Protocol + Client Namespace + Multimethod

You could use a protocol to define the abstraction, except connect, which is a multimethod:

1 (ns stadig.storage.protocol
2   (:refer-clojure :exclude [get]))
3
4 (defprotocol IStorage
5   (get [this bucket key])
6   (put [this bucket key value])
7   (delete [this bucket key])
8   (close [this]))
9
10 (defmulti connect :backend)

Implement the protocol and multimethod for each backend:

1 (ns stadig.storage.s3
2   (:require
3    [aws.sdk.s3 :as s3]
4    [stadig.storage.protocol :as proto]))
5
6 (defrecord S3Storage
7     [access-key secret-key]
8   proto/IStorage
9   (get [this bucket key]
10     (s3/get-object this bucket key))
11   (put [this bucket key value]
12    ...