In-Memory Storage Engine内存存储引擎

Specify In-Memory Storage Engine指定内存中存储引擎

To select the in-memory storage engine, specify:要选择内存存储引擎，请指定：

• inMemory for the --storageEngine option, or the storage.engine setting if using a configuration file.inMemory用于--storageEngine选项，或storage.engine设置（如果使用配置文件）。
• --dbpath, or storage.dbPath if using a configuration file. Although the in-memory storage engine does not write data to the filesystem, it maintains in the --dbpath small metadata files and diagnostic data as well temporary files for building large indexes.--dbpath或storage.dbPath（如果使用配置文件）。尽管内存存储引擎不将数据写入文件系统，但它在--dbpath中维护小型元数据文件和诊断数据，以及用于构建大型索引的临时文件。

For example, from the command line:例如，从命令行：

mongod --storageEngine inMemory --dbpath <path>

Or, if using the YAML configuration file format:或者，如果使用YAML配置文件格式：

storage:
   engine: inMemory
   dbPath: <path>

See inMemory Options for configuration options specific to this storage engine. Most mongod configuration options are available for use with in-memory storage engine except for those options that are related to data persistence, such as journaling or encryption at rest configuration.有关特定于此存储引擎的配置选项，请参阅内存选项。除了那些与数据持久性相关的选项外，大多数mongod配置选项都可用于内存存储引擎，例如日志记录或静态加密配置。

Concurrency并发

The in-memory storage engine uses document-level concurrency control for write operations. As a result, multiple clients can modify different documents of a collection at the same time.内存中存储引擎使用文档级并发控制进行写操作。因此，多个客户端可以同时修改集合的不同文档。

Memory Use内存使用

In-memory storage engine requires that all its data (including indexes, oplog if mongod instance is part of a replica set, etc.) must fit into the specified --inMemorySizeGB command-line option or storage.inMemory.engineConfig.inMemorySizeGB setting in the YAML configuration file.内存存储引擎要求其所有数据（包括索引、oplog（如果mongod实例是副本集的一部分）等）必须符合YAML配置文件中指定的--inMemorySizeGB命令行选项或storage.inMemory.engineConfig.inMemorySizeGB设置。

By default, the in-memory storage engine uses 50% of physical RAM minus 1 GB.默认情况下，内存中存储引擎使用50%的物理RAM减去1 GB。

If a write operation would cause the data to exceed the specified memory size, MongoDB returns with the error:如果写操作会导致数据超过指定的内存大小，MongoDB会返回错误：

"WT_CACHE_FULL: operation would overflow cache"

To specify a new size, use the storage.inMemory.engineConfig.inMemorySizeGB setting in the YAML configuration file format:要指定新的大小，请使用YAML配置文件格式中的storage.inMemory.engineConfig.inMemorySizeGB设置：

storage:
   engine: inMemory
   dbPath: <path>
   inMemory:
      engineConfig:
         inMemorySizeGB: <newSize>

Or use the command-line option --inMemorySizeGB:或者使用命令行选项--inMemorySizeGB：

mongod --storageEngine inMemory --dbpath <path> --inMemorySizeGB <newSize>

Durability耐久性

The in-memory storage engine is non-persistent and does not write data to a persistent storage. Non-persisted data includes application data and system data, such as users, permissions, indexes, replica set configuration, sharded cluster configuration, etc.内存中存储引擎是非持久性的，不会将数据写入持久性存储器。非持久化数据包括应用程序数据和系统数据，如用户、权限、索引、副本集配置、分片集群配置等。

As such, the concept of journal or waiting for data to become durable does not apply to the in-memory storage engine.因此，日志或等待数据变得持久的概念不适用于内存存储引擎。

If any voting member of a replica set uses the in-memory storage engine, you must set writeConcernMajorityJournalDefault to false.

With writeConcernMajorityJournalDefault set to false, MongoDB does not wait for w: "majority" writes to be written to the on-disk journal before acknowledging the writes. As such, "majority" write operations could possibly roll back in the event of a transient loss (e.g. crash and restart) of a majority of nodes in a given replica set.

Write operations that specify a write concern journaled are acknowledged immediately. When an mongod instance shuts down, either as result of the shutdown command or due to a system error, recovery of in-memory data is impossible.

Transactions事务

Starting in MongoDB 4.2, transactions are supported on replica sets and sharded clusters where:从MongoDB 4.2开始，在副本集和分片集群上支持事务，其中：

• the primary uses the WiredTiger storage engine, and主存储使用WiredTiger存储引擎，并且
• the secondary members use either the WiredTiger storage engine or the in-memory storage engines.次要成员使用WiredTiger存储引擎或内存中存储引擎。

In MongoDB 4.0, only replica sets using the WiredTiger storage engine supported transactions.在MongoDB 4.0中，只有使用WiredTiger存储引擎的副本集支持事务。

Deployment Architectures部署体系结构

In addition to running as standalones, mongod instances that use in-memory storage engine can run as part of a replica set or part of a sharded cluster.除了作为单机运行外，使用内存存储引擎的mongod实例还可以作为副本集的一部分或分片集群的一部分运行。

sh.addShardTag("shardC", "inmem")

sh.addShardTag("shardA", "persisted")
sh.addShardTag("shardB", "persisted")

sh.addTagRange("test.analytics", { shardKey: MinKey }, { shardKey: MaxKey }, "inmem")

sh.addTagRange("salesdb.orders", { shardKey: MinKey }, { shardKey: MaxKey }, "persisted")