Dynamo.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 background.
Membership and failure detection
Gossip-based membership protocol
and failure detection.
Preserves symmetry and avoids
having a centralized registry for
storing membership and node
liveness information.

Partition Algorithm
 Consistent hashing: the output
range of a hash function is treated as a
fixed circular space or “ring”.
 ”Virtual Nodes”: Each node can
be responsible for more than one
virtual node.

Advantages of using virtual nodes
 If a node becomes unavailable the
load handled by this node is evenly
dispersed across the remaining
available nodes.
 When a node becomes available
again, the newly available node
accepts a roughly equivalent
amount of load from each of the
other available nodes.
 The number of virtual nodes that a
node is responsible can decided
based on its capacity, accounting
for heterogeneity in the physical
infrastructure.

Replication
 Each data item is
replicated at N hosts.
 “preference list”: The list of
nodes that is responsible
for storing a particular key.

Data Versioning
 A put() call may return to its caller before the
update has been applied at all the replicas
 A get() call may return many versions of the
same object.
 Challenge: an object having distinct version sub-histories,
which the system will need to reconcile in the future.
 Solution: uses vector clocks in order to capture causality
between different versions of the same object.

Vector Clock
 A vector clock is a list of (node, counter)
pairs.
 Every version of every object is associated
with one vector clock.
 If the counters on the first object’s clock are
less-than-or-equal to all of the nodes in the
second clock, then the first is an ancestor of
the second and can be forgotten.

Execution of get () and put ()
operations
1. Route its request through a generic load
balancer that will select a node based on
load information.
2. Use a partition-aware client library that
routes requests directly to the appropriate
coordinator nodes.

Sloppy Quorum
 R/W is the minimum number of nodes that
must participate in a successful read/write
operation.
 Setting R + W > N yields a quorum-like
system.
 In this model, the latency of a get (or put)
operation is dictated by the slowest of the R
(or W) replicas. For this reason, R and W are
usually configured to be less than N, to
provide better latency.

Hinted handoff
 Assume N = 3. When A
is temporarily down or
unreachable during a
write, send replica to D.
 D is hinted that the
replica is belong to A and
it will deliver to A when A
is recovered.
 Again: “always writeable”

Other techniques
 Replica synchronization:
 Merkle hash tree.
 Membership and Failure Detection:
 Gossip

Implementation
 Java
 Local persistence component allows for
different storage engines to be plugged in:
 Berkeley Database (BDB) Transactional Data
Store: object of tens of kilobytes
 MySQL: object of > tens of kilobytes
 BDB Java Edition, etc.

Dynamo.ppt

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