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Distributed k-ary System Algorithms for Distributed Hash Tables Ali Ghodsi aligh@kth.se http://www.sics.se/~ali/thesis/ PhD Defense, 7th December 2006, KTH/Royal Institute of Technology 1 Distributed k-ary System Algorithms for Distributed Hash Tables Ali Ghodsi aligh@kth.se http://www.sics.se/~ali/thesis/ PhD Defense, 7th December 2006, KTH/Royal Institute of Technology 2 Presentation Overview • Gentle introduction to DHTs • Contributions • The future 3 What’s a Distributed Hash Table (DHT)? • An ordinary hash table , which is distributed Key Value Alexander Berlin Ali Stockholm Marina Gothenburg Peter Louvain la neuve Seif Stockholm Stefan Stockholm • Every node provides a lookup operation •Provide the value associated with a key • Nodes keep routing pointers •If item not found, route to another node 4 So what? •Characteristic propertiesSelf-management routing info: Time to find data is Store number of items logarithmic •Scalability proportional to number of • Ensure routing information Size of routing tables is •Number of nodes can be huge isnodes up-to-date logarithmic •Number of items can be hugeTypically: Example: Self-management of items: With D items and n nodes • Ensure that data is always •Self-manage in presence joins/leaves/failures log2(1000000)≈20 •Routing information Store D/n itemsavailable replicated and per node EFFICIENT! •Data items Move D/n items when nodes join/leave/fail EFFICIENT! 5 Presentation Overview •… •… • What’s been the general motivation for DHTs? •… •… 6 Traditional Motivation (1/2) • Peer-to-Peer filesharing very popular • Napster central index • Completely centralized • Central server knows who has what • Judicial problems decentralized index • Gnutella • Completely decentralized • Ask everyone you know to find data • Very inefficient 7 Traditional Motivation (2/2) • Grand vision of DHTs • Provide efficient file sharing • Quote from Chord: ”In particular, [Chord] can help avoid single points of failure or control that systems like Napster possess, and the lack of scalability that systems like Gnutella display because of their widespread use of broadcasts.” [Stoica et al. 2001] • Hidden assumptions • Millions of unreliable nodes • User can switch off computer any time (leave=failure) • Extreme dynamism (nodes joining/leaving/failing) • Heterogeneity of computers and latencies • Unstrusted nodes 8 Our philosophy • DHT is a useful data structure • Assumptions might not be true • Moderate amount of dynamism • Leave not same thing as failure • Dedicated servers • Nodes can be trusted • Less heterogeneity • Our goal is to achieve more given stronger assumptions 9 Presentation Overview •… •… • How to construct a DHT? •… •… 10 How to construct a DHT (Chord)? • Use a logical name space, called the identifier space, consisting of identifiers {0,1,2,…, N-1} • Identifier space is a logical ring modulo N • Every node picks a random identifier 0 15 1 • Example: 14 2 • Space N=16 {0,…,15} 13 3 • Five nodes a, b, c, d 12 4 • a picks 6 • b picks 5 11 5 • c picks 0 10 6 • d picks 5 9 7 • e picks 2 8 11 Definition of Successor • The successor of an identifier is the first node met going in clockwise direction starting at the identifier 0 15 1 • Example 14 2 • succ(12)=14 13 3 • succ(15)=2 12 4 • succ(6)=6 11 5 10 6 9 7 8 12 Where to store data (Chord) ? •Use globally known hash function, H Store number of items •Each item <key,value> gets proportional to number of Key Value Alexander Berlin nodes identifier H(key) Marina Gothenburg Louvain la Peter Typically: neuve •Store each item at its successor Seif Stockholm With D items and nn is responsible for item k •Node nodes Stefan Stockholm 0 Store D/n items per node 15 1 14 2 Move D•Example /n items when 13 3 nodes join/leave/fail • H(“Marina”)=12 12 4 • H(“Peter”)=2 