Redis xlen 命令
Returns the number of entries inside a stream. If the specified key does not
exist the command returns zero, as if the stream was empty.
However note that unlike other Redis types, zero-length streams are
possible, so you should call TYPE or EXISTS in order to check if
a key exists or not.
Streams are not auto-deleted once they have no entries inside (for instance
after an XDEL call), because the stream may have consumer groups
associated with it.
Integer reply: the number of entries of the stream at
redis> XADD mystream * item 2
redis> XADD mystream * item 3
redis> XLEN mystream