feat: Compress data backup using zstd (#264)

The backup turns out to be highly compressible!

I chose zstd. Although gzip is standard, zstd is faster, and compresses
better.

Example:

4.6M sloop-cluster-0.bak.zst
9.5M sloop-cluster-0.bak.gz
155M sloop-cluster-0.bak

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/golang/glog v1.0.0
 	github.com/golang/protobuf v1.5.2
 	github.com/jteeuwen/go-bindata v3.0.7+incompatible
+	github.com/klauspost/compress v1.16.5
 	github.com/nsf/jsondiff v0.0.0-20190712045011-8443391ee9b6
 	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.14.0

go.sum

@@ -205,6 +205,8 @@ github.com/julienschmidt/httprouter v1.2.0/go.mod h1:SYymIcj16QtmaHHD7aYtjjsJG7V
 github.com/julienschmidt/httprouter v1.3.0/go.mod h1:JR6WtHb+2LUe8TCKY3cZOxFyyO8IZAc4RVcycCCAKdM=
 github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI2bnpBCr8=
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
+github.com/klauspost/compress v1.16.5 h1:IFV2oUNUzZaz+XyusxpLzpzS8Pt5rh0Z16For/djlyI=
+github.com/klauspost/compress v1.16.5/go.mod h1:ntbaceVETuRiXiv4DpjP66DpAtAGkEQskQzEyD//IeE=
 github.com/konsorten/go-windows-terminal-sequences v1.0.1/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/konsorten/go-windows-terminal-sequences v1.0.3/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/kr/logfmt v0.0.0-20140226030751-b84e30acd515/go.mod h1:+0opPa2QZZtGFBFZlji/RkVcI2GknAs/DXo4wKdlNEc=

pkg/sloop/webserver/webserver.go

@@ -23,6 +23,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/klauspost/compress/zstd"
 	"github.com/salesforce/sloop/pkg/sloop/common"
 	"github.com/spf13/afero"
 
@@ -116,11 +117,25 @@ func backupHandler(db badgerwrap.DB, currentContext string) http.HandlerFunc {
 			return
 		}
 
-		w.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=sloop-%s-%d.bak", currentContext, since))
-		w.Header().Set("Content-Type", "application/octet-stream")
+		w.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=sloop-%s-%d.bak.zst", currentContext, since))
+		// The 'Content-Length' header is not set, because we do not know the size of the backup before we write it to the body.
+		w.Header().Set("Content-Encoding", "zstd")
+		w.Header().Set("Content-Type", "application/zstd")
 		w.Header().Set("Transfer-Encoding", "chunked")
 
-		_, err = db.Backup(w, since)
+		zw, err := zstd.NewWriter(w)
+		if err != nil {
+			logWebError(err, "Error configuring compression", r, w)
+			return
+		}
+
+		_, err = db.Backup(zw, since)
+		if err != nil {
+			logWebError(err, "Error writing backup", r, w)
+			return
+		}
+
+		err = zw.Close()
 		if err != nil {
 			logWebError(err, "Error writing backup", r, w)
 			return

pkg/sloop/webserver/webserver_test.go

@@ -5,6 +5,8 @@ import (
 	"net/http/httptest"
 	"testing"
 
+	badger "github.com/dgraph-io/badger/v2"
+	"github.com/salesforce/sloop/pkg/sloop/store/untyped/badgerwrap"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -55,3 +57,19 @@ func TestWebFileHandler(t *testing.T) {
 	assert.Equal(t, http.StatusOK, rr.Code)
 	assert.NotNil(t, rr.Body.String())
 }
+
+func TestBackupHandler(t *testing.T) {
+	req, err := http.NewRequest("GET", "/clusterContext/data/backup", nil)
+	assert.Nil(t, err)
+
+	db, err := (&badgerwrap.MockFactory{}).Open(badger.DefaultOptions(""))
+	assert.Nil(t, err)
+
+	// Create a ResponseRecorder (which satisfies http.ResponseWriter) to record the response.
+	rr := httptest.NewRecorder()
+	handler := http.HandlerFunc(backupHandler(db, "clusterContext"))
+	handler.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusOK, rr.Code)
+	assert.NotNil(t, rr.Body.String())
+}