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Mobius

Library for localized telemetry metrics

Installation

def deps do
  [
    {:mobius, "~> 0.6.1"}
  ]
end

Usage

Add Mobius to your supervision tree and pass in the metrics you want to track.

def start(_type, _args) do
  metrics = [
    Metrics.last_value("my.telemetry.event"),
  ]

  children = [
    # ... other children ....
    {Mobius, metrics: metrics}
    # ... other children ....
  ]

  # See https://hexdocs.pm/elixir/Supervisor.html
  # for other strategies and supported options
  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)
end

Quick tips

To see a view of the current metrics you can use Mobius.info/0:

iex> Mobius.info()
Metric Name: vm.memory.total
Tags: %{}
last_value: 83952736

To plot a metric measurement over time you can use Mobius.Exports.plot/4:

iex> Mobius.Exports.plot("vm.memory.total", :last_value, %{})
                Metric Name: vm.memory.total, Tags: %{}

34355808.00 ┤
34253736.73 ┤                                    ╭──╮    ╭────╮
34151665.45 ┤                               ╭────╯  ╰────╯    ╰───
34049594.18 ┤         ╭────╮    ╭────╮ ╭────╯
33947522.91 ┤         │    ╰────╯    ╰─╯
33845451.64 ┤         │
33743380.36 ┤    ╭────╯
33641309.09 ┤    │
33539237.82 ┤    │
33437166.55 ┤    │
33335095.27 ┼────╯

Configure persistence directory

By default Mobius will try to save metric data for all resolutions and the current value when the erlang system exits gracefully. This makes Mobius useful for Nerves devices that have to reboot after doing a planned firmware update. The default direction Mobius will try to persist data to is the /data directory as this is friendly to Nerves devices. If you want Mobius to store data in a different location you can pass that into Mobius when you start it:


children = [
   # ... other children ...
   {Mobius, metrics: metrics(), persistence_dir: "/tmp"}
   # ... other children ...
]

def metrics() do
  [
    Metrics.last_value("vm.memory.total", unit: {:byte, :kilobyte})
  ]
end

Saving / Autosaving metrics data

By default the metrics data is persisted on a normal shutdown. However, data will not be persisted during a sudden shutdown, eg Control-C in IEX, kill, sudden power off.

It's possible to manually call Mobius.save/1 to force an interim write of the persistence data.

This can be automated by passing autosave_interval to Mobius

def start(_type, _args) do
  metrics = [
    Metrics.last_value("my.telemetry.event.measurement"),
  ]

  children = [
    # ... other children ....
    {Mobius, metrics: metrics, autosave_interval: 60} # auto save every 60 seconds
    # ... other children ....
  ]

  # See https://hexdocs.pm/elixir/Supervisor.html
  # for other strategies and supported options
  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)
end

Exporting data

The Mobius.Exports module provide functions for exporting the data in a couple different formats.

CSV
Series
Line plot
Mobius Binary Format

# export as CSV string
Mobius.Exports.csv("vm.memory.total", :last_value, %{})

# export as series list
Mobius.Exports.series("vm.memory.total", :last_value, %{})

# export as mobius binary format
Mobius.Exports.mbf()

The Mobius Binary Format (MBF) is a binary string that has encoded and compressed all the historical metrics that mobius current has. This is most useful for preparing metrics to send off to another system. To parse the binary format you can use Mobius.Exports.parse_mbf/1.

For each of these you can see the Mobius.Exports module for more details.

Report metrics to a remote server

Mobius allows sending metrics to a remote server. You can do this by passing the :remote_reporter option to Mobius. This is a module that implements the Mobius.RemoteReporter behaviour. Optionally, you can pass the :remote_report_interval option to specify how often to report metrics, by default this is every 1 minute.

Events

In a system we want to track metrics and events. Metrics are measurements tracked at a regular interval. These could in cloud CPU and memory usage or something like bytes transmitted over an LTE connection. Events are things tracked at irregular intervals that might not have a measurement. Events are necessary to pin point moments in time that something of interest happens that isn't necessarily a measurement. For example, firmware updates and interface connections.

Events are good to track things that happen at particular time that are enriched by extra data, whereas metrics are good for understand single piece of data over time.

You can listen for raw telemetry events by passing a list of event names to Mobius.

def start(_type, _args) do
  events = [
    "a.button.was.pressed"
  ]

  children = [
    # ... other children ....
    {Mobius, events: events, autosave_interval: 60} # auto save every 60 seconds
    # ... other children ....
  ]

  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)
end

The above example will listen for that telemetry event and save that into Mobius's event log.

Event configurations can take different options:

:tags - list of tag names to save with the event
:measurement_values - a function that will receive each measurement that allows for data processing before storing the event in the event log
:group - an atom that defines the event group, this will allow for filtering on particular types of events for example: :network. Default is :default

For example if a measurement is reported in naive time and you want to convert that to seconds you can do that this way:

def start(_type, _args) do
  events = [
    {"a.button.was.pressed", measurement_values: &process_button_measurements/1}
  ]

  children = [
    # ... other children ....
    {Mobius, events: events, autosave_interval: 60} # auto save every 60 seconds
    # ... other children ....
  ]

  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)
end

defp process_button_measurement({:system_time, sys_time}) do
  System.convert_time(sys_time, :naive, :second)
end

defp process_button_measurement({_, value}), do: value

A good resource for understanding telemetry data and some of the differences between events and metrics is New Relic's MELT 101 post.

Clocks

For systems that lack battery-backed real-time clock which will advance the clock at startup to a reasonable guess, the early events will have a timestamp that do not make much sense. Mobius allows you to pass the clock argument which is a module that implements the Mobius.Clock behaviour. This behavior has one callback: synchronized?/0 which returns a boolean.

If a clock implementation is provide, Mobius will wait for the clock to synchronize before including any events into the logs. Once the clock is synchronized Mobius will make a best effort attempt to adjust early event timestamps to reflect the actual time the event occurred and will then include the events into the event log.

If no clock implementation is provided, Mobius will assume the clock is synchronized and that it can trust the provided timestamps of early events.

For Nerves devices, the NervesTime package can be used:

{Mobius, metrics: my_metrics, events: my_events, clock: NervesTime}

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