EFFICIENT! • H(“Seif”)=9 11 5 • H(“Stefan”)=14 6 10 9 7 8 13 Where to point (Chord) ? •Each node points to its successor •The successor of a node n is succ(n+1) •Known as a node’s succ pointer •Each node points to its predecessor •First node met in anti-clockwise direction starting at n-1 •Known as a node’s pred pointer 0 15 1 •Example 14 2 • 0’s successor is succ(1)=2 13 3 • 2’s successor is succ(3)=5 • 5’s successor is succ(6)=6 12 4 • 6’s successor is succ(7)=11 • 11’s successor is succ(12)=0 11 5 10 6 9 7 8 14 DHT Lookup Key Value Alexander Berlin •To lookup a key k Marina Gothenburg • Calculate H(k) Peter Louvain la neuve • Follow succ pointers until Seif Stockholm item k is found Stefan Stockholm •Example 15 0 1 • Lookup ”Seif” at node 2 14 2 13 3 • H(”Seif”)=9 12 4 • Traverse nodes: • 2, 5, 6, 11 (BINGO) 11 5 • Return “Stockholm” to initiator 10 6 9 7 8 15 Speeding up lookups • If only pointer to succ(n+1) is used • Worst case lookup time is N, for N nodes Time to find data is • Improving lookup time logarithmic • Point to is Size of routing tables succ(n+1) 15 0 1 • Point to succ(n+2) logarithmic 14 2 • Point to succ(n+4) Example: 13 • Point to succ(n+8) 3 log2(1000000)≈20 •… 12 4 • Point EFFICIENT! to succ(n+2 ) M 11 5 • Distance always halved to the destination 10 6 9 7 8 16 Dealing with failures • Each node keeps a successor-list • Pointer to f closest successors 15 0 1 • succ(n+1) 14 2 • succ(succ(n+1)+1) 13 3 • succ(succ(succ(n+1)+1)+1) • ... 12 4 11 5 • If successor fails 10 6 • Replace with closest alive successor 9 8 7 • If predecessor fails • Set pred to nil 17 Handling Dynamism • Periodic stabilization used to make pointers eventually correct • Try pointing succ to closest alive successor • Try pointing pred to closest alive predecessor 18 Presentation Overview • Gentle introduction to DHTs • Contributions • The future 19 Outline •… •… • Lookup consistency •… •… 20 Problems with periodic stabilization • Joins and leaves can result in inconsistent lookup results • At node 12, lookup(14)=14 • At node 10, lookup(14)=15 12 14 10 15 21 Problems with periodic stabilization • Leaves can result in routing failures 13 10 16 22 Problems with periodic stabilization • Too many leaves destroy the system • #leaves+#failures/round < |successor-list| 11 12 14 10 15 23 Outline •… •… • Atomic Ring Maintenance •… •… 24 Atomic Ring Maintenance • Differentiate leaves from failures • Leave is a synchronized departure • Failure is a crash-stop • Initially assume no failures • Build a ring initially 25 Atomic Ring Maintenance • Separate parts of the problem • Concurrency control • Serialize neighboring joins/leaves • Lookup consistency 26 Naïve Approach • Each node i hosts a lock called Li • For p to join or leave: • First acquire Lp.pred • Second acquire Lp • Third acquire Lp.succ • Thereafter update relevant pointers • Can lead to deadlocks 27 Our Approach to Concurrency Control • Each node i hosts a lock called Li • For p to join or leave: • First acquire Lp • Thereafter acquire Lp.succ • Thereafter update relevant pointers • Each lock has a lock queue • Nodes waiting to acquire the lock 28 Safety • Non-interference theorem: • When node p acquires both locks: • Node p’s successor cannot leave • Node p’s ”predecessor” cannot leave • Other joins cannot affect ”relevant” pointers 29 Dining Philosophers • Problem similar to the Dining philosophers’ problem • Five philosophers around a table • One fork between each philosopher (5) • Philosophers eat and think • To eat: • grab left fork • then grab right fork 30 Deadlocks • Can result in a deadlock • If all nodes acquire their first lock • Every node waiting indefinitely for second lock • Solution from Dining philosophers’ • Introduce asymmetry • One node acquires locks in reverse order • Node with highest identifier reverses • If n<n.succ, then n has highest