# telemetry_poller
[](https://codecov.io/gh/beam-telemetry/telemetry_poller/branch/master/graphs/badge.svg)
Allows to periodically collect measurements and dispatch them as Telemetry events.
`telemetry_poller` by default runs a poller to perform VM measurements:
* `[vm, memory]` - contains the total memory, process memory, and all other keys in `erlang:memory/0`
* `[vm, total_run_queue_lengths]` - returns the run queue lengths for CPU and IO schedulers. It contains the `total`, `cpu` and `io` measurements
* `[vm, system_counts]` - returns the current process, atom and port count as per `erlang:system_info/1`
You can directly consume those events after adding `telemetry_poller` as a dependency.
Poller also provides a convenient API for running custom pollers.
## Defining custom measurements
Poller also includes conveniences for performing process-based measurements as well as custom ones.
### Erlang
First define the poller with the custom measurements. The first measurement is the built-in `process_info` measurement and the second one is given by a custom module-function-args defined by you:
```erlang
telemetry_poller:start_link(
[{measurements, [
{process_info, [{name, my_app_worker}, {event, [my_app, worker]}, {keys, [memory, message_queue_len]}]},
{example_app_measurements, dispatch_session_count, []}
]},
{period, timer:seconds(10)}, % configure sampling period - default is timer:seconds(5)
{init_delay, timer:seconds(600)}, % configure sampling initial delay - default is 0
{name, my_app_poller}
]).
```
Now define the custom measurement and you are good to go:
```erlang
-module(example_app_measurements).
dispatch_session_count() ->
% emit a telemetry event when called
telemetry:execute([example_app, session_count], #{count => example_app:session_count()}, #{}).
```
### Elixir
You typically start the poller as a child in your supervision tree:
```elixir
children = [
{:telemetry_poller,
# include custom measurement as an MFA tuple
measurements: [
{:process_info, name: :my_app_worker, event: [:my_app, :worker], keys: [:memory, :message_queue_len]},
{ExampleApp.Measurements, :dispatch_session_count, []},
],
period: :timer.seconds(10), # configure sampling period - default is :timer.seconds(5)
init_delay: :timer.seconds(600), # configure sampling initial delay - default is 0
name: :my_app_poller}
]
Supervisor.start_link(children, strategy: :one_for_one)
```
The poller above has two periodic measurements. The first is the built-in `process_info` measurement that will gather the memory and message queue length of a process. The second is given by a custom module-function-args defined by you, such as below:
```elixir
defmodule ExampleApp.Measurements do
def dispatch_session_count() do
# emit a telemetry event when called
:telemetry.execute([:example_app, :session_count], %{count: ExampleApp.session_count()}, %{})
end
end
```
## Documentation
See [documentation](https://hexdocs.pm/telemetry_poller/) for more concrete examples and usage
instructions.
## VM metrics example
### Erlang
Find, in `examples/telemetry_poller_vm.erl`, an example on how to retrieve to VM measurements,
mentioned above.
To see it in action, fire up `rebar3 shell`, then
```erlang
{ok, telemetry_poller_vm} = c("examples/telemetry_poller_vm").
ok = file:delete("telemetry_poller_vm.beam"). % Deletes generated BEAM
ok = telemetry_poller_vm:attach().
```
### Elixir
Find, in `examples/TelemetryPollerVM.ex`, an example on how to retrieve to VM measurements,
mentioned above.
To see it in action, first compile the Erlang sources with `rebar3 compile`.
Then fire up `iex -pa "_build/default/lib/*/ebin"`, then
```elixir
{:ok, _} = Application.ensure_all_started(:telemetry_poller)
[TelemetryPollerVM] = c("examples/TelemetryPollerVM.ex")
:ok = TelemetryPollerVM.attach()
```
## Copyright and License
Copyright (c) 2019 Erlang Ecosystem Foundation and Erlang Solutions.
telemetry_poller source code is released under Apache License, Version 2.0.
See [LICENSE](LICENSE) and [NOTICE](NOTICE) files for more information.
