Telemetry.Metrics (Telemetry.Metrics v0.6.0) View Source
Common interface for defining metrics based on
:telemetry
events.
Metrics are aggregations of Telemetry events with specific name, providing a view of the system's behaviour over time.
To give a more concrete example, imagine that somewhere in your code there is a function which sends an HTTP request, measures the time it took to get a response, and emits an event with the information:
:telemetry.execute([:http, :request, :stop], %{duration: duration})
You could define a counter metric, which counts how many HTTP requests were completed:
Telemetry.Metrics.counter("http.request.stop.duration")
or you could use a summary metric to see statistics about the request duration:
Telemetry.Metrics.summary("http.request.stop.duration")
This documentation is going to cover all the available metrics and how to use them, as well as options, and how to integrate those metrics with reporters.
Metrics
There are five metric types provided by Telemetry.Metrics
:
counter/2
which counts the total number of emitted eventssum/2
which keeps track of the sum of selected measurementlast_value/2
holding the value of the selected measurement from the most recent eventsummary/2
calculating statistics of the selected measurement, like maximum, mean, percentiles etc.distribution/2
which builds a histogram of selected measurement
The first argument to all metric functions is the metric name. Metric
name can be provided as a string (e.g. "http.request.stop.duration"
) or a
list of atoms ([:http, :request, :stop, :duration]
). The metric name is
automatically used to infer the telemetry event and measurement. For example,
In the "http.request.stop.duration"
example, the source event name is
[:http, :request, :stop]
and metric values are drawn from :duration
measurement. Like this:
[:http , :request, :stop] :duration
<----- event name ------> <-- measurement -->
You can also explicitly specify the event name and measurement if you prefer.
The second argument is a list of options. Below is the description of the options common to all metric types:
:event_name
- the source event name. Can be represented either as a string (e.g."http.request"
) or a list of atoms ([:http, :request]
). By default the event name is all but the last segment of the metric name.:measurement
- the event measurement used as a source of a metric values. By default it is the last segment of the metric name. It can be:- an arbitrary term
- a key in the event's measurements map
- an unary function accepting the whole measurements map and returning the actual value to be used.
- a binary function accepting the measurements and metadata maps and returning the actual value to be used.
:tags
- a subset of metadata keys by which aggregations will be broken down. Defaults to an empty list.:tag_values
- a function that receives the metadata and returns a map with the tags as keys and their respective values. Defaults to returning the metadata itself.:keep
- a predicate function that evaluates the metadata to conditionally record a given event.:keep
and:drop
cannot be combined. Defaults tonil
.:drop
- a predicate function that evaluates the metadata to conditionally skip recording a given event.:keep
and:drop
cannot be combined. Defaults tonil
.:description
- human-readable description of the metric. Might be used by reporters for documentation purposes. Defaults tonil
.:unit
- an atom describing the unit of selected measurement, typically in singular, such as:millisecond
,:byte
,:kilobyte
, etc. It may also be a tuple indicating that a measurement should be converted from one unit to another before a metric is updated. Currently, only time and byte unit conversions are supported. We discuss those in detail in the "Converting Units" section.:reporter_options
- a keyword list of reporter-specific options for the metric.
Breaking down metric values by tags
Sometimes it's not enough to have a global overview of all HTTP requests received
or all DB queries made. It's often more helpful to break down this data, for example,
we might want to have separate metric values for each unique database table and
operation name (select
, insert
etc.) to see how these particular queries behave.
This is where tagging comes into play. All metric definitions accept a :tags
option:
counter("db.query.count", tags: [:table, :operation])
The above definition means that we want to keep track of the number of queries, but
we want a separate counter for each unique pair of table and operation. Tag values are
fetched from event metadata - this means that in this example, [:db, :query]
events
need to include :table
and :operation
keys in their payload:
:telemetry.execute([:db, :query], %{duration: 198}, %{table: "users", operation: "insert"})
:telemetry.execute([:db, :query], %{duration: 112}, %{table: "users", operation: "select"})
:telemetry.execute([:db, :query], %{duration: 201}, %{table: "sessions", operation: "insert"})
:telemetry.execute([:db, :query], %{duration: 212}, %{table: "sessions", operation: "insert"})
The result of aggregating the events above looks like this:
table | operation | count |
---|---|---|
users | insert | 1 |
users | select | 1 |
sessions | insert | 2 |
The approach where we create a separate metric for some unique set of properties is called a multi-dimensional data model.
