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Babashka pods

Clojars Project

Babashka pods are programs that can be used as Clojure libraries by babashka.

This is the library to load babashka pods. It is used by babashka but also usable from the JVM and sci-based projects other than babashka.

Below Golden Gate Bridge
The word pod means bridge in Romanian.


Pods are standalone programs that can expose namespaces with vars to babashka or a JVM. Pods can be built in Clojure, but also in languages that don't run on the JVM.

Some terminology:

  • pod: a program that exposes namespaces with vars via the pod protocol.
  • pod client: the program invoking a pod. When babashka invokes a pod, babashka is the pod client. When a JVM invokes a pod, the JVM is the pod client.
  • message: a message sent from the pod client to the pod or vice versa, encoded in bencode format.
  • payload: a particular field of a message encoded in a payload format (currently JSON, EDN or Transit JSON). Examples are args, value and ex-data. _
  • pod protocol: the documented way of exchanging messages between a pod client and pod.

Pods can be created independently from pod clients. Any program can be invoked as a pod as long as it implements the pod protocol. This protocol is influenced by and built upon battle-tested technologies:

  • the nREPL and LSP protocols
  • bencode
  • JSON
  • EDN
  • composition of UNIX command line tools in via good old stdin and stdout

The name pod is inspired by boot's pod feature. It means underneath or below in Polish and Russian. In Romanian it means bridge (source).

Available pods

For a list of available pods, take a look here.


The protocol should be considered alpha. Breaking changes may occur at this phase and will be documented in


Using pod-babashka-hsqldb as an example pod.

On the JVM:

(require '[babashka.pods :as pods])
(pods/load-pod "pod-babashka-hsqldb")
(require '[pod.babashka.hsqldb :as sql])

(def db "jdbc:hsqldb:mem:testdb;sql.syntax_mys=true")
(sql/execute! db ["create table foo ( foo int );"])
;;=> [#:next.jdbc{:update-count 0}]

Where does the pod come from?

When calling load-pod with a string or vector of strings (or declaring it in your bb.edn), the pod is looked up on the local file system (either using the PATH, or using an absolute path). When it is called with a qualified symbol and a version - like (load-pod 'org.babashka/aws "0.0.5") then it will be looked up in and downloaded from the pod-registry. You can customize the file system location that load-pod will use by setting the BABASHKA_PODS_DIR environment variable.

By default babashka will search for a pod binary matching your system's OS and arch. If you want to download pods for a different OS / arch (e.g. for deployment to servers), you can set one or both of the following environment variables:

  • BABASHKA_PODS_OS_NAME=Linux (or Mac OS X or any other value returned by Java's property)
  • BABASHKA_PODS_OS_ARCH=aarch64 (or amd64 or any other value returned by Java's os.arch property)

In a babashka project

As of babashka 0.8.0 you can declare the pods your babashka project uses in your bb.edn file like so:

:pods {org.babashka/hsqldb {:version "0.1.0"} ; will be downloaded from the babashka pod registry
       my.local/pod {:path "../pod-my-local/my-pod-binary"
                     :cache false}} ; optionally disable namespace caching if you're actively working on this pod

Then you can just require the pods in your code like any other clojure lib:

(ns my.project
  (:require [pod.babashka.hsqldb :as sql]
            [my.local.pod :as my-pod]))

(def db "jdbc:hsqldb:mem:testdb;sql.syntax_mys=true")
(sql/execute! db ["create table foo ( foo int );"])
;;=> [#:next.jdbc{:update-count 0}]

(my-pod/do-a-thing "foo")
;;=> "something"

The pods will then be loaded on demand when you require them. No need to call load-pod explicitly.


To use pods in a sci based project, see test/babashka/pods/sci_test.clj.

Why JVM support?

  • Babashka pods allow you to leverage functionality from other programs regardless of the technology they were implemented in. As such, pods can be a light weight replacement for native interop (JNI, JNA, etc.).

  • When developing pods, this library can be used to test them on the JVM.

Implementing your own pod


Beyond the already available pods mentioned above, eductional examples of pods can be found here:


If you are looking for libraries to deal with bencode, JSON or EDN, take a look at the existing pods or nREPL implementations for various languages.


When choosing a name for your pod, we suggest the following naming scheme:


where <user-id> is your Github or Gitlab handle and <pod-name> describes what your pod is about.


Pods created by the babashka maintainers use the identifier babashka:

The protocol

Message and payload format

Exchange of messages between pod client and the pod happens in the bencode format. Bencode is a bare-bones format that only has four types:

  • integers
  • lists
  • dictionaries (maps)
  • byte strings

Additionally, payloads like args (arguments) or value (a function return value) are encoded in either EDN, JSON or Transit JSON.

So remember: messages are in bencode, payloads (particular fields in the message) are in either EDN, JSON or Transit JSON.

Bencode is chosen as the message format because it is a light-weight format which can be implemented in 200-300 lines of code in most languages. If pods are implemented in Clojure, they only need to depend on the bencode library and use pr-str and edn/read-string for encoding and decoding payloads.

