> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt # hello (database command) ## Definition **New in version 4.2** [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) returns a document that describes the role of the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) instance. If the optional field `saslSupportedMechs` is specified, the command also returns an [`array of SASL mechanisms`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.saslSupportedMechs) used to create the specified user's credentials. If the instance is a member of a replica set, then [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) returns a subset of the replica set configuration and status including whether or not the instance is the [primary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-primary) of the replica set. When sent to a [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) instance that is not a member of a replica set, [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) returns a subset of this information. MongoDB [drivers](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-driver) and [clients](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-client) use [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) to determine the state of the replica set members and to discover additional members of a [replica set.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set) ## Compatibility This command is available in deployments hosted in the following environments: - [MongoDB Atlas](https://www.mongodb.com/docs/atlas): The fully managed service for MongoDB deployments in the cloud **Note:** This command is supported in all MongoDB Atlas clusters. For information on Atlas support for all commands, see [Unsupported Commands.](https://www.mongodb.com/docs/atlas/unsupported-commands/) - [MongoDB Enterprise](https://www.mongodb.com/docs/manual/administration/install-enterprise.md#std-label-install-mdb-enterprise): The subscription-based, self-managed version of MongoDB - [MongoDB Community](https://www.mongodb.com/docs/manual/administration/install-community.md#std-label-install-mdb-community-edition): The source-available, free-to-use, and self-managed version of MongoDB ## Syntax The command has the following syntax: ```javascript db.runCommand( { hello: 1 } ) ``` The [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) command accepts optional fields `saslSupportedMechs: ` to return an additional field [`hello.saslSupportedMechs`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.saslSupportedMechs) in its result and `comment ` to add a log comment associated with the command. ```javascript db.runCommand( { hello: 1, saslSupportedMechs: "", comment: } ) ``` The [`db.hello()`](https://www.mongodb.com/docs/manual/reference/method/db.hello.md#mongodb-method-db.hello) method in [`mongosh`](https://www.mongodb.com/docs/mongodb-shell.md#mongodb-binary-bin.mongosh) provides a wrapper around [`hello`.](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) ## Behavior ### Client Disconnection If the client that issued [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) disconnects before the operation completes, MongoDB marks [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) for termination using [`killOp`.](https://www.mongodb.com/docs/manual/reference/command/killOp.md#mongodb-dbcommand-dbcmd.killOp) ## Output ### All Instances The following [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) fields are common across all roles: A boolean value that reports when this node is writable. If `true`, then this instance is a [primary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-primary) in a [replica set](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set), or a [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance, or a standalone [`mongod`.](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) This field will be `false` if the instance is a [secondary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-secondary) member of a replica set or if the member is an [arbiter](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-arbiter) of a replica set. *For internal use by MongoDB.* The maximum permitted size of a [BSON](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-BSON) object in bytes for this [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) process. If not provided, clients should assume a max size of "`16 * 1024 * 1024`". The maximum permitted size of a [BSON](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-BSON) wire protocol message. The default value is `48000000` bytes. The maximum number of write operations permitted in a write batch. If a batch exceeds this [limit](https://www.mongodb.com/docs/manual/reference/limits.md#mongodb-limit-Write-Command-Batch-Limit-Size), the client driver divides the batch into smaller groups each with counts less than or equal to the value of this field. The value of this limit is `100,000` writes. Returns the local server time in UTC. This value is an [ISO date.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-ISODate) The time in minutes that a [session](https://www.mongodb.com/docs/manual/core/read-isolation-consistency-recency.md#std-label-sessions) remains active after its most recent use. Sessions that have not received a new read/write operation from the client or been refreshed with [`refreshSessions`](https://www.mongodb.com/docs/manual/reference/command/refreshSessions.md#mongodb-dbcommand-dbcmd.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. Only available when `featureCompatibilityVersion` is `"3.6"` or greater. An identifier for the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) / [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance's outgoing connection to the client. The earliest version of the wire protocol that this [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance is capable of using to communicate with clients. Clients may use [`minWireVersion`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.minWireVersion) to help negotiate compatibility with MongoDB. The latest version of the wire protocol that this [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance is capable of using to communicate with clients. Clients may use [`maxWireVersion`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.maxWireVersion) to help negotiate compatibility with MongoDB. A boolean value that, when `true`, indicates that the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) is running in read-only mode. An array listing the compression algorithms used or available for use (i.e. common to both the client and the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance) to compress the communication between the client and the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance. The field is only available if compression is used. For example: - If the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) is enabled to use both the `snappy,zlib` compressors and a client has specified `zlib`, the [`compression`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.compression) field would contain: ```javascript "compression": [ "zlib" ] ``` - If the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) is enabled to use both the `snappy,zlib` compressors and a client has specified `zlib,snappy`, the [`compression`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.compression) field would contain: ```javascript "compression": [ "zlib", "snappy" ] ``` - If the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) is enabled to use the `snappy` compressor and a client has specified `zlib,snappy`, the [`compression`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.compression) field would contain : ```javascript "compression": [ "snappy" ] ``` - If the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.