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Mobile Computing - Replication Schemes For Mobile Environment, Study notes of Mobile Computing

Detail Summery about Replication Schemes For Mobile Environment, Eager Replication, Lazy Replication, Replication Schemes For Mobile Environment, Replication achieved through dynamic data allocation, Active Replication .

Typology: Study notes

2010/2011

Uploaded on 09/04/2011

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Replication Schemes For Mobile Environment
Eager Replication: It attains replication by keeping all replicas
exactly synchronized at all nodes. This in turn is achieved by
updating all replicas in one transaction. No concurrency
anomalies occur since eager replication gives serializable
execution. Eager replication is not a proper choice for a
mobile environment since eager replication reduced update
performance with extra updates, and in a mobile environment
the nodes are normally disconnected.
Lazy Replication: These algorithms are more suited to the
mobile environment . The synchronization occurs after the
updating transactions commits. That is, replication is
asynchronous in nature. The problem with lazy replication is
that it may allow a transaction to see a very old committed
value.
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Replication Schemes For Mobile Environment

  • (^) Eager Replication: It attains replication by keeping all replicas exactly synchronized at all nodes. This in turn is achieved by updating all replicas in one transaction. No concurrency anomalies occur since eager replication gives serializable execution. Eager replication is not a proper choice for a mobile environment since eager replication reduced update performance with extra updates, and in a mobile environment the nodes are normally disconnected.
  • (^) Lazy Replication: These algorithms are more suited to the mobile environment. The synchronization occurs after the updating transactions commits. That is, replication is asynchronous in nature. The problem with lazy replication is that it may allow a transaction to see a very old committed value.

Replication Schemes For Mobile Environment

  • (^) Several replication schemes for mobile environments have been proposed are in use. Some of those are : 1. Replication achieved through dynamic data allocation : In this method the switching takes place between one-copy and two-copy schemes 2. Adaptive Data Replication : Adaptive replication is a generalization of the principle of caching. In adaptive replication the number of copies of an object, the location of these copies, and which updates are propagated to each copy, change dynamically depending on a given cost function. 3. Active Replication : Active replication is performed by processing the same request at every replica. 4. Optimistic Replication: Optimistic replication (also known as lazy replication) is a strategy for replication in which replicas are allowed to diverge. Traditional pessimistic replication systems are based on the principle of single-copy consistency. that is, users should observe the system to behave as if there was only one copy of the data. Optimistic replication does away with this in favor of eventual consistency, meaning that replicas are guaranteed to converge only when a system is idle. 5. User Profile Replication:- A dynamic replication scheme which employs user profiles for recording user' mobility schedules, access behavior and read/write patterns, and actively reconfigures the replicas to adjust to the changes in user locations and system status. Simulation results demonstrate that the scheme can accurately predict the data requirement to facilitate effective replication, reduce response time, and increase data availability.

Location Data Mgmt. - Solutions

  • (^) Stores the profiles of users currently located in that area for whom the area is **not their home
  1. Home Location Register (HLR)
  2. Improved: Visitor Location Register (VLR)**
  • (^) Decomposing PCS (Personal Communication System) network into regions and use regional directories.
  • (^) Per-user caching where geographical areas cache the last known location of certain users for fast lookup.
  • (^) **Hierarchical distributed database architecture.
  1. each leaf database covers a specific geographical region
  2. maintains location information for all users currently residing in that region**
  • (^) assignment of location databases to the nodes of a signaling network.
  • (^) It uses dynamic programming to optimize the mapping.
  • (^) maps a database hierarchy to the network configuration, based on fixed calling and mobility patterns.