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Troubleshooting Loki

“Loki: Bad Gateway. 502”

This error can appear in Grafana when Grafana Loki is added as a data source, indicating that Grafana in unable to connect to Loki. There may one of many root causes:

  • If Loki is deployed with Docker, and Grafana and Loki are not running in the same node, check your firewall to make sure the nodes can connect.
  • If Loki is deployed with Kubernetes:
    • If Grafana and Loki are in the same namespace, set the Loki URL as http://$LOKI_SERVICE_NAME:$LOKI_PORT
    • Otherwise, set the Loki URL as http://$LOKI_SERVICE_NAME.$LOKI_NAMESPACE:$LOKI_PORT

“Data source connected, but no labels received. Verify that Loki and Promtail is configured properly.”

This error can appear in Grafana when Loki is added as a data source, indicating that although Grafana has connected to Loki, Loki hasn’t received any logs from Promtail yet. There may be one of many root causes:

  • Promtail is running and collecting logs but is unable to connect to Loki to send the logs. Check Promtail’s output.
  • Promtail started sending logs to Loki before Loki was ready. This can happen in test environment where Promtail has already read all logs and sent them off. Here is what you can do:
    • Start Promtail after Loki, e.g., 60 seconds later.
    • To force Promtail to re-send log messages, delete the positions file (default location /tmp/positions.yaml).
  • Promtail is ignoring targets and isn’t reading any logs because of a configuration issue.
    • This can be detected by turning on debug logging in Promtail and looking for dropping target, no labels or ignoring target messages.
  • Promtail cannot find the location of your log files. Check that the scrape_configs contains valid path settings for finding the logs on your worker nodes.
  • Your pods are running with different labels than the ones Promtail is configured to read. Check scrape_configs to validate.

Loki timeout errors

Loki 504 errors, context canceled, and error processing requests can have many possible causes.

  • Review Loki configuration

    • Loki configuration limits_config.query_timeout
    • server.http_server_read_timeout
    • server.http_server_write_timeout
    • server.http_server_idle_timeout

  • Check your Loki deployment. If you have a reverse proxy in front of Loki, that is, between Loki and Grafana, then check any configured timeouts, such as an NGINX proxy read timeout.

  • Other causes. To determine if the issue is related to Loki itself or another system such as Grafana or a client-side error, attempt to run a LogCLI query in as direct a manner as you can. For example, if running on virtual machines, run the query on the local machine. If running in a Kubernetes cluster, then port forward the Loki HTTP port, and attempt to run the query there. If you do not get a timeout, then consider these causes:

    • Adjust the Grafana dataproxy timeout. Configure Grafana with a large enough dataproxy timeout.
    • Check timeouts for reverse proxies or load balancers between your client and Grafana. Queries to Grafana are made from the your local browser with Grafana serving as a proxy (a dataproxy). Therefore, connections from your client to Grafana must have their timeout configured as well.

Cache Generation errors

Loki cache generation number errors(Loki >= 2.6)

error loading cache generation numbers

  • Symptom:

    • Loki exposed errors on log with msg="error loading cache generation numbers" err="unexpected status code: 403" or msg="error getting cache gen numbers from the store"

  • Investigation:

    • Check the metric loki_delete_cache_gen_load_failures_total on /metrics, which is an indicator for the occurrence of the problem. If the value is greater than 1, it means that there is a problem with that component.

    • Try Http GET request to route: /loki/api/v1/cache/generation_numbers

      • If response is equal as "deletion is not available for this tenant", this means the deletion API is not enabled for the tenant. To enable this api, set allow_deletes: true for this tenant via the configuration settings. Check more deletion docs

Troubleshooting targets

Promtail exposes two web pages that can be used to understand how its service discovery works.

The service discovery page (/service-discovery) shows all discovered targets with their labels before and after relabeling as well as the reason why the target has been dropped.

The targets page (/targets) displays only targets that are being actively scraped and their respective labels, files, and positions.

