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If Linkerd reports that a given service is in the failfast state, it means that the proxy has determined that there are no available endpoints for that service. In this situation there’s no point in the proxy trying to actually make a connection to the service - it already knows that it can’t talk to it - so it reports that the service is in failfast and immediately returns an error from the proxy.

The error will be either a 503 or a 504; see below for more information, but if you already know that the service is in failfast because you saw it in the logs, that’s the important part.

To get out of failfast, some endpoints for the service have to become available.