4,843 research outputs found
Atomic commitment in transactional DHTs
We investigate the problem of atomic commit in transactional database systems
built on top of Distributed Hash Tables. DHTs provide a decentralized way to
store and look up data. To solve the atomic commit problem we propose to
use an adaption of Paxos commit as a non-blocking algorithm. We exploit the
symmetric replication technique existing in the DKS DHT to determine which
nodes are necessary to execute the commit algorithm. By doing so we achieve a
lower number of communication rounds and a reduction of meta-data in contrast
to traditional Three-Phase-Commit protocols. We also show how the proposed
solution can cope with dynamism due to churn in DHTs. Our solution works
correctly relying only on an inaccurate failure detection of node failure which is
necessary for systems running over the Internet
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)
NCC: Natural Concurrency Control for Strictly Serializable Datastores by Avoiding the Timestamp-Inversion Pitfall
Strictly serializable datastores greatly simplify the development of correct
applications by providing strong consistency guarantees. However, existing
techniques pay unnecessary costs for naturally consistent transactions, which
arrive at servers in an order that is already strictly serializable. We find
these transactions are prevalent in datacenter workloads. We exploit this
natural arrival order by executing transaction requests with minimal costs
while optimistically assuming they are naturally consistent, and then leverage
a timestamp-based technique to efficiently verify if the execution is indeed
consistent. In the process of designing such a timestamp-based technique, we
identify a fundamental pitfall in relying on timestamps to provide strict
serializability, and name it the timestamp-inversion pitfall. We find
timestamp-inversion has affected several existing works.
We present Natural Concurrency Control (NCC), a new concurrency control
technique that guarantees strict serializability and ensures minimal costs --
i.e., one-round latency, lock-free, and non-blocking execution -- in the best
(and common) case by leveraging natural consistency. NCC is enabled by three
key components: non-blocking execution, decoupled response control, and
timestamp-based consistency check. NCC avoids timestamp-inversion with a new
technique: response timing control, and proposes two optimization techniques,
asynchrony-aware timestamps and smart retry, to reduce false aborts. Moreover,
NCC designs a specialized protocol for read-only transactions, which is the
first to achieve the optimal best-case performance while ensuring strict
serializability, without relying on synchronized clocks. Our evaluation shows
that NCC outperforms state-of-the-art solutions by an order of magnitude on
many workloads
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