hello (database command数据库命令)

Client Disconnection客户端断开连接

If the client that issued hello disconnects before the operation completes, MongoDB marks hello for termination using killOp.如果发出hello的客户端在操作完成之前断开连接，MongoDB将使用killOp标记hello以终止。

All Instances所有实例

The following hello fields are common across all roles:以下hello字段在所有角色中都很常见：

hello.isWritablePrimary

A boolean value that reports when this node is writable. 一个布尔值，用于报告此节点何时可写。If true, then this instance is a primary in a replica set, or a mongos instance, or a standalone mongod.如果为true，则此实例是副本集中的primary、mongos实例或独立mongod。

This field will be false if the instance is a secondary member of a replica set or if the member is an arbiter of a replica set.如果该实例是副本集的次要成员，或者该成员是副本集中的仲裁器，则此字段将为false。

hello.topologyVersion

For internal use by MongoDB.供MongoDB内部使用。

hello.maxBsonObjectSize

The maximum permitted size of a BSON object in bytes for this mongod process. If not provided, clients should assume a max size of "16 * 1024 * 1024".此mongod进程允许的BSON对象的最大大小（以字节为单位）。如果没有提供，客户端应假设最大大小为“16*1024*1024”。

hello.maxMessageSizeBytes

The maximum permitted size of a BSON wire protocol message. The default value is 48000000 bytes.BSON有线协议消息的最大允许大小。默认值为48000000字节。

hello.maxWriteBatchSize

The maximum number of write operations permitted in a write batch. If a batch exceeds this limit, the client driver divides the batch into smaller groups each with counts less than or equal to the value of this field.写入批处理中允许的最大写入操作数。如果批次超过此限制，客户端驱动程序会将批次分成更小的组，每组的计数小于或等于此字段的值。

The value of this limit is 100,000 writes.此限制的值为100000次写入。

hello.localTime

Returns the local server time in UTC. This value is an ISO date.返回以UTC为单位的本地服务器时间。此值是ISO日期。

hello.logicalSessionTimeoutMinutes

The time in minutes that a session remains active after its most recent use. 会话在最近一次使用后保持活动状态的时间（分钟）。Sessions that have not received a new read/write operation from the client or been refreshed with refreshSessions within this threshold are cleared from the cache. State associated with an expired session may be cleaned up by the server at any time.未从客户端接收到新的读/写操作或在此阈值内使用refreshSessions刷新的会话将从缓存中清除。服务器可以随时清理与过期会话相关联的状态。

Only available when featureCompatibilityVersion is "3.6" or greater.仅当featureCompatibilityVersion为"3.6"或更高版本时可用。

hello.connectionId

An identifier for the mongod / mongos instance's outgoing connection to the client.mongod/mongos实例到客户端的传出连接的标识符。

hello.minWireVersion

The earliest version of the wire protocol that this mongod or mongos instance is capable of using to communicate with clients.此mongod或mongos实例能够用于与客户端通信的最早版本的有线协议。

Clients may use minWireVersion to help negotiate compatibility with MongoDB.客户端可以使用minWireVersion来帮助协商与MongoDB的兼容性。

hello.maxWireVersion

The latest version of the wire protocol that this mongod or mongos instance is capable of using to communicate with clients.此mongod或mongos实例能够用于与客户端通信的最新版本的有线协议。

Clients may use maxWireVersion to help negotiate compatibility with MongoDB.客户端可以使用maxWireVersion来帮助协商与MongoDB的兼容性。

hello.readOnly

A boolean value that, when true, indicates that the mongod or mongos is running in read-only mode.一个布尔值，当为true时，表示mongod或mongos正在以只读模式运行。

hello.compression

An array listing the compression algorithms used or available for use (i.e. common to both the client and the mongod or mongos instance) to compress the communication between the client and the mongod or mongos instance.一个数组，列出了用于压缩客户端和mongod或mongos实例之间通信的压缩算法（即客户端和mongood或mongos实例共有的压缩算法）。

The field is only available if compression is used. For example:该字段仅在使用压缩时可用。例如：

• If the mongod is enabled to use both the snappy,zlib compressors and a client has specified zlib, the compression field would contain:如果mongod启用了snappy和zlib压缩器，并且客户端指定了zlib，则压缩字段将包含：

  "compression": [ "zlib" ]

• If the mongod is enabled to use both the snappy,zlib compressors and a client has specified zlib,snappy, the compression field would contain:如果mongod启用了snappy,zlib压缩器，并且客户端指定了snappy,zlib，则compression字段将包含：

  "compression": [ "zlib", "snappy" ]

• If the mongod is enabled to use the snappy compressor and a client has specified zlib,snappy, the compression field would contain :如果mongod启用了snappy压缩器，并且客户端指定了zlib,snappy，则compression字段将包含：

  "compression": [ "snappy" ]

• If the mongod is enabled to use the snappy compressor and a client has specified zlib or the client has specified no compressor, the field is omitted.如果启用了mongod来使用snappy压缩器，并且客户端指定了zlib，或者客户端没有指定压缩器，则省略该字段。

That is, if the client does not specify compression or if the client specifies a compressor not enabled for the connected mongod or mongos instance, the field does not return.也就是说，如果客户端没有指定压缩，或者客户端指定了一个未为连接的mongod或mongos实例启用的压缩器，则该字段不会返回。

hello.saslSupportedMechs

An array of SASL mechanisms used to create the user's credential or credentials. Supported SASL mechanisms are:一系列用于创建用户凭据的SASL机制。支持的SASL机制包括：

