分布式系统之Dynamo算法.ppt

* * * * * * * * Dynamo: Amazon’s Highly Available Key-value Store Giuseppe DeCandia, Deniz Hastorun, Madan Jampani, Gunavardhan Kakulapati, Avinash Lakshman, Alex Pilchin, Swaminathan Sivasubramanian, Peter Vosshall and Werner Vogels Motivation Build a distributed storage system: Scale Simple: key-value Highly available Guarantee Service Level Agreements (SLA) System Assumptions and Requirements Query Model: simple read and write operations to a data item that is uniquely identified by a key. ACID Properties: Atomicity, Consistency, Isolation, Durability. Efficiency: latency requirements which are in general measured at the 99.9th percentile of the distribution. Other Assumptions: operation environment is assumed to be non-hostile and there are no security related requirements such as authentication and authorization. Service Level Agreements (SLA) Application can deliver its functionality in abounded time: Every dependency in the platform needs to deliver its functionality with even tighter bounds. Example: service guaranteeing that it will provide a response within 300ms for 99.9% of its requests for a peak client load of 500 requests per second. Service-oriented architecture of Amazon’s platform Design Consideration Sacrifice strong consistency for availability Conflict resolution is executed during read instead of write, i.e. “always writeable”. Other principles: Incremental scalability. Symmetry. Decentralization. Heterogeneity. Summary of techniques used in Dynamo and their advantages Problem Technique Advantage Partitioning Consistent Hashing Incremental Scalability High Availability for writes Vector clocks with reconciliation during reads Version size is decoupled from update rates. Handling temporary failures Sloppy Quorum and hinted handoff Provides high availability and durability guarantee when some of the replicas are not available. Recovering from permanent failures Anti-entropy using Merkle trees Synchronizes divergent replicas in the