2 research outputs found
Improving the Latency and Throughput of ZooKeeper Atomic Broadcast
ZooKeeper is a crash-tolerant system that offers fundamental services to Internet-scale applications, thereby reducing the development and hosting of the latter. It consists of >3 servers that form a replicated state machine. Maintaining these replicas in a mutually consistent state requires executing an Atomic Broadcast Protocol, Zab, so that concurrent requests for state changes are serialised identically at all replicas before being acted upon.
Thus, ZooKeeper performance for update operations is determined by Zab performance. We contribute by presenting two easy-to-implement Zab variants, called ZabAC and ZabAA. They are designed to offer small atomic-broadcast latencies and to reduce the processing load on the primary node that plays a leading role in Zab. The former improves ZooKeeper performance and the latter enables ZooKeeper
to face more challenging load conditions
Mechanisms for improving ZooKeeper Atomic Broadcast performance
PhD ThesisCoordination services are essential for building higher-level primitives that are often
used in today’s data-center infrastructures, as they greatly facilitate the operation of
distributed client applications. Examples of typical functionalities offered by coordination
services include the provision of group membership, support for leader election,
distributed synchronization, as well as reliable low-volume storage and naming.
To provide reliable services to the client applications, coordination services in general
are replicated for fault tolerance and should deliver high performance to ensure that
they do not become bottlenecks for dependent applications. Apache ZooKeeper, for
example, is a well-known coordination service and applies a primary-backup approach
in which the leader server processes all state-modifying requests and then forwards
the corresponding state updates to a set of follower servers using an atomic broadcast
protocol called Zab.
Having analyzed state-of-the-art coordination services, we identified two main
limitations that prevent existing systems such as Apache ZooKeeper from achieving a
higher write performance: First, while this approach prevents the data stored by client
applications from being lost as a result of server crashes, it also comes at the cost of a
performance penalty. In particular, the fact that it relies on a leader-based protocol,
means that its performance becomes bottlenecked when the leader server has to handle
an increased message traffic as the number of client requests and replicas increases.
Second, Zab requires significant communication between instances (as it entails three
communication steps). This can potentially lead to performance overhead and uses up
more computer resources, resulting in less guarantees for users who must then build
more complex applications to handle these issues.
To this end, the work makes four contributions. First, we implement ZooKeeper
atomic broadcast, extracting from ZooKeeper in order to make it easier for other
developers to build their applications on top of Zab without the complexity of integrating
the entire ZooKeeper codebase. Second, we propose three variations of Zab, which
are all capable of reaching an agreement in fewer communication steps than Zab. The
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variations are built with restriction assumptions that server crashes are independent
and a server quorum remains operative at all times. The first variation offers excellent
performance but can only be used for 3-server systems; the other two are built without
this limitation. Then, we redesigned the latest two Zab variations to operate under the
least-restricted Zab fault assumptions. Third, we design and implement a ZooKeeper
coin-tossing protocol, called ZabCT which addresses the above concerns by having the
other, non-leader server replicas toss a coin and broadcast their acknowledgment of a
leader’s proposal only if the toss results in an outcome of Head. We model the ZabCT
process and derive analytical expressions for estimating the coin-tossing probability
of Head for a given arrival rate of service requests such that the dual objectives of
performance gains and traffic reduction can be accomplished. If a coin-tossing protocol,
ZabCT is judged not to offer performance benefits over Zab, processes should be able to
switch autonomously to Zab. We design protocol switching by letting processes switch
between ZabCT and Zab without stopping message delivery. Finally, an extensive
performance evaluation is provided for Zab and Zab-variant protocols