identity 31 Pitfalls • Join adds node/“philosopher” • Solution: some requests in the lock queue forwarded to new node 12 14 14, 12 12 12 14 10 15 32 Pitfalls • Leave removes a node/“philosopher” • Problem: if leaving node gives lock queue to its successor, nodes can get worse position in queue: starvation • Use forwarding to avoid starvation • Lock queue empty after local leave request 33 Correctness • Liveness Theorem: • Algorithm is starvation free • Also free from deadlocks and livelocks • Every joining/leaving node will eventually succeed getting both locks 34 Performance drawbacks • If many neighboring nodes leaving • All grab local lock • Sequential progress 12 14 10 15 • Solution • Randomized locking • Release locks and retry • Liveness with high probability 35 Lookup consistency: leaves • So far dealt with concurrent joins/leaves • Look at concurrent join/leaves/lookups • Lookup consistency (informally): • At any time, only one node responsible for any key • Joins/leaves should “not affect” functionality of lookups 36 Lookup consistency • Goal is to make joins and leaves appear as if they happened instantaneously • Every leave has a leave point • A point in global time, where the whole system behaves as if the node instantaneously left • Implemented with a LeaveForward flag • The leaving node forwards messages to successor if LeaveForward is true 37 Leave Algorithm Node p Node q (leaving) Node r LeaveForward=true leave point pred:=p succ:=r LeaveForward=false 38 Lookup consistency: joins • Every join has a join point • A point in global time, where the whole system behaves as if the node instantaneously joined • Implemented with a JoinForward flag • The successor of a joining node forwards messages to new node if JoinForward is true 39 Join Algorithm Node p Node q (joining) Node r Join Point JoinForward=true oldpred=pred pred=q pred:=p succ:=r succ:=q JoinForwarding=false 40 Outline •… •… • What about failures? •… •… 41 Dealing with Failures • We prove it is impossible to provide lookup consistency on the Internet • Assumptions • Availability (always eventually answer) • Lookup consistency • Partition tolerance • Failure detectors can behave as if the networked partitioned 42 Dealing with Failures • We provide fault-tolerant atomic ring • Locks leased • Guarantees locks are always released • Periodic stabilization ensures • Eventually correct ring • Eventual lookup consistency 43 Contributions • Lookup consistency in presence of joins/leaves • System not affected by joins/leaves • Inserts do not “disappear” • No routing failures when nodes leave • Number of leaves not bounded 44 Related Work • Li, Misra, Plaxton (’04, ’06) have a similar solution • Advantages • Assertional reasoning • Almost machine verifiable proofs • Disadvantages • Starvation possible • Not used for lookup consistency • Failure-free environment assumed 45 Related Work • Lynch, Malkhi, Ratajczak (’02), position paper with pseudo code in appendix • Advantages • First to propose atomic lookup consistency • Disadvantages • No proofs • Message might be sent to a node that left • Does not work for both joins and leaves together • Failures not dealt with 46 Outline •… •… • Additional Pointers on the Ring •… •… 47 Routing • Generalization of Chord to provide arbitrary arity • Provide logk(n) hops per lookup • k being a configurable parameter • n being the number of nodes • Instead of only log2(n) 48 Achieving logk(n) lookup • Each node logk(N) levels, N=kL • Each level contains k intervals, • Example, k=4, N=64 (43), node 0 0 4 8 Node 0 I0 I1 I2 I3 Interval 3 Interval 0 12 16…3 32…4 48…6 Level 1 0…15 1 7 3 48 16 Interval 2 Interval 1 32 49 Achieving logk(n) lookup • Each node logk(N) levels, N=kL • Each level contains k intervals, • Example, k=4, N=64 (43), node 0 0 4 Interval 0 8 Interval 1 Node 0 I0 I1 I2 I3 12 16…3 