How to specify backups only if primary provider fails in FallbackProvider?

[mrwillis]

Hi Richard,

This is very useful https://docs.ethers.io/v5/api/providers/other/#FallbackProvider. However, I'm wondering if there can be something simpler with no quorum requirements, i.e., quorum = 0? For example a primary provider and n backup providers, where we only call the backup providers if the primary providers don't return in time.

Thanks,

Julian

[ricmoo]

A quorum of 0 won’t make much sense (it would return immediately, since we successfully have 0 things agreeing ;)), but you can set the quorum to 1 without issue.

You can also set up your backends to have a priority, so you can make the one you want to have priority a lower number (e.g. 1) and set the rest higher (e.g. 2) and that will use your preferred one first.

And finally, you can specify a longer stallTimeout per provider, which will determine how long it will wait before kicking off the next provider. :)

So, with a properly configured FallbackProvider, you can do that today. :)

I will be adding a few more options to the FallbackProvider over the next while, but nothing that should impact this use case.

Make sense?

[ricmoo]

(Moving to discussions)

[paouvrard]

@ricmoo I'm experimenting with FallbackProvider, but not getting the result I would expect when using it with InfuraProvider.
For example:

  new ethers.providers.FallbackProvider([
    {
      provider: new ethers.providers.InfuraProvider('mainnet', INFURA_ID),
      priority: 2,
      weight: 1
    },
    {
      provider: new ethers.providers.JsonRpcProvider('http://localhost:1111'),
      priority: 1,
      weight: 1
    }
  ], 1)

Looking at the browser network console, the localhost JsonRpcProvider is called correctly (and fails as expected with ERR_CONNECTION_REFUSED: no rpc running there).
But the InfuraProvider is never called and there is no console error.

[paouvrard]

I think had I misunderstood how the FallbackProvider quorum works. If quorum = 1 and the 1st provider with weight = 1 fails, then quorum is already reached and other providers are not queried.
So it doesn't appear possible to implement the use case described above where we want to query other providers only if the 1st one failed.