notify: Add options to specify sender, recipient, filter(s), etc.

[scruzin]

It is proposed to change notifier.Init from:

func (n *Notifier) Init(ctx context.Context, projectID string, sender string, store TimeStore) error

to

func (n *Notifier) Init(ctx context.Context, projectID string, option... Option) error where Option would be defined as:

type Option func(*Notifier,string) error

The following four options are envisioned:

• WithStore - specifies using a datastore for notification persistence
• WithSender - specifies which sender to use
• WithRecipient - specifies which recipient to use
• WithFilter - specifies a substring that is used to filter messages. More than one WithFilter can be applied to achieve a compound filter, i.e, A AND B.

Clients may instantiate multiple Notifiers, each configured with its own options.

[saxon-milton]

Nice; the functional options pattern would have to be one of my favourites. It'll make it really easy to add new options to the same interface as we develop further :)

I'm wondering what the string parameter is for in the Option type however ? The way that I have seen this used (and used personally), is where you provide parameters for the option through the factories for the Options e.g.

type Option func(*Notifier) error

func WithSender(sender string) Option {
    return func(n *Notifier) error {
        n.sender = sender
        return nil
    }
}

[scruzin]

You're right. The Option doesn't need to take a string. I was conflating the fact that all option params are of type string.

[scruzin]

With this approach, notify.Send no longer takes a recipient, as the latter is now specified as an option to notify.Init. If necessary, different Notifiers can/should be instantiated for different recipients. For example, we might define broadcast-ops@, device-ops@, networkblue-ops@, etc. for different operations needs (which could be handled by different teams).

[scruzin]

Implemented in Pull Request #80