32…4 48…6 Interval 2 Level 1 0…15 1 7 3 48 Interval 3 16 12…1 Level 2 0…3 4…7 8…11 5 32 50 Achieving logk(n) lookup • Each node logk(N) levels, N=kL • Each level contains k intervals, • Example, k=4, N=64 (43), node 0 0 4 8 Node 0 I0 I1 I2 I3 12 16…3 32…4 48…6 Level 1 0…15 1 7 3 48 16 12…1 Level 2 0…3 4…7 8…11 5 Level 3 0 1 2 3 32 51 Arity important • Maximum number of hops can be configured 1 k N r 1 r log k ( N ) log 1 ( N ) log 1 N r r r N r N • Example, a 2-hop system k N log N (N ) 2 52 Placing pointers •Each node has (k-1)logk(N) pointers • Node p’s pointers point at i 1 k 1 f (i ) p (1 (( i 1) mod (k 1))) k 0 4 Node 0’s pointers 8 f(1)=1 f(2)=2 12 f(3)=3 f(4)=4 f(5)=8 48 16 f(6)=12 f(7)=16 f(8)=32 f(9)=48 32 53 Greedy Routing • lookup(i) algorithm • Use pointer closest to i, without “overshooting” i • If no such pointer exists, succ is responsible for i i 54 Routing with Atomic Ring Maintenance • Invariant of lookup • Last hop is always predecessor of responsible node • Last step in lookup • If JoinForward is true, forward to pred • If LeaveForward is true, forward to succ 55 Avoiding Routing Failures • If nodes leave, routing failures can occur • Accounting algorithm • Simple Algorithm • No routing failures of ordinary messages • Fault-free Algorithm • No routing failures • Many cases and interleavings • Concurrent joins and leaves, pointers in both directions 56 General Routing • Three lookup styles • Recursive • Iterative • Transitive 57 Reliable Routing • Reliable lookup for each style • If initiator doesn’t crash, responsible node reached • No redundant delivery of messages • General strategy • Repeat operation until success • Filter duplicates using unique identifiers • Iterative lookup • Reliability easy to achieve • Recursive lookup • Several algorithms possible • Transitive lookup • Efficient reliability hard to achieve 58 Outline •… •… • One-to-many Communication •… •… 59 Group Communication on an Overlay • Use existing routing pointers • Group communication • DHT only provides key lookup • Complex queries by searching the overlay • Limited horizon broadcast • Iterative deepening • More efficient than Gnutella-like systems • No unintended graph partitioning • Cheaper topology maintenance [castro04] 60 Group Communication on an Overlay • DHT builds a graph • Why not use general graph algorithms? • Can use the specific structure of DHTs • More efficient • Avoids redundant messages 61 Broadcast Algorithms • Correctness conditions: • Termination • Algorithm should eventually terminate • Coverage • All nodes should receive the broadcast message • Non-redundancy • Each node receives the message at most once • Initially assume no failures 62 Naïve Broadcast • Naive Broadcast Algorithm send message to succ until: initiator reached or overshooted initiator 0 15 1 14 2 13 3 12 4 11 5 10 6 9 7 8 63 Naïve Broadcast • Naive Broadcast Algorithm send message to succ until: initiator reached or overshooted initiator • Improvement 0 15 1 • Initiator delegates half 14 2 the space to neighbor 13 3 12 4 • Idea applied recursively • log(n) time and n messages 11 5 10 6 9 7 8 64 Simple Broadcast in the Overlay • Dissertation assumes general DHT model event n.SimpleBcast(m, limit) % initially limit = n for i:=M downto 1 do if u(i) ∈ (n,limit) then sendto u(i) : SimpleBcast(m, limit) limit := u(i) 65 ”Advanced” Broadcast • Old algorithm on k-ary trees 66 Getting responses • Getting a reply • Nodes send directly back to initiator • Not scalable • Simple Broadcast with Feedback • Collect responses back to initiator • Broadcast induces a tree, feedback in reverse direction • Similar to simple broadcast algorithm • Keeps track of parent (par) • Keeps track of children (Ack) • Accumulate feedback from children, send to parent • Atomic ring maintenance • Acquire local lock to ensure nodes do not leave 67 