lein REPL server launch timed out

set timeout value in field :repl-options in project.clj,the default is 30000(30s). for example :

:repl-options{:timeout 120000}

It can sometimes take a while for all the ducks to be in order. Specify :repl-options in your project.clj file with a longer time-out like this:

:repl-options {
             ;; If nREPL takes too long to load it may timeout,
             ;; increase this to wait longer before timing out.
             ;; Defaults to 30000 (30 seconds)
             :timeout 120000
             }

Hopefully that should do the trick.

The reason was the multiple redefinition and evaluation of the Datomic connection among several files

(def conn (d/connect DATOMIC_URI))

The solution to this topic has been already discussed at How to deal with a variable in a library that needs to be set outside of it?