Table of Contents

Querier/Query #

The Querier component (also known as “Query”) implements the Prometheus HTTP v1 API to query data in a Thanos cluster via PromQL.

In short, it gathers the data needed to evaluate the query from underlying StoreAPIs, evaluates the query and returns the result.

Querier is fully stateless and horizontally scalable.

Example command to run Querier:

$ thanos query \
    --http-address     "" \
    --store            "<store-api>:<grpc-port>" \
    --store            "<store-api2>:<grpc-port>"

Querier use cases, why do I need this component? #

Thanos Querier essentially allows to aggregate and optionally deduplicate multiple metrics backends under single Prometheus Query endpoint.

Global View #

Since for Querier “a backend” is anything that implements gRPC StoreAPI we can aggregate data from any number of the different storages like:

  • Prometheus (see Sidecar)
  • Object Storage (see Store Gateway)
  • Global alerting/recording rules evaluations (see Ruler)
  • Metrics received from Prometheus remote write streams (see Thanos Receiver)
  • Another Querier (you can stack Queriers on top of each other)
  • Non-Prometheus systems!

Thanks to that, you can run queries (manually, from Grafana or via Alerting rule) that aggregate metrics from mix of those sources.

Some examples:

  • sum(cpu_used{cluster=~"cluster-(eu1|eu2|eu3|us1|us2|us3)", job="service1"}) that will give you sum of CPU used inside all listed clusters for service service1. This will work even if those clusters runs multiple Prometheus servers each. Querier will know which data sources to query.

  • In single cluster you shard Prometheus functionally or have different Prometheus instances for different tenants. You can spin up Querier to have access to both within single Query evaluation.

Run-time deduplication of HA groups #

Prometheus is stateful and does not allow replicating its database. This means that increasing high availability by running multiple Prometheus replicas is not very easy to use. Simple loadbalancing will not work as for example after some crash, replica might be up but querying such replica will result in small gap during the period it was down. You have a second replica that maybe was up, but it could be down in other moment (e.g rolling restart), so load balancing on top of those is not working well.

Thanos Querier instead pulls the data from both replicas, and deduplicate those signals, filling the gaps if any, transparently to the Querier consumer.

Metric Query Flow Overview #


Overall QueryAPI exposed by Thanos is guaranteed to be compatible with Prometheus 2.x. API. The above diagram shows what Querier does for each Prometheus query request.

See here on how to connect Querier with desired StoreAPIs.

Deduplication #

The query layer can deduplicate series that were collected from high-availability pairs of data sources such as Prometheus. A fixed single or multiple replica labels must be chosen for the entire cluster and can then be passed to query nodes on startup.

Two or more series that are only distinguished by the given replica label, will be merged into a single time series. This also hides gaps in collection of a single data source.

An example with a single replica labels: #

  • Prometheus + sidecar “A”: cluster=1,env=2,replica=A
  • Prometheus + sidecar “B”: cluster=1,env=2,replica=B
  • Prometheus + sidecar “A” in different cluster: cluster=2,env=2,replica=A

If we configure Querier like this:

$ thanos query \
    --http-address        "" \
    --query.replica-label "replica" \
    --store               "<store-api>:<grpc-port>" \
    --store               "<store-api2>:<grpc-port>" \

And we query for metric up{job="prometheus",env="2"} with this option we will get 2 results:

  • up{job="prometheus",env="2",cluster="1"} 1
  • up{job="prometheus",env="2",cluster="2"} 1

WITHOUT this replica flag (deduplication turned off), we will get 3 results:

  • up{job="prometheus",env="2",cluster="1",replica="A"} 1
  • up{job="prometheus",env="2",cluster="1",replica="B"} 1
  • up{job="prometheus",env="2",cluster="2",replica="A"} 1

The same output will be present for this example with multiple replica labels: #

  • Prometheus + sidecar “A”: cluster=1,env=2,replica=A,replicaX=A
  • Prometheus + sidecar “B”: cluster=1,env=2,replica=B,replicaX=B
  • Prometheus + sidecar “A” in different cluster: cluster=2,env=2,replica=A,replicaX=A
$ thanos query \
    --http-address        "" \
    --query.replica-label "replica" \
    --query.replica-label "replicaX" \
    --store               "<store-api>:<grpc-port>" \
    --store               "<store-api2>:<grpc-port>" \

This logic can also be controlled via parameter on QueryAPI. More details below.

Query API Overview #

As mentioned, Query API exposed by Thanos is guaranteed to be compatible with Prometheus 2.x. API. However for additional Thanos features on top of Prometheus, Thanos adds:

  • partial response behaviour
  • several additional parameters listed below
  • custom response fields.

Let’s walk through all of those extensions:

Partial Response #

QueryAPI and StoreAPI has additional behaviour controlled via query parameter called PartialResponseStrategy.

This parameter controls tradeoff between accuracy and availability.

Partial response is a potentially missed result within query against QueryAPI or StoreAPI. This can happen if one of StoreAPIs is returning error or timeout whereas couple of others returns success. It does not mean you are missing data, you might lucky enough that you actually get the correct data as the broken StoreAPI did not have anything for your query.

If partial response happen QueryAPI returns human readable warnings explained here.

NOTE: Having warning does not necessary means partial response (e.g no store matched query warning).

See this on how to control this behaviour.

