vmalert: add support for round_digits param in datasource package (#1278)

Starting from v1.56.0 VM supports `round_digits` which allows to limit
the number of digits after the decimal point in response value. The feature
can be used to reduce entropy of produced by recording rules values
and significantly improve the compression. See more details in link below.

https://github.com/VictoriaMetrics/VictoriaMetrics/issues/525

app/vmalert/datasource/init.go

@@ -4,6 +4,7 @@ import (
 	"flag"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmalert/utils"
 )
@@ -26,6 +27,8 @@ var (
 		"For example, if datasource.queryStep=15s then param \"step\" with value \"15s\" will be added to every query."+
 		"If queryStep isn't specified, rule's evaluationInterval will be used instead.")
 	maxIdleConnections = flag.Int("datasource.maxIdleConnections", 100, `Defines the number of idle (keep-alive connections) to each configured datasource. Consider setting this value equal to the value: groups_total * group.concurrency. Too low a value may result in a high number of sockets in TIME_WAIT state.`)
+	roundDigits        = flag.Int("datasource.roundDigits", 0, `Adds \"round_digits\" GET param to datasource requests. 
+		In VM \"round_digits\" limits the number of digits after the decimal point in response values.`)
 )
 
 // Init creates a Querier from provided flag values.
@@ -33,11 +36,27 @@ func Init() (QuerierBuilder, error) {
 	if *addr == "" {
 		return nil, fmt.Errorf("datasource.url is empty")
 	}
+
 	tr, err := utils.Transport(*addr, *tlsCertFile, *tlsKeyFile, *tlsCAFile, *tlsServerName, *tlsInsecureSkipVerify)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create transport: %w", err)
 	}
 	tr.MaxIdleConns = *maxIdleConnections
-	c := &http.Client{Transport: tr}
-	return NewVMStorage(*addr, *basicAuthUsername, *basicAuthPassword, *lookBack, *queryStep, *appendTypePrefix, c), nil
+
+	var rd string
+	if *roundDigits > 0 {
+		rd = fmt.Sprintf("%d", *roundDigits)
+	}
+
+	return &VMStorage{
+		c:                &http.Client{Transport: tr},
+		basicAuthUser:    *basicAuthUsername,
+		basicAuthPass:    *basicAuthPassword,
+		datasourceURL:    strings.TrimSuffix(*addr, "/"),
+		appendTypePrefix: *appendTypePrefix,
+		lookBack:         *lookBack,
+		queryStep:        *queryStep,
+		roundDigits:      rd,
+		dataSourceType:   NewPrometheusType(),
+	}, nil
 }

app/vmalert/datasource/vm.go

@@ -81,6 +81,7 @@ type VMStorage struct {
 	appendTypePrefix bool
 	lookBack         time.Duration
 	queryStep        time.Duration
+	roundDigits      string
 
 	dataSourceType     Type
 	evaluationInterval time.Duration
@@ -218,6 +219,9 @@ func (s *VMStorage) setPrometheusReqParams(r *http.Request, query string, timest
 		// override step with user-specified value
 		q.Set("step", s.queryStep.String())
 	}
+	if s.roundDigits != "" {
+		q.Set("round_digits", s.roundDigits)
+	}
 	r.URL.RawQuery = q.Encode()
 }
 

app/vmalert/datasource/vm_test.go

@@ -243,6 +243,16 @@ func TestPrepareReq(t *testing.T) {
 				checkEqualString(t, exp, r.URL.RawQuery)
 			},
 		},
+		{
+			"round digits",
+			&VMStorage{
+				roundDigits: "10",
+			},
+			func(t *testing.T, r *http.Request) {
+				exp := fmt.Sprintf("query=%s&round_digits=10&time=%d", query, timestamp.Unix())
+				checkEqualString(t, exp, r.URL.RawQuery)
+			},
+		},
 	}
 
 	for _, tc := range testCases {