Caching OGSI Grid Service Data to Allow

Disconnected State Retrieval

Alastair Hampshire

University of Nottingham

Overview

• Motivation:– Why would intermittently connected devices use the grid?

• The problem:– Lack of support in OGSI for intermittent network

connectivity.

• A set of recommendations– Using caching to improve access to the state of

intermittently connected services

• Conclusion

Intermittently Connected Devices on the Grid

• Rationale:– Allows devices to exploit a wealth of

grid service functionality, e.g. for data processing and archival

• Intermittently Connected Sensors– Mobile Sensors:

– E.g. Wearable Medical Sensing

– Remote Sensors:– E.g. Antarctic Sensing Probe

• Ubiquitous Computing

The MIAS-Equator Toolkit

• Aims– Explore the extent to which the grid could be used to

support mobile sensing devices

• Prototype Toolkit– MIAS-Equator toolkit: toolkit designed to expose sensing

devices as grid services

• Deployment and Trials– Wearable Medical Sensing Device– Antarctic Sensing Probe

• Partners:- EQUATOR eScience programme in collaboration with MIAS IRC.

Lack of Support in OGSI for Intermittent Network Connectivity

• Invocations failure– Extended network disconnections cause grid

service invocation failure

• Failed Service Data Requests– Best case: delayed data retrieval– Worst case: data loss

Approach

• Caching Grid Service Data (SD)– Allow access to the SD of a temporarily disconnected

service

– Allow access SD from a temporarily disconnected client

• Improved access to SD in intermittently connected network environments

Caching OGSI Service Data

• OGSI findServiceData requests allow easy identification of SD requests

• SD marked as static or constant does not change for the lifetime of the service and is therefore ideal for caching

• Paper covers recommendations for:– Which SD to cache?

– How to populate the SD cache?

– How to check the consistency of the cache?

Which SD to cache?

• Caching all SD would be excessive• Strategies:

– Client selects which SD to cache

– Service selects which SD to cache

– Notification request triggers SD caching

– SD request triggers SD cache

How to populate the SD cache?

• Approaches to populating the cache– Speculative Caching

• A cache retains a copy of the response from all successful find SD requests

– Proactive• A cache actively maintains any up-to-date copy of cacheable SD

elements

Checking the cache consistency

• Difficult to monitor changes to SD– Internal factors may effect the service state

• Static of Constant SD never changes and need not be updated

• Extendable or mutable SD:– Cache SD elements alongside a timestamp– When a cached item is retrieved, calculate the age: items under a

given age a considered valid

• Cache validity determined by:– The client– The cache

Conclusions

• Demonstrated a requirement for the grid to better support intermittently connected devices– E.g. mobile or remote sensing devices

• Demonstrated how OGSI does not adequately support intermittently connected devices.

• Proposed caching as a mechanism to improve access to the state (service data) of intermittently connected devices

• Provided a set of recommendations for the use of cache in OGSI to support disconnected state retrieval

Questions?