So we use bencode as the first encoding and choose one of multiple richer encodings on top of this, similar to how the nREPL protocol is implemented. More payload formats might be added in the future. Other languages typically use a bencode library + a JSON library to encode payloads.

When calling the babashka.pods/load-pod function, the pod client will start the pod and leave the pod running throughout the duration of a babashka script.


The first message that the pod client will send to the pod on its stdin is:

{"op" "describe"}

Encoded in bencode this looks like:

(bencode/write-bencode System/out {"op" "describe"})
;;=> d2:op8:describee

The pod should reply to this request with a message similar to:

{"format" "json"
 [{"name" "pod.lispyclouds.sqlite"
   "vars" [{"name" "execute!"}]}]
 "ops" {"shutdown" {}}}

In this reply, the pod declares that payloads will be encoded and decoded using JSON. It also declares that the pod exposes one namespace, pod.lispyclouds.sqlite with one var execute!.

To encode payloads in EDN use "edn" and for Transit JSON use "transit+json".

The pod encodes the above map to bencode and writes it to stdoud. The pod client reads this message from the pod's stdout.

Upon receiving this message, the pod client creates these namespaces and vars.

The optional ops value communicates which ops the pod supports, beyond describe and invoke. It is a map of op names to option maps. In the above example the pod declares that it supports the shutdown op. Since the shutdown op does not need any additional options right now, the value is an empty map.

As a pod user, you can load the pod with:

(require '[babashka.pods :as pods])
(pods/load-pod "pod-lispyclouds-sqlite")
(some? (find-ns 'pod.lispyclouds.sqlite)) ;;=> true
;; yay, the namespace exists!

;; let's give the namespace an alias
(require '[pod.lispyclouds.sqlite :as sql])


When invoking a var that is related to the pod, let's call it a proxy var, the pod client reaches out to the pod with the arguments encoded in EDN, JSON or Transit JSON. The pod will then respond with a return value encoded in EDN, JSON or Transit JSON. The pod client will then decode the return value and present the user with that.

Example: the user invokes (sql/execute! "select * from foo"). The pod client sends this message to the pod:

{"id" "1d17f8fe-4f70-48bf-b6a9-dc004e52d056"
 "var" "pod.lispyclouds.sqlite/execute!"
 "args" "[\"select * from foo\"]"

The id is unique identifier generated by the pod client which correlates this request with a response from the pod.

An example response from the pod could look like:

{"id" "1d17f8fe-4f70-48bf-b6a9-dc004e52d056"
 "value" "[[1] [2]]"
 "status" "[\"done\"]"}

Here, the value payload is the return value of the function invocation. The field status contains "done". This tells the pod client that this is the last message related to the request with id 1d17f8fe-4f70-48bf-b6a9-dc004e52d056.

Now you know most there is to know about the pod protocol!


When the pod client is about to exit, it sends an {"op" "shutdown"} message, if the pod has declared that it supports it in the describe response. Then it waits for the pod process to end. This gives the pod a chance to clean up resources before it exits. If the pod does not support the shutdown op, the pod process is killed by the pod client.

out and err

Pods may send messages with an out and err string value. The Pod Client prints these messages to *out* and *err*. Stderr from the pod is redirected to System/err.

{"id" "1d17f8fe-4f70-48bf-b6a9-dc004e52d056"
 "out" "hello"}
{"id" "1d17f8fe-4f70-48bf-b6a9-dc004e52d056"
 "err" "debug"}


If format is edn then the pod may describe reader functions:

{"readers" {"my/tag" "clojure.core/identity"}}

so payloads containing tagged values like #my/tag[1 2 3] are read correctly as [1 2 3].

Error handling

Responses may contain an ex-message string and ex-data payload string (JSON or EDN) along with an "error" value in status. This will cause the pod client to throw an ex-info with the associated values.


{"id" "1d17f8fe-4f70-48bf-b6a9-dc004e52d056"
 "ex-message" "Illegal input"
 "ex-data" "{\"input\": 10}
 "status" "[\"done\", \"error\"]"}


To debug your pod, you can write to stderr of the pod's process or write to a log file. Currently, stderr is sent to stderr of the pod client.


The pod client will set the BABASHKA_POD environment variable to true when invoking the pod. This can be used by the invoked program to determine whether it should behave as a pod or not.

Added in v0.0.94.

Client side code

Pods may implement functions and macros by sending arbitrary code to the pod client in a "code" field as part of a "var" section. The code is evaluated by the pod client inside the declared namespace.

