Add feature flag for remote config (#739)

Author: felixbarny

apm-agent-core/src/main/java/co/elastic/apm/agent/configuration/ApmServerConfigurationSource.java

@@ -151,6 +151,10 @@ private void pollConfig(ConfigurationRegistry configurationRegistry) {
      */
     @Nullable
     String fetchConfig(final ConfigurationRegistry configurationRegistry) {
+        if (!configurationRegistry.getConfig(CoreConfiguration.class).isCentralConfigEnabled()) {
+            logger.debug("Remote configuration is disabled");
+            return null;
+        }
         try {
             return apmServerClient.execute("/config/v1/agents", new ApmServerClient.ConnectionHandler<String>() {
                 @Override
@@ -177,11 +181,11 @@ private String tryFetchConfig(ConfigurationRegistry configurationRegistry, HttpU
         payloadSerializer.setOutputStream(connection.getOutputStream());
         payloadSerializer.serializeMetadata(metaData);
         payloadSerializer.flush();
+        etag = connection.getHeaderField("ETag");
 
         final int status = connection.getResponseCode();
         switch (status) {
             case SC_OK:
-                etag = connection.getHeaderField("ETag");
                 InputStream is = connection.getInputStream();
                 final JsonReader<Object> reader = dslJson.newReader(is, buffer);
                 reader.startObject();
@@ -202,16 +206,7 @@ private String tryFetchConfig(ConfigurationRegistry configurationRegistry, HttpU
                 logger.debug("Configuration did not change");
                 break;
             case SC_NOT_FOUND:
-                etag = null;
-                // means that there either is no configuration for this agent
-                // or that this is an APM Server < 7.3 which does not have the config endpoint
-                logger.debug("No remote config found for this agent");
-                // makes sure to remove the configuration if all configs are deleted for this agent in Kibana
-                if (!config.isEmpty()) {
-                    logger.info("Received new configuration from APM Server: <empty>");
-                }
-                config = Collections.emptyMap();
-                configurationRegistry.reloadDynamicConfigurationOptions();
+                logger.debug("This APM Server does not support central configuration. Update to APM Server 7.3+");
                 break;
             case SC_FORBIDDEN:
                 logger.debug("Central configuration is disabled. Set kibana.enabled: true in your APM Server configuration.");

apm-agent-core/src/main/java/co/elastic/apm/agent/configuration/CoreConfiguration.java

@@ -354,6 +354,13 @@ public class CoreConfiguration extends ConfigurationOptionProvider {
             "----\n")
         .buildWithDefault("co.elastic.apm.agent.*");
 
+    private final ConfigurationOption<Boolean> centralConfig = ConfigurationOption.booleanOption()
+        .key("central_config")
+        .configurationCategory(CORE_CATEGORY)
+        .description("When enabled, the agent will make periodic requests to the APM Server to fetch updated configuration.")
+        .dynamic(true)
+        .buildWithDefault(true);
+
     private final ConfigurationOption<Boolean> breakdownMetrics = ConfigurationOption.booleanOption()
         .key("breakdown_metrics")
         .configurationCategory(CORE_CATEGORY)
@@ -447,6 +454,10 @@ public Map<String, String> getGlobalLabels() {
         return globalLabels.get();
     }
 
+    boolean isCentralConfigEnabled() {
+        return centralConfig.get();
+    }
+
     public boolean isBreakdownMetricsEnabled() {
         return breakdownMetrics.get();
     }

apm-agent-core/src/test/java/co/elastic/apm/agent/configuration/ApmServerConfigurationSourceTest.java

@@ -50,6 +50,7 @@
 import static com.github.tomakehurst.wiremock.client.WireMock.equalTo;
 import static com.github.tomakehurst.wiremock.client.WireMock.get;
 import static com.github.tomakehurst.wiremock.client.WireMock.notFound;
+import static com.github.tomakehurst.wiremock.client.WireMock.okJson;
 import static com.github.tomakehurst.wiremock.client.WireMock.post;
 import static com.github.tomakehurst.wiremock.client.WireMock.postRequestedFor;
 import static com.github.tomakehurst.wiremock.client.WireMock.serverError;
@@ -62,6 +63,7 @@
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 public class ApmServerConfigurationSourceTest {
 
@@ -100,18 +102,26 @@ public void testLoadRemoteConfig() throws Exception {
         }
     }
 
+    @Test
+    public void testRemoteConfigDisabled() {
+        when(config.getConfig(CoreConfiguration.class).isCentralConfigEnabled()).thenReturn(false);
+        configurationSource.fetchConfig(config);
+        assertThat(configurationSource.getValue("foo")).isNull();
+        mockApmServer.verify(0, postRequestedFor(urlEqualTo("/config/v1/agents")));
+    }
+
     @Test
     public void testNotFound() {
         mockApmServer.stubFor(post(urlEqualTo("/config/v1/agents")).willReturn(notFound()));
         assertThat(configurationSource.fetchConfig(config)).isNull();
-        verify(mockLogger, times(1)).debug(contains("No remote config found for this agent"));
+        verify(mockLogger, times(1)).debug(contains("This APM Server does not support central configuration"));
     }
 
     @Test
     public void configDeleted() {
         configurationSource.fetchConfig(config);
         assertThat(configurationSource.getValue("foo")).isEqualTo("bar");
-        mockApmServer.stubFor(post(urlEqualTo("/config/v1/agents")).willReturn(notFound()));
+        mockApmServer.stubFor(post(urlEqualTo("/config/v1/agents")).willReturn(okJson("{}")));
         configurationSource.fetchConfig(config);
         assertThat(configurationSource.getValue("foo")).isNull();
     }