Generalize Modules Service to make it extensible

Author: @annanay25
Reviewers: @jtlisi, @pstibrany, @cyriltovena, @pracucci
Date: April 2020
Status: Accepted

Overview

Cortex uses modules to start and operate services with dependencies. Inter-service dependencies are specified in a map and passed to a module manager which ensures that they are initialised in the right order of dependencies. While this works really well, the implementation is tied in specifically to the Cortex struct and is not flexible for use with other projects like Loki, which also require similar forms of dependency management.

We would like to extend modules in cortex to a generic dependency management framework, that can be used by any project with no ties to cortex.

Specific goals

Framework should allow for reusing cortex modules and allow us to:
- Add new modules
- Overwrite the implementation of a current module
- Manage dependencies
Framework should allow for building an application from scratch using the modules package, with no dependencies on Cortex. For ex: Remove code from Loki that was copied from pkg/cortex/cortex.go.

Proposed Design

Modules package

To make the modules package extensible, we need to abstract away any Cortex specific details from the module manager. The proposed design is to:

Make a new component Manager, which is envisioned to be a central manager for all modules of the application. It stores modules & dependencies, and will be housed under a new package pkg/util/modules. Manager has the following methods for interaction:
```
func (m *Manager) RegisterModule(name string, initFn func() (Service, error))
func (m *Manager) AddDependency(name string, dependsOn... string) error
func (m *Manager) InitModuleServices(target string) (map[string]services.Service, error)
```
Modules can be created by the application and registered with modules.Manager using RegisterModule. The parameters are:
- name: Name of the module
- initFn: A function that will be used to start the module. If it returns nil, and other modules depend on it, InitModuleServices will return an error.
Dependencies between modules can be added using AddDependency. The parameters to the function are:
- name: Name of the module
- dependsOn: A variadic list of modules that the module depends on.

These need to be added before the call to InitModuleServices.

The application can be initialized by running initFn’s of all the modules in the right order of dependencies by invoking InitModuleServices with the target module name.

Changes to `pkg/cortex`:

WrappedService present in the current module design will be deprecated. All initFn’s will be wrapped into WrappedService by default.
While the process of loading modules into modules.Manager should be remain as part of the Cortex.New() function, InitModuleServices should be part of Cortex.Run() and to enable this, modules.Manager would be made a member of the Cortex struct.

Usage

Following these changes, the Modules package will be a generic dependency management framework that can be used by any project.

To use the modules framework:

Import the pkg/util/modules package, and initialize a new instance of the Manager using modules.NewManager()
Create components in the system that implement the services interface (present in pkg/util/services).
Register each of these components as a module using Manager.RegisterModule() by passing name of the module and initFn for the module.
To add dependencies between modules, use Manager.AddDependency()
Once all modules are added into modules.Manager, initialize the application by calling Manager.InitModuleServices() which initializes modules in the right order of dependencies.

Future work

Extend the module manager to allow specifying multiple targets as opposed to a single target name supported currently.
Factor out Run() method to make it independent of Cortex. This will help reduce replicated code in the Loki project as well as help manage modules.Manager outside of the Cortex struct.