Caching OGSI Grid Service Data to Allow Disconnected State Retrieval

1. Caching OGSI Grid Service Data to AllowDisconnected State Retrieval Alastair Hampshire University of Nottingham

2. 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

3. 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

4. The MIAS-Equator Toolkit • Partners: • - EQUATOR eScience programme in collaboration with MIAS IRC. • 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

5. 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

6. 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

7. 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?

8. 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

9. 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

10. 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

11. 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

12. Questions?