• GSSAPI
• SCRAM-SHA-256
• SCRAM-SHA-1

The field is returned only when the command is run with the saslSupportedMechs field:仅当使用saslSupportedMechs字段运行命令时，才会返回该字段：

db.runCommand( { hello: 1, saslSupportedMechs: "<db.username>" } )

Sharded Instances分片实例

mongos instances add the following field to the hello response document:实例将以下字段添加到hello响应文档中：

hello.msg

Contains the value isdbgrid when hello returns from a mongos instance.包含从mongos实例返回hello时的值isdbgrid。

Replica Sets复制集

hello contains these fields when returned by a member of a replica set:当副本集的成员返回时，包含以下字段：

hello.hosts

An array of strings in the format of "[hostname]:[port]" that lists all members of the replica set that are neither hidden, passive, nor arbiters.一个字符串数组，格式为"[hostname]:[port]"，列出了副本集中既不是隐藏的、被动的也不是仲裁器的所有成员。

Drivers use this array and the hello.passives to determine which members to read from.驱动程序使用此数组和hello.passives来确定从哪些成员读取。

hello.setName

The name of the current :replica set.当前副本集的名称。

hello.setVersion

The current replica set config version.当前副本集配置版本。

hello.secondary

A boolean value that, when true, indicates if the mongod is a secondary member of a replica set.一个布尔值，当为true时，表示mongod是否是副本集的次要成员。

hello.passives

An array of strings in the format of "[hostname]:[port]" listing all members of the replica set which have a members[n].priority of 0.一个字符串数组，格式为"[hostname]:[port]"，列出了副本集中members[n].priority为0的所有成员。

This field only appears if there is at least one member with a members[n].priority of 0.仅当至少有一个members[n].priority为0时，才会显示此字段。

Drivers use this array and the hello.hosts to determine which members to read from.驱动程序使用此数组和hello.hosts来确定从哪些成员读取。

hello.arbiters

An array of strings in the format of "[hostname]:[port]" listing all members of the replica set that are arbiters.一个字符串数组，格式为"[hostname]:[port]"，列出了副本集中作为仲裁器的所有成员。

This field only appears if there is at least one arbiter in the replica set.仅当副本集中至少有一个仲裁器时，才会显示此字段。

hello.primary

A string in the format of "[hostname]:[port]" listing the current primary member of the replica set.一个格式为"[hostname]:[port]"的字符串，列出了副本集的当前primary成员。

hello.arbiterOnly

A boolean value that , when true, indicates that the current instance is an arbiter. The arbiterOnly field is only present, if the instance is an arbiter.一个布尔值，当为true时，表示当前实例是仲裁器。只有当实例是仲裁器时，arbiterOnly字段才存在。

hello.passive

A boolean value that, when true, indicates that the current instance is passive. 一个布尔值，当为true时，表示当前实例是被动的。The passive field is only present for members with a members[n].priority of 0.passive字段仅适用于members[n].priority为0的成员。

hello.hidden

A boolean value that, when true, indicates that the current instance is hidden. The hidden field is only present for hidden members.一个布尔值，当为true时，表示当前实例已隐藏。hidden字段仅对隐藏成员存在。

hello.tags

A tags document contains user-defined tag field and value pairs for the replica set member.tags文档包含用户定义的标记字段和副本集成员的值对。

{ "<tag1>": "<string1>", "<tag2>": "<string2>",... }

• For read operations, you can specify a tag set in the read preference to direct the operations to replica set member(s) with the specified tag(s).对于读取操作，您可以在读取首选项中指定一个标记集，以将操作定向到具有指定标记的副本集成员。
• For write operations, you can create a customize write concern using settings.getLastErrorModes and settings.getLastErrorDefaults.对于写入操作，您可以使用settings.getLastErrorModes和settings.getLastErrorDefaults创建自定义写入关注。

For more information, see Configure Replica Set Tag Sets.有关详细信息，请参阅配置副本集标记集。

Tip

members[n].tags

hello.me

The [hostname]:[port] of the member that returned hello.返回hello的成员的[hostname]:[port]。

hello.electionId

A unique identifier for each election. Included only in the output of hello for the primary. Used by clients to determine when elections occur.每次选举的唯一标识符。仅包含在primary的hello输出中。客户使用它来确定选举何时发生。

hello.lastWrite

A document containing optime and date information for the database's most recent write operation.包含数据库最近写入操作的最佳值和日期信息的文档。

hello.lastWrite.opTime

An object giving the optime of the last write operation.给出上次写入操作optime（最优值）的对象。

hello.lastWrite.lastWriteDate

A date object containing the time of the last write operation.包含上次写入操作时间的日期对象。

hello.lastWrite.majorityOpTime

An object giving the optime of the last write operation readable by majority reads.一个对象，给出majority读取可读的最后一次写入操作的optime（最优值）。

hello.lastWrite.majorityWriteDate

A date object containing the time of the last write operation readable by majority reads.一个日期对象，包含majority读取可读的最后一次写入操作的时间。

For details on the ok status field, the operationTime field, and the $clusterTime field, see Command Response.有关ok状态字段、operationTime字段和$clusterTime字段的详细信息，请参阅命令响应。