Where do you create application logs of app running in Docker?

Question

Where do you create application logs of app running in Docker so that I can map them later to host system ?

It's unclear in documentation for me whether I should create logs in some directory accessible without root permissions (then which one?) or I somehow can chown directory I need.

I tried using /var/log/case and /opt/case/logs with no success. Here is my minified SBT script

object build extends Build {

  lazy val root = Project(
    id = "case-server",
    base = file("."),
    settings = Defaults.coreDefaultSettings ++ graphSettings ++ Revolver.settings ++ Seq(

      version := "1.15",
      scalaVersion := "2.11.7",
      libraryDependencies ++= {
        val akkaV = "2.4.1"
        val akkaStreamV = "2.0.1"
        val scalaTestV = "2.2.5"
        Seq(
          "com.typesafe.scala-logging" %% "scala-logging-slf4j" % "2.1.2",
          "ch.qos.logback" % "logback-classic" % "1.1.+",
          "org.scalatest" %% "scalatest" % scalaTestV % "test"
        )
      },

      dockerBaseImage := "develar/java",
      dockerCommands := dockerCommands.value.flatMap {
        case cmd@Cmd("FROM", _) => List(cmd, Cmd("RUN", "apk update && apk add bash"))
        case other => List(other)
      },
      dockerExposedVolumes := Seq("/opt/case/logs"),
      version in Docker := version.value
    )
  ).enablePlugins(AssemblyPlugin).enablePlugins(JavaAppPackaging).enablePlugins(DockerPlugin)
}

What is the correct approach to do this ?

Answer 1

You don't want your application log to be written on files on the container filesystem. The other solutions are:

• write log files to a docker volume
• write log entries to stdout (so that they will be forwarded to the Docker engine and available with the docker logs command)

Furthermore, if your application is writing its log entries to stdout, you can rely on Docker logging drivers to send those logs to syslog, journald, gelf, fluentd, awslogs, json files, or any log collecting system that provides a docker logging driver.

A trick to make an application write to stdout instead of to a file is to configure it to write to the special file /proc/self/fd/1. Anything written to this special file will be sent to stdout.