event_bus

v1.0.0-beta1

EventBus

Build Status

Traceable, simple event bus library.

Event Bus

Installation

The package can be installed by adding event_bus to your list of dependencies in mix.exs:

def deps do
  [{:event_bus, "~> 1.0.0-beta1"}]
end

Usage

Register event topics in config.exs

config :event_bus, topics: [:message_received, :another_event_occured]

You can also register/unregister event topics on demand

# register
EventBus.register_topic(:webhook_received)
> :ok

# unregister topic
# Warning: It also deletes the related topic tables!
EventBus.unregister_topic(:webhook_received)
> :ok

Subscribe to the ‘event bus’ with a listener and list of given topics, EventManager will match with Regex

# to catch every event topic
EventBus.subscribe({MyEventListener, [".*"]})
> :ok

# to catch specific topics
EventBus.subscribe({MyEventListener, ["purchase_", "booking_confirmed$", "flight_passed$"]})
> :ok

# if your processor has a config
config = %{}
EventBus.subscribe({{MyEventListener, config}, [".*"]})
> :ok

Unsubscribe from the ‘event bus’

EventBus.unsubscribe(MyEventListener)
> :ok

# if your processor has a config
config = %{}
EventBus.unsubscribe({MyEventListener, config})
> :ok

List subscribers

EventBus.subscribers()
> [{MyEventListener, [".*"]}, {{AnotherListener, %{}}, [".*"]]

List subscribers of a specific event

EventBus.subscribers(:hello_received)
> [MyEventListener, {{AnotherListener, %{}}}]

EventBus.Model.Event structure

%EventBus.Model.Event{
  id: String.t | integer(), # required
  transaction_id: String.t | integer(), # optional
  topic: atom(), # required
  data: any() # required,
  initialized_at: integer(), # optional, might be seconds, milliseconds or microseconds even nano seconds since Elixir does not have a limit on integer size
  occurred_at: integer(), # optional, might be seconds, milliseconds or microseconds even nano seconds since Elixir does not have a limit on integer size
  source: String.t, # optional, source of the event, who created it
  ttl: integer() # optional, might be seconds, milliseconds or microseconds even nano seconds since Elixir does not have a limit on integer size. If `tll` field is set, it is recommended to set `occurred_at` field too.
}

transaction_id attribute

Firstly, transaction_id attribute is an optional field, if you need to store any meta identifier related to event transaction, it is the place to store. Secondly, transaction_id is one of the good ways to track events related to the same transaction on a chain of events. If you have time, have a look to the story.

initialized_at attribute

Optional, but good to have field for all events to track when the event processor started to process generating this event.

occurred_at attribute

Optional, but good to have field for all events to track when the event occurred with unixtimestamp value. The library does not automatically set this value since the value depends on the timing choice.

ttl attribute

Optional, but might to have field for all events to invalidate an event after a certain amount of time. Currently, the event_bus library does not do any specific thing using this field. If you need to discard an event in a certain amount of time, that field would be very useful.

Note: If you set this field, then occurred_at field is required.

Define an event struct

alias EventBus.Model.Event
event = %Event{id: "123", transaction_id: "1",
  topic: :hello_received, data: %{message: "Hello"}}
another_event = %Event{id: "124", transaction_id: "1",
  topic: :bye_received, data: [user_id: 1, goal: "exit"]}

Important Note: It is important to have unique identifier for each event struct per topic. I recommend to use a unique id generator like {:uuid, "~> 1.1"}.

Notify all subscribers with EventBus.Model.Event data

EventBus.notify(event)
> :ok
EventBus.notify(another_event)
> :ok

Fetch an event from the store

topic = :bye_received
id = "124"
EventBus.fetch_event({topic, id})
> %EventBus.Model.Event{data: [user_id: 1, goal: "exit"], id: "124", topic: :bye_received, transaction_id: "1"}

Mark as completed on Event Watcher

processor = MyEventListener
# If your processor has config then pass tuple
processor = {MyEventListener, config}
EventBus.mark_as_completed({processor, :bye_received, id})
> :ok

Mark as skipped on Event Watcher

processor = MyEventListener
# If your processor has config then pass tuple
processor = {MyEventListener, config}
EventBus.mark_as_skipped({processor, :bye_received, id})
> :ok

Check if a topic exists?

EventBus.topic_exist?(:metrics_updated)
> false

Use block builder to build Event struct

Builder automatically sets initialized_at and occured_at attributes

use EventBus.EventSource

id = "some unique id"
topic = :user_created
transaction_id = "tx" # optional
ttl = 600_000 # optional
source = "my event creator" # optional

params = %{id: id, topic: topic, transaction_id: transaction_id, ttl: ttl, source: source}
EventSource.build(params) do
  # do some calc in here
  Process.sleep(1)
  # as a result return only the event data
  %{email: "jd@example.com", name: "John Doe"}
end
> %EventBus.Model.Event{data: %{email: "jd@example.com", name: "John Doe"},
 id: "some unique id", initialized_at: 1515274599140,
 occurred_at: 1515274599141, source: "my event creator", topic: :user_created,
 transaction_id: "tx", ttl: 600000}


# Without optional params
params = %{id: id, topic: topic}
EventSource.build(params) do
  %{email: "jd@example.com", name: "John Doe"}
end
> %EventBus.Model.Event{data: %{email: "jd@example.com", name: "John Doe"},
 id: "some unique id", initialized_at: 1515274599140,
 occurred_at: 1515274599141, source: nil, topic: :user_created,
 transaction_id: nil, ttl: nil}

