Providing a notification when a plurality of users are...

Electrical computers and digital processing systems: multicomput – Computer conferencing – Cooperative computer processing

Reexamination Certificate

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Details

C709S232000, C707S793000

Reexamination Certificate

active

06701345

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to system management and more particularly to a data safeguarding mechanism.
BACKGROUND
Computerized databases are commonly used to store large amounts of data for easy access and manipulation by multiple users. In a centralized computer system, there is a single copy of the data stored at one location, typically a computer. By maintaining a single, centralized database, such a system avoids inconsistencies which might otherwise occur with more than one copy of the data. Nevertheless, the centralized database approach has several drawbacks. First, since only one copy of the data exists, if the data becomes corrupted or inaccessible, the entire system becomes unavailable. Second, with only one copy of data available for read and update purposes, the system may appear slow and time-consuming, especially to multiple users.
Consequently, many of today's organizations, especially those dispersed over several locations, utilize some type of distributed database system. In a distributed system, an organization's data is spread across the storage facilities of several computers or processors. These storage facilities may be located throughout a single building, across several adjacent buildings or at different locations across the country or around the world. These computers or processors are interconnected via a communications network and are referred to as sites or nodes. Each site, moreover, is able to process local transactions which access data retained only at that local storage facility as well as distributed transactions which access data stored on more than one computer.
Computerized databases, both centralized and distributed, are often used to execute transactions. A transaction is a set of data-dependent operations requested by a user of the system. For example, a user may request some combination of retrieval, update, deletion or insertion operations. The completion of a transaction is called a commitment and the cancellation of a transaction prior to its completion is referred to as an abort. If a transaction is aborted, then any partial results (i.e., updates from those operations that were performed prior to the abort decision) must be undone. This process of returning the data items to their original values is also referred to as a roll back. An important aspect of a transaction is atomicity. Atomicity means that all of the operations associated with a particular transaction must be performed or none of them can be performed. That is, if a transaction is interrupted by a failure, the transaction must be aborted so that its partial results are undone (i.e., rolled back) and, if the transaction is completed, the results are preserved (i.e., committed) despite subsequent failures. The classic example of atomicity concerns a transfer of bank funds from account A to account B. Clearly, the system must either perform both the withdrawal and the deposit operations of the transaction or neither operation.
To protect against disruptions caused by the failure of any particular site, most distributed database systems allow additional copies or “replicas” of the data to be made at other sites. That is, a copy of each data item stored on one of the system's database facilities may also exist at the database facilities of other sites. By replicating the data across multiple instances of database facilities, a certain degree of fault-tolerance may be obtained. Furthermore, by having a locally available replica of the database available, the response time of certain transactions may be improved.
Although replicated systems provide the above advantages over non-replicated systems, there are nonetheless inherent costs associated with the replication of databases. To update a single data item, at least one message must be propagated to every replica of that data item, consuming substantial communications resources. Furthermore, in order to manage multiple databases and handle the execution of concurrent transactions, a complicated administrative support mechanism is required. In addition, if the replicated system cannot guarantee consistent updates at all replicas, data integrity may be compromised.
Most commercially available replicated database systems utilize either a distributed transaction approach or a primary-backup approach to replicate the data. In the distributed transaction approach, all database replicas are updated with a single, distributed transaction. That is, whenever a data item is updated by a transaction, all copies or replicas of that data item are updated as part of the same transaction. This approach results in completely synchronized replicas. To ensure atomicity, distributed transaction-based systems must employ an atomic commit protocol, such as the well-known 2 Phase Commit (“2PC”) protocol. The basic idea behind 2PC is to determine a unique decision for all replicas with respect to either committing or aborting a transaction and then executing that decision at all replicas. If a single replica is unable to commit, then the transaction must be aborted at all replicas.
More specifically, under the 2PC protocol, a single database manager associated with a single database facility is chosen as the coordinator of the transaction. The coordinator first asks all of the participants (i.e., the other replicas) including itself (if the coordinator is a participant) to prepare for the commitment of a transaction. Each participant replies to the coordinator with either a READY message, signaling that the participant is ready and willing to commit the transaction, or an ABORT message, signaling that the participant is unable to commit the transaction. Before sending the first prepare message, the coordinator typically enters a record in a log stored on stable storage, identifying all of the replicas participating in the transaction. The coordinator also activates a time-out mechanism. Based on the replies received from the participants, the coordinator decides whether to commit or abort the transaction. If all participants answer READY, the coordinator decides to commit the transaction. Otherwise, if at least one participant replies with an ABORT message or has not yet answered when the time-out expires, the coordinator decides to abort the transaction.
The coordinator begins the second phase of 2PC by recording its decision (i.e., commit or abort) in the log. The coordinator then informs all of the participants, including itself, of its decision by sending them a command message, i.e., COMMIT or ABORT. In response, all of the participants write a commit or abort record in their own logs. Finally, all participants send a final acknowledgment message to the coordinator and execute the relevant procedures for either committing or aborting the transaction. The acknowledgment message, moreover, is not simply an acknowledgment that a command has been received, but is a message informing the coordinator that the command has been recorded by the participant in its stable log record. When the coordinator receives the acknowledgment messages from the participants, it enters a “complete” record in its log.
Although widely implemented, the 2PC protocol nonetheless has several disadvantages. First, as set forth above, the protocol requires each replicated database facility to submit a READY message before the transaction can be committed. Thus, in a fully replicated environment, any site or link failure brings all activity to a complete halt until the site or link is repaired, since that site cannot transmit a READY message. That is, until the failed site is recovered, no further transactions may be executed by a system relying on 2PC. Second, 2PC requires the transmission of at least three messages per replicated database per transaction. The protocol thus consumes substantial communications resources and reduces the system's response time and throughput. Third, 2PC requires both the coordinator and all participants to record the commit/abort decision and the final outcome to stable storage. This involves two

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