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WFL Logging⚓︎

TL;DR⚓︎

Import clojure.tools.logging :as log and use log/error / log/info etc.

clojure.tools.logging uses SLF4J under the hood which in turn uses Log4j2 as its implementation.

Below is more detailed information for those interested.

Usage⚓︎

We use clojure.tools.logging aliased to log.

Require it like any other dependency: 

(ns "..."
  (:require
    ...
    [clojure.tools.logging :as log]
    ...))

(log/info "Hello!")

Full documentation is available here.

Behavior⚓︎

Currently, all error-level messages are routed to STDERR, and everything else is routed to STDOUT.

Thus, to have WFL log everything to a file you'd want to use something like 

my-command >output.log 2>&1
That'll capture both STDOUT and STDERR to the same file. Note that this specific syntax is more universal than just &>output.log.

Testing⚓︎

clojure.tools.logging provides a test namespace.

An example of usage is in test/wfl/unit/logging_test.clj, which exists to test that the service loaders are correctly resolving our dependencies.

Under the Hood⚓︎

This section might be a bit verbose but hopefully it won't be too out-of-date since logging setup doesn't change all that much.

The key takeaway here is that JVM logging libraries use service loaders and other runtime configuration to find each other.

WFL's logging works as follows:

  1. clojure.tools.logging is imported and used directly
  2. Why clojure.tools.logging? It is Clojure-native so it has intuitive syntax
  3. Why not wrap or delegate to it? It already works just as a wrapper to any other logging implementation so wrapping it would duplicate its purpose
  4. clojure.tools.logging delegates to SLF4J
  5. Why SLF4J? Jetty already uses it, so configuring it ourselves helps keep it quiet
  6. SLF4J delegates to Log4j 2
  7. Why delegate? SLF4J is a facade that still needs an implementation to actually log things
  8. Why Log4j 2? It is a fair default implementation: highly configurable, well-tested, well-supported

Even without making our own wrapper around clojure.tools.logging, we have a lot of flexibility. Suppose Jetty removes their dependency on SLF4J: we could remove our own dependency on SLF4J and clojure.tools.logging would immediately begin interacting directly with Log4j 2.