# Opus
[](https://travis-ci.org/Zorbash/opus)
[](https://hex.pm/packages/opus)
[](https://coveralls.io/github/Zorbash/opus?branch=master)
A framework for pluggable business logic components.
## Installation
The package can be installed by adding `opus` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[{:opus, "~> 0.3"}]
end
```
## Conventions
* Each Opus pipeline module has a single entry point and returns tagged tuples
`{:ok, value} | {:error, error}`
* A pipeline is a composition of stateless stages
* A stage returning `{:error, _}` halts the pipeline
* A stage may be skipped based on a condition function (`:if` option)
* Exceptions are converted to `{:error, error}` tuples by default
* An exception may be left to raise using the `:raise` option
* Each stage of the pipeline is instrumented. Metrics are captured
automatically (but can be disabled).
* Errors are meaningful and predictable
## Usage
```elixir
defmodule ArithmeticPipeline do
use Opus.Pipeline
step :add_one, with: &(&1 + 1)
check :even?, with: &(rem(&1, 2) == 0), error_message: :expected_an_even
tee :publish_number, if: &Publisher.publishable?/1, raise: [ExternalError]
step :double, if: :lucky_number?
step :randomize, with: &(&1 * :rand.uniform)
link JSONPipeline
def double(n), do: n * 2
def lucky_number?(n) when n in 42..1337, do: true
def lucky_number?(_), do: false
end
ArithmeticPipeline.call(41)
# {:ok, %{number: 84.13436750126804}}
```
## Pipeline
The core aspect of this library is defining pipeline modules. As in the
example above you need to add `use Opus.Pipeline` to turn a module into
a pipeline. A pipeline module is a composition of stages executed in
sequence.
## Stages
There are a few different types of stages for different use-cases.
All stage functions, expect a single argument which is provided either
from initial `call/1` of the pipeline module or the return value of the
previous stage.
An error value is either `:error` or `{:error, any}` and anything else
is considered a success value.
### Step
This stage processes the input value and with a success value the next
stage is called with that value. With an error value the pipeline is
halted and an `{:error, any}` is returned.
### Check
This stage is intended for validations.
This stage calls the stage function and unless it returns `true` it
halts the pipeline.
Example:
```elixir
defmodule CreateUserPipeline do
use Opus.Pipeline
check :valid_params?, with: &match? %{email: email} when is_bitstring(email), &1
# other stages to actually create the user
end
```
### Tee
This stage is intended for side effects, such as a notification or a
call to an external system where the return value is not meaningful.
It never halts the pipeline.
### Link
This stage is to link with another Opus.Pipeline module. It calls
`call/1` for the provided module. If the module is not an
`Opus.Pipeline` it is ignored.
### Available options
The behaviour of each stage can be configured with any of the available
options:
* `:with`: The function to call to fulfill this stage. It can be an Atom
referring to a public function of the module, an anonymous function or
a function reference.
* `:if`: Makes a stage conditional, it can be either an Atom referring
to a public function of the module, an anonymous function or a
function reference. For the stage to be executed, the condition *must*
return `true`. When the stage is skipped, the input is forwarded to
the next step if there's one.
* `:raise`: A list of exceptions to not rescue. Defaults to `false`
which converts all exceptions to `{:error, %Opus.PipelineError{}}`
values halting the pipeline.
* `:error_message`: A String or Atom to replace the original error when
a stage fails.
* `:retry_times`: How many times to retry a failing stage, before
halting the pipeline.
* `:retry_backoff`: A backoff function to provide delay values for
retries. It can be an Atom referring to a public function in the
module, an anonymous function or a function reference. It must return
an `Enumerable.t` yielding at least as many numbers as the
`retry_times`.
* `:instrument?`: A boolean which defaults to `true`. Set to `false` to
skip instrumentation for a stage.
### Retries
```elixir
defmodule ExternalApiPipeline do
use Opus.Pipeline
step :http_request, retry_times: 8, retry_backoff: fn -> lin_backoff(10, 2) |> cap(100) end
def http_request(_input) do
# code for the actual request
end
end
```
The above module, will retry be retried up to 8 times, each time
applying a delay from the next value of the retry_backoff function, which returns a
Stream.
All the functions from the [:retry][hex-retry] package will be available to be used in `retry_backoff`.
## Stage Filtering
You can select the stages of a pipeline to run using `call/2` with the `:except` and `:only` options.
Example:
```elixir
# Runs only the stage with the :validate_params name
CreateUserPipeline.call(params, only: [:validate_params]
# Runs all the stages except the selected ones
CreateUserPipeline.call(params, except: :send_notification)
```
## Instrumentation
Instrumentation hooks which can be defined:
* `:before_stage`: Called before each stage
* `:stage_skipped`: Called when a conditional stage was skipped
* `:stage_completed`: Called after each stage
You can disable all instrumentation callbacks for a stage using `instrument?: false`.
```elixir
defmodule ArithmeticPipeline do
use Opus.Pipeline
step :double, instrument: false
end
```
You can define module specific instrumentation callbacks using:
```elixir
defmodule ArithmeticPipeline do
use Opus.Pipeline
step :double, with: &(&1 * 2)
step :triple, with: &(&1 * 3)
instrument :before_stage, fn %{input: input} ->
IO.inspect input
end
# Will be called only for the matching stage
instrument :stage_completed, %{stage: %{name: :triple}}, fn %{time: time} ->
# send to the monitoring tool of your choice
end
end
```
You can define a default instrumentation module for all your pipelines
by adding in your `config/*.exs`:
```elixir
config :opus, :instrumentation, YourModule
# but you may choose to provide a list of modules
config :opus, :instrumentation, [YourModuleA, YourModuleB]
```
An instrumentation module has to export `instrument/3` functions like:
```elixir
defmodule CustomInstrumentation do
def instrument(:stage_completed, %{stage: %{pipeline: ArithmeticPipeline}}, %{time: time}) do
# publish the metrics to specific backend
end
def instrument(:stage_completed, _metadata, %{time: time}) do
# publish the metrics to common backend
end
end
```
## Graph
You may visualise your pipelines using `Opus.Graph`:
```elixir
Opus.Graph.generate(:your_app)
# => {:ok, "Graph file has been written to your_app_opus_graph.png"}
```
`Opus.Graph` is in fact a pipeline and its visualisation is:
You can customise the visualisation:
```elixir
Opus.Graph.generate(:your_app, %{filetype: :svg})
# => {:ok, "Graph file has been written to your_app_opus_graph.svg"}
```
Read the available visualisation options [here][hexdocs-graph].
## License
Copyright (c) 2018 Dimitris Zorbas, MIT License.
See [LICENSE.txt](https://github.com/zorbash/opus/blob/master/LICENSE.txt) for further details.
[hex-retry]: https://github.com/safwank/ElixirRetry/blob/master/lib/retry/delay_streams.ex
[hexdocs-graph]: https://hexdocs.pm/opus/Opus.Graph.html
