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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