On Kubernetes, you can access those two pages by port-forwarding the Promtail port (9080 or 3101 if using Helm) locally:

bash 
$ kubectl port-forward loki-promtail-jrfg7 9080

Then, in a web browser, visit http://localhost:9080/service-discovery

Debug output

Both Loki and Promtail support a log level flag with the addition of a command-line option:

bash 
loki -log.level=debug
bash 
promtail -log.level=debug

Failed to create target, ioutil.ReadDir: readdirent: not a directory

The Promtail configuration contains a __path__ entry to a directory that Promtail cannot find.

Connecting to a Promtail Pod to troubleshoot

First check Troubleshooting targets section above. If that doesn’t help answer your questions, you can connect to the Promtail Pod to investigate further.

If you are running Promtail as a DaemonSet in your cluster, you will have a Promtail Pod on each node, so figure out which Promtail you need to debug first:

shell 
$ kubectl get pods --all-namespaces -o wide
NAME                                   READY   STATUS    RESTARTS   AGE   IP             NODE        NOMINATED NODE
...
nginx-7b6fb56fb8-cw2cm                 1/1     Running   0          41d   10.56.4.12     node-ckgc   <none>
...
promtail-bth9q                         1/1     Running   0          3h    10.56.4.217    node-ckgc   <none>

That output is truncated to highlight just the two pods we are interested in, you can see with the -o wide flag the NODE on which they are running.

You’ll want to match the node for the Pod you are interested in, in this example NGINX, to the Promtail running on the same node.

To debug you can connect to the Promtail Pod:

shell 
kubectl exec -it promtail-bth9q -- /bin/sh

Once connected, verify the config in /etc/promtail/promtail.yml has the contents you expect.

Also check /var/log/positions.yaml (/run/promtail/positions.yaml when deployed by Helm or whatever value is specified for positions.file) and make sure Promtail is tailing the logs you would expect.

You can check the Promtail log by looking in /var/log/containers at the Promtail container log.

Enable tracing for Loki

Loki can be traced using Jaeger by setting the environment variable JAEGER_AGENT_HOST to the hostname and port where Jaeger is running.

If you deploy with Helm, use the following command:

bash 
$ helm upgrade --install loki loki/loki --set "loki.tracing.enabled=true"
  --set "read.extraEnv[0].name=JAEGER_AGENT_HOST"    --set "read.extraEnv[0].value=<JAEGER_AGENT_HOST>"
  --set "write.extraEnv[0].name=JAEGER_AGENT_HOST"   --set "write.extraEnv[0].value=<JAEGER_AGENT_HOST>"
  --set "backend.extraEnv[0].name=JAEGER_AGENT_HOST" --set "backend.extraEnv[0].value=<JAEGER_AGENT_HOST>"
  --set "gateway.extraEnv[0].name=JAEGER_AGENT_HOST" --set "gateway.extraEnv[0].value=<JAEGER_AGENT_HOST>"

Running Loki with Istio Sidecars

An Istio sidecar runs alongside a Pod. It intercepts all traffic to and from the Pod. When a Pod tries to communicate with another Pod using a given protocol, Istio inspects the destination’s service using Protocol Selection. This mechanism uses a convention on the port name (for example, http-my-port or grpc-my-port) to determine how to handle this outgoing traffic. Istio can then do operations such as authorization and smart routing.

This works fine when one Pod communicates with another Pod using a hostname. But, Istio does not allow pods to communicate with other pods using IP addresses, unless the traffic type is tcp.

Loki internally uses DNS to resolve the IP addresses of the different components. Loki attempts to send a request to the IP address of those pods. The Loki services have a grpc (:9095/:9096) port defined, so Istio will consider this to be grpc traffic. It will not allow Loki components to reach each other using an IP address. So, the traffic will fail, and the ring will remain unhealthy.

The solution to this issue is to add appProtocol: tcp to all of the grpc (:9095) and grpclb (:9096) service ports of Loki components. This overrides the Istio protocol selection, and it force Istio to consider this traffic raw tcp, which allows pods to communicate using raw ip addresses.

This disables part of the Istio traffic interception mechanism, but still enables mTLS. This allows pods to communicate between themselves using IP addresses over grpc.