query-frontend: make HTTP downstream tripper configurable

CHANGELOG.md

@@ -21,6 +21,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#4506](https://github.com/thanos-io/thanos/pull/4506) `Baidu BOS` object storage, see [documents](docs/storage.md#baidu-bos) for further information.
 - [#4552](https://github.com/thanos-io/thanos/pull/4552) Compact: Adds `thanos_compact_downsample_duration_seconds` histogram.
 - [#4594](https://github.com/thanos-io/thanos/pull/4594) reloader: Expose metrics in config reloader to give info on the last operation.
+- [#4623](https://github.com/thanos-io/thanos/pull/4623) query-frontend: made HTTP downstream tripper (client) configurable via parameters `--query-range.downstream-tripper-config` and `--query-range.downstream-tripper-config-file`. If your downstream URL is localhost or 127.0.0.1 then it is strongly recommended to bump `max_idle_conns_per_host` to at least 100 so that `query-frontend` could properly use HTTP keep-alive connections and thus reduce the latency of `query-frontend` by about 20%.
 
 ### Fixed
 

cmd/thanos/query_frontend.go

@@ -19,10 +19,12 @@ import (
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/weaveworks/common/user"
+	"gopkg.in/yaml.v2"
 
 	extflag "github.com/efficientgo/tools/extkingpin"
 	"github.com/thanos-io/thanos/pkg/api"
 	"github.com/thanos-io/thanos/pkg/component"
+	"github.com/thanos-io/thanos/pkg/exthttp"
 	"github.com/thanos-io/thanos/pkg/extkingpin"
 	"github.com/thanos-io/thanos/pkg/extprom"
 	extpromhttp "github.com/thanos-io/thanos/pkg/extprom/http"
@@ -118,6 +120,8 @@ func registerQueryFrontend(app *extkingpin.App) {
 	cmd.Flag("query-frontend.downstream-url", "URL of downstream Prometheus Query compatible API.").
 		Default("http://localhost:9090").StringVar(&cfg.DownstreamURL)
 
+	cfg.DownstreamTripperConfig.CachePathOrContent = *extflag.RegisterPathOrContent(cmd, "query-frontend.downstream-tripper-config", "YAML file that contains downstream tripper configuration. If your downstream URL is localhost or 127.0.0.1 then it is highly recommended to increase max_idle_conns_per_host to at least 100.", extflag.WithEnvSubstitution())
+
 	cmd.Flag("query-frontend.compress-responses", "Compress HTTP responses.").
 		Default("false").BoolVar(&cfg.CompressResponses)
 
@@ -142,6 +146,41 @@ func registerQueryFrontend(app *extkingpin.App) {
 	})
 }
 
+func parseTransportConfiguration(downstreamTripperConfContentYaml []byte) (*http.Transport, error) {
+	downstreamTripper := exthttp.NewTransport()
+
+	if len(downstreamTripperConfContentYaml) > 0 {
+		tripperConfig := &queryfrontend.DownstreamTripperConfig{}
+		if err := yaml.UnmarshalStrict(downstreamTripperConfContentYaml, tripperConfig); err != nil {
+			return nil, errors.Wrap(err, "parsing downstream tripper config YAML file")
+		}
+
+		if tripperConfig.IdleConnTimeout > 0 {
+			downstreamTripper.IdleConnTimeout = time.Duration(tripperConfig.IdleConnTimeout)
+		}
+		if tripperConfig.ResponseHeaderTimeout > 0 {
+			downstreamTripper.ResponseHeaderTimeout = time.Duration(tripperConfig.ResponseHeaderTimeout)
+		}
+		if tripperConfig.TLSHandshakeTimeout > 0 {
+			downstreamTripper.TLSHandshakeTimeout = time.Duration(tripperConfig.TLSHandshakeTimeout)
+		}
+		if tripperConfig.ExpectContinueTimeout > 0 {
+			downstreamTripper.ExpectContinueTimeout = time.Duration(tripperConfig.ExpectContinueTimeout)
+		}
+		if tripperConfig.MaxIdleConns != nil {
+			downstreamTripper.MaxIdleConns = *tripperConfig.MaxIdleConns
+		}
+		if tripperConfig.MaxIdleConnsPerHost != nil {
+			downstreamTripper.MaxIdleConnsPerHost = *tripperConfig.MaxIdleConnsPerHost
+		}
+		if tripperConfig.MaxConnsPerHost != nil {
+			downstreamTripper.MaxConnsPerHost = *tripperConfig.MaxConnsPerHost
+		}
+	}
+
+	return downstreamTripper, nil
+}
+
 func runQueryFrontend(
 	g *run.Group,
 	logger log.Logger,
@@ -191,7 +230,16 @@ func runQueryFrontend(
 	}
 
 	// Create a downstream roundtripper.
-	roundTripper, err := cortexfrontend.NewDownstreamRoundTripper(cfg.DownstreamURL, http.DefaultTransport)
+	downstreamTripperConfContentYaml, err := cfg.DownstreamTripperConfig.CachePathOrContent.Content()
+	if err != nil {
+		return err
+	}
+	downstreamTripper, err := parseTransportConfiguration(downstreamTripperConfContentYaml)
+	if err != nil {
+		return err
+	}
+
+	roundTripper, err := cortexfrontend.NewDownstreamRoundTripper(cfg.DownstreamURL, downstreamTripper)
 	if err != nil {
 		return errors.Wrap(err, "setup downstream roundtripper")
 	}

docs/components/query-frontend.md

@@ -103,6 +103,22 @@ Query Frontend supports `--query-frontend.log-queries-longer-than` flag to log q
 
 Naming is hard :) Please check [here](https://github.com/thanos-io/thanos/pull/2434#discussion_r408300683) to see why we chose `query-frontend` as the name.
 
