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Appenders

An appender is a map. Write your own by returning a map of the same shape.

{:name      :my-appender
 :min-level :warn               ; optional per-appender threshold
 :formatter :line               ; :line | :json | (fn [event] string)
 :write!    (fn [event line] ...)} ; side-effecting writer, mandatory

Built-in appenders

console-appender

(log/console-appender)
(log/console-appender {:formatter :json :min-level :warn})
(log/console-appender {:color :always})   ; force ANSI colours
(log/console-appender {:color :never})    ; force plain text

:info and quieter go to stdout. :warn and louder go to stderr.

:color defaults to :auto, which emits ANSI escapes only when the destination stream is a TTY — so piping into a file or CI capture stays plain text. Per-level colours live in log/level->ansi; override the map to pick your own palette.

file-appender

(log/file-appender {:path "/var/log/my-app.log"})
(log/file-appender {:path "/var/log/audit.log"
                    :formatter :json
                    :min-level :report})

Append-only, LOCK_EX on each write. No rotation; use logrotate, rotating-file-appender below, or pair with the Monolog RotatingFileHandler via the bridge.

rotating-file-appender

(log/rotating-file-appender
  {:path-template "/var/log/my-app/{date}/{level}.log"})

(log/rotating-file-appender
  {:path-template "/var/log/my-app/{date}-{hour}.log"
   :formatter     :json})

The path template is rendered per event from the event's :time-ms, not the host wall-clock. Substitutions: {date} (UTC YYYY-MM-DD), {hour} (UTC HH), {ns} (caller namespace), {level} (lowercase level name). Parent directories are created on demand; pass :ensure-dir? false to skip the mkdir.

memory-appender

(let [[app events] (log/memory-appender)]
  (log/set-config! {:appenders [app] :min-level :debug})
  (log/info "captured" {:k 1})
  (println (deref events)))
;; => [{:level :info :ns "..." :message "captured" :data {:k 1} ...}]

Useful for tests and REPL exploration.

monolog-appender

See monolog-bridge.md.

Custom appender

(defn syslog-appender []
  {:name :syslog
   :formatter :line
   :write! (fn [event line]
             (php/syslog
               (case (get event :level)
                 :error (php/-> \LOG_ERR)
                 :warn  (php/-> \LOG_WARNING)
                 (php/-> \LOG_INFO))
               line))})

(log/update-config! {:appenders [(syslog-appender)]})

The event passed to :write! is the same plain map every other appender sees, so processors and :base-data apply identically to custom appenders.