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FAQ What is an advisory lock? An “advisory lock” is simply a tool/API provided by Postgres to create arbitrary locks that can be acquired by applications. These locks, however, are not enforced in any meaningful way by the database – it’s up to application code to give them meaning (the same way any other non-database distributed lock would work). What is a write-through cache? A write-through cache is a caching strategy where data is simultaneously written into the cache and the corresponding database or backing store. ...

FAQ What is a single-row transaction? 在 Bigtable 变更（例如读取、写入和删除请求）中，行级更改始终属于原子操作。这包括对单行中的多个列进行的变更，前提是它们包含在同一变更操作中。Bigtable 不支持以原子方式更新多个行的事务。 但是，Bigtable 支持某些需要在其他数据库中执行事务的写入操作。实际上，Bigtable 使用单行事务来完成这些操作。这些操作包括读取和写入操作，且所有读取和写入均以原子方式执行，但仍然只是在行级的原子操作： 读取-修改-写入 (Read-modify-write) 操作（包括增量和附加）。在读取-修改-写入 (read-modify-write) 操作中，Cloud Bigtable 会读取现有值，对现有值进行增量或附加操作，并将更新后的值写入表中。 检查并更改 (Check-and-mutate) 操作（也称为条件更改或条件写入）。在检查并更改 (check-and-mutate) 操作中，Bigtable 会对行进行检查以了解其是否符合指定条件。如果符合条件，Bigtable 则会将新值写入该行中。 What is SSTable format? SSTable (Sorted Strings Table) is a file format used in Apache Cassandra, a popular NoSQL database. An SSTable is a data structure that provides a persistent, ordered immutable map from keys to values, where both keys and values are arbitrary byte streams. It contains a series of key-value pairs sorted by keys, which enables efficient lookup and range queries. It’s used to store and retrieve data in a highly optimized manner. The SSTable also supports internal indexing, which makes accessing a particular data point faster. ...

Analyze Key Features Processing and generating large data sets. Exploits a restricted programming model to parallelize the user program automatically and to provide transparent fault-tolerance. Details By distributing the workload to many machines and let them execute the tasks in parallel. Specifically, input files are split into $M$ pieces and master assign each one to an idle worker. Worker process it with the map function and save each key/value pair to one of the $R$ files according to the partioning function. When finishing processing all $M$ pieces, $R$ reduce workers will read data from the corresponding intermediate files, process it with the reduce function and save to output file eventually. ...

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. ...