[OpAMP] enable OpAMP endpoint by default (#6657) (#6662)

* feat: enable OpAMP feature flag by default

Set EnableOpAMP to true in InitDefaults() so the OpAMP endpoint
is enabled out of the box without requiring explicit configuration.



* test: update config test to use defaultServer() for input-config case

Replace the manually built Server struct with defaultServer() plus
overrides so it picks up the new EnableOpAMP default. Remove unused
helpers (defaultServerBulk, defaultServerGC, defaultPBKDF2).



* test: clarify feature flag test case names

Rename test cases to "explicitly disabled/enabled" to reflect that
the flag now defaults to true.



* docs: update reference config to reflect OpAMP enabled by default



* docs: update OpAMP setup guide for default-enabled flag

The feature flag is now enabled by default, so the docs now explain
how to disable it rather than how to enable it.



* e2e: remove redundant enable_opamp flag from OpAMP test template

The flag now defaults to true, so explicitly setting it is unnecessary.



* changelog: add fragment for OpAMP feature flag default change



* chore: fix gofmt formatting in config_test.go



---------


(cherry picked from commit fee63f0)

Co-authored-by: Shaunak Kashyap <ycombinator@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

changelog/fragments/1774388797-opamp-ff-default-enabled.yaml

@@ -0,0 +1,45 @@
+# REQUIRED
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: enhancement
+
+# REQUIRED for all kinds
+# Change summary; a 80ish characters long description of the change.
+summary: Enable OpAMP feature flag by default
+
+# REQUIRED for breaking-change, deprecation, known-issue
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# description:
+
+# REQUIRED for breaking-change, deprecation, known-issue
+# impact:
+
+# REQUIRED for breaking-change, deprecation, known-issue
+# action:
+
+# REQUIRED for all kinds
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: fleet-server
+
+# AUTOMATED
+# OPTIONAL to manually add other PR URLs
+# PR URL: A link the PR that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+# pr: https://github.com/owner/repo/1234
+
+# AUTOMATED
+# OPTIONAL to manually add other issue URLs
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+# issue: https://github.com/owner/repo/1234

docs/opamp.md

@@ -9,12 +9,12 @@ This section describes how to connect a OpenTelemetry Collector instance to Flee
    ```
    POST /_security/service/elastic/fleet-server/credential/token/opamp
    ```
-3. Enable the OpAMP feature flag in the `fleet-server.yml` file by adding the following snippet to the `fleet-server` input
+3. The OpAMP endpoint is enabled by default. If you need to disable it, add the following to the `fleet-server` input
    section, as a sibling of the `policy.id` key:
    ```yml
    server:
      feature_flags:
-       enable_opamp: true
+       enable_opamp: false
    ```
 
 4. Build the Fleet Server binary for your platform.

fleet-server.reference.yml

@@ -277,8 +277,8 @@ fleet:
 #        // POLICY_CHANGE actions need an explicit ack if this is set.
 #        ignore_checkin_policy_id: false
 #
-#        // enable OpAMP endpoint
-#        enable_opamp: false
+#        // enable OpAMP endpoint (enabled by default)
+#        enable_opamp: true
 #
 #    # monitor options are advanced configuration and should not be adjusted is most cases
 #    monitor:

internal/pkg/api/handleOpAMP_test.go

@@ -16,11 +16,11 @@ func TestFeatureFlag(t *testing.T) {
 		FeatureFlagEnabled bool
 		WantEnabled        bool
 	}{
-		"feature flag is disabled": {
+		"feature flag is explicitly disabled": {
 			FeatureFlagEnabled: false,
 			WantEnabled:        false,
 		},
-		"feature flag is enabled": {
+		"feature flag is explicitly enabled": {
 			FeatureFlagEnabled: true,
 			WantEnabled:        true,
 		},

internal/pkg/config/config_test.go

@@ -24,6 +24,14 @@ import (
 )
 