Transforming event metadata for tagging
Finally, sometimes there is a need to modify event metadata before it's used for
tagging. Each metric definition accepts a function in :tag_values
option which
transforms the metadata into desired shape. Note that this function is called for
each event, so it's important to keep it fast if the rate of events is high.
Converting Units
It might happen that the unit of measurement we're tracking is not the desirable unit for the metric values, e.g. events are emitted by a 3rd-party library we do not control, or a reporter we're using requires specific unit of measurement.
For these scenarios, each metric definition accepts a :unit
option in a form of a tuple:
summary("http.request.stop.duration", unit: {from_unit, to_unit})
This means that the measurement will be converted from from_unit
to to_unit
before
being used for updating the metric. Currently, only time and byte conversions are
supported.
Time Conversions
Most time measurements in the Erlang VM are done in the :native
unit, which we need
to convert to the desired precision. The supported time units are: :second
, :millisecond
,
:microsecond
, :nanosecond
and :native
.
For example, to convert HTTP request duration from :native
time unit to milliseconds
you'd write:
summary("http.request.stop.duration", unit: {:native, :millisecond})
Byte Conversions
Some metrics, like VM memory's usage are reported in bytes. You might want to convert this
to megabytes, for example. The supported byte units are: :byte
, :kilobyte
and :megabyte
.
In order to convert a metric value from bytes to megabytes, you can write the following:
last_value("vm.memory.total", unit: {:byte, :megabyte})
VM metrics
Telemetry.Metrics
doesn't have a special treatment for the VM metrics - they need
to be based on the events like all other metrics.
:telemetry_poller
package (http://hexdocs.pm/telemetry_poller) exposes a bunch of
VM-related metrics and also provides custom periodic measurements. You can add
telemetry poller as a dependency:
{:telemetry_poller, "~> 0.4"}
By simply adding :telemetry_poller
as a dependency, two events will become available:
[:vm, :memory]
- contains the total memory, as well as the memory used for binaries, processes, etc. Seeerlang:memory/0
for all keys;[:vm, :total_run_queue_lengths]
- returns the run queue lengths for CPU and IO schedulers. It contains thetotal
,cpu
andio
measurements;
You can consume those events with Telemetry.Metrics
with the following sample metrics:
last_value("vm.memory.total", unit: :byte)
last_value("vm.total_run_queue_lengths.total")
last_value("vm.total_run_queue_lengths.cpu")
last_value("vm.total_run_queue_lengths.io")
If you want to change the frequency of those measurements, you can set the following configuration in your config file:
config :telemetry_poller, :default, period: 5_000 # the default
Or disable it completely with:
config :telemetry_poller, :default, false
The :telemetry_poller
package also allows you to run your own poller, which is
useful to retrieve process information or perform custom measurements periodically.
For example, to keep track of the number of users, inside a supervision tree, you could do:
measurements = [
{:process_info,
event: [:my_app, :worker],
name: MyApp.Worker,
keys: [:message_queue_len, :memory]},
{MyApp, :measure_users, []}
]
Supervisor.start_link([
# Run the given measurements every 10 seconds
{:telemetry_poller, measurements: measurements(), period: 10_000}
], strategy: :one_for_one)
Where MyApp.measure_users/0
could be written like this:
defmodule MyApp do
def measure_users do
:telemetry.execute([:my_app, :users], %{total: MyApp.users_count()}, %{})
end
end
Now with measurements in place, you can define the metrics for the events above:
last_value("my_app.worker.memory", unit: :byte)
last_value("my_app.worker.message_queue_len")
last_value("my_app.users.total")
Optionally Recording Metric Events
There may be occasions where you want to record metrics differently or not at all based upon metadata. Rather than depending on changing event names with prefixes, you can instead provide a predicate function which returns true when you want the metric to be processed for this event and false when you do not.
Let's examine some a few examples where optional recording can be helpful.
Filtering on Metadata
Let's assume you are using an HTTP client library in your application which has the following
event: [:http_client, :request, :stop]
. You use this library in multiple places and
you'd like to monitor the request duration.
You can use the event provided by the library but you have very different acceptable
performance requirements for a critical request, so it would be better to provide a different
metric name for monitoring. The client library includes a user configured :name
option which
you can set and is passed in the event metadata.
