Table of Contents

Rule (aka Ruler) #

NOTE: It is recommended to keep deploying rules inside the relevant Prometheus servers locally. Use ruler only on specific cases. Read details below why.

The rule component should in particular not be used to circumvent solving rule deployment properly at the configuration management level.

The rule component evaluates Prometheus recording and alerting rules against chosen query API via repeated --query (or FileSD via If more than one query is passed, round robin balancing is performed.

Rule results are written back to disk in the Prometheus 2.0 storage format. Rule nodes at the same time participate in the system as source store nodes, which means that they expose StoreAPI and upload their generated TSDB blocks to an object store.

You can think of Rule as a simplified Prometheus that does not require a sidecar and does not scrape and do PromQL evaluation (no QueryAPI).

The data of each Rule node can be labeled to satisfy the clusters labeling scheme. High-availability pairs can be run in parallel and should be distinguished by the designated replica label, just like regular Prometheus servers. Read more about Ruler in HA here

$ thanos rule \
    --data-dir             "/path/to/data" \
    --eval-interval        "30s" \
    --rule-file            "/path/to/rules/*.rules.yaml" \
    --alert.query-url      "" \ # This tells what query URL to link to in UI.
    --alertmanagers.url    "" \
    --query                "" \
    --query                "" \
    --objstore.config-file "bucket.yml" \
    --label                'monitor_cluster="cluster1"'
    --label                'replica="A"'

Risk #

Ruler has conceptual tradeoffs that might not be favorable for most use cases. The main tradeoff is its dependence on query reliability. For Prometheus it is unlikely to have alert/recording rule evaluation failure as evaluation is local.

For Ruler the read path is distributed, since most likely Ruler is querying Thanos Querier which gets data from remote Store APIs.

This means that query failure are more likely to happen, that’s why clear strategy on what will happen to alert and during query unavailability is the key.

Partial Response #

See this on initial info.

Rule allows you to specify rule groups with additional fields that control PartialResponseStrategy e.g:

- name: "warn strategy"
  partial_response_strategy: "warn"
  - alert: "some"
    expr: "up"
- name: "abort strategy"
  partial_response_strategy: "abort"
  - alert: "some"
    expr: "up"
- name: "by default strategy is abort"
  - alert: "some"
    expr: "up"

It is recommended to keep partial response as abort for alerts and that is the default as well.

Essentially, for alerting, having partial response can result in symptoms being missed by Rule’s alert.

Must have: essential Ruler alerts! #

To be sure that alerting works it is essential to monitor Ruler and alert from another Scraper (Prometheus + sidecar) that sits in same cluster.

The most important metrics to alert on are:

  • thanos_alert_sender_alerts_dropped_total. If greater than 0, it means that alerts triggered by Rule are not being sent to alertmanager which might indicate connection, incompatibility or misconfiguration problems.

  • prometheus_rule_evaluation_failures_total. If greater than 0, it means that that rule failed to be evaluated, which results in either gap in rule or potentially ignored alert. Alert heavily on this if this happens for longer than your alert thresholds. strategy label will tell you if failures comes from rules that tolerate partial response or not.

  • prometheus_rule_group_last_duration_seconds < prometheus_rule_group_interval_seconds If the difference is large, it means that rule evaluation took more time than the scheduled interval. It can indicate that your query backend (e.g Querier) takes too much time to evaluate the query, i.e. that it is not fast enough to fill the rule. This might indicate other problems like slow StoreAPis or too complex query expression in rule.

  • thanos_rule_evaluation_with_warnings_total. If you choose to use Rules and Alerts with partial response strategy’s value as “warn”, this metric will tell you how many evaluation ended up with some kind of warning. To see the actual warnings see WARN log level. This might suggest that those evaluations return partial response and might not be accurate.

Those metrics are important for vanilla Prometheus as well, but even more important when we rely on (sometimes WAN) network.

// TODO(bwplotka): Rereview them after recent changes in metrics.

See alerts for more example alerts for ruler.

NOTE: It is also recommended to set a mocked Alert on Ruler that checks if Query is up. This might be something simple like vector(1) query, just to check if Querier is live.

Performance. #

As rule nodes outsource query processing to query nodes, they should generally experience little load. If necessary, functional sharding can be applied by splitting up the sets of rules between HA pairs. Rules are processed with deduplicated data according to the replica label configured on query nodes.

External labels #

It is mandatory to add certain external labels to indicate the ruler origin (e.g label='replica="A"' or for cluster). Otherwise running multiple ruler replicas will be not possible, resulting in clash during compaction.

NOTE: It is advised to put different external labels than labels given by other sources we are recording or alerting against.

For example:

  • Ruler is in cluster mon1 and we have Prometheus in cluster eu1
  • By default we could try having consistent labels so we have cluster=eu1 for Prometheus and cluster=mon1 for Ruler.
  • We configure ScraperIsDown alert that monitors service from work1 cluster.
  • When triggered this alert results in ScraperIsDown{cluster=mon1} since external labels always replace source labels.