 func TestConfig(t *testing.T) {
+	// Expected server config for the "input-config" test case.
+	inputCfgServer := defaultServer()
+	inputCfgServer.Host = "localhost"
+	inputCfgServer.Port = 8888
+	inputCfgServer.Timeouts.Read = 20 * time.Second
+	inputCfgServer.Timeouts.Write = 5 * time.Second
+	inputCfgServer.Timeouts.CheckinMaxPoll = 10 * time.Minute
+
 	testcases := map[string]struct {
 		err string
 		cfg *Config
@@ -109,38 +117,9 @@ func TestConfig(t *testing.T) {
 				},
 				Inputs: []Input{
 					{
-						Type: "fleet-server",
-						Server: Server{
-							Host:         "localhost",
-							Port:         8888,
-							InternalPort: 8221,
-							Timeouts: ServerTimeouts{
-								Read:             20 * time.Second,
-								ReadHeader:       5 * time.Second,
-								Idle:             35 * time.Second,
-								Write:            5 * time.Second,
-								CheckinTimestamp: 30 * time.Second,
-								CheckinLongPoll:  5 * time.Minute,
-								CheckinJitter:    30 * time.Second,
-								CheckinMaxPoll:   10 * time.Minute,
-								Drain:            10 * time.Second,
-							},
-							Profiler: ServerProfiler{
-								Enabled: false,
-								Bind:    "localhost:6060",
-							},
-							CompressionLevel:  1,
-							CompressionThresh: 1024,
-							Limits:            generateServerLimits(0),
-							Bulk:              defaultServerBulk(),
-							GC:                defaultServerGC(),
-							PGP: PGP{
-								UpstreamURL: defaultPGPUpstreamURL,
-								Dir:         filepath.Join(retrieveExecutableDir(), defaultPGPDirectoryName),
-							},
-							PDKDF2: defaultPBKDF2(),
-						},
-						Cache: generateCache(0),
+						Type:   "fleet-server",
+						Server: inputCfgServer,
+						Cache:  generateCache(0),
 						Monitor: Monitor{
 							FetchSize:          defaultFetchSize,
 							PollTimeout:        defaultPollTimeout,
@@ -542,24 +521,6 @@ func generateServerLimits(maxAgents int) ServerLimits {
 	return d
 }
 
-func defaultServerBulk() ServerBulk {
-	var d ServerBulk
-	d.InitDefaults()
-	return d
-}
-
-func defaultServerGC() GC {
-	var d GC
-	d.InitDefaults()
-	return d
-}
-
-func defaultPBKDF2() PBKDF2 {
-	var d PBKDF2
-	d.InitDefaults()
-	return d
-}
-
 func defaultLogging() Logging {
 	var d Logging
 	d.InitDefaults()

internal/pkg/config/input.go

@@ -99,7 +99,7 @@ type (
 		// This setting restores previous behaviour where all POLICY_CHANGE actions need an explicit ack.
 		IgnoreCheckinPolicyID bool `config:"ignore_checkin_policy_id"`
 
-		// EnableOpAMP when true will enable the OpAMP endpoint.
+		// EnableOpAMP controls whether the OpAMP endpoint is enabled. Defaults to true.
 		EnableOpAMP bool `config:"enable_opamp"`
 	}
 )
@@ -119,6 +119,7 @@ func (c *Server) InitDefaults() {
 	c.GC.InitDefaults()
 	c.PGP.InitDefaults()
 	c.PDKDF2.InitDefaults()
+	c.Features.EnableOpAMP = true
 }
 
 // BindEndpoints returns the binding address for the all HTTP server listeners.

testing/e2e/testdata/stand-alone-opamp.tpl

@@ -8,8 +8,6 @@ fleet.agent.id: e2e-test-id
 inputs:
 - type: fleet-server
   server:
-    feature_flags:
-      enable_opamp: true
     static_policy_tokens:
       enabled: true
       policy_tokens: