3,349 research outputs found
PaRiS: Causally Consistent Transactions with Non-blocking Reads and Partial Replication
Geo-replicated data platforms are at the backbone of several large-scale
online services. Transactional Causal Consistency (TCC) is an attractive
consistency level for building such platforms. TCC avoids many anomalies of
eventual consistency, eschews the synchronization costs of strong consistency,
and supports interactive read-write transactions. Partial replication is
another attractive design choice for building geo-replicated platforms, as it
increases the storage capacity and reduces update propagation costs. This paper
presents PaRiS, the first TCC system that supports partial replication and
implements non-blocking parallel read operations, whose latency is paramount
for the performance of read-intensive applications. PaRiS relies on a novel
protocol to track dependencies, called Universal Stable Time (UST). By means of
a lightweight background gossip process, UST identifies a snapshot of the data
that has been installed by every DC in the system. Hence, transactions can
consistently read from such a snapshot on any server in any replication site
without having to block. Moreover, PaRiS requires only one timestamp to track
dependencies and define transactional snapshots, thereby achieving resource
efficiency and scalability. We evaluate PaRiS on a large-scale AWS deployment
composed of up to 10 replication sites. We show that PaRiS scales well with the
number of DCs and partitions, while being able to handle larger data-sets than
existing solutions that assume full replication. We also demonstrate a
performance gain of non-blocking reads vs. a blocking alternative (up to 1.47x
higher throughput with 5.91x lower latency for read-dominated workloads and up
to 1.46x higher throughput with 20.56x lower latency for write-heavy
workloads)
Efficient middleware for database replication
Dissertação de Mestrado em Engenharia InformáticaDatabase systems are used to store data on the most varied applications, like Web
applications, enterprise applications, scientific research, or even personal applications.
Given the large use of database in fundamental systems for the users, it is necessary that database systems are efficient e reliable. Additionally, in order for these systems to serve a large number of users, databases must be scalable, to be able to process large numbers of transactions. To achieve this, it is necessary to resort to data replication. In a
replicated system, all nodes contain a copy of the database. Then, to guarantee that
replicas converge, write operations must be executed on all replicas. The way updates
are propagated leads to two different replication strategies. The first is known as
asynchronous or optimistic replication, and the updates are propagated asynchronously
after the conclusion of an update transaction. The second is known as synchronous or pessimistic replication, where the updates are broadcasted synchronously during the transaction.
In pessimistic replication, contrary to the optimistic replication, the replicas remain
consistent. This approach simplifies the programming of the applications, since the
replication of the data is transparent to the applications. However, this approach
presents scalability issues, caused by the number of exchanged messages during
synchronization, which forces a delay to the termination of the transaction. This leads
the user to experience a much higher latency in the pessimistic approach.
On this work is presented the design and implementation of a database replication
system, with snapshot isolation semantics, using a synchronous replication approach.
The system is composed by a primary replica and a set of secondary replicas that fully
replicate the database- The primary replica executes the read-write transactions, while
the remaining replicas execute the read-only transactions. After the conclusion of a read-write transaction on the primary replica the updates are propagated to the
remaining replicas. This approach is proper to a model where the fraction of read
operations is considerably higher than the write operations, allowing the reads load to be
distributed over the multiple replicas.
To improve the performance of the system, the clients execute some operations
speculatively, in order to avoid waiting during the execution of a database operation.
Thus, the client may continue its execution while the operation is executed on the
database. If the result replied to the client if found to be incorrect, the transaction will be aborted, ensuring the correctness of the execution of the transactions
An Alternative Model to Overcoming Two Phase Commit Blocking Problem
In distributed transactions, the atomicity requirement of the atomic commitment protocol should be preserved. The two phased commit protocol algorithm is widely used to ensure that transactions in distributed environment are atomic. However, a main drawback was attributed to the algorithm, which is a blocking problem. The system will get in stuck when participating sites or the coordinator itself crashes. To address this problem a number of algorithms related to 2PC protocol were proposed such as back up coordinator and a technique whereby the algorithm passes through 3PC during failure. However, both algorithms face limitations such as multiple site and backup coordinator failures. Therefore, we proposed an alternative model to overcoming the blocking problem in combined form. The algorithm was simulated using Britonix transaction manager (BTM) using Eclipse IDE and MYSQL. In this paper, we assessed the performance of the alternative model and found that this algorithm handled site and coordinator failures in distributed transactions using hybridization (combination of two algorithms) with minimal communication messages
Database recovery
Recovery techniques are an important aspect of database systems. They are essential to ensure that data integrity is maintained after any type of failure occurs. The recovery mechanism must be designed so that the availability and performance of the system are not unacceptably impacted by the recovery algorithms running during normal execution. On the other hand, enough information must be stored so that the database can be restored or transactions backed out in a reasonable amount of time. Concepts, techniques, and problems associated with database recovery will be presented in this thesis. The recovery issues for both centralized and distributed systems will be discussed, along with the tradeoffs of different recovery tools. The database recovery schemes in IMS/VS, DB2 and SDD-1 will be described to show approaches in existing systems
- …