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A very thin wrapper around Log4j2, optimised for logging Clojure maps (via MapMessage).

Should support the upcoming log4j-3 release, but only versions starting from 0.3.0 have actually been tested against it.


Log4j2 puts strong emphasis on structured-logging. does provide a layer of integration with it, but it has (traditionally) been geared towards logging Strings. It is virtually impossible, to send a MapMessage to log4j2 via, and have it be recognised as an actual MapMessage. That's because the bottom call to .log happens inside (i.e. you don't control it, and therefore you can't type-hint it). This library completely sidesteps - the only thing it uses from it is the *logging-agent* (in its async api).


Clojars Project


There are two identical api-variants (sync VS async) to choose from. The async variant uses the same function as sync one, but dispatches it on*logging-agent*(making sure it doesn't lose the context along the way).

;; pick one
(require '[flog.api.sync  :as l]
         '[flog.api.async :as l])

Map VS String

We can log plain String like so:

(l/info "I am a log-message") ;; will emit a `SimpleMessage`

or a map like so:

;; you are encouraged to use `:log/message` as the key for the main log-message 
(l/info {:log/message "I am a log-message" :foo 1 :bar 2}) ;; will emit a `MapMessage`

or a map more conveniently like so:

(l/info "I am a log-message" :foo 1 :bar 2) ;; will emit the same `MapMessage` as above

All level-specific calls can also take a Throwable as the first arg in which case the following map will be merged into the rest of the provided args:

{:error/message (ex-message throwable)
 :error/data    (ex-data throwable)
 :error/cause   (ex-cause throwable)
 :error/class   (class throwable)}

You can find more examples in test/flog/demo.clj.

Location info

As Log4j2 puts it:

Generating location information is an expensive operation and may impact performance. Use with caution.

This is particularly true when you leave it up to the layout (as it has to walk the stack). If you do want location-info in your logs, it's much better to provide it at the LogBuilder level, as explained here.

By default, flog will use a builder w/o location-info. However, you have the option of overriding that (at compile time) via the flog.builder/include-location-info? system property.

FYI, the usefulness of location-info in Clojure programs is somewhat limited, when compared to Java ones
(especially if going via The method field will always be .invoke, and the file/line fields may not even be your own code (i.e. Moreover, if dispatching on an agent (like flog does), the thread field will always be some sort of clojure-agent-send-off-pool-N. With flog, you should at least expect correct file/line info.

ThreadContext (ex MDC/NDC)

Care has been taken to carry the ThreadContext, and any thread-local bindings when logging asynchronously (see flog.context/inherit-fn).


The standard Log4j2 rules apply. See here. flog comes with a very basic log4j2.xml (the last choice in the aforementioned rules), which is nothing more than a copy of the DefaultConfiguration, which includes printing the MDC.

Macro expansions


(walk/macroexpand-all '(sync.log/debug "Hi there" :a 1 :b 2))
(* "Hi there"
   (flog.builder/ns-logger #object[clojure.lang.Namespace 0x49575f89 "flog.demo"])
 [:a 1 :b 2])


(walk/macroexpand-all '(async.log/debug "Hi there" :a 1 :b 2))
    (. org.apache.logging.log4j.ThreadContext getImmutableContext)
    (. (. org.apache.logging.log4j.ThreadContext getImmutableStack) asList)]
      [___181__auto__ (. org.apache.logging.log4j.CloseableThreadContext putAll kvs__192__auto__)]
         [___175__auto__ (. org.apache.logging.log4j.CloseableThreadContext pushAll vs__193__auto__)]
           ;; synchronous call omitted as it can be seen above
          (finally (. ___175__auto__ clojure.core/close)))))
       (finally (. ___181__auto__ clojure.core/close))))))))

Async options

If you want async-logging you have 3 options:

  1. Use flog.api.async, which sends off on an agent - (easy)
  2. Use an async api of your own which submits to some ExecutorService that you control - (trivial)
  3. Let log4j do the work - (requires extra config/deps)


flog expects the following two JARs on the classpath (preferably version 2.17.2, or higher):

  • org.apache.logging.log4j/log4j-core
  • org.apache.logging.log4j/log4j-api


Copyright © 2021 Dimitrios Piliouras

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at

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