diff --git a/x-pack/plugins/actions/server/lib/action_executor.ts b/x-pack/plugins/actions/server/lib/action_executor.ts
index 101e18f2583e39..3e9262c05efac4 100644
--- a/x-pack/plugins/actions/server/lib/action_executor.ts
+++ b/x-pack/plugins/actions/server/lib/action_executor.ts
@@ -17,7 +17,7 @@ import {
 import { EncryptedSavedObjectsPluginStart } from '../../../encrypted_saved_objects/server';
 import { SpacesServiceSetup } from '../../../spaces/server';
 import { EVENT_LOG_ACTIONS } from '../plugin';
-import { IEvent, IEventLogger } from '../../../event_log/server';
+import { IEvent, IEventLogger, SAVED_OBJECT_REL_PRIMARY } from '../../../event_log/server';
 
 export interface ActionExecutorContext {
   logger: Logger;
@@ -110,7 +110,16 @@ export class ActionExecutor {
     const actionLabel = `${actionTypeId}:${actionId}: ${name}`;
     const event: IEvent = {
       event: { action: EVENT_LOG_ACTIONS.execute },
-      kibana: { saved_objects: [{ type: 'action', id: actionId, ...namespace }] },
+      kibana: {
+        saved_objects: [
+          {
+            rel: SAVED_OBJECT_REL_PRIMARY,
+            type: 'action',
+            id: actionId,
+            ...namespace,
+          },
+        ],
+      },
     };
 
     eventLogger.startTiming(event);
diff --git a/x-pack/plugins/alerting/server/task_runner/create_execution_handler.test.ts b/x-pack/plugins/alerting/server/task_runner/create_execution_handler.test.ts
index 0e46ef4919626a..a564b87f2ca50c 100644
--- a/x-pack/plugins/alerting/server/task_runner/create_execution_handler.test.ts
+++ b/x-pack/plugins/alerting/server/task_runner/create_execution_handler.test.ts
@@ -95,6 +95,7 @@ test('calls actionsPlugin.execute per selected action', async () => {
             "saved_objects": Array [
               Object {
                 "id": "1",
+                "rel": "primary",
                 "type": "alert",
               },
               Object {
diff --git a/x-pack/plugins/alerting/server/task_runner/create_execution_handler.ts b/x-pack/plugins/alerting/server/task_runner/create_execution_handler.ts
index 5c3e36b88879dd..16fadc8b06cd59 100644
--- a/x-pack/plugins/alerting/server/task_runner/create_execution_handler.ts
+++ b/x-pack/plugins/alerting/server/task_runner/create_execution_handler.ts
@@ -9,7 +9,7 @@ import { AlertAction, State, Context, AlertType } from '../types';
 import { Logger } from '../../../../../src/core/server';
 import { transformActionParams } from './transform_action_params';
 import { PluginStartContract as ActionsPluginStartContract } from '../../../../plugins/actions/server';
-import { IEventLogger, IEvent } from '../../../event_log/server';
+import { IEventLogger, IEvent, SAVED_OBJECT_REL_PRIMARY } from '../../../event_log/server';
 import { EVENT_LOG_ACTIONS } from '../plugin';
 
 interface CreateExecutionHandlerOptions {
@@ -96,7 +96,7 @@ export function createExecutionHandler({
             instance_id: alertInstanceId,
           },
           saved_objects: [
-            { type: 'alert', id: alertId, ...namespace },
+            { rel: SAVED_OBJECT_REL_PRIMARY, type: 'alert', id: alertId, ...namespace },
             { type: 'action', id: action.id, ...namespace },
           ],
         },
diff --git a/x-pack/plugins/alerting/server/task_runner/task_runner.test.ts b/x-pack/plugins/alerting/server/task_runner/task_runner.test.ts
index 26d8a1d1777c04..35a0018049c335 100644
--- a/x-pack/plugins/alerting/server/task_runner/task_runner.test.ts
+++ b/x-pack/plugins/alerting/server/task_runner/task_runner.test.ts
@@ -172,6 +172,7 @@ describe('Task Runner', () => {
             Object {
               "id": "1",
               "namespace": undefined,
+              "rel": "primary",
               "type": "alert",
             },
           ],
@@ -234,6 +235,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
               ],
@@ -254,6 +256,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
               ],
@@ -274,6 +277,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
                 Object {
@@ -351,6 +355,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
               ],
@@ -371,6 +376,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
               ],
@@ -568,6 +574,7 @@ describe('Task Runner', () => {
                 Object {
                   "id": "1",
                   "namespace": undefined,
+                  "rel": "primary",
                   "type": "alert",
                 },
               ],
diff --git a/x-pack/plugins/alerting/server/task_runner/task_runner.ts b/x-pack/plugins/alerting/server/task_runner/task_runner.ts
index 26970dc6b2b0d9..bf005301adc07c 100644
--- a/x-pack/plugins/alerting/server/task_runner/task_runner.ts
+++ b/x-pack/plugins/alerting/server/task_runner/task_runner.ts
@@ -25,7 +25,7 @@ import { promiseResult, map, Resultable, asOk, asErr, resolveErr } from '../lib/
 import { taskInstanceToAlertTaskInstance } from './alert_task_instance';
 import { AlertInstances } from '../alert_instance/alert_instance';
 import { EVENT_LOG_ACTIONS } from '../plugin';
-import { IEvent, IEventLogger } from '../../../event_log/server';
+import { IEvent, IEventLogger, SAVED_OBJECT_REL_PRIMARY } from '../../../event_log/server';
 import { isAlertSavedObjectNotFoundError } from '../lib/is_alert_not_found_error';
 
