Telemere includes a number of signal handlers out-the-box.
[Writing your own handlers](#writing-handlers) is also often straight-forward, and [PRs](https://github.com/taoensso/telemere/pulls) are **very welcome** for additions to Telemere's included handlers, or to Telemere's [community resources](./8-Community).
It helps to know what people need! You can [vote on](https://www.taoensso.com/roadmap/vote) additional handlers to add, [ping me](https://github.com/taoensso/telemere/issues), or ask on the [`#telemere` Slack channel](https://www.taoensso.com/telemere/slack).
Current handlers, alphabetically (see linked docstrings below for features and usage):
| Name | Platform | Output target | Output format |
| [`handler:console-raw`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#handler:console-raw) | Cljs | Browser console | Raw signals for [cljs-devtools](https://github.com/binaryage/cljs-devtools), etc. |
| [`handler:file`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#handler:file) | Clj | File/s on disk | [edn/JSON](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#pr-signal-fn) or [human-readable](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#format-signal-fn) |
This is all specified when calling [`add-handler!`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#add-handler!) - and documented there.
Note that handler middleware in particular is an often overlooked but powerful feature, allowing you to arbitrarily transform and/or filter every [signal map](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#help:signal-content) before it is given to the handler.
## Handler-specific opts
Handler-specific opts are specified when calling a particular **handler constructor** (like [`handler:console`](https://cljdoc.org/d/com.taoensso/telemere/CONSOLE/api/taoensso.telemere#handler:console)) - and documented by the constructor.
Note that it's common for Telemere handlers to be customized by providing *Clojure/Script functions* to the relevant handler constructor call.
See the [utils namespace](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere.utils) for tools useful for customizing and writing signal handlers.
### Example
The standard Clj/s console handler ([`handler:console`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#handler:console)) writes signals **as strings** to `*out*`/`*err` or browser console.
By default it writes formatted strings intended for human consumption:
```clojure
;; Create a test signal
(def my-signal
(t/with-signal
(t/log! {:id ::my-id, :data {:x1 :x2}} "My message")))
;; Create console handler with default opts (writes formatted string)
(def my-handler (t/handler:console))
;; Test handler, remember it's just a (fn [signal])
(my-handler my-signal) ; =>
;; 2024-04-11T10:54:57.202869Z INFO LOG Schrebermann.local examples(56,1) ::my-id - My message
Note that when writing JSON with Clojure, you *must* provide an appropriate `pr-fn`. This lets you plug in the JSON serializer of your choice ([jsonista](https://github.com/metosin/jsonista) is my default recommendation).
Telemere includes a handy mechanism for including arbitrary user-level data/opts in individual signals for use by custom middleware and/or handlers.
Any *non-standard* (user) keys you include in your signal constructor opts will automatically be included in created signals, e.g.:
```clojure
(t/with-signal
(t/event! ::my-id
{:my-middleware-data "foo"
:my-handler-data "bar"}))
;; %>
;; {;; User kvs included inline (assoc'd to signal root)
;; :my-middleware-data "foo"
;; :my-handler-data "bar"
;; :kvs ; And also collected together under ":kvs" key
;; {:my-middleware-data "foo"
;; :my-handler-data "bar"}
;; ... }
```
These user-level data/opts are typically NOT included by default in handler output, making them a great way to convey data/opts to custom middleware/handlers.
See [`help:signal-handlers`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#help:signal-handlers) for info on handler management.
## Managing handlers on startup
Want to add or remove a particular handler when your application starts?
Just make an appropriate call to [`add-handler!`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#add-handler!) or [`remove-handler!`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#remove-handler!).
If you want to manage handlers **conditionally** based on **environmental config** (JVM properties, environment variables, or classpath resources) - Telemere provides the highly flexible [`get-env`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#get-env) util.
Writing your own signal handlers for Telemere is straightforward, and a reasonable choice if you prefer customizing behaviour that way, or want to write signals to a DB/format/service for which a ready-made handler isn't available.
Remember that signals are just plain Clojure/Script [maps](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#help:signal-content), and handlers just plain Clojure/Script functions that do something with those maps.
Here's a simple Telemere handler:
```clojure
(fn my-handler [signal] (println signal))
```
For more complex cases, or for handlers that you want to make available for use by other folks, here's the general template that Telemere uses for all its included handlers:
- See [`help:signal-content`](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere#help:signal-content) for signal map content.
- See the [utils namespace](https://cljdoc.org/d/com.taoensso/telemere/CURRENT/api/taoensso.telemere.utils) for tools useful for customizing and writing signal handlers.
- [PRs](https://github.com/taoensso/telemere/pulls) are **very welcome** for additions to Telemere's included handlers, or to Telemere's [community resources](./8-Community)
