Q. Populating a part of config param from environment variables

[mayankpahwa]

I have a config paramter that stores the HDFS location of a file. The parameter has HDFS IP, port and the file location, something like hdfs://172.21.1.11:54310/schema/schema.txt.
Since IPs and ports are always changing, I do not want to hard code their values. I tried setting the HDFS IP and port as environment variables and passing them to the value of config param in the configuration file, ex: hdfs://$HDFS_IP:$HDFS_PORT/schema/schema.txt, but it didn't work.

I know this can be achieved at the code level by reading the IP and port from the environment, and concatenating them with the file location, but is there a way to do it at the configuration level?

[alexander-yakushev]

It's possible to do this using the :delayed-transform option (see here). You'd do something like this in the config map:

{:hdfs-ip {:type :string}
 :hdfs-port {:type :number}
 :hdfs-url {:type :string
            :delayed-transform #(or % ;; If HDFS_URL was set explicitly, just use it.
                                    (format "hdfs://%s:%s/schema/schema.txt"
                                            (cfg/get :hdfs-ip) (cfg/get :hdfs-port)))}}

Then, the first time you do (cfg/get :hdfs-url) it will resolve into either the explicit value or the formatted string.

[mayankpahwa]

Thanks a lot @alexander-yakushev for your response. I tried the above with multiple cases and below is what I have found

;; Configuration definition
(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :delayed-transform #(or %
                                      (str "hdfs://" (cfg/get :hdfs-ip) ":"
                                           (cfg/get :hdfs-port) "/schema/schema.txt"))}})

;; Main function
(defn -main
  "Omniconf Demo"
  [& args]
  (cfg/populate-from-cmd args)
  (cfg/verify :quit-on-error true)
  (prn (cfg/get :hdfs-url)))

1. I ran lein run --hdfs-ip 192.168.1.124 --hdfs-port 54310 and got the following output

Omniconf configuration:
 {:hdfs-ip "192.168.1.124", :hdfs-port 54310}

nil

Couldn't understand why nil was printed instead of the concatenated string.

2. Then I ran lein run --hdfs-ip 192.168.1.124 --hdfs-port 54321 --hdfs-url hdfs://192.168.1.124:54310/schema/schema.txt and got the output

Omniconf configuration:
 {:hdfs-ip "192.168.1.124",
 :hdfs-port 54310,
 :hdfs-url #<Delay@294a6b8e: :not-delivered>}

"hdfs://192.168.1.124:54310/schema/schema.txt"

This works as expected

3. After playing around for a while, I changed the configuration definition to

;; Configuration definition
(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :delayed-transform #(or
                                   (str "hdfs://" (cfg/get :hdfs-ip) ":"
                                        (cfg/get :hdfs-port) "/schema/schema.txt") %)}})

Then I ran lein run --hdfs-ip 192.168.1.124 --hdfs-port 54310 --hdfs-url and got the ouput

Omniconf configuration:
 {:hdfs-ip "192.168.1.124",
 :hdfs-port 54310,
 :hdfs-url #<Delay@294a6b8e: :not-delivered>}

"hdfs://192.168.1.124:54310/schema/schema.txt"

Couldn't understand why this works fine

[alexander-yakushev]

This is a bug, sorry. :delayed-transform was originally intended for values that you do provide but want to do some post-processing on them. I thought it also supported what I've described, but I was wrong.

I'll try to think of a fix soon. Meanwhile, you can set a default value to :hdfs-url. The working example:

(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :default nil ;; Needed for delayed transform to work
              :delayed-transform #(or %
                                      (str "hdfs://" (cfg/get :hdfs-ip) ":"
                                           (cfg/get :hdfs-port) "/schema/schema.txt"))}})

lein run --hdfs-ip 192.168.1.124 --hdfs-port 54310

[mayankpahwa]

I am trying with the following

(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :default nil ;; Needed for delayed transform to work
              :delayed-transform #(or %
                                      (str "hdfs://" (cfg/get :hdfs-ip) ":"
                                           (cfg/get :hdfs-port) "/schema/schema.txt"))}})

(defn -main
  "Omniconf Demo"
  [& args]
  (cfg/populate-from-cmd args)
  (cfg/verify :quit-on-error true)
  (prn (cfg/get :hdfs-url)))

And lein run --hdfs-ip 192.168.1.124 --hdfs-port 54310

Still getting

Omniconf configuration:
 {:hdfs-ip "192.168.1.124", :hdfs-port 54310}

nil

Am I doing something wrong?

[alexander-yakushev]

Damn, I must have had unclean state when testing. This seems to work though:

(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :default ::none ;; Needed for delayed transform to work
              :delayed-transform #(if-not (= % ::none) ;; To allow setting the URL explicitly
                                    %
                                    (str "hdfs://" (cfg/get :hdfs-ip) ":"
                                         (cfg/get :hdfs-port) "/schema/schema.txt"))}})

[mayankpahwa]

Thanks. Works fine now.

[mayankpahwa]

I have another question

My configuration definition is:

(cfg/define
  {:hdfs-ip {:type :string}
   :hdfs-port {:type :number}
   :hdfs-url {:type :string
              :default "nil"
              :delayed-transform (fn [v] (str (cfg/get [:hdfs-ip]) ":"
                                              (cfg/get [:hdfs-port])))}
   :hdfs-url-nested {:nested
                     {:url {:type :string
                            :default "nil"
                            :delayed-transform (fn [v] (str (cfg/get [:hdfs-ip]) ":"
                                                            (cfg/get [:hdfs-port])))}}}})

Main function is:

(defn -main
  "Omniconf Demo"
  [& args]
  (cfg/populate-from-cmd args)
  (cfg/verify :quit-on-error true)
  (prn (cfg/get :hdfs-url))
  (prn (cfg/get :hdfs-url-nested)))

And I run lein run --hdfs-ip 192.168.1.1 --hdfs-port 9200

I am getting the output

Omniconf configuration:
{:hdfs-ip "192.168.1.1",
 :hdfs-port 9200,
 :hdfs-url #<Delay@6fb365ed: :not-delivered>,
 :hdfs-url-nested {:url "nil"}}

"192.168.1.1:9200"
{:url "nil"}

Looks like delayed-transform isn't working for nested params. Do you know of any workaround?

I am using version 0.2.7.

[alexander-yakushev]

Sorry! That is another bug. I've just pushed a fix to it as 0.2.8-SNAPSHOT, please try :).

[mayankpahwa]

Works fine, thanks :)

I just wanted to ask one more question. My configuration definition is present in a separate file rather than in the core.clj. In the file, I have the same configuration as mentioned above. For defining the configuration, I am doing something like
(cfg/define (read-string (slurp (io/resource "conf.def"))))

When I run lein run --hdfs-ip 192.168.1.1 --hdfs-port 54310, I get the error clojure.lang.PersistentList cannot be cast to clojure.lang.IFn

Looks like the function is not evaluated after reading from the file. Is there any way to solve this?

[alexander-yakushev]

That's because you put a function definition into the file. When you do read-string it does not get compiled but treated as plain s-expressions. You may eval it, but I strongly advise against that – that's a security hole.

Honestly, at this point, I think that the delayed-transform approach caused you more harm than good. Perhaps, it would be easier just to do

(cfg/set :hdfs-url (str (cfg/get :hdfs-ip) ":" (cfg/get :hdfs-port))

before calling (cfg/verify).

[mayankpahwa]

Makes sense. Thanks a lot :)