Let's create a default distribution metric and one for the high performance call.
distribution(
"http.client.request.duration",
event: [:http_client, :request, :stop],
drop: &(match?(%{name: :fast_client}, &1))
)
distribution(
"http.fast.client.request.duration",
event: [:http_client, :request, :stop],
keep: &(match?(%{name: :fast_client}, &1))
)
With this configuration, you can now monitor these requests separately. In the first example,
any events where the client's name is :fast_client
will be dropped for that metric.
Conversely, any matching events in the second example metric will be kept.
Note: only :keep
OR :drop
may be set, never both.
Other Uses
Another potential use case for :keep | :drop
could be per metric sampling rates. As long
as your function returns a boolean, it can determine if the event should be processed.
Reporter Support
Event optional recording must be supported by the reporter you are using. Check your reporter's documentation before relying on this functionality.
The keep
function should be evaluated by reporters prior to tag_values
using the
raw :telemetry.medatadata()
values from the event.
Reporters
So far, we have talked about metrics and how to describe them, but we haven't discussed how those metrics are consumed and published to a system that provides data visualization, aggregation, and more. The job of subscribing to events and processing the actual metrics is a responsibility of reporters.
Generally speaking, a reporter is a process that you would start in your supervision
tree with a list of metrics as input. For example, Telemetry.Metrics
ships with a
Telemetry.Metrics.ConsoleReporter
module, which prints data to the terminal as an
example. You would start it as follows:
metrics = [
last_value("my_app.worker.memory", unit: :byte),
last_value("my_app.worker.message_queue_len"),
last_value("my_app.users.total")
]
Supervisor.start_link([
{Telemetry.Metrics.ConsoleReporter, metrics: metrics}
], strategy: :one_for_one)
Reporters take metric definitions as an input, subscribe to relevant events and
aggregate data when the events are emitted. Reporters may push metrics to StatsD,
some time-series database, or exposing a HTTP endpoint for Prometheus to scrape.
In a nutshell, Telemetry.Metrics
defines only how metrics of particular type
should behave and reporters provide the actual implementation for these aggregations.
Official reporters, maintained by the Observability Working Group of the Erlang Ecosystem Foundation, can be found on the BEAM Telemetry organization on GitHub. You may also find community reporters on hex.pm. You can also read the Writing Reporters page for general information on how to write a reporter.
Wiring it all up
Over the previous sections we discussed how to setup metrics and pass them to reporters and how to configure a poller for measurements. We can wire it all up into a single module as shown below. The example below would be used in the context of a Phoenix application, where we have web metrics, database metrics (through Ecto) as well as from the database, Phoenix metrics as well as VM metrics.
The first step is to add both :telemetry_metrics
and :telemetry_poller
as
dependencies:
[
{:telemetry_poller, "~> 0.4"},
{:telemetry_metrics, "~> 0.4"}
]
Then you could define a module that wires everything up:
defmodule MyAppWeb.Telemetry do
use Supervisor
import Telemetry.Metrics
def start_link(arg) do
Supervisor.start_link(__MODULE__, arg, name: __MODULE__)
end
def init(_arg) do
children = [
{:telemetry_poller,
measurements: periodic_measurements(),
period: 10_000},
# Or TelemetryMetricsPrometheus or TelemetryMetricsFooBar
{TelemetryMetricsStatsd, metrics: metrics()}
]
Supervisor.init(children, strategy: :one_for_one)
end
defp metrics do
[
# VM Metrics
last_value("vm.memory.total", unit: :byte),
last_value("vm.total_run_queue_lengths.total"),
last_value("vm.total_run_queue_lengths.cpu"),
last_value("vm.total_run_queue_lengths.io"),
last_value("my_app.worker.memory", unit: :byte),
last_value("my_app.worker.message_queue_len"),
# Database Time Metrics
summary("my_app.repo.query.total_time", unit: {:native, :millisecond}),
summary("my_app.repo.query.decode_time", unit: {:native, :millisecond}),
summary("my_app.repo.query.query_time", unit: {:native, :millisecond}),
summary("my_app.repo.query.idle_time", unit: {:native, :millisecond}),
summary("my_app.repo.query.queue_time", unit: {:native, :millisecond}),
# Phoenix Time Metrics
summary("phoenix.endpoint.stop.duration",
unit: {:native, :millisecond}),
summary(
"phoenix.router_dispatch.stop.duration",
unit: {:native, :millisecond},
tags: [:plug]
)
]
end
defp periodic_measurements do
[
{:process_info,
event: [:my_app, :worker],
name: Rumbl.Worker,
keys: [:message_queue_len, :memory]}
]
end
end
Link to this section Summary
Types
The name of the metric, either as string or a list of atoms.
