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| <h1 id="Overview">Overview</h1> |
| |
| <p>Apache Pegasus is a distributed key-value storage system which is designed to be:</p> |
| |
| <ul> |
| <li><strong>horizontally scalable</strong>: distributed using hash-based partitioning</li> |
| <li><strong>strongly consistent</strong>: ensured by <a href="https://www.microsoft.com/en-us/research/publication/pacifica-replication-in-log-based-distributed-storage-systems/">PacificA</a> consensus protocol</li> |
| <li><strong>high-performance</strong>: using <a href="https://rocksdb.org/">RocksDB</a> as underlying storage engine</li> |
| <li><strong>simple</strong>: well-defined, easy-to-use APIs</li> |
| </ul> |
| |
| <h2 id="background">Background</h2> |
| |
| <p>Pegasus targets to fill the gap between Redis and <a href="https://hbase.apache.org/">HBase</a>. As the former |
| is in-memory, low latency, but does not provide a strong-consistency guarantee. |
| And unlike the latter, Pegasus is entirely written in C++ and its write-path |
| relies merely on the local filesystem.</p> |
| |
| <p>Apart from the performance requirements, we also need a storage system |
| to ensure multiple-level data safety and support fast data migration |
| between data centers, automatic load balancing, and online partition split.</p> |
| |
| <h2 id="features">Features</h2> |
| |
| <ul> |
| <li> |
| <p><strong>Persistence of data</strong>: Each write is replicated three-way to different ReplicaServers before responding to the client. Using PacificA protocol, Pegasus has the ability for strong consistent replication and membership changes.</p> |
| </li> |
| <li> |
| <p><strong>Automatic load balancing over ReplicaServers</strong>: Load balancing is a builtin function of MetaServer, which manages the distribution of replicas. When the cluster is in an inbalance state, the administrator can invoke a simple rebalance command that automatically schedules the replica migration.</p> |
| </li> |
| <li> |
| <p><strong>Cold Backup</strong>: Pegasus supports an extensible backup and restore mechanism to ensure data safety. The location of snapshot could be a distributed filesystem like HDFS or local filesystem. The snapshot storing in the filesystem can be further used for analysis based on <a href="https://github.com/pegasus-kv/pegasus-spark">pegasus-spark</a>.</p> |
| </li> |
| <li> |
| <p><strong>Eventually-consistent intra-datacenter replication</strong>: This is a feature we called <em>duplication</em>. It allows a change made in the local cluster accesible after a short time period by the remote cluster. It help achieving higher availability of your service and gaining better performance by accessing only local cluster.</p> |
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| <li><a href="#Overview">Overview</a> |
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| <li><a href="#background">Background</a></li> |
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