grafana/contribute/architecture/backend/communication.md

Communication

Grafana uses a bus to pass messages between different parts of the application. All communication over the bus happens synchronously.

There are three types of messages: events, commands, and queries.

Events

An event is something that happened in the past. Since an event has already happened, you can't change it. Instead, you can react to events by triggering additional application logic to be run, whenever they occur.

Because they happened in the past, event names are written in past tense, such as UserCreated, and OrgUpdated.

Subscribe to an event

In order to react to an event, you first need to subscribe to it.

To subscribe to an event, register an event listener in the service's Init method:

func (s *MyService) Init() error {
    s.bus.AddEventListener(s.UserCreated)
    return nil
}

func (s *MyService) UserCreated(event *events.UserCreated) error {
    // ...
}

Tip: Browse the available events in the events package.

Publish an event

If you want to let other parts of the application react to changes in a service, you can publish your own events:

event := &events.StickersSentEvent {
    UserID: "taylor",
    Count:   1,
}
if err := s.bus.Publish(event); err != nil {
    return err
}

Commands

A command is a request for an action to be taken. Unlike an event's fire-and-forget approach, a command can fail as it is handled. The handler will then return an error.

Because we request an operation to be performed, command are written in imperative mood, such as CreateFolderCommand, and DeletePlaylistCommand.

Dispatch a command

To dispatch a command, pass the context.Context and object to the DispatchCtx method:

// context.Context from caller
ctx := req.Request.Context()
cmd := &models.SendStickersCommand {
    UserID: "taylor",
    Count: 1,
}
if err := s.bus.DispatchCtx(ctx, cmd); err != nil {
  if err == bus.ErrHandlerNotFound {
    return nil
  }
  return err
}

Note: DispatchCtx will return an error if no handler is registered for that command.

Note: Dispatch currently exists and requires no context.Context to be provided, but it's strongly suggested to not use this since there's an ongoing refactoring to remove usage of non-context-aware functions/methods and use context.Context everywhere.

Tip: Browse the available commands in the models package.

Handle commands

Let other parts of the application dispatch commands to a service, by registering a command handler:

To handle a command, register a command handler in the Init function.

func (s *MyService) Init() error {
    s.bus.AddHandlerCtx(s.SendStickers)
    return nil
}

func (s *MyService) SendStickers(ctx context.Context, cmd *models.SendStickersCommand) error {
    // ...
}

Note: The handler method may return an error if unable to complete the command.

Note: AddHandler currently exists and requires no context.Context to be provided, but it's strongly suggested to not use this since there's an ongoing refactoring to remove usage of non-context-aware functions/methods and use context.Context everywhere.

Queries

A command handler can optionally populate the command sent to it. This pattern is commonly used to implement queries.

Making a query

To make a query, dispatch the query instance just like you would a command. When the DispatchCtx method returns, the Results field contains the result of the query.

// context.Context from caller
ctx := req.Request.Context()
query := &models.FindDashboardQuery{
    ID: "foo",
}
if err := bus.Dispatch(ctx, query); err != nil {
    return err
}
// The query now contains a result.
for _, item := range query.Results {
    // ...
}

Note: Dispatch currently exists and requires no context.Context to be provided, but it's strongly suggested to not use this since there's an ongoing refactoring to remove usage of non-context-aware functions/methods and use context.Context everywhere.

Return query results

To return results for a query, set any of the fields on the query argument before returning:

func (s *MyService) FindDashboard(ctx context.Context, query *models.FindDashboardQuery) error {
    // ...
    query.Result = dashboard
    return nil
}