Design principles of scalable, distributed systems


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Key algorithms of scalable, distributed systems

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Design principles of scalable, distributed systems

  1. 1. Design Principles of Scalable, Distributed Systems Tinniam V Ganesh tvganesh.85@gmail.com03/28/12 Tinniam V Ganesh - 1
  2. 2. Distributed SystemsThere are two classes of systems- Monolithic- Distributed03/28/12 Tinniam V Ganesh - 2
  3. 3. Traditional Client Server Architecture Client Server03/28/12 Tinniam V Ganesh - 3
  4. 4. Properties of Distributed SystemsDistributed Systems are made up of 100s of commodity servers• No machine has complete information about the system state• Machines make decisions based on local information• Failure of one machine does not cause any problems• There is no implicit assumption about a global clock03/28/12 Tinniam V Ganesh - 4
  5. 5. Characteristics of Distributed SystemsDistributed Systems are made up of• Commodity Servers• Large number of servers• Servers crash, there network failures, messages not sent, received• New Servers can join without changing behavior03/28/12 Tinniam V Ganesh - 5
  6. 6. Examples of Distributed Systems• Amazon’s e-retail store• Google• Yahoo• Facebook• Twitter• YoutubeEtc03/28/12 Tinniam V Ganesh - 6
  7. 7. Key principles of distributed systems• Incremental scalability• Symmetry – All nodes are equal• Decentralization – No central control• Work distribution heterogenity03/28/12 Tinniam V Ganesh - 7
  8. 8. Transaction Processing System• Traditional databases have to ensure that transactions are consistent. Transaction must be fully complete or not at all.• Successful transactions are committed.• Otherwise transactions are rolled back03/28/12 Tinniam V Ganesh - 8
  9. 9. ACID postulateTransactions in traditional system have to have the following propertiesEarlier Systems were designed for ACID propertiesA – AtomicC – ConsistentI – IsolatedD - Durable03/28/12 Tinniam V Ganesh - 9
  10. 10. ACIDAtomic – This property ensures that each transaction happens completely or not at allConsistent - The transaction should maintain system invariants. For e.g. an internal bank transfer should result in the total amount in the bank before and after the transaction to be same. It may be temporarily differentIsolated – Different transactions should be isolated or serializable. It must appear that transactions happen sequentially in some particular orderDurable – Once the transaction commits the effect is complete and durable going forward.03/28/12 Tinniam V Ganesh - 10
  11. 11. ScalingThere are 2 types of scalingVertical scaling – This method scales by adding faster CPU , more memory and a larger database. Does not scale beyond a particular pointHorizontal scalability – This method scales laterally by adding more servers with the same capacity03/28/12 Tinniam V Ganesh - 11
  12. 12. System behavior on Scaling Response ResponseTransactions Throughput TimePer Second Load 03/28/12 Tinniam V Ganesh - 12
  13. 13. Consistency and ReplicationIn order to increase reliability against failures data has to be replicated across multiple servers.The problem with replicas is the need to keep the data consistent03/28/12 Tinniam V Ganesh - 13
  14. 14. Reasons for ReplicationData is replicated in distributed systems for two reasons- Reliability – Ensuring that there is a consistency in data in a majority of the replicas- Performance – Performance can be improved by accessing a replica that is closer to the user. Geographical resiliency03/28/12 Tinniam V Ganesh - 14
  15. 15. Downside of Replication• Replication of data has several advantages but the downside is the issue maintaining consistency• A modification of a copy makes it different from the rest and this update has to be propagated to all copies to ensure consistency03/28/12 Tinniam V Ganesh - 15
  16. 16. SynchronizationNo machine has a view of the global system state• Problems with distributed systems• How can processes synchronize ?• Clocks on different systems will be slightly different• Is there a way to maintain a global view of the clock• Can we order events causally?03/28/12 Tinniam V Ganesh - 16
  17. 17. Hypothetical situationConsider a hypothetical situation with banks - Man deposits Rs 55,000/- at 10.00 am- Man withdraws Rs 20,000/- at 10.02 amWhat will happen if the updates happen in different order- Operations must be idempotent. Idempotency refers to getting the same result no matter how many times the operation is performed.eCommerce Site – Amazon-add to shopping cart-delete from shopping cart03/28/12 Tinniam V Ganesh - 17