This effectively drops the important metadata and makes it impossible to tell in what exactly cluster the ScraperIsDown alert found problem without falling back to manual query.

Ruler UI #

On HTTP address Ruler exposes its UI that shows mainly Alerts and Rules page (similar to Prometheus Alerts page). Each alert is linked to the query that the alert is performing, which you can click to navigate to the configured alert.query-url.

Ruler HA #

Ruler aims to use a similar approach to the one that Prometheus has. You can configure external labels, as well as simple relabelling.

In case of Ruler in HA you need to make sure you have the following labelling setup:

  • Labels that identify the HA group ruler and replica label with different value for each ruler instance, e.g: cluster="eu1", replica="A" and cluster=eu1, replica="B" by using --label flag.
  • Labels that need to be dropped just before sending to alermanager in order for alertmanager to deduplicate alerts e.g --alert.label-drop="replica".

Full relabelling is planned to be done in future and is tracked here:

Flags #

usage: thanos rule [<flags>]

ruler evaluating Prometheus rules against given Query nodes, exposing Store API
and storing old blocks in bucket

  -h, --help                     Show context-sensitive help (also try
                                 --help-long and --help-man).
      --version                  Show application version.
      --log.level=info           Log filtering level.
      --log.format=logfmt        Log format to use.
                                 Path to YAML file that contains tracing
                                 Alternative to 'tracing.config-file' flag.
                                 Tracing configuration in YAML.
                                 Listen host:port for HTTP endpoints.
                                 Listen ip:port address for gRPC endpoints
                                 (StoreAPI). Make sure this address is routable
                                 from other components.
      --grpc-server-tls-cert=""  TLS Certificate for gRPC server, leave blank to
                                 disable TLS
      --grpc-server-tls-key=""   TLS Key for the gRPC server, leave blank to
                                 disable TLS
                                 TLS CA to verify clients against. If no client
                                 CA is specified, there is no client
                                 verification on server side. (tls.NoClientCert)
      --label=<name>="<value>" ...
                                 Labels to be applied to all generated metrics
                                 (repeated). Similar to external labels for
                                 Prometheus, used to identify ruler and its
                                 blocks as unique source.
      --data-dir="data/"         data directory
      --rule-file=rules/ ...     Rule files that should be used by rule manager.
                                 Can be in glob format (repeated).
      --eval-interval=30s        The default evaluation interval to use.
      --tsdb.block-duration=2h   Block duration for TSDB block.
      --tsdb.retention=48h       Block retention time on local disk.
      --alertmanagers.url=ALERTMANAGERS.URL ...
                                 Alertmanager replica URLs to push firing
                                 alerts. Ruler claims success if push to at
                                 least one alertmanager from discovered
                                 succeeds. The scheme may be prefixed with
                                 'dns+' or 'dnssrv+' to detect Alertmanager IPs
                                 through respective DNS lookups. The port
                                 defaults to 9093 or the SRV record's value. The
                                 URL path is used as a prefix for the regular
                                 Alertmanager API path.
                                 Timeout for sending alerts to alertmanager
                                 The external Thanos Query URL that would be set
                                 in all alerts 'Source' field
      --alert.label-drop=ALERT.LABEL-DROP ...
                                 Labels by name to drop before sending to
                                 alertmanager. This allows alert to be
                                 deduplicated on replica label (repeated).
                                 Similar Prometheus alert relabelling
      --web.route-prefix=""      Prefix for API and UI endpoints. This allows
                                 thanos UI to be served on a sub-path. This
                                 option is analogous to --web.route-prefix of
      --web.external-prefix=""   Static prefix for all HTML links and redirect
                                 URLs in the UI query web interface. Actual
                                 endpoints are still served on / or the
                                 web.route-prefix. This allows thanos UI to be
                                 served behind a reverse proxy that strips a URL
      --web.prefix-header=""     Name of HTTP request header used for dynamic
                                 prefixing of UI links and redirects. This
                                 option is ignored if web.external-prefix
                                 argument is set. Security risk: enable this
                                 option only if a reverse proxy in front of
                                 thanos is resetting the header. The
                                 --web.prefix-header=X-Forwarded-Prefix option
                                 can be useful, for example, if Thanos UI is
                                 served via Traefik reverse proxy with
                                 PathPrefixStrip option enabled, which sends the
                                 stripped prefix value in X-Forwarded-Prefix
                                 header. This allows thanos UI to be served on a
                                 Path to YAML file that contains object store
                                 Alternative to 'objstore.config-file' flag.
                                 Object store configuration in YAML.
      --query=<query> ...        Addresses of statically configured query API
                                 servers (repeatable). The scheme may be
                                 prefixed with 'dns+' or 'dnssrv+' to detect
                                 query API servers through respective DNS
                                 lookups.<path> ...
                                 Path to file that contain addresses of query
                                 peers. The path can be a glob pattern
                                 (repeatable).     Refresh interval to re-read file SD files.
                                 (used as a fallback)
                                 Interval between DNS resolutions.