Analyze Key Features Coordination in large-scale distributed systems by providing general “coordination kernel” APIs that support a lot of use cases. Good performance Fault tolerance / high availability Details ZooKeeper can be used for group messaging, shared registers, and distributed lock services. Reliability By replicating the ZooKeeper data on each server that compses the service. The data is periodically snapshotted. Fuzzy Snapshot We do not lock the ZooKeeper state to take the snapshot. ...

A transaction is a way for an application to group several reads and writes together into a logical unit. Transactions are not a law of nature; they were created with a purpose, namely to simplify the programming model for applications accessing a database. 相当于数据库提供了一层重要的抽象，在编写应用程序时不用再去考虑那些能被事务处理的错误与问题了。 The Meaning of ACID The safety guarantees provided by transactions are often described by the well known acronym ACID, which stands for Atomicity, Consistency, Isolation, and Durability. ...

The main reason for wanting to partition data is scalability. Normally, partitions are defined in such a way that each piece of data (each record, row, or document) belongs to exactly one partition. Partitioning and Replication Partitioning is usually combined with replication so that copies of each partition are stored on multiple nodes. This means that, even though each record belongs to exactly one partition, it may still be stored on several different nodes for fault tolerance. ...

Replication Versus Partitioning There are two common ways data is distributed across multiple nodes: Replication Keeping a copy of the same data on several different nodes, potentially in different locations. Replication provides redundancy and can also help improve performance. Partitioning Splitting a big database into smaller subsets called partitions so that different partitions can be assigned to different nodes (also known as sharding). These are separate mechanisms, but they often go hand in hand: ...

Formats for Encoding Data 这里提到了两种兼容性，后面分析数据编码格式时都会用到： In order for the system to continue running smoothly, we need to maintain compatibility in both directions: Backward compatibility Newer code can read data that was written by older code. Forward compatibility Older code can read data that was written by newer code. 直译有一个问题, 英语的"前后"在时间和空间上统一, 而汉语却是相反. 比如 forward 在空间上指前进, 在时间上指未来. 但是汉语中的"前"在空间上指前进, 在时间上却指过去. 向后兼容很好理解：指新的版本的软／硬件可以使用老版本的软／硬件产生的数据。 Forward compatibility 译为向前兼容极容易混乱，这里可以想成向未来兼容：指老的版本的软／硬件可以使用新版本的软／硬件产生的数据。 以下是几个例子： Intel 的 x86指令集 CPU 是向后兼容的，因为新款 CPU 依然可以运行老版本的软件。Intel 保证老版本 CPU 有的指令集新版本一定还保留着，这种只增加不删除的策略，保证了我们换 CPU 时，不需要更换很多软件。 ...