+## Recommended Downstream Tripper Configuration
+
+You can configure the parameters of the HTTP client that `query-frontend` uses for the downstream URL with parameters `--query-range.downstream-tripper-config` and `--query-range.downstream-tripper-config-file`. If it is pointing to a single host, most likely a load-balancer, then it is highly recommended to increase `max_idle_conns_per_host` via these parameters to at least 100 because otherwise `query-frontend` will not be able to leverage HTTP keep-alive connections, and the latency will be 10 - 20% higher. By default, the Go HTTP client will only keep two idle connections per each host.
+
+Keys which denote a duration are strings that can end with `s` or `m` to indicate seconds or minutes respectively. All of the other keys are integers. Supported keys are:
+
+* `idle_conn_timeout` - timeout of idle connections (string);
+* `response_header_timeout` - maximum duration to wait for a response header (string);
+* `tls_handshake_timeout` - maximum duration of a TLS handshake (string);
+* `expect_continue_timeout` - [Go source code](https://github.com/golang/go/blob/912f0750472dd4f674b69ca1616bfaf377af1805/src/net/http/transport.go#L220-L226) (string);
+* `max_idle_conns` - maximum number of idle connections to all hosts (integer);
+* `max_idle_conns_per_host` - maximum number of idle connections to each host (integer);
+* `max_conns_per_host` - maximum number of connections to each host (integer);
+
+You can find the default values [here](https://github.com/thanos-io/thanos/blob/55cb8ca38b3539381dc6a781e637df15c694e50a/pkg/exthttp/transport.go#L12-L27).
+
 ## Flags
 
 ```$ mdox-exec="thanos query-frontend --help"
@@ -170,6 +186,21 @@ Flags:
                                  Disable request logging.
       --query-frontend.compress-responses  
                                  Compress HTTP responses.
+      --query-frontend.downstream-tripper-config=<content>  
+                                 Alternative to
+                                 'query-frontend.downstream-tripper-config-file'
+                                 flag (mutually exclusive). Content of YAML file
+                                 that contains downstream tripper configuration.
+                                 If your downstream URL is localhost or
+                                 127.0.0.1 then it is highly recommended to
+                                 increase max_idle_conns_per_host to at least
+                                 100.
+      --query-frontend.downstream-tripper-config-file=<file-path>  
+                                 Path to YAML file that contains downstream
+                                 tripper configuration. If your downstream URL
+                                 is localhost or 127.0.0.1 then it is highly
+                                 recommended to increase max_idle_conns_per_host
+                                 to at least 100.
       --query-frontend.downstream-url="http://localhost:9090"  
                                  URL of downstream Prometheus Query compatible
                                  API.

pkg/queryfrontend/config.go

@@ -17,6 +17,7 @@ import (
 	"gopkg.in/yaml.v2"
 
 	extflag "github.com/efficientgo/tools/extkingpin"
+	prommodel "github.com/prometheus/common/model"
 	"github.com/thanos-io/thanos/pkg/cacheutil"
 	"github.com/thanos-io/thanos/pkg/model"
 )
@@ -138,10 +139,24 @@ func NewCacheConfig(logger log.Logger, confContentYaml []byte) (*cortexcache.Con
 	}
 }
 
+// DownstreamTripperConfig stores the http.Transport configuration for query-frontend's HTTP downstream tripper.
+type DownstreamTripperConfig struct {
+	IdleConnTimeout       prommodel.Duration `yaml:"idle_conn_timeout"`
+	ResponseHeaderTimeout prommodel.Duration `yaml:"response_header_timeout"`
+	TLSHandshakeTimeout   prommodel.Duration `yaml:"tls_handshake_timeout"`
+	ExpectContinueTimeout prommodel.Duration `yaml:"expect_continue_timeout"`
+	MaxIdleConns          *int               `yaml:"max_idle_conns"`
+	MaxIdleConnsPerHost   *int               `yaml:"max_idle_conns_per_host"`
+	MaxConnsPerHost       *int               `yaml:"max_conns_per_host"`
+
+	CachePathOrContent extflag.PathOrContent
+}
+
 // Config holds the query frontend configs.
 type Config struct {
 	QueryRangeConfig
 	LabelsConfig
+	DownstreamTripperConfig
 
 	CortexHandlerConfig    *transport.HandlerConfig
 	CompressResponses      bool