Mcrouter will interpret certain parts of the client request's key in a special way.
A memcached key is typically a short (mcrouter limit is 250 characters) ASCII string which does not contain any whitespace or control characters.
Consider the request get /region/cluster/foo:key|#|etc. The prefix /region/cluster/ is [[used to lookup the route|Routing prefix]], and is normally stripped before forwarding the request to the destination (there's a keep_routing_prefix option in the pool config that allows preserving this prefix, which is useful when setting up multiple mcrouters in series).
The string |#| is known as a "hash stop". The suffix up to and including the hash stop is ignored when performing consistent hashing, but it's not stripped. So this request is hashed as foo:key, and the destination server will see get foo:key|#|etc. This is useful when you want to group certain keys together on one box.
A key that starts with a special prefix __mcrouter__. is interpreted as an [[admin request|Admin requests]] and is not sent to any destination. For example, get __mcrouter__.version will cause mcrouter to return its package string (like mcrouter 1.0) as value.
/region/cluster/foo:key|#|etc
Full key: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Routing prefix: ^^^^^^^^^^^^^^^^
Routing key: ^^^^^^^
Key with stripped routing prefix: ^^^^^^^^^^^^^