This command is used in order to manage the consumer groups associated
with a stream data structure. Using `XGROUP` you can:

* Create a new consumer group associated with a stream.
* Destroy a consumer group.
* Remove a specific consumer from a consumer group.
* Set the consumer group *last delivered ID* to something else.

To create a new consumer group, use the following form:

    XGROUP CREATE mystream consumer-group-name $

The last argument is the ID of the last item in the stream to consider already
delivered. In the above case we used the special ID '$' (that means: the ID
of the last item in the stream). In this case the consumers fetching data
from that consumer group will only see new elements arriving in the stream.

If instead you want consumers to fetch the whole stream history, use
zero as the starting ID for the consumer group:

    XGROUP CREATE mystream consumer-group-name 0

Of course it is also possible to use any other valid ID. If the specified
consumer group already exists, the command returns a `-BUSYGROUP` error.
Otherwise, the operation is performed and a @simple-string-reply `OK` is returned.
There are no hard limits to the number of consumer groups you can associate with a given stream.

If the specified stream doesn't exist when creating a group, an error will be
returned. You can use the optional `MKSTREAM` subcommand as the last argument
after the `ID` to automatically create the stream, if it doesn't exist. Note
that if the stream is created in this way it will have a length of 0:

    XGROUP CREATE mystream consumer-group-name $ MKSTREAM

A consumer group can be destroyed completely by using the following form:

    XGROUP DESTROY mystream consumer-group-name

The consumer group will be destroyed even if there are active consumers
and pending messages, so make sure to call this command only when really
needed.
This form returns an @integer-reply with the number of destroyed consumer groups (0 or 1).

Consumers in a consumer group are auto-created every time a new consumer
name is mentioned by some command. They can also be explicitly created
by using the following form:

    XGROUP CREATECONSUMER mystream consumer-group-name myconsumer123

This form returns an @integer-reply with the number of created consumers (0 or 1).

To just remove a given consumer from a consumer group, the following
form is used:

    XGROUP DELCONSUMER mystream consumer-group-name myconsumer123

Sometimes it may be useful to remove old consumers since they are no longer
used.
This form returns an @integer-reply with the number of pending messages that the consumer had before it was deleted.

Finally it possible to set the next message to deliver using the
`SETID` subcommand. Normally the next ID is set when the consumer is
created, as the last argument of `XGROUP CREATE`. However using this form
the next ID can be modified later without deleting and creating the consumer
group again. For instance if you want the consumers in a consumer group
to re-process all the messages in a stream, you may want to set its next
ID to 0:

    XGROUP SETID mystream consumer-group-name 0

This form returns a @simple-string-reply `OK` or an error.

Finally to get some help if you don't remember the syntax, use the
HELP subcommand:

    XGROUP HELP

@history

  * `>= 6.2.0`: Supports the `CREATECONSUMER` subcommand.
