Presentation prepared by: Nikos Chondros May 21, 2010 For MDE519. Distributed Systems

1. UsenetDHT: A low-overhead design for UsenetEmil Sit, Robert Morris and FransKaashoek (MIT)NSDI 08 Presentation prepared by: Nikos Chondros May 21, 2010 For MDE519. Distributed Systems Prof. MemaRoussopoulos

2. What is Usenet • Usenet is a worldwide distributed Internet discussion system • Operating since 1981 • Content creators post messages to newsgroups • Content consumers “subscribe” to newsgroups, download all “new” message headers and finally download specific messages to read

3. How Usenet works

4. Usenet message • Header • Subject • Author • Date • Newsgroups to be posted to • Body • Actual text of message • UUencoded binary attachments

5. The problem • Usenet places large overhead to participating servers • Each server hosts a (large) subset of the groups • However, it has a complete copy of the headers and message bodies in these groups • Binary objects (multimedia mostly) contained in the bodies are large and have to be replicated everywhere • As a result, content is saved for a very short period of time. Giganews.com claims approx 2 years currently and that’s not a free service. Free servers from ISPs are limited to a (very) few months.

6. The central idea of the paper • Let’s make Usenet distributed • Each server to host part of the data-set • All servers to cooperate in a DHT to provide the service to clients • Let each server host the complete database of headers • Size is small (a few hundred bytes per message) • Quick response to clients building their list • Each server to “cache” popular content locally to avoid accessing the DHT repeatedly

7. What’s the potential gain? • Each message (body) stored in two places (and the second is for replication) • Scalability (simply add more nodes) • Traffic of incoming updates severely reduced to headers only • Storage requirements on each server reduced to 2/n of the full load

8. Approach • Use a DHT to distribute the content to servers • DHash (details coming) • Based on Chord • Replicate the messages to more than one server for safety • DHash already touched that but authors contribute to it with a finer-grain solution

9. UsenetDHT client operation Figure 1: Messages exchanged during duringUsenetDHT reads and writes. 1. Client A posts an article to his local NNTP front-end. 2. The front-end stores the article in DHash (via a DHash gateway, not shown). 3. After successful writes, the front-end propagates the article's metadata to other front-ends. 4. Client B checks for new news and asks for the article. 5. Client B's front-end retrieves the article from the DHT and returns it to her.

10. So what’s DHash • Published at SOSP 01 • Wide-area cooperative storage with CFS (MIT) • DHash uses Chord to organize nodes and resolve “keys” • DHash itself stores unstructured objects reliably (more to come) • FYI, CFS uses the SFS file system toolkit to provide a filesystem that splits file contents to blocks and stores each one seperately on DHash.

11. DHash replication (for safety) • With a parameter k, store the content to the k consecutive Chord nodes starting from the “owner”. • Why? After a block’s successor fails, the block is immediately available at its new successor. Key Key’s owner Key owner’s successor Example for k=2

12. What’s wrong with that? • Efficiency! • How do you manage these copies when new nodes arrive and old ones leave? How do you maintain k copies only (optimal) • DHash targeted small blocks, parts of the files. These guys want to store a complete Usenet message with it, which can be quite large

13. Here comes “Passing Tone” • Ideally: • Generate repairs only for those objects with fewer than k remaining replicas • But that’s a tough nut to crack as no global “directory” exists! • Actually, on each node: • Keep a synchronization data structure for objects that “belong” to me, using Merkle trees (more soon) • Make sure I have a copy of all objects I am responsible for • Any objects stored here, for which I am not responsible for anymore, make them available to whoever of my immediate neighbors now is. • Ask Chord to maintain O(logn) predecessors • Now the node knows the range of objects it is required to replicate (the ones owned by its k predecessors)

14. Merkle trees you said? • As we use hashing, build a tree were the leaves are the hashes of the local objects and, as you go up, the inner nodes hash the hashes below them • What’s so special? • When comparing two different sub-trees, if their roots match, they are equal! (with very high probability when cryptographic hashes like SHA-1 are used)

15. Passing Tone “Local maintenance” (objects that a node is responsible for but does not currently “have”) • Each server synchronizes only with its direct predecessor and successor • Walk down the relevant branches of the Merkle tree and efficiently identify objects the current node is missing that are available at its neighbors. • n.local_maintenance(): • a, b = n.pred_k, n # k-th predecessor to n • for partner in n.succ, n.pred: • diffs = partner.synchronize(a, b) • for o in diffs: • data = partner.fetch(o) • n.db.insert(o, data)

16. Passing Tone “Global maintenance” (objects that a node “has” but is not currently responsible for) • Iterate over the database and identify objects no longer responsible for (not in its replica set) • Contact the current successor and synchronize for the set that he is responsible for • Still, does not delete objects (it lets them expire)

17. Dealing with expiration • Eventually, even in UsenetDHT, articles will expire • Handle this by not including expired object in the Merkle trees • Requires clocks to be synchronized among peers so that they expire articles together

18. Evaluation (simulation from a real 13 month trace of PlanetLab failures) Figure 5: Durability of objects in Passing Tone: the graph shows a CDF of the number of replicas per object after a one year PlanetLab trace. No objects are lost during this simulation.

19. Why so many replicas for some? Figure 6: Number of crashes within an eight hour window over the course of the PlanetLab trace. Eight hours represents the approximate time needed to replicate the data of one PlanetLab server to another.

20. Wide area performance (12 machines at universities across USA) • Figure 7: Articles received, posted and deferred by the CSAIL UsenetDHT deployment over a twelve hour period. During normal operation, deferrals are very rare. • Figure 8: UsenetDHT aggregate read throughput. 100 hosts (distributed on PlanetLab) started article reads continuously against 12 servers.

21. Passing Tone Efficiency • Figure 9: Number of repairs and number of writes over time, where we introduce a simple transient failure, a server addition, and a second transient failure. The second failure demonstrates behavior under recovery and also what would have happened in a permanent failure.

22. DHashmicrobenchmark • DHash Read and write microbenchmark performance on a cluster of 8 Xeaon 2.80, 1GB RAM, dual Maxtor SATA and FreeBSD 5.4p22. However, no replication, maintenance or expiration! Proves scalability.

23. UsenetDHT: potential savings Storage Net bandwidth 10 Terabyte/week Usenet 12 Megabyte/s 120 Kbyte/s 60 Gbyte/week UsenetDHT • Suppose 300 site network • Each site reads 1% of all articles

24. Sources • Emil Sit, Robert Morris, and M. FransKaashoek, “UsenetDHT: A low-overhead design for Usenet”, NSDI08 • http://www.sscnet.ucla.edu/soc/csoc/papers/sunbelt97/Usenet%20Technical%20Architecture.jpg • Wikipedia article on Usenet • Dabek, F., Kaashoek, M. F., Karger, D., Morris, R., and Stoica, I. “Wide-area cooperative storage with CFS”. In Proc. of the 18th ACM Symposium on Operating System Principles (Oct. 2001) • Josh Cates , Robust and Efficient Data Management for a Distributed Hash Table, Master’s thesis MIT 2003 • Ross Anderson, “Future Peer-to-Peer Systems” presentation