Querier also allows to configure different timeouts:

  • --query.timeout
  • --store.response-timeout

If you prefer availability over accuracy you can set tighter timeout to underlying StoreAPI than overall query timeout. If partial response strategy is NOT abort, this will “ignore” slower StoreAPIs producing just warning with 200 status code response.

Deduplication replica labels. #

HTTP URL/FORM parameterTypeDefaultExample
replicaLabels[]stringquery.replica-label flag (default: empty).replicaLabels=replicaA&replicaLabels=replicaB

This overwrites the query.replica-label cli flag to allow dynamic replica labels at query time.

Deduplication Enabled #

HTTP URL/FORM parameterTypeDefaultExample
dedupBooleanTrue, but effect depends on query.replica configuration flag.1, t, T, TRUE, true, True for “True”

This controls if query results should be deduplicated using the replica labels.

Auto downsampling #

HTTP URL/FORM parameterTypeDefaultExample
max_source_resolutionFloat64/time.Duration/model.Durationstep / 5 or 0 if is false (default: False)5m

Max source resolution is max resolution in seconds we want to use for data we query for. This means that for value:

  • 0 -> we will use only raw data.
  • 5m -> we will use max 5m downsampling.
  • 1h -> we will use max 1h downsampling.

Partial Response Strategy #

// TODO(bwplotka): Update. This will change to “strategy” soon as PartialResponseStrategy enum here

HTTP URL/FORM parameterTypeDefaultExample
partial_responseBooleanquery.partial-response flag (default: True)1, t, T, TRUE, true, True for “True”

If true, then all storeAPIs that will be unavailable (and thus return no data) will not cause query to fail, but instead return warning.

Custom Response Fields #

Any additional field does not break compatibility, however there is no guarantee that Grafana or any other client will understand those.

Currently Thanos UI exposed by Thanos understands

type queryData struct {
	ResultType promql.ValueType `json:"resultType"`
	Result     promql.Value     `json:"result"`

	// Additional Thanos Response field.
	Warnings []error `json:"warnings,omitempty"`

Additional field is Warnings that contains every error that occurred that is assumed non critical. partial_response option controls if storeAPI unavailability is considered critical.

Expose UI on a sub-path #

It is possible to expose thanos-query UI and optionally API on a sub-path. The sub-path can be defined either statically or dynamically via an HTTP header. Static path prefix definition follows the pattern used in Prometheus, where web.route-prefix option defines HTTP request path prefix (endpoints prefix) and web.external-prefix prefixes the URLs in HTML code and the HTTP redirect responses.

Additionally, Thanos supports dynamic prefix configuration, which is not yet implemented by Prometheus. Dynamic prefixing simplifies setup when thanos query is exposed on a sub-path behind a reverse proxy, for example, via a Kubernetes ingress controller Traefik or nginx. If PathPrefixStrip: /some-path option or traefik.frontend.rule.type: PathPrefixStrip Kubernetes Ingress annotation is set, then Traefik writes the stripped prefix into X-Forwarded-Prefix header. Then, thanos query --web.prefix-header=X-Forwarded-Prefix will serve correct HTTP redirects and links prefixed by the stripped path.

Flags #

usage: thanos query [<flags>]

query node exposing PromQL enabled Query API with data retrieved from multiple
store nodes

  -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 with tracing configuration.
                                 See format details:
                                 Alternative to 'tracing.config-file' flag
                                 (lower priority). Content of YAML file with
                                 tracing configuration. See format details:
                                 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)
      --grpc-client-tls-secure   Use TLS when talking to the gRPC server
      --grpc-client-tls-cert=""  TLS Certificates to use to identify this client
                                 to the server
      --grpc-client-tls-key=""   TLS Key for the client's certificate
      --grpc-client-tls-ca=""    TLS CA Certificates to use to verify gRPC
                                 Server name to verify the hostname on the
                                 returned gRPC certificates. See
      --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
      --query.timeout=2m         Maximum time to process query by query node.
      --query.max-concurrent=20  Maximum number of queries processed
                                 concurrently by query node.
      --query.replica-label=QUERY.REPLICA-LABEL ...
                                 Labels to treat as a replica indicator along
                                 which data is deduplicated. Still you will be
                                 able to query without deduplication using
                                 'dedup=false' parameter.
      --selector-label=<name>="<value>" ...
                                 Query selector labels that will be exposed in
                                 info endpoint (repeated).
      --store=<store> ...        Addresses of statically configured store API
                                 servers (repeatable). The scheme may be
                                 prefixed with 'dns+' or 'dnssrv+' to detect
                                 store API servers through respective DNS
                                 lookups.<path> ...
                                 Path to files that contain addresses of store
                                 API servers. The path can be a glob pattern
                                 (repeatable).     Refresh interval to re-read file SD files. It
                                 is used as a resync fallback.
                                 Interval between DNS resolutions.
                                 Timeout before an unhealthy store is cleaned
                                 from the store UI page.  Enable automatic adjustment (step / 5) to what
                                 source of data should be used in store gateways
                                 if no max_source_resolution param is specified.
      --query.partial-response   Enable partial response for queries if no
                                 partial_response param is specified.
                                 Set default evaluation interval for sub
                                 If a Store doesn't send any data in this
                                 specified duration then a Store will be ignored
                                 and partial data will be returned if it's
                                 enabled. 0 disables timeout.