Outline •… •… • Advanced One-to-many Communication •… •… 68 Motivation for Bulk Operation • Building MyriadStore in 2005 • Distributed backup using the DKS DHT • Restoring a 4mb file • Each block (4kb) indexed in DHT • Requires 1000 items in DHT • Expensive • One node making 1000 lookups • Marshaling/unmarshaling 1000 requests 69 Bulk Operation • Define a bulk set: I • A set of identifiers • bulk_operation(m, I) • Send message m to every node i ∈ I • Similar correctness to broadcast • Coverage: all nodes with identifier in I • Termination • Non-redundancy 70 Bulk Owner Operation with Feedback • Define a bulk set: I • A set of identifiers • bulk_own(m, I) • Send m to every node responsible for an identifier i∈I • Example • Bulk set I={4} • Node 4 might not exist • Some node is responsible for identifier 4 71 Bulk Operation with Feedback • Define a bulk set: I • A set of identifiers • bulk_feed(m, I) • Send message m to every node i ∈ I • Accumulate responses back to initiator • bulk_own_feed(m, I) • Send message m to every node responsible for i ∈ I • Accumulate responses back to initiator 72 Bulk Properties (1/2) • No redundant messages • Maximum log(n) messages per node 73 Bulk Properties (2/2) • Two extreme cases • Case 1 • Bulk set is all identifiers • Identical to simple broadcast • Message complexity is n • Time complexity is log(n) • Case 2 • Bulk set is a singleton with one identifier • Identical to ordinary lookup • Message complexity is log(n) • Time complexity is in log(n) 74 Pseudo Reliable Broadcast • Pseudo-reliable broadcast to deal with crash failures • Coverage property • If initiator is correct, every node gets the message • Similar to broadcast with feedback • Use failure detectors on children • If child with responsibility to cover I fails • Use bulk to retry covering interval I • Filter redundant messages using unique identifiers • Eventually perfect failure detector for termination • Inaccuracy results in redundant messages 75 Applications of bulk operation • Bulk operation • Topology maintenance: update nodes in bulk set • Pseudo-reliable broadcast: re-covering intervals • Bulk owner • Multiple inserts into a DHT • Bulk owner with feedback • Multiple lookups in a DHT • Range queries 76 Outline •… •… • Replication •… •… 77 Successor-list replication • Successor-list replication • Replicate a node’s item on its f successors • DKS, Chord, Pastry, Koorde etcetera. • Was abandoned in favor of symmetric replication because … 78 Motivation: successor-lists • If a node joins or leaves • f replicas need to be updated Color represents data item Replication degree 3 Every color replicated three times 79 Motivation: successor-lists • If a node joins or leaves • f replicas need to be updated Color represents data item Node leaves Yellow, green, red, blue need to be re-distributed 80 Multiple hashing • Rehashing • Store each item <k,v> at • succ( H(k) ) • succ( H(H(k)) ) • succ( H(H(H(k))) ) • … • Multiple hash functions • Store each item <k,v> at • succ( H1(k) ) • succ( H2(k) ) • succ( H3(k) ) • … • Advocated by CAN and Tapestry 81 Motivation: multiple hashing • Example • Item <”Seif”, ”Stockholm”> • H(”Seif”)=7 • succ(7)=9 • Node 9 crashes • Node 12 should get item from replica • Need hash inverse H-1(7)=”Seif” (impossible) • Items dispersed all over nodes (inefficient) 9 12 7 Seif, 5 Stockholm 82 Symmetric Replication •Basic Idea •Replicate identifiers, not nodes •Associate each identifier i with f other identifiers: N • r (k ) i k , for 0 k f f •Identifier space partitioned into m equivalence classes •Cardinality of each class is f, m=N/f •Each node replicates the equivalence class of all identifiers it is responsible for 83 Symmetric replication Replication degree f=4, Space={0,…,15} • Congruence classes modulo 4: • {0, 4, 8, 12} • {1, 5, 9, 13} • {2, 6, 10, 14} Data: 15, 0 • {3, 