For example, a pod can define a macro called do-twice:

{"format" "json"
 [{"name" "pod.babashka.demo"
   "vars" [{"name" "do-twice" "code" "(defmacro do-twice [x] `(do ~x ~x))"}]}]}

In the pod client:

(pods/load-pod "pod-babashka-demo")
(require '[pod.babashka.demo :as demo])
(demo/do-twice (prn :foo))


From pod to pod client

Fixed Metadata on vars

Pods may attach metadata to functions and macros by sending data to the pod client in a "meta" field as part of a "var" section. The metadata must be an appropriate map, encoded as an EDN string. This is only applicable to vars in the pod and will be ignored if the var refers to Client-side code, since metadata can already be defined in those code blocks (see 'Dynamic Metadata' below to enable the encoding of metadata).

For example, a pod can define a function called add:

{"format" "json"
 [{"name" "pod.babashka.demo"
   "vars" [{"name" "add"
            "meta" "{:doc \"arithmetic addition of 2 arguments\" :arglists ([a b])}"}]}]}

Dynamic Metadata

Pods may send metadata on values returned to the client if metadata encoding is enabled for the particular transport format used by the pod.

For example, if your pod uses :transit+json as its format, you can enable metadata encoding by adding :transform transit/write-meta (or whatever transit is aliased to) to the optional map passed to transit/writer. e.g.:

(transit/writer baos :json {:transform transit/write-meta})
From pod client to pod

Currently sending metadata on arguments passed to a pod function is available only for the transit+json format and can be enabled on a per var basis.

A pod can enable metadata to be read on arguments by sending the "arg-meta" field to "true" for the var representing that function. For example:

{:format :transit+json
    :namespaces [{:name "pod.babashka.demo"
                  :vars [{"name" "round-trip" "arg-meta" "true"}]}]}

Deferred namespace loading

When your pod exposes multiple namespaces that can be used independently from each other, consider implementing the load-ns op which allows the pod client to load the namespace and process the client side code when it is loaded using require. This will speed up the initial setup of the pod in load-pod.

In describe the pod will mark the namespaces as deferred:

{"name" "pod.lispyclouds.deferred-ns"
 "defer" "true"}

When the user requires the namespace with (require '[pod.lispyclouds.deferred-ns]) the pod client will then send a message:

{"op" "load-ns"
 "ns" "pod.lispyclouds.deferred-ns"
 "id  "..."}

upon which the pod will reply with the namespace data:

{"name" "pod.lispyclouds.deferred-ns"
 "vars" [{"name" "myfunc" "code" "(defn my-func [])"}]
 "id" "..."}

If a deferred namespace depends on another deferred namespace, provide explicit requires in code segments:

{"name" "pod.lispyclouds.another-deferred-ns"
 [{"name" "myfunc"
   "code" "(require '[pod.lispyclouds.deferred-ns :as dns])
           (defn my-func [] (dns/x))"}]
 "id" "..."}


Asynchronous functions can be implemented using callbacks.

The pod will first declare a wrapper function accepting user provided callbacks as client side code. An example from the filewatcher pod:

(defn watch
  ([path cb] (watch path cb {}))
  ([path cb opts]
    [path opts]
    {:handlers {:success (fn [event] (cb (update event :type keyword)))
                :error (fn [{:keys [:ex-message :ex-data]}]
                         (binding [*out* *err*]
                           (println "ERROR:" ex-message)))}})

The wrapper function will then invoke babashka.pods/invoke, a lower level function to invoke a pod var with callbacks.

The arguments to babashka.pods/invoke are:

  • a pod identifier string derived from the first described namespace.
  • the symbol of the var to invoke
  • the arguments to the var
  • an opts map containing :handlers containing callback functions: :success, :error and :done

The return value of babashka.pods/invoke is a map containing :result. When not using callbacks, this is the return value from the pod var invocation. When using callbacks, this value is undefined.

The callback :success is called with a map containing a return value from the pod invocation. The pod can potentially return multiple values. The callback will be called with every value individually.

The callback :error is called in case the pod sends an error, a map containing:

  • :ex-message: an error message
  • :ex-data: an arbitrary additional error data map. Typically it will contain :type describing the type of exception that happened in the pod.

If desired, :ex-message and :ex-data can be reified into a java.lang.Exception using ex-info.

The callback :done is a 0-arg function. This callback can be used to determine if the pod is done sending values, in case it wants to send multiple. The callback is only called if no errors were sent by the pod.

In the above example the wrapper function calls the pod identified by "pod.babashka.filewatcher". It calls the var pod.babashka.filewatcher/watch*. In :success it pulls out received values, passing them to the user-provided callback. Additionally, it prints any errors received from the pod library in :error to *err*.

A user will then use pod.babashka.filewatcher/watch like this:

$ clj
Clojure 1.10.1
user=> (require '[babashka.pods :as pods])
user=> (pods/load-pod "pod-babashka-filewatcher")
user=> (require '[pod.babashka.filewatcher :as fw])
user=> (fw/watch "/tmp" (fn [result] (prn "result" result)))
user=> (spit "/tmp/foobar123.txt" "foo")
user=> "result" {:path "/private/tmp/foobar123.txt", :type :create}

Run tests

To run the tests for the pods library:

$ script/test


Copyright © 2020 Michiel Borkent

Distributed under the EPL License. See LICENSE.