  18. 18. Vector ClocksVector clocks are used to capture causality between different versions of the same object.Amazon’s Dynamo uses vector clocks to reconcile different versions of the objects and determine the causal ordering of events.03/28/12 Tinniam V Ganesh - 18
  19. 19. Vector Clocks 2 OK 5 8 4 10 16 6 15 24 8 20 32 10 25 Adjust 40 12 30 48 14 41 56 16 46 64 18 51 6803/28/12 Tinniam V Ganesh - 19
  20. 20. Dynamo’s reconciliation process03/28/12 Tinniam V Ganesh - 20
  21. 21. Problem with Relational DatabasesRDBMS databases provide the user the ability to construct complex queries but they do not scale well.ProblemPerformance deteriorates as the number of records reach several millionSolutionTo partition the database horizontally and distribute records across several servers.03/28/12 Tinniam V Ganesh - 21
  22. 22. No SQL Databases• Databases horizontally partitioned• Simple queries based on gets() and sets()• Access are made on key/value pairs• Cannot do complex queries like joins• Database can contain several hundred million records03/28/12 Tinniam V Ganesh - 22
  23. 23. Databases that use Consistent Hashing1. Cassandra2. Amazon’s Dynamo3. NoSQL4. HBASE5. CouchDB6. MongoDB03/28/12 Tinniam V Ganesh - 23
  24. 24. Hash Tables • Distribute records among many servers • Distribution based on keys which is hashed • Key – 128 bit or 160 bits • Hash values fall into a range servers visualized to lie on the circumference of a circle going clockwise.03/28/12 Tinniam V Ganesh - 24
  25. 25. Distributed Hash Table• Hashing the keys results in reaching servers are assumed to reside on the circumference of a circle• The highest key coincides back to the beginning of this circle• The movement is clockwise03/28/12 Tinniam V Ganesh - 25
  26. 26. Distributed Hash TableAn entity with key K falls under the jurisdiction of the node with the smallest id >= K• For e.g. if we have two nodes, one at position 50 and another at position 200.• If we want to store a key / value pair in the DHT and the key hash is 100, would go to node 200.• Another key hash of 30 would go to the node 5003/28/12 Tinniam V Ganesh - 26
  27. 27. Consistent HashingA naïve approach with 8 nodes and 100 keys could use a simple modulo algorithm.So key 18 would end up on node 2 and key 63 on node 7.But how do we handle servers crashing or new servers joining the system.Consistent Hashing handles this issue03/28/12 Tinniam V Ganesh - 27
  28. 28. Consistent HashingSource: Tinniam V Ganesh - 28
  29. 29. Distributed Hash Table03/28/12 Tinniam V Ganesh - 29
  30. 30. Consistent Hashing Source: http://horicky.blogspot.in03/28/12 Tinniam V Ganesh - 30
  31. 31. 1 4 Chord System 1 3 4 9 Resolving K = 26 4 9 5 18 1 1 1 2 1 1 3 3 1 4 4 28 4 5 141 281 28 23 284 1 21 95 9 1 21 20 1 28 3 28 1 20 4 28 1 20 18 FTp[i]=succ(p+2 i-1) 5 4 3 28 14 4 28 03/28/12 Tinniam V Ganesh - 31 5 4
  32. 32. Process of determining nodeTo look up a key k node p will forward request to node q with index j in p’s finger table such thatq = FTp[j] <= k < FTp[j+1]To resolve k =264. 26> FT1[5] = 18. Hence forwarded to Node 185. FT18[2] <= 26 < FT 18[3]6. FT20[1] <=26 < FT20[2]7. 26 > FT21[1] = 28 Hence Node 28 is responsible for key 2603/28/12 Tinniam V Ganesh - 32
  33. 33. Hashing efficiency of Chord SystemThe Chord System gets to the node in O (log n) stepsThere are other hashing techniques that get in O(1) but use a larger local table. For example attains a O(1) hashing method.03/28/12 Tinniam V Ganesh - 33
  34. 34. Joining the Chord SystemSuppose node p wants to join. It performs the following steps- Requests lookup for succ (p+1)- Inserts itself before this node03/28/12 Tinniam V Ganesh - 34
  35. 35. Maintaining consistencyPeriodically each node checks its successor’s predecessor.Node ‘q’ contacts succ(q+1) and requests it to return pred(succ(q+1))If q = pred(succ(q+1)) then nothing has changed. If the node passes another value then q knows that a new node ‘p’ has joined the systemq < p < succ (q+1)so it updates its Finger table so qWill set FTq[1] = p03/28/12 Tinniam V Ganesh - 35
  36. 36. CAP TheoremDatabases that are designed based on ACID properties have poor availability.Postulated by Eric Brewer of University of BerkeleyAt most only 2 of 3 properties are possible in distributed systemsC – ConsistencyA – AvailabilityP – Partition Tolerance03/28/12 Tinniam V Ganesh - 36
  37. 37. CAP Theorem• Consistency – Ability for repeated reads to provide the same value• Availability – Ability to be resilient to server crashes• Partition Tolerance – Ability to partition data between servers and always be able to get the data03/28/12 Tinniam V Ganesh - 37