7, 11, 15} Data: 14, 13, 12, 11 15 0 1 14 2 Data: 1, 2, 3 13 3 12 4 11 5 Data: 4, 5 10 6 9 7 Data: 6, 7, 8, 9, 10 8 84 Ordinary Chord Replication degree f=4, Space={0,…,15} • Congruence classes modulo 4 Data: 3, 4 • {0, 4, 8, 12} Data: 7, 8 Data: 2, 1, 0, 15 • {1, 5, 9, 13} Data: 11, 12 Data: 6, 5, 4, 3 • {2, 6, 10, 14} Data: 15, 0 • {3, 7, 11, 15} Data: 10, 9, 8, 7 Data: 5, 6, 7 Data: 14, 13, 12, 11 Data: 9, 10, 11 15 0 1 Data: 13, 14, 15 14 2 Data: 1, 2, 3 13 3 Data: 8, 9 Data: 12, 13 12 4 Data: 0, 1 Data: 10, 11, 12, 13, 14 Data: 14, 15, 0, 1, 2 11 5 Data: 4, 5 Data: 2, 3, 4, 5, 6 10 6 9 7 Data: 6, 7, 8, 9, 10 8 85 Cheap join/leave Replication degree f=4, Space={0,…,15} • Congruence classes modulo 4 Data: 3, 4 • {0, 4, 8, 12} Data: 7, 8 Data: 2, 1, 0, 15 • {1, 5, 9, 13} Data: 11, 12 Data: 6, 5, 4, 3 • {2, 6, 10, 14} Data: 15, 0 • {3, 7, 11, 15} Data: 10, 9, 8, 7 Data: 5, 6, 7 Data: 14, 13, 12, 11 Data: 9, 10, 11 15 0 1 Data: 13, 14, 15 Data: 0, 15 14 2 Data: 1, 2, 3 Data: 3, 4 13 3 Data: 7, 8 Data: 11, 12, 7, Data: 8, 9 8, 3, 4, 0, 15 Data: 12, 13 Data: 11, 12 12 4 Data: 0, 1 Data: 10, 11, 12, 13, 14 Data: 14, 15, 0, 1, 2 11 5 Data: 4, 5 Data: 2, 3, 4, 5, 6 10 6 9 7 Data: 6, 7, 8, 9, 10 8 86 Contributions • Message complexity for join/leave O(1) • Bit complexity remains unchanged • Handling failures more complex • Bulk operation to fetch data • On average log(n) complexity • Can do parallel lookups • Decreasing latencies • Increasing robustness • Distributed voting • Erasure codes 87 Presentation Overview •… •… • Summary •… •… 88 Summary (1/3) • Atomic ring maintenance • Lookup consistency for j/l • No routing failures as nodes j/l • No bound on number of leaves • Eventual consistency with failures • Additional routing pointers • k-ary lookup • Reliable lookup • No routing failures with additional pointers 89 Summary (2/3) • Efficient Broadcast • log(n) time and n message complexity • Used in overlay multicast • Bulk operations • Efficient parallel lookups • Efficient range queries 90 Summary (3/3) • Symmetric Replication • Simple, O(1) message complexity for j/l • O(log f) for failures • Enables parallel lookups • Decreasing latencies • Increasing robustness • Distributed voting 91 Presentation Overview • Gentle introduction to DHTs • Contributions • The future 92 Future Work (1/2) • Periodic stabilization • Prove it is self-stabilizing 93 Future Work (2/2) • Replication Consistency • Atomic consistency impossible in asynchronous systems • Assume partial synchrony • Weaker consistency models? • Using virtual synchrony 94 Speculative long-term agenda • Overlay today provides • Dynamic membership • Identities (max/min avail) • Only know subset of nodes • Shared memory registers • Revisit distributed computing • Assuming an overlay as basic primitive • Leader election • Consensus • Shared memory consistency (started) • Transactions • Wave algorithms (started) • Implement middleware providing these… 95 Acknowledgments • Seif Haridi • Luc Onana Alima • Cosmin Arad • Per Brand • Sameh El-Ansary • Roland Yap 96 THANK YOU 97 98 Handling joins • When n joins • Find n’s successor with lookup(n) • Set succ to n’s successor 15 • Stabilization fixes the rest 13 11 Periodically at n: When receiving notify(p) at n: 1. set v:=succ.pred 1. if pred=nil or p is in (pred,n] 2. if v≠nil and v is in (n,succ] 2. set pred:=p 3. set succ:=v 4. send a notify(n) to succ 99 Handling leaves • When n leaves • Just dissappear (like failure) • When pred detected failed 15 • Set pred to nil 13 • When succ detected failed • Set succ to closest alive in successor list 11 Periodically at n: When receiving notify(p) at n: 1. set v:=succ.pred 1. if pred=nil or p is in (pred,n] 2. if v≠nil and v is in (n,succ] 2. set pred:=p 3. set succ:=v 4. send a notify(n) to succ 100

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