designing a replication system cs 188 distributed...

Lecture 10 Page 1 CS 188,Winter 2015

Designing a Replication System CS 188

Distributed Systems February 12, 2015

Lecture 10 Page 2 CS 188,Winter 2015

Introduction

•  There are many situations in which we might use replicated data

•  Let’s look at another, different one •  And design a system to work well in

that situation

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Challenging Network Environments

•  Some environments have a hard time maintaining: –  low-loss –  low-latency – End-to-end communications

•  Examples: – Mobile ad hoc networks – Wireless sensor nets – Situations where wireless connectivity to

Internet is costly

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Internet Protocols in These Environments

•  Not what the Internet was designed for •  E.g., TCP

– Poor throughput if you get lots of losses or highly varying delays

•  Basic architecture also a poor match: – Packet switching – End-to-end retransmission on loss

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An Alternative: Delay Tolerant Networking

•  A message switching approach •  Using store-and-forward at message level •  Designed to handle seriously changing

conditions – Like mobility changing which links are

available – Or long-term failures of network

elements

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Delay Tolerant Networking and Mobile Computing

•  Essentially, use device mobility to establish new network links

•  Those links allow access to devices not easily reachable before

•  As devices move around, connectivity changes

•  “Carry” messages on the mobile devices, in the hopes of finding ways to deliver them

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Using Delay Tolerant Networking for Replication

•  Use the delayed message delivery to handle update propagation

•  Apply updates at one place and propagate them quickly, when possible

•  When not, attach them to mobile devices – Which may not hold replicas themselves

•  Use device mobility to get updates to the other places they must go

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A Specific Scenario •  A sensor network in a forest •  Sensors hold replicated files related to their task

– E.g., schedules of measurements – Any copy updatable

•  Some sensors can communicate with each other – But most have no connectivity to the Internet,

directly or indirectly •  Forest rangers carrying mobile devices

periodically move through the forest – But not specifically to visit sensors – Occasionally coming into range of some sensor

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Making Use of Mobility

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Some Replication Constraints •  We want all replicas updatable •  We want to achieve the highest degree

of consistency as fast as feasible •  We want to avoid conflicts, when

possible – And at least know when they happen

•  Avoid requiring extra hardware – Especially out in the woods

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A Couple of Very Important Questions

1.  What is insufficiently specified about this scenario to allow good design?

– In particular, things that it’s reasonable to know

– Not oracular knowledge of the future 2.  Are there constraints we should put in

place?

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So, What’s Our Replication System Going To Be Like?

•  Optimistic or conservative? •  Server oriented or peer oriented?

– Replicas only or caching? •  How will updates occur? •  How will updates be propagated? •  What is our replication granularity? •  How can we achieve fast consistency? •  How do we handle concurrency?

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A Final Question

•  Is file system replication even suitable here?

•  Is there some other type of distributed data management we should use instead?