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  • noun Perception; understanding.
  • noun View; sight.
  • intransitive verb To have knowledge or an understanding.

This library provides a set of general-purpose observability tools which can be used to instrument your code in order to better understand it.

In particular, it attempts to satisfy the following goals:

  • Define a general "observability event" shape decoupled from any one consumer.
  • Be able to report events from anywhere in the code, without worrying about wiring in a component dependency.
  • Provide an open extension point for collecting rich context data from an event's source. Don't couple to sources.
  • Provide an open extension point for subscribing to observability events. Don't couple to sinks.
  • Support distributed tracing by tracking trace/span identifiers and context.
  • Build up events over time with annotations.


There are a few concepts in ken to understand in order to use the tools effectively.


An event is a map of Clojure keys and values which represent a thing that happened in your code. Events will typically contain a selection of keys from ken.event to provide a basic foundation:

{:ken.event/label "the thing",
 :ken.event/time #inst "2020-03-27T21:22:27.003Z",
 :ken.event/duration 44.362681,
 :ken.event/message "Perform some routine activity",

Here we have an event about some labeled process which started at the given time, lasted 44.3 milliseconds, and had some associated human-friendly metadata. Events may have many other attributes as well, including authentication context, custom identifiers, inputs and outputs, and more.


A context collector in ken is a source of contextual information used to enrich the events being observed. Useful data sources could include properties about the running process such as the build number and deployment environment, dynamic information about request-specific properties like the authenticated user, and even local attributes that are breadcrumbs for later deugging.

;; Register a static context which is known at process startup:
(let [build-info {:build-version (System/getProperty "")}]
  (ken.context/register-static! :build build-info))

;; Context collection happens at the call location, so you can also
;; pull dynamic properties:
(defn user-context
  (select-keys my.web.auth/*user-info* [:user/id :user/name :user/email]))

(ken.context/register! :user-info user-context)

Contexts can be removed with unregister! and the keyword id, or reset entirely with clear!.

If you have a service-oriented architecture, a rich source of additional data can be the broadcast context which is transmitted through the whole request graph.


This library adopts the idea of the tap introduced in Clojure 1.10. This is a global queue which may be sent events from anywhere in the code. Note that ken has its own queue, it does not utilize the built-in tap in order to avoid polluting local debugging usage.

In order for those events to be useful, they must be handled by functions which are subscribed to the tap. As an example, we can pretty-print all events to our console for inspection:

(ken.tap/subscribe! :cprint puget.printer/cprint)

Subscribed functions will be called with every event sent to the tap and should not block for significant periods of time or they may cause event loss. Subscriptions can be removed with unsubscribe! or reset entirely with clear!.


Finally, ken uses a standard model for distributed tracing of events. Events are grouped together under a top-level trace which identifies an entire unit of work. Each event within this may be a span which represents a subunit of work covering some duration. Spans may be children of other spans, meaning they represent more fine-grained bits of work in turn. Linking all the spans in a trace together forms a tree, sometimes called a "call graph" which represents the observations collected about the unit of work which was traced.

Using the library will automatically capture and extend the tracing identifiers where needed, which show up in the observed events:

{:ken.event/label "A"
 :ken.event/time #inst "2020-03-27T21:22:27.003Z",
 :ken.event/duration 44.362681,
 :ken.trace/trace-id "bplzs2gajkfcbojkspx7",
 :ken.trace/span-id "cuoclyafu4",

{:ken.event/label "B"
 :ken.event/time #inst "2020-03-27T21:22:27.005Z",
 :ken.event/duration 41.805794,
 :ken.trace/trace-id "bplzs2gajkfcbojkspx7",
 :ken.trace/parent-id "cuoclyafu4",
 :ken.trace/span-id "cjatpftw5j",

Above are two related spans A and B, the second nested inside the first.


Releases are published on Clojars; to use the latest version with Leiningen, add the following to your project dependencies:

Clojars Project

Enough theory, how do you actually use this?

(require '[ken.core :as ken])

Direct Observation

The most direct way to use the library is to call the observe macro in your code in order to send events.

(ken/observe {:ken.event/label "a thing", ::my/key 123})

This will collect the available context and trace data, add it to the event, then send it to the tap for publishing. By default, this returns immediately (non-blocking) but you can specify a timeout in milliseconds if you would like to wait for the event to be accepted. Either way, this returns true if the event was queued and false if it was rejected.

Watching Spans

The most common way to generate events is by describing spans which cover some work happening. You can use the watch macro for this:

(ken/watch "a thing happening"
  (crunch-numbers 2.17 3.14)
  (think-heavily "what am I?"))

This will instrument the body of expressions and observes an event at the end which includes tracing and timing information. The watch form will return the value of the final expression. This also works for asynchronous values, so the following code will only record the event once the deferred chain completes:

(ken/watch "another thing"
      (crunch-numbers 8675309))

For richer event data, you can specify a map - the string versions above automatically expand into :ken.event/label entries:

(ken/watch {:ken.event/label "foo the bar"
            ::foo 123
            ::bar 'baz}
  (foo-bar! bar))

If you nest calls to watch, the nested spans will automatically link to the enclosing watch as the parent span.


For data you don't necessarily know until you've started doing some work, you can annotate spans by adding additional properties to the events. When code is executing inside a watch, you can use the annotate, time, and error tools:

(ken/watch "a thing"
    (when (foo? x)
      (ken/annotate {::foo? true}))
    (ken/time ::thinking
      (think-heavily "what is consciousness?"))
    (catch Exception ex
      (ken/error ex))))

This would produce a span event labeled "a thing" with a few potential additional attributes - a ::foo? key set to true, a :ken.event/error key with the caught exception, and a ::thinking key holding the number of milliseconds spent in the think-heavily call.

Local Context

In addition to the registered context collector functions, ken comes with a built-in local context which you can bind anywhere in your code to provide additional information. Unlike adding keys directly to observe and watch calls, these values are propagated to all events inside the block:

;; all events observed inside this block will include the :foo and :bar keys
(ken/with-context {:foo 123
                   :bar "baz"}
  (something-with-foos x y)


Sampling is the act of selecting a subset of events from a large collection of events. Not everything needs to be sampled, but if you have high frequency events and most of them are very similar, sampling them can be a good way to reducing your total event volume.

Sampling is controlled by two tracing keys, which can be specified in the initial ken/watch or in a later ken/annotate call.

In order to opt into sampling for a specific span, you can set the :ken.event/sample-rate key. This is an integer value n that will cause, on average, about 1/n of the events to be sampled. The rest will be marked to be discarded by consumers.

When a span has been marked for sampling, it will set the second tracing key :ken.trace/keep? on the resulting event. The keep key can have one of three possible states:

  • nil or absent: The span will be kept and forwarded along. This is the default behavior.
  • false: The span will be marked to be dropped and the decision will propagate down to its child spans.
  • true: The span and its children will be kept. This decision overrides sampling logic in child spans.

The :ken.trace/keep? key can also be set directly; for example, if you encounter an error and want to ensure that a span and its (subsequent) children are recorded, you can use annotate to set the flag to true.

For additional reading on sampling best practices, see Honeycomb's article on the topic.

NOTE: events which have been "sampled away" are still reported to tap subscribers! It is up to the individual subscribed functions to decide to drop the events or not.



Copyright © 2021 Amperity, Inc.

Distributed under the MIT License.

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