[RFC][skip-ci] Prevent plugins from blocking Kibana startup

rfcs/text/0006_lifecycle_unblocked.md

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+- Start Date: 2019-09-11
+- RFC PR: (leave this empty)
+- Kibana Issue: (leave this empty)
+
+# Summary
+
+Prevent plugin lifecycle methods from blocking Kibana startup by making the
+following changes:
+1. Synchronous lifecycle methods 
+2. Synchronous context provider functions
+3. Core should not expose API's as observables
+
+# Motivation
+Plugin lifecycle methods and context provider functions are async
+(promise-returning) functions. Core runs these functions in series and waits
+for each plugin's lifecycle function to resolve before calling the next. This
+allows plugins to depend on the API's returned from other plugins.
+
+With the current design, a single lifecycle method or context provider that
+blocks will block all of Kibana from starting up.
+
+We should make it impossible for a single plugin lifecycle function to
+stall all of kibana.
+
+# Detailed design
+
+### 1. Synchronous lifecycle methods
+Lifecycle methods are synchronous functions, they can perform asynchronous
+operations but Core doesn't wait for these to complete. This guarantees that
+no plugin lifecycle function can block other plugins or core from starting up. 
+
+### 2. Synchronous Context Provider functions
+Making context provider functions synchronous guarantees that a context
+handler will never be blocked by registered context providers. They can expose
+asynchronous API's which could potentially have blocking behaviour. 
+
+```ts
+export type IContextProvider<
+  TContext extends Record<string, any>,
+  TContextName extends keyof TContext,
+  TProviderParameters extends any[] = []
+> = (
+  