 const FALLBACK_RETRY_INTERVAL: IntervalSchedule = { interval: '5m' };
@@ -174,7 +174,16 @@ export class TaskRunner {
     const alertLabel = `${this.alertType.id}:${alertId}: '${name}'`;
     const event: IEvent = {
       event: { action: EVENT_LOG_ACTIONS.execute },
-      kibana: { saved_objects: [{ type: 'alert', id: alertId, namespace }] },
+      kibana: {
+        saved_objects: [
+          {
+            rel: SAVED_OBJECT_REL_PRIMARY,
+            type: 'alert',
+            id: alertId,
+            namespace,
+          },
+        ],
+      },
     };
     eventLogger.startTiming(event);
 
@@ -393,7 +402,14 @@ function generateNewAndResolvedInstanceEvents(params: GenerateNewAndResolvedInst
         alerting: {
           instance_id: id,
         },
-        saved_objects: [{ type: 'alert', id: params.alertId, namespace: params.namespace }],
+        saved_objects: [
+          {
+            rel: SAVED_OBJECT_REL_PRIMARY,
+            type: 'alert',
+            id: params.alertId,
+            namespace: params.namespace,
+          },
+        ],
       },
       message,
     };
diff --git a/x-pack/plugins/event_log/generated/mappings.json b/x-pack/plugins/event_log/generated/mappings.json
index f487e9262e50e7..0a858969c4f6af 100644
--- a/x-pack/plugins/event_log/generated/mappings.json
+++ b/x-pack/plugins/event_log/generated/mappings.json
@@ -86,6 +86,10 @@
                 },
                 "saved_objects": {
                     "properties": {
+                        "rel": {
+                            "type": "keyword",
+                            "ignore_above": 1024
+                        },
                         "namespace": {
                             "type": "keyword",
                             "ignore_above": 1024
diff --git a/x-pack/plugins/event_log/generated/schemas.ts b/x-pack/plugins/event_log/generated/schemas.ts
index 9c923fe77d0358..57fe90a8e876ed 100644
--- a/x-pack/plugins/event_log/generated/schemas.ts
+++ b/x-pack/plugins/event_log/generated/schemas.ts
@@ -65,6 +65,7 @@ export const EventSchema = schema.maybe(
         saved_objects: schema.maybe(
           schema.arrayOf(
             schema.object({
+              rel: ecsString(),
               namespace: ecsString(),
               id: ecsString(),
               type: ecsString(),
diff --git a/x-pack/plugins/event_log/scripts/mappings.js b/x-pack/plugins/event_log/scripts/mappings.js
index 8cc2c74b60e57b..fd149d132031e1 100644
--- a/x-pack/plugins/event_log/scripts/mappings.js
+++ b/x-pack/plugins/event_log/scripts/mappings.js
@@ -24,6 +24,11 @@ exports.EcsKibanaExtensionsMappings = {
     saved_objects: {
       type: 'nested',
       properties: {
+        // relation; currently only supports "primary" or not set
+        rel: {
+          type: 'keyword',
+          ignore_above: 1024,
+        },
         // relevant kibana space
         namespace: {
           type: 'keyword',
@@ -58,6 +63,7 @@ exports.EcsEventLogProperties = [
   'user.name',
   'kibana.server_uuid',
   'kibana.alerting.instance_id',
+  'kibana.saved_objects.rel',
   'kibana.saved_objects.namespace',
   'kibana.saved_objects.id',
   'kibana.saved_objects.name',
diff --git a/x-pack/plugins/event_log/server/es/cluster_client_adapter.test.ts b/x-pack/plugins/event_log/server/es/cluster_client_adapter.test.ts
index f79962a3241317..66c16d0ddf3838 100644
--- a/x-pack/plugins/event_log/server/es/cluster_client_adapter.test.ts
+++ b/x-pack/plugins/event_log/server/es/cluster_client_adapter.test.ts
@@ -236,6 +236,13 @@ describe('queryEventsBySavedObject', () => {
                   query: {
                     bool: {
                       must: [
+                        {
+                          term: {
+                            'kibana.saved_objects.rel': {
+                              value: 'primary',
+                            },
+                          },
+                        },
                         {
                           term: {
                             'kibana.saved_objects.type': {
@@ -319,6 +326,13 @@ describe('queryEventsBySavedObject', () => {
                   query: {
                     bool: {
                       must: [
+                        {
+                          term: {
+                            'kibana.saved_objects.rel': {
+                              value: 'primary',
+                            },
+                          },
+                        },
                         {
                           term: {
                             'kibana.saved_objects.type': {
@@ -388,6 +402,13 @@ describe('queryEventsBySavedObject', () => {
                   query: {
                     bool: {
                       must: [
+                        {
+                          term: {
+                            'kibana.saved_objects.rel': {
+                              value: 'primary',
+                            },
+                          },
+                        },
                         {
                           term: {
                             'kibana.saved_objects.type': {
diff --git a/x-pack/plugins/event_log/server/es/cluster_client_adapter.ts b/x-pack/plugins/event_log/server/es/cluster_client_adapter.ts
index 47d273b9981e32..c0ff87234c09db 100644
--- a/x-pack/plugins/event_log/server/es/cluster_client_adapter.ts
+++ b/x-pack/plugins/event_log/server/es/cluster_client_adapter.ts
@@ -7,7 +7,7 @@
 import { reject, isUndefined } from 'lodash';
 import { SearchResponse, Client } from 'elasticsearch';
 import { Logger, ClusterClient } from '../../../../../src/core/server';
-import { IEvent } from '../types';
+import { IEvent, SAVED_OBJECT_REL_PRIMARY } from '../types';
 import { FindOptionsType } from '../event_log_client';
 