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. That is, if the client does not specify compression or if the client specifies a compressor not enabled for the connected [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) or [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance, the field does not return. An array of SASL mechanisms used to create the user's credential or credentials. Supported SASL mechanisms are: - `GSSAPI` - `SCRAM-SHA-256` - `SCRAM-SHA-1` The field is returned only when the command is run with the `saslSupportedMechs` field: ```javascript db.runCommand( { hello: 1, saslSupportedMechs: "" } ) ``` ### Sharded Instances [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instances add the following field to the [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) response document: Contains the value `isdbgrid` when [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) returns from a [`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) instance. ### Replica Sets [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) contains these fields when returned by a member of a replica set: An array of strings in the format of `"[hostname]:[port]"` that lists all members of the [replica set](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set) that are neither [hidden](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-hidden-member), [passive](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-passive-member), nor [arbiters.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-arbiter) Drivers use this array and the [`hello.passives`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.passives) to determine which members to read from. The name of the current :replica set. The current replica set config version. A boolean value that, when `true`, indicates if the [`mongod`](https://www.mongodb.com/docs/manual/reference/program/mongod.md#mongodb-binary-bin.mongod) is a [secondary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-secondary) member of a [replica set.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set) An array of strings in the format of `"[hostname]:[port]"` listing all members of the [replica set](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set) which have a [`members[n].priority`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.members-n-.priority) of `0`. This field only appears if there is at least one member with a [`members[n].priority`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.members-n-.priority) of `0`. Drivers use this array and the [`hello.hosts`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.hosts) to determine which members to read from. An array of strings in the format of `"[hostname]:[port]"` listing all members of the [replica set](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-replica-set) that are [arbiters.](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-arbiter) This field only appears if there is at least one arbiter in the replica set. A string in the format of `"[hostname]:[port]"` listing the current [primary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-primary) member of the replica set. A boolean value that , when `true`, indicates that the current instance is an [arbiter](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-arbiter). The [`arbiterOnly`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.arbiterOnly) field is only present, if the instance is an arbiter. A boolean value that, when `true`, indicates that the current instance is [passive](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-passive-member). The [`passive`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.passive) field is only present for members with a [`members[n].priority`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.members-n-.priority) of `0`. A boolean value that, when `true`, indicates that the current instance is [hidden](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-hidden-member). The [`hidden`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-data-hello.hidden) field is only present for hidden members. A `tags` document contains user-defined tag field and value pairs for the replica set member. ```javascript { "": "", "": "",... } ``` - For read operations, you can specify a tag set in the [read preference](https://www.mongodb.com/docs/manual/core/read-preference-tags.md#std-label-replica-set-read-preference-tag-sets) to direct the operations to replica set member(s) with the specified tag(s). - For write operations, you can create a customize [write concern](https://www.mongodb.com/docs/manual/reference/write-concern.md) using [`settings.getLastErrorModes`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.settings.getLastErrorModes) and [`settings.getLastErrorDefaults`.](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.settings.getLastErrorDefaults) For more information, see [Configure Replica Set Tag Sets.](https://www.mongodb.com/docs/manual/tutorial/configure-replica-set-tag-sets.md#std-label-replica-set-configuration-tag-sets) **See also:** [`members[n].tags`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.members-n-.tags) The `[hostname]:[port]` of the member that returned [`hello`.](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) A unique identifier for each election. Included only in the output of [`hello`](https://www.mongodb.com/docs/manual/reference/command/hello.md#mongodb-dbcommand-dbcmd.hello) for the [primary](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-primary). Used by clients to determine when elections occur. A document containing [optime](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-optime) and date information for the database's most recent write operation. An object giving the [optime](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-optime) of the last write operation. A date object containing the time of the last write operation. An object giving the [optime](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-optime) of the last write operation readable by [`majority`](https://www.mongodb.com/docs/manual/reference/read-concern-majority.md#mongodb-readconcern-readconcern.-majority-) reads. A date object containing the time of the last write operation readable by [`majority`](https://www.mongodb.com/docs/manual/reference/read-concern-majority.md#mongodb-readconcern-readconcern.-majority-) reads. For details on the `ok` status field, the `operationTime` field, and the `$clusterTime` field, see [Command Response.](https://www.mongodb.com/docs/manual/reference/method/db.runCommand.md#std-label-command-response)