[RFC] 0001 Configurable console output

rfcs/0001-configurable-console-output.md

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+# RFC 0001 Configurable console output
+
+- Start Date: 2019-07-18
+- RFC PR: [#42](https://github.com/SAP/ui5-migration/pull/42) 
+- Issue: [#39](https://github.com/SAP/ui5-migration/issues/39)
+
+## Summary
+
+The output of the findings of a migration-run should be configurable. We want to support use-cases like machine-readability, summaries, eslint-like output. The final output should facilitate integration into CI environments.
+
+## Motivation
+
+See [Summary](#Summary).
+
+The internal refactoring efforts should aim to support integration scenarios, where the output needs to be concise to minimize the time to identify/fix the problem.
+
+## Detailed design
+
+### Refactoring
+
+In order to achieve an easy configuration and extension of the output, a deeper refactoring of the existing `Reporter` mechanism is required.
+
+### Favor composition over inheritance
+
+The current implementation is heavily using inheritance, which makes it rather rigid and inconvenient to extend/maintain.  
+
+The new approach should be flexible and use composition instead of inheritance. We aim for better extensibility and maintainability.
+
+### Single Responsibility Principle
+
+The current concept makes the Reporters (`JSONReporter`, `ConsoleReporter` and `MetaConsoleReporter`) does too many things intermingled:
+
+- collect Findings
+- format output
+- manage other reporters
+- instantiate other reporters
+
+We aim to break this feature *lump* down into classes with only one single responsibility.  
+In the following [section](#Solution) we will explain our simplified class design.
+
+### Solution
+
+The following image shows our intended simplified class structure.
+
+![concept](./../docs/images/ui5-migration-reporter-concept.png)
+
+#### `Exporter`
+
+`Exporter` is an interface.  
+An `Exporter` is responsible for exporting the given findings. The findings will be exported in the format defined by the implementing class.  
+
+We currently want to provide two predefined `Exporter` implementations:
+
+1. `ConsoleExporter` (*implements `Exporter` interface*)  
+Dumps content to the console in human readable format.
+
+1. `JSONExporter` (*implements `Exporter` interface*)  
+For automation and visualization purposes we also provide a JSON export.
+
+#### `ReporterManager`
+
+- The `ReporterManager` is a Singleton
+- It serves as a `Reporter` factory and holds references on all instantiated Reporters.  
+- In the future the `ReporterManager` also might implement grouping and filtering mechanisms on all retained `Reporter` instances to allow for eslint-like "per-file" output.  
+- It uses then an `Exporter` instance for exporting the different findings.
+
+#### `Reporter`
+
+A `Reporter` collects findings and logs issued by the migration processors.
+
+### Configuration
+
+There should be a configuration option such that the user can configure the way the output is reported with option `--export`.
+
+Currently the available export formats will be *console-logs* and *JSON*.
+
+## Open Points
+
+- Extend design to include the Migration `Task` class.  
+  Show how the `Task` classes can create a new `Reporter` via the `ReporterManager` factory.