 export type EsClusterClient = Pick<ClusterClient, 'callAsInternalUser' | 'asScoped'>;
@@ -155,6 +155,13 @@ export class ClusterClientAdapter {
                       query: {
                         bool: {
                           must: [
+                            {
+                              term: {
+                                'kibana.saved_objects.rel': {
+                                  value: SAVED_OBJECT_REL_PRIMARY,
+                                },
+                              },
+                            },
                             {
                               term: {
                                 'kibana.saved_objects.type': {
diff --git a/x-pack/plugins/event_log/server/event_logger.test.ts b/x-pack/plugins/event_log/server/event_logger.test.ts
index 6a745931420c0c..2bda194a65d133 100644
--- a/x-pack/plugins/event_log/server/event_logger.test.ts
+++ b/x-pack/plugins/event_log/server/event_logger.test.ts
@@ -150,6 +150,35 @@ describe('EventLogger', () => {
     message = await waitForLogMessage(systemLogger);
     expect(message).toMatch(/invalid event logged.*action.*undefined.*/);
   });
+
+  test('logs warnings when writing invalid events', async () => {
+    service.registerProviderActions('provider', ['action-a']);
+    eventLogger = service.getLogger({});
+
+    eventLogger.logEvent(({ event: { PROVIDER: 'provider' } } as unknown) as IEvent);
+    let message = await waitForLogMessage(systemLogger);
+    expect(message).toMatch(/invalid event logged.*provider.*undefined.*/);
+
+    const event: IEvent = {
+      event: {
+        provider: 'provider',
+        action: 'action-a',
+      },
+      kibana: {
+        saved_objects: [
+          {
+            rel: 'ZZZ-primary',
+            namespace: 'default',
+            type: 'event_log_test',
+            id: '123',
+          },
+        ],
+      },
+    };
+    eventLogger.logEvent(event);
+    message = await waitForLogMessage(systemLogger);
+    expect(message).toMatch(/invalid rel property.*ZZZ-primary.*/);
+  });
 });
 
 // return the next logged event; throw if not an event
diff --git a/x-pack/plugins/event_log/server/event_logger.ts b/x-pack/plugins/event_log/server/event_logger.ts
index bcfd7bd45a6f5c..1a710a6fa48653 100644
--- a/x-pack/plugins/event_log/server/event_logger.ts
+++ b/x-pack/plugins/event_log/server/event_logger.ts
@@ -19,6 +19,7 @@ import {
   ECS_VERSION,
   EventSchema,
 } from './types';
+import { SAVED_OBJECT_REL_PRIMARY } from './types';
 
 type SystemLogger = Plugin['systemLogger'];
 
@@ -118,6 +119,8 @@ const RequiredEventSchema = schema.object({
   action: schema.string({ minLength: 1 }),
 });
 
+const ValidSavedObjectRels = new Set([undefined, SAVED_OBJECT_REL_PRIMARY]);
+
 function validateEvent(eventLogService: IEventLogService, event: IEvent): IValidatedEvent {
   if (event?.event == null) {
     throw new Error(`no "event" property`);
@@ -137,7 +140,17 @@ function validateEvent(eventLogService: IEventLogService, event: IEvent): IValid
   }
 
