Routing for multiple uplinks/providers

Routing for multiple uplinks/providers
A common configuration is the following, in which there are two providers that connect a local network (or even a single machine) to the big Internet.
                                                                 ________
                                          +————+        /
                                          |            |       |
                            +————-+ Provider 1 +——-
        __                  |             |            |     /
    ___/  \_         +——+——-+     +————+    |
  _/        \__      |     if1      |                      /
 /             [...]