services/horizon: Add deprecation warning for --captive-core-use-db

services/horizon/internal/flags.go

@@ -242,6 +242,11 @@ func Flags() (*Config, support.ConfigOptions) {
 			Usage:       `when enabled, Horizon ingestion will instruct the captive core invocation to use an external db url for ledger states rather than in memory(RAM). Will result in several GB of space shifting out of RAM and to the external db persistence. The external db url is determined by the presence of DATABASE parameter in the captive-core-config-path or if absent, the db will default to sqlite and the db file will be stored at location derived from captive-core-storage-path parameter.`,
 			CustomSetValue: func(opt *support.ConfigOption) error {
 				if val := viper.GetBool(opt.Name); val {
+					stdLog.Printf(
+						"DEPRECATED - The usage of the flag --captive-core-use-db has been deprecated. This in-memory " +
+							"functionality will soon be removed in the future. We recommend keeping the default value of True, " +
+							"to use the external DB rather than in-memory mode. ",
+					)
 					config.CaptiveCoreConfigUseDB = val
 					config.CaptiveCoreTomlParams.UseDB = val
 				}

services/horizon/internal/integration/parameters_test.go

@@ -541,6 +541,39 @@ func TestDeprecatedOutputs(t *testing.T) {
 			"Configuring section in the developer documentation on how to use them - "+
 			"https://developers.stellar.org/docs/run-api-server/configuring")
 	})
+	t.Run("deprecated output for --captive-core-use-db", func(t *testing.T) {
+		originalStderr := os.Stderr
+		r, w, _ := os.Pipe()
+		os.Stderr = w
+		stdLog.SetOutput(os.Stderr)
+
+		testConfig := integration.GetTestConfig()
+		testConfig.HorizonIngestParameters = map[string]string{"captive-core-use-db": "false"}
+		test := integration.NewTest(t, *testConfig)
+		err := test.StartHorizon()
+		assert.NoError(t, err)
+		test.WaitForHorizon()
+
+		// Use a wait group to wait for the goroutine to finish before proceeding
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if err := w.Close(); err != nil {
+				t.Errorf("Failed to close Stdout")
+				return
+			}
+		}()
+
+		outputBytes, _ := io.ReadAll(r)
+		wg.Wait() // Wait for the goroutine to finish before proceeding
+		_ = r.Close()
+		os.Stderr = originalStderr
+
+		assert.Contains(t, string(outputBytes), "DEPRECATED - The usage of the flag --captive-core-use-db has been deprecated. This in-memory "+
+			"functionality will soon be removed in the future. We recommend keeping the default value of True, "+
+			"to use the external DB rather than in-memory mode. ")
+	})
 }
 
 func TestGlobalFlagsOutput(t *testing.T) {