The name of the metric represented as a list of atoms.
One of the base metric definitions.
Functions
Returns a definition of counter metric.
Returns a definition of distribution metric.
Returns a definition of last value metric.
Returns a definition of sum metric.
Returns a definition of summary metric.
Link to this section Types
Specs
byte_unit() :: :megabyte | :kilobyte | :byte
Specs
Specs
description() :: nil | String.t()
Specs
drop() :: (:telemetry.event_metadata() -> boolean())
Specs
keep() :: (:telemetry.event_metadata() -> boolean())
Specs
Specs
metric_name() :: String.t() | normalized_metric_name()
The name of the metric, either as string or a list of atoms.
Specs
metric_option() :: {:event_name, :telemetry.event_name()} | {:measurement, measurement()} | {:tags, tags()} | {:tag_values, tag_values()} | {:keep, keep()} | {:drop, drop()} | {:description, description()} | {:unit, unit() | time_unit_conversion() | byte_unit_conversion()} | {:reporter_options, reporter_options()}
Specs
metric_options() :: [metric_option()]
Specs
normalized_metric_name() :: [atom(), ...]
The name of the metric represented as a list of atoms.
Specs
reporter_options() :: keyword()
Specs
t() :: Telemetry.Metrics.Counter.t() | Telemetry.Metrics.LastValue.t() | Telemetry.Metrics.Sum.t() | Telemetry.Metrics.Summary.t() | Telemetry.Metrics.Distribution.t()
One of the base metric definitions.
Specs
tag() :: term()
Specs
tag_values() :: (:telemetry.event_metadata() -> :telemetry.event_metadata())
Specs
tags() :: [tag()]
Specs
time_unit() :: :second | :millisecond | :microsecond | :nanosecond | :native
Specs
Specs
unit() :: atom()
Link to this section Functions
Specs
counter(metric_name(), metric_options()) :: Telemetry.Metrics.Counter.t()
Returns a definition of counter metric.
Counter metric keeps track of the total number of specific events emitted.
The value of the counter is always incremented by one, regardless of the value of the measurement. However, note the measurement must still be available in the event, otherwise the event is not accounted for.
See the "Metrics" section in the top-level documentation of this module for more information.
Example
counter(
"http.request.count",
tags: [:controller, :action]
)
Specs
distribution(metric_name(), metric_options()) :: Telemetry.Metrics.Distribution.t()
Returns a definition of distribution metric.
Distribution metric builds a histogram of selected measurement's values. It is up to the reporter
to decide how the boundaries of the distribution buckets are configured - via :reporter_options
,
configuration of the aggregating system, or other means.
See the "Metrics" section in the top-level documentation of this module for more information.
Example
distribution(
"http.request.duration",
tags: [:controller, :action],
)
Specs
last_value(metric_name(), metric_options()) :: Telemetry.Metrics.LastValue.t()
Returns a definition of last value metric.
Last value keeps track of the selected measurement found in the most recent event.
See the "Metrics" section in the top-level documentation of this module for more information.
Example
last_value(
"vm.memory.total",
description: "Total amount of memory allocated by the Erlang VM", unit: :byte
)
Specs
sum(metric_name(), metric_options()) :: Telemetry.Metrics.Sum.t()
Returns a definition of sum metric.
Sum metric keeps track of the sum of selected measurement's values carried by specific events.
See the "Metrics" section in the top-level documentation of this module for more information.
Example
sum(
"user.session_count",
event_name: "user.session_count",
measurement: :delta,
tags: [:role]
)
Specs
summary(metric_name(), metric_options()) :: Telemetry.Metrics.Summary.t()
Returns a definition of summary metric.
This metric aggregates measurement's values into statistics, e.g. minimum and maximum, mean, or percentiles. It is up to the reporter to decide which statistics exactly are exposed.
See the "Metrics" section in the top-level documentation of this module for more information.
Example
summary(
"db.query.duration",
tags: [:table],
unit: {:native, :millisecond}
)