1. Designing a Replication System CS 188 Distributed Systems February 12, 2015

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

10. Making Use of Mobility

11. 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

12. A Couple of Very Important Questions
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
Are there constraints we should put in place?

13. 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?

14. A Final Question Is file system replication even suitable here?
Is there some other type of distributed data management we should use instead?