Transcript The Rover Toolkit and Beyond

The Rover Toolkit
Anthony D. Joseph
Computer Science Division
University of California at Berkeley
BARWAN Retreat
January 15, 1998
MIT Rover People
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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)
Outline
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Harsh mobile environment
Rover toolkit benefits and techniques
Rover architecture
Use and benefits
Summary and future directions
Mobile Applications are Hard to Build
• Environment varies over an app’s lifetime
– Connectivity, bandwidth, latency, and cost vary
– Technological improvements won’t fix variability
• Mobile computers are limited & fragile
• Network partitions are frequent ($$ & shadows)
– Most dist. algorithms and apps assume they’re not
– Availability and consistency tradeoff
Application-adaptation is complex
– When/where to move data and code
– System provides mechanisms and policy engine
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
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
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
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
Outline
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Harsh mobile environment
Rover toolkit benefits and techniques
Rover architecture
Use and benefits
Summary and future directions
Rover Architecture
E-mail
GUI
RDOs: Tcl/Tk Code
E-mail
Filesys
QRPCs: Log to Disk
Rover
O/S
Rover
Transport: HTTP & SMTP
Links: Wired and wireless
Client
O/S
Server
• Client-server object-based model (code/data shipping)
• Programming model:
– Import RDOs, invoke local methods, export operations
• FSM-based implementation
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
Queued Remote Procedure Call
QRPC = RPC + Split phase + Stable Q + Scheduler
GUI
Filesys
Request
Stable queue
Scheduler
Stable queues
Callback
Client
Server
• Hides unpredictable remote access time
• Tolerates network, client, and server failures
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
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
Example: WWW Browser Proxy
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Proxy Cache Queue
Rover
Server
Rover
TranSend
Server
WWW
Server
WWW
Server
Client
• “Click-ahead” for unchanged clients
• Prefetches & compresses inlined objects
WWW
Server
– HTTP/1.1 (1/97): compression and pipelining (no prefetching)
• Separate cache display (simple paradigm)
Better overall performance
– Benefits for workstation users
Rover Toolkit Summary
• Rover makes building mobile apps easier
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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
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]
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
Advertisement: Spring ‘98 Seminar
• Mobile computing and wireless networking
– CS 294: WF 12:30-2:00 (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)