There is a REST API, yes [0]. Currently you send snappy-compressed protobufs[1] containing yours log to a HTTP endpoint.

We initially started using fluentd as the agent, but we found its metadata "enrichment" facilities weren't reliable enough - we'd get log lines without the pod tags, for instance. For something like Loki, which depends really heavily on the metadata for index, this was super important. So we wrote promtail.

[0] https://github.com/grafana/loki/blob/master/docs/api.md [1] https://github.com/grafana/loki/blob/master/pkg/logproto/log...