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.
整数: the number of entries of the stream at
"1621392774866-0"redis> XADD mystream * item 2
"1621392774867-0"redis> XADD mystream * item 3
"1621392774867-1"redis> XLEN mystream