by traversing, e.g., “replaying”, the log. • Logging is fundamental to database systems and other storage systems. home image

17. The Problem of Distributed Recovery • In a distributed system, a recovered node’s state must also be consistent with the states of other nodes. • E.g., what if a recovered node has forgotten an important event that others have remembered? • A functioning node may need to respond to a peer’s recovery. • rebuild the state of the recovering node, and/or • discard local state, and/or • abort/restart operations/interactions in progress • e.g., two-phase commit protocol • How to know if a peer has failed and recovered?

18. Example: Session Verifier “Do A for me.” “OK, my verifier is x.” S “B” “x” oops... “C” “OK, my verifier is y.” S´ “A and B” “y” What if y == x? How to guarantee that y != x? What is the implication of re-executing A and B, and after C? Some uses: NFS V3 write commitment, RPC sessions, NFS V4 and DAFS (client).

19. Availability • The basic technique for achieving availability is replication. • replicate hardware components • replicate functions • replicate data • replicate servers • e.g., primary/backup, hot standby, process pairs, etc. • e.g., RAID parity for available storage • Build decentralized systems that eliminate single points of failure. • If a component fails, select a replica and redirect requests there. • fail over

20. The Meaning of Scalability • Scalability is now part of the “enhanced standard litany” [Fox]; everybody claims their system is “scalable”. What does it really mean? Pay as you go: expand capacity by spending more money, in proportion to the new capacity. total cost of capacity scalable unscalable cost Note: watch out for “hockey sticks”! marginal cost of capacity capacity How do we measure or validate claims of scalability?

21. Scalability II: Manageability • Today, “cost” has a broader meaning than it once did: • growth in administrative overhead with capacity • no interruption of service to upgrade capacity • “24 * 7 * 365 * .9999” Where does the money go? [Borrowed from Jim Grey] New World Old World

22. Scalability III: Adaptability, Agility, etc. This is a graph of request traffic to download the Starr Report on Pres. Clinton’s extracurricular pursuits, released in 9/98. Ideally, systems could self-organize and adapt to absorb workload bursts and/or to spread load to new resources.

23. Coordination • If the solution to availability and scalability is to replicate functions and data, how do we coordinate the replicas? • data consistency • update propagation • mutual exclusion • consistent global states • group membership • group communication • event ordering • distributed consensus • quorum consensus

24. Example: Time • Can the nodes of a distributed system agree on what time it is? • This is necessary to impose a global ordering on events... • ...which can ensure that nodes observe important events in the same order. • Distributed/decentralized systems have no central notion of time. • physical clocks (“wall clock”) • induces a total order, but must account for drift • synchronous distributed systems assume a bound on drift • synchronize clocks using e.g., Network Time Protocol • “logical” clocks • order events that are logically related, i.e., one might have caused the other (potential causality); induces a partial order • vector clocks • stronger partial order: determine if e1did or did not causally precede e2

25. 1970 1980 1990 1992 1995 A Brief History (viewed from 1998) distributed objects CORBA COM client/server DCE OMG Wintel PCs AOL Viper Wolfpack Falcon, etc. NT The Web Mosaic Ethernet LANs Berkeley Unix r* TCP/IP AppleTalk ISPs Netscape commercial Internet Dark Ages The Future DECnet SNA 1960 Spring Network Objects search engines workstations RPC, NFS, NIS Apollo NSFnet DSM PVM/MPI Coda ARPAnet Cedar Grapevine (Xerox PARC) IBM Java Java Java The Web The Web The Web Beans RMI servlets JINI ... V and Mach Emerald Argus ISIS AFS Tuxedo Encina DES RSA