The Rover Toolkit

1
The Rover Toolkit

• Anthony D. Joseph
• Computer Science Division
• University of California at Berkeley
• BARWAN Retreat
• January 15, 1998

2
MIT Rover People

• Profs. Frans Kaashoek and David Gifford
• Anthony D. Joseph (Toolkit arch/impl apps)
• Joshua A. Tauber (Toolkit arch Webcal)
• George M. Candea (Rover File System)
• Constantine Cristakos (Rover NNTP)
• Alan deLespinasse (Rover Web Browser proxy)
• Michael Shurpik (Webcal)

3
Outline

• Harsh mobile environment
• Rover toolkit benefits and techniques
• Rover architecture
• Use and benefits
• Summary and future directions

4
Mobile Applications are Hard to Build

• Environment varies over an apps lifetime
• Connectivity, bandwidth, latency, and cost vary
• Technological improvements wont fix variability
• Mobile computers are limited fragile
• Network partitions are frequent ( shadows)
• Most dist. algorithms and apps assume theyre not
• Availability and consistency tradeoff
• Application-adaptation is complex
• When/where to move data and code
• System provides mechanisms and policy engine

5
How the Rover Toolkit Helps

• Simplifies application development
• Provides reusable building blocks
• Alternative to special-purpose/single-use
  approaches
• Helps developers cope with changes
• Varying environment and computational resources
• Mobile-transparent/mobile-aware support
• Allows incremental transition transparent to
  aware

6
Transparent versus Aware

• Mobile-transparent
• Hides mobility from application
• Allows the use of existing unmodified apps
• Examples Coda, Ficus, Little Work
• Mobile-aware
• Exposes environmental information to apps
• Yields better functionality and performance
• Examples InfoPad, Daedalus, GloMop, Bayou

7
Applications Targeted by Rover

• Groupware (client-server) apps
• Applications with shared state
• E-mail and distributed calendar
• File-level conflicts occur all the time
• May not be real conflicts
• Need semantic information
• Leverage intermittent connectivity to reduce
  conflict window

8
Techniques Used in Rover

• Queued Remote Procedure Call
• Isolate applications from network limitations
• Relocatable Dynamic Objects
• Adapt to changing environment
• App-specific, optimistic concurrency control
• Reduce number of unresolvable conflicts
• Fault-Tolerant RDOs
• Protect long running client computation at servers

9
Outline

• Harsh mobile environment
• Rover toolkit benefits and techniques
• Rover architecture
• Use and benefits
• Summary and future directions

10
Rover Architecture
RDOs Tcl/Tk Code
QRPCs Log to Disk
Rover
Rover
Transport HTTP SMTP
Links Wired and wireless
Client
Server

• Client-server object-based model (code/data
  shipping)
• Programming model
• Import RDOs, invoke local methods, export
  operations
• FSM-based implementation

11
Relocatable Dynamic Objects
RDO Code Data Interface Outcalls

• Rover provides execution environment
• Application starts w/ core functionality
• Important for small devices / large applications
• Dynamically load additional functionality
• Reduce communication
• Application-specific compression
• Service user requests locally
• Current limitation rudimentary security

12
Queued Remote Procedure Call
QRPC RPC Split phase Stable Q Scheduler

• Hides unpredictable remote access time
• Tolerates network, client, and server failures

13
QRPC Benefits

• Request/reply can use different connections
• Asymmetric or one-way networks
• Exposes queues to applications
• Applications can determine QRPC state/progress
• Log compression/request absorption
• Network scheduler
• Allows prioritizing, batching, and compression
• Reduces transmission power, time, and cost

14
Rover is Easy to Use

• Create server and client proxies (transparent)
• Port applications to Rover
• Not a difficult process change 10-15 of code
• Easy to improve responsiveness
• Use QRPC for stability and to make GUI
  non-blocking
• Group metadata with objects
• Change granularity of data
• Better responsiveness and performance (2x)
• Perform useful work instead of waiting
• Service user requests locally

15
Example WWW Browser Proxy
Client

• Click-ahead for unchanged clients
• Prefetches compresses inlined objects
• HTTP/1.1 (1/97) compression and pipelining (no
  prefetching)
• Separate cache display (simple paradigm)
• Better overall performance
• Benefits for workstation users

16
Rover Toolkit Summary

• Rover makes building mobile apps easier
• Provides useful set of abstractions
• RDOs reduce latency and bandwidth needs
• QRPC matches mobile environment challenges
• Reliable RDOs simplify the protection of client
  code
• Application-specific concurrency control reduces
  unresolvable conflicts
• Application performance is excellent
• Pre-alpha release - Dec 97

17
Future Directions

• Java-based implementation
• Provide portable QRPC operator for ProActive
  Infrastructure
• Policies for adaptive mechanisms
• When to open/close a network connection
• Which network to use (b/w, latency, cost)
• Security for mobile code
• Code generated at clients and servers runs
  everywhere
• How to protect private client and server data and
  resources
• Highly-available servers
• Providing consistent client views across servers
  Bayou

18
More Future Directions

• Peer-peer workgroups
• Islands of high bandwidth (e.g., this retreat)
• Where is the canonical copy / commit point?
• Multi-layer client-server hierarchy
• Similar to peer-peer workgroups
• Dismounted warfighter with PDA to laptop to server

19
Advertisement Spring 98 Seminar

• Mobile computing and wireless networking
• CS 294 WF 1230-200 (for now) in 405 Soda
• Local- and wide-area wireless networks
• WaveLAN, Metricom, CDPD, and GSM
• Problems and issues
• Mobile computing Adapting to harsh env.
• Disconnected operation/intermittent connectivity
• Sharing Consistency versus availability
• Projects
• From ProActive Infrastructure Viking (BT3GC)