Alloy panic: discovery.File.readFile: unhandled file extension ".log"

Grafana Alloy
1

I’m trying to scrape a caddy access.log file with the following basic config (I have checked, the alloy container can access /caddy-logs/access.log)

discovery.file "caddy_logs" {
  files = ["/caddy-logs/*.log"]
}

discovery.relabel "caddy_logs_relabel" {
  targets = discovery.file.caddy_logs.targets
  rule {
    target_label = "job"
    replacement  = "caddy-access"
  }
  rule {
    target_label = "host"
    replacement  = "dtsc-iwu"
  }
  rule {
    target_label = "platform"
    replacement  = "docker-volume"
  }
}

loki.source.file "caddy_logs" {
  targets        = discovery.relabel.caddy_logs_relabel.output
  forward_to     = [loki.process.caddy_parser.receiver]
  tail_from_end  = true
}

loki.process "caddy_parser" {
  forward_to = [loki.write.default.receiver]
  stage.logfmt {
    mapping = {
      "extra" = "",
    }
  }
}

When starting the alloy container, I get this:

alloy  | panic: discovery.File.readFile: unhandled file extension ".log"
alloy  | 
alloy  | goroutine 406 [running]:
alloy  | github.com/prometheus/prometheus/discovery/file.(*Discovery).readFile(0xc001dfeee0, {0xc001d8c798, 0x16})
alloy  |        /go/pkg/mod/github.com/grafana/prometheus@v1.8.2-0.20250312141819-a2b6722387bf/discovery/file/file.go:406 +0x745
alloy  | github.com/prometheus/prometheus/discovery/file.(*Discovery).refresh(0xc001dfeee0, {0xd04c840, 0xc001e02c30}, 0xc001f12620)
alloy  |        /go/pkg/mod/github.com/grafana/prometheus@v1.8.2-0.20250312141819-a2b6722387bf/discovery/file/file.go:338 +0x125
alloy  | github.com/prometheus/prometheus/discovery/file.(*Discovery).Run(0xc001dfeee0, {0xd04c840, 0xc001e02c30}, 0xc001f12620)
alloy  |        /go/pkg/mod/github.com/grafana/prometheus@v1.8.2-0.20250312141819-a2b6722387bf/discovery/file/file.go:250 +0xe5
alloy  | github.com/grafana/alloy/internal/component/discovery.(*discovererWithMetrics).Run(0xc001dea6d0?, {0xd04c840?, 0xc001e02c30?}, 0xc001e45c80?)
alloy  |        /src/alloy/internal/component/discovery/discovery_metrics.go:46 +0x25
alloy  | github.com/grafana/alloy/internal/component/discovery.(*Component).runDiscovery.func1()
alloy  |        /src/alloy/internal/component/discovery/discovery.go:177 +0x31
alloy  | created by github.com/grafana/alloy/internal/component/discovery.(*Component).runDiscovery in goroutine 361
alloy  |        /src/alloy/internal/component/discovery/discovery.go:176 +0x185

What am I missing?

2

Ok, so here’s the solution:

local.file_match "caddy_logs" {
  path_targets = [{"__path__" = "/caddy-logs/*.log"}]
  sync_period = "5s"
}

loki.source.file "log_scrape" {
  targets    = local.file_match.caddy_logs.targets
  forward_to = [loki.process.caddy_parser.receiver]
  tail_from_end = true
}

loki.process "caddy_parser" {
  forward_to = [loki.write.default.receiver]
  stage.drop {
    source = ""
    expression  = ".*\"remote_ip\":\"::1\".*"
  }
  stage.json {
    expressions = {request = "request", duration = "duration", status = "status"}
  }
  stage.json {
    source = "request"
      expressions = {
        caddy_remote_ip = "remote_ip",
        caddy_method = "method",
        caddy_host = "host",
        caddy_uri = "uri",
      }
  }
  stage.labels {
    values = {
      caddy_remote_ip = "",
      caddy_method = "",
      caddy_host = "",
      caddy_uri = "",
      caddy_status = "status",
      caddy_duration = "duration",
    }
  }
  stage.static_labels {
      values = {
        service_name = "caddy",
        host = "myhost",
      }
  }  
}