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From OZAWA Tsuyoshi <ozawa.tsuyo...@lab.ntt.co.jp>
Subject [announce] Accord: A high-performance coordination service for write-intensive workloads
Date Fri, 23 Sep 2011 13:08:44 GMT

Sending zookeeper-users and hbase-users ml since there may be some
cluster developers interested in participating in this project there.

I am pleased to announce the initial release of Accord, yet another
coordination service like Apache ZooKeeper.
ZooKeeper is a de facto standard coordination kernel as you know at present.
Accord provides ZK-like features as a coordination service. Concretely
speaking, it features:
- Accord is a distributed, transactional, and fully-replicated (No SPoF)
Key-Value Store with strong consistency.
- Accord can be scale-out up to tens of nodes.
- Accord servers can handle tens or thousands of clients.
- The changes for a write request from a client can be notified to the
other clients.
- Accord detects events of client's joining/leaving, and notifies
joined/left client information to the other clients.

There are some problems in ZK, however, as follows:
- ZK cannot handle write-intensive workloads well. ZK forwards all write
requests to a master server. It may be bottleneck in write-intensive
- ZK is optimized for disk-persistence mode, not for in-memory mode.
ZOOKEEPER-866 shows that ZK has the other bottleneck outside disk
persistence, though there are some needs of a fully-replicated storage
with both strong consistency and low latency.
- Limited Transaction APIs. ZK can only issue write operations (write,
del) in a transaction(multi-update).

These restriction limit the capability of the coordination kernel.
Accord solves such problems.
1. Accord uses Corosync Cluster Engine as a total-order messaging
infrastructure instead of Zab, an atomic broadcast protocol ZK uses. The
engine enable any servers to accept and process requests.
2. Accord supports in-memory mode.
3. More flexible transaction support. Not only write, del operations,
but also cmp, copy, read operations are supported in transaction operation.

These differences of the core engine (1, 2) enable us to avoid master
bottleneck. Benchmark demonstrates that the write-operation throughput
of Accord is much higher than one of ZooKeeper
(up to 20 times better throughput at persistent mode, and up to 18 times
better throughput at in-memory mode).

The high performance kernel can extend the application ranges. Assumed
applications are as follows, for instance :
- Distributed Lock Manager whose lock operations occur at a high
frequency from thousands of clients.
I assume that the lock manager for Hbase in particluar. The coordination
service enables HBase to update multiple rows with ACID properties.
Hbase acts as distributed DB with ACID properties until the coordination
service becomes the bottleneck. The new coordination kernel, Accord, can
handle 18 times better throughput than ZK. As a result, Accord can
dramatically improve the scalability of Hbase with ACID properties.
- Metadata management service for large-scale distributed storage,
including HDFS, Ceph and Sheepdog etc.
Replicated-master can be implemented easily.
- Replicated Message Queue or logger (For instance, replicated RabbitMQ).
and so on.

The other distributed systems can use Accord features easily because
Accord provides general-purpose APIs (read/write/del/more flexible

More information including getting started, benchmarks, and API docs are
available from our project page :

and all code is available from:

Please try it out, and let me know any opinions or problems.

Best regards,
OZAWA Tsuyoshi

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