  38. 38. Real world examples of CAP TheoremAmazon’s Dynamo chooses availability over consistency. Dynamo implements eventual consistency where data become consistent over timeGoogle’s BigTable chooses consistency over availabilityConsistentcy, Partition Tolerance (CP)Big TableHbaseAvailability, Partition Tolerance (AP)DynamoVoldemortCassandra03/28/12 Tinniam V Ganesh - 38
  39. 39. Consistency issuesData replication used in many commercial systems perform synchronous replica coordination to provide strongly consistent data.The downside of this approach is the poor availabilityThese systems maintain that the data is unavailable if they are not able to ensure consistencyFor e.g.If data is replicated on 5 servers and an update needs to be made then the following has to be done- Update all 5 copies- Ensure all of them are successful- If one of them fails roll back the updates on the other 4If a read is done when one of the server fail a strongly consistent system would return “data unavailable” when correctness is undetermined.03/28/12 Tinniam V Ganesh - 39
  40. 40. Quorum ProtocolTo maintain consistency data is replicated in many servers.For e.g. let us assume there are N servers in the systemTypical algorithms maintain at least writes to > N/2 => N/2 +1Usually Nw> N/2A write is successful if it has been successfully committed in N/2 +1 serversThis is known as write quorum03/28/12 Tinniam V Ganesh - 40
  41. 41. Quorum ProtocolSimilarly reads are done from an arbitrary number of server replicas Nr. This is known as a read quorumReads from different servers are comparedA consistent design requires that Nw + Nr > NWith this you are assured of reading your writes03/28/12 Tinniam V Ganesh - 41
  42. 42. Election AlgorithmMany distributed systems usually have one process to act as a coordinator. If the coordinator crashes then an election takes place to identify the new coordinator2. P sends a ELECTION message to all higher numbered processes3. If no one responds P becomes coordinator4. If a higher number process answers, it takes over the election process03/28/12 Tinniam V Ganesh - 42
  43. 43. Traditional Fault ToleranceTraditional systems use redundancy to handle failures and be tolerant to fault as shown below Active Standby Active Standby03/28/12 Tinniam V Ganesh - 43
  44. 44. Process ResilienceHandling failures in distributed systems is much more difficult as no system has any view of the global state.03/28/12 Tinniam V Ganesh - 44
  45. 45. Byzantine FailuresByzantine refers to Byzantine General Problem where an army must unanimously decide whether to attack another army. The problem is complicated because the generals must use messengers to communicate and by the presence of traitorsDistributed Systems are prone to a type of failures known as Byzantine failuresOmission failures – Disk crashes, network congestion, failure to receive request etcCommission failures – Failures when the server behaves incorrectly, corrupting local state etcSolution: To be able to handle Byzantine Failures where k processes are sick is to have a minimum 2k+1 processes so that we are left with k+1 replies given that k process are behaving incorrectly03/28/12 Tinniam V Ganesh - 45
  46. 46. CheckpointingIn fault tolerant distributed computing backward error recovery requires that the system regularly save its state at periodic intervals. We need to create a consistent global state called a distributed snapshot.In a distributed snapshot if a process P has recorded the receipt of a message then there should be a process Q that has sent a corresponding message.Each process saves its state from time to time.To recover we need to construct a consistent global state from these local states03/28/12 Tinniam V Ganesh - 46
  47. 47. Gossip ProtocolUsed to handle server crashes and server or servers joining into the systemChanges to the distributed system like membership changes are spread similar to gossiping- A server picks another random server and sends a message regarding a server crash or a server joining- If the receiver has already received this message it is dropped.- The receiving server similarly gossips to other servers and the system reaches a steady state soon03/28/12 Tinniam V Ganesh - 47
  48. 48. Sloppy QuorumQuorum protocol is applied on first N healthy nodes rather than N nodes walking clockwise in the ring.Data meant for Node A is sent to Node D if A is temporarily down.Node D has a hinted handoff in its metadata that updates Node A when it is up.03/28/12 Tinniam V Ganesh - 48
  49. 49. Thank You ! Tinniam V Ganesh Read my blogs: Tinniam V Ganesh - 49