Use block notifier to notify event data to given topic

Builder automatically sets initialized_at and occured_at attributes

use EventBus.EventSource

id = "some unique id"
topic = :user_created
transaction_id = "tx" # optional
ttl = 600_000 # optional
source = "my event creator" # optional
EventBus.register_topic(topic) # incase you didn't register it in `config.exs`

params = %{id: id, topic: topic, transaction_id: transaction_id, ttl: ttl, source: source}
EventSource.notify(metadata) do
  # do some calc in here
  # as a result return only the event data
  %{email: "mrsjd@example.com", name: "Mrs Jane Doe"}
end
> # it automatically calls notify method with event data and return only event data as response
> %{email: "mrsjd@example.com", name: "Mrs Jane Doe"}

Sample Processor/Listener Implementation 

defmodule MyEventListener do
  ...

  # if your listener does not have a config
  def process({topic, id} = event_shadow) do
    GenServer.cast(__MODULE__, event_shadow)
    :ok
  end

  ...

  # if your listener/processor has a config
  def process({config, topic, id} = event_shadow_with_conf) do
    GenServer.cast(__MODULE__, event_shadow_with_conf)
    :ok
  end

  ...


  # if your processor does not have a config
  def handle_cast({:bye_received, id}, state) do
    event = EventBus.fetch_event({:bye_received, id})
    # do sth with event

    # update the watcher!
    EventBus.mark_as_completed({__MODULE__, :bye_received, id})
    ...
    {:noreply, state}
  end
  def handle_cast({:hello_received, id}, state) do
    event = EventBus.fetch_event({:hello_received, id})
    # do sth with EventBus.Model.Event

    # update the watcher!
    EventBus.mark_as_completed({__MODULE__, :hello_received, id})
    ...
    {:noreply, state}
  end
  def handle_cast({topic, id}, state) do
    EventBus.mark_as_skipped({__MODULE__, topic, id})
    {:noreply, state}
  end

  ...

  # if your processor has a config
  def handle_cast({config, :bye_received, id}, state) do
    event = EventBus.fetch_event({:bye_received, id})
    # do sth with event

    # update the watcher!
    EventBus.mark_as_completed({{__MODULE__, config}, :bye_received, id})
    ...
    {:noreply, state}
  end
  def handle_cast({config, :hello_received, id}, state) do
    event = EventBus.fetch_event({:hello_received, id})
    # do sth with EventBus.Model.Event

    # update the watcher!
    EventBus.mark_as_completed({{__MODULE__, config}, :hello_received, id})
    ...
    {:noreply, state}
  end
  def handle_cast({config, topic, id}, state) do
    EventBus.mark_as_skipped({{__MODULE__, config}, topic, id})
    {:noreply, state}
  end

  ...
end

Event Storage Details

When an event configured in config file, 2 ETS tables will be created for the event on app start.

All event data is temporarily saved to the ETS tables with the name :eb_es_<<topic>> until all subscribers processed the data. This table is a read heavy table. When a subscriber needs to process the event data, it queries this table to fetch event data.

To watch event status, a separate watcher table is created for each event type with the name :eb_ew_<<topic>>. This table is used for keeping the status of the event. EventWatcher updates this table frequently with the notification of the event processors/subscribers.

When all subscribers process the event data, data in the event store and watcher, automatically deleted by the EventWatcher. If you need to see the status of unprocessed events, event watcher table is one of the good places to query.

For example; to get the list unprocessed events for :hello_received event:

# The following command will return a list of tuples with the `id`, and `event_subscribers_list` where `subscribers` is the list of event subscribers, `completers` is the subscribers those processed the event and notified EventWatcher, and lastly `skippers` is the subscribers those skipped the event without processing.

# Assume you have an event with the name ':hello_received'
:ets.tab2list(:eb_ew_hello_received)
> [{id, {subscribers, completers, skippers}}, ...]

ETS storage SHOULD NOT be considered as a persistent storage. If you need to store events to a persistant data store, then subscribe to all event types by a module with [".*"] event topic then save every event data.

For example;

EventBus.subscribe({MyDataStore, [".*"]})

# then in your data store save the event
defmodule MyDataStore do
  ...

  def process({topic, id} = event_shadow) do
    GenServer.cast(__MODULE__, event_shadow)
    :ok
  end

  ...

  def handle_cast({topic, id}, state) do
    event = EventBus.fetch_event({topic, id})
    # write your logic to save event_data to a persistant store

    EventBus.mark_as_completed({__MODULE__, topic, id})
    {:noreply, state}
  end
end

Traceability

EventBus comes with a good enough data structure to track the event life cycle with its optional parameters. For a traceable system, it is highly recommend to fill optional fields on event data. It is also encouraged to use Event.nofify block/yield to automatically set the initialized_at and occurred_at values.

Documentation

Module docs can be found at https://hexdocs.pm/event_bus.

Implementation details can be found at: https://medium.com/@mustafaturan/event-bus-implementation-s-d2854a9fafd5

Contributing

Issues, Bugs, Documentation, Enhancements

  1. Fork the project

  2. Make your improvements and write your tests(make sure you covered all the cases).

  3. Make a pull request.

License

MIT