diff --git a/file/readfile.go b/file/readfile.go
index 7fb8d3b82..93054c8e3 100644
--- a/file/readfile.go
+++ b/file/readfile.go
@@ -158,12 +158,18 @@ func toFloat(key string) (float64, error) {
 	return strconv.ParseFloat(key, 64)
 }
 
+func indent(spaces int, v string) string {
+	pad := strings.Repeat(" ", spaces)
+	return strings.Replace(v, "\n", "\n"+pad, -1)
+}
+
 func renderTemplate(content string) (string, error) {
 	t := template.New("state").Funcs(template.FuncMap{
 		"env":     getPrefixedEnvVar,
 		"toBool":  toBool,
 		"toInt":   toInt,
 		"toFloat": toFloat,
+		"indent":  indent,
 	}).Delims("${{", "}}")
 	t, err := t.Parse(content)
 	if err != nil {
diff --git a/file/readfile_test.go b/file/readfile_test.go
index 385132763..df2a49285 100644
--- a/file/readfile_test.go
+++ b/file/readfile_test.go
@@ -172,6 +172,17 @@ func Test_getContent(t *testing.T) {
 			args: args{[]string{"testdata/file.yaml"}},
 			envVars: map[string]string{
 				"DECK_SVC2_HOST": "2.example.com",
+				"DECK_FILE_LOG_FUNCTION": `
+function parse_traceid(str)str = string.sub(str,1,8)
+  local uint = 0
+  for i = 1, #str do
+    uint = uint + str:byte(i) * 0x100^(i-1)
+  end
+  return string.format("%.0f", uint)
+end
+
+kong.log.set_serialize_value("trace_id", parse_traceid(ngx.ctx.KONG_SPANS[1].trace_id))
+kong.log.set_serialize_value("span_id", parse_traceid(ngx.ctx.KONG_SPANS[1].span_id))`,
 			},
 			want: &Content{
 				Services: []FService{
@@ -197,6 +208,25 @@ func Test_getContent(t *testing.T) {
 							Name: kong.String("prometheus"),
 						},
 					},
+					{
+						Plugin: kong.Plugin{
+							Name: kong.String("pre-function"),
+							Config: kong.Configuration{
+								"log": `
+function parse_traceid(str)str = string.sub(str,1,8)
+  local uint = 0
+  for i = 1, #str do
+    uint = uint + str:byte(i) * 0x100^(i-1)
+  end
+  return string.format("%.0f", uint)
+end
+
+kong.log.set_serialize_value("trace_id", parse_traceid(ngx.ctx.KONG_SPANS[1].trace_id))
+kong.log.set_serialize_value("span_id", parse_traceid(ngx.ctx.KONG_SPANS[1].span_id))
+`,
+							},
+						},
+					},
 				},
 			},
 			wantErr: false,
@@ -220,7 +250,8 @@ func Test_getContent(t *testing.T) {
 			name: "multiple files",
 			args: args{[]string{"testdata/file.yaml", "testdata/file.json"}},
 			envVars: map[string]string{
-				"DECK_SVC2_HOST": "2.example.com",
+				"DECK_SVC2_HOST":         "2.example.com",
+				"DECK_FILE_LOG_FUNCTION": "kong.log.set_serialize_value('trace_id', 1))",
 			},
 			want: &Content{
 				Services: []FService{
@@ -246,6 +277,14 @@ func Test_getContent(t *testing.T) {
 							Name: kong.String("prometheus"),
 						},
 					},
+					{
+						Plugin: kong.Plugin{
+							Name: kong.String("pre-function"),
+							Config: kong.Configuration{
+								"log": "kong.log.set_serialize_value('trace_id', 1))\n",
+							},
+						},
+					},
 				},
 				Consumers: []FConsumer{
 					{
diff --git a/file/testdata/file.yaml b/file/testdata/file.yaml
index 4d9d0ff62..05d6fa8b6 100644
--- a/file/testdata/file.yaml
+++ b/file/testdata/file.yaml
@@ -9,3 +9,7 @@ services:
     - '<' # verifies that the templating engine does not perform character escaping
 plugins:
 - name: prometheus
+- name: pre-function
+  config:
+    log: |
+      ${{ env "DECK_FILE_LOG_FUNCTION" | indent 8 }}