   // could throw an error
-  return EventSchema.validate(event);
+  const result = EventSchema.validate(event);
+
+  if (result?.kibana?.saved_objects?.length) {
+    for (const so of result?.kibana?.saved_objects) {
+      if (!ValidSavedObjectRels.has(so.rel)) {
+        throw new Error(`invalid rel property in saved_objects: "${so.rel}"`);
+      }
+    }
+  }
+
+  return result;
 }
 
 export const EVENT_LOGGED_PREFIX = `event logged: `;
diff --git a/x-pack/plugins/event_log/server/index.ts b/x-pack/plugins/event_log/server/index.ts
index b7fa25cb6eb9cc..0612b5319c15b2 100644
--- a/x-pack/plugins/event_log/server/index.ts
+++ b/x-pack/plugins/event_log/server/index.ts
@@ -8,6 +8,12 @@ import { PluginInitializerContext } from 'src/core/server';
 import { ConfigSchema } from './types';
 import { Plugin } from './plugin';
 
-export { IEventLogService, IEventLogger, IEventLogClientService, IEvent } from './types';
+export {
+  IEventLogService,
+  IEventLogger,
+  IEventLogClientService,
+  IEvent,
+  SAVED_OBJECT_REL_PRIMARY,
+} from './types';
 export const config = { schema: ConfigSchema };
 export const plugin = (context: PluginInitializerContext) => new Plugin(context);
diff --git a/x-pack/plugins/event_log/server/types.ts b/x-pack/plugins/event_log/server/types.ts
index baf53ef4479141..58be6707b03730 100644
--- a/x-pack/plugins/event_log/server/types.ts
+++ b/x-pack/plugins/event_log/server/types.ts
@@ -13,6 +13,8 @@ import { IEvent } from '../generated/schemas';
 import { FindOptionsType } from './event_log_client';
 import { QueryEventsBySavedObjectResult } from './es/cluster_client_adapter';
 
+export const SAVED_OBJECT_REL_PRIMARY = 'primary';
+
 export const ConfigSchema = schema.object({
   enabled: schema.boolean({ defaultValue: true }),
   logEntries: schema.boolean({ defaultValue: false }),
diff --git a/x-pack/test/plugin_api_integration/plugins/event_log/server/init_routes.ts b/x-pack/test/plugin_api_integration/plugins/event_log/server/init_routes.ts
index c5f3e65581df97..9622715e87e55f 100644
--- a/x-pack/test/plugin_api_integration/plugins/event_log/server/init_routes.ts
+++ b/x-pack/test/plugin_api_integration/plugins/event_log/server/init_routes.ts
@@ -40,7 +40,7 @@ export const logEventRoute = (router: IRouter, eventLogger: IEventLogger, logger
       } catch (ex) {
         logger.info(`log event error: ${ex}`);
         await context.core.savedObjects.client.create('event_log_test', {}, { id });
-        logger.info(`created saved object`);
+        logger.info(`created saved object ${id}`);
       }
       eventLogger.logEvent(event);
       logger.info(`logged`);
diff --git a/x-pack/test/plugin_api_integration/test_suites/event_log/public_api_integration.ts b/x-pack/test/plugin_api_integration/test_suites/event_log/public_api_integration.ts
index d7bbc29bd861e2..f3a3d58336b1db 100644
--- a/x-pack/test/plugin_api_integration/test_suites/event_log/public_api_integration.ts
+++ b/x-pack/test/plugin_api_integration/test_suites/event_log/public_api_integration.ts
@@ -205,6 +205,7 @@ export default function({ getService }: FtrProviderContext) {
         kibana: {
           saved_objects: [
             {
+              rel: 'primary',
               namespace: 'default',
               type: 'event_log_test',
               id,
diff --git a/x-pack/test/plugin_api_integration/test_suites/event_log/service_api_integration.ts b/x-pack/test/plugin_api_integration/test_suites/event_log/service_api_integration.ts
index 31668e8345275e..361d80aaedd419 100644
--- a/x-pack/test/plugin_api_integration/test_suites/event_log/service_api_integration.ts
+++ b/x-pack/test/plugin_api_integration/test_suites/event_log/service_api_integration.ts
@@ -101,7 +101,7 @@ export default function({ getService }: FtrProviderContext) {
       const eventId = uuid.v4();
       const event: IEvent = {
         event: { action: 'action1', provider: 'provider4' },
-        kibana: { saved_objects: [{ type: 'event_log_test', id: eventId }] },
+        kibana: { saved_objects: [{ rel: 'primary', type: 'event_log_test', id: eventId }] },
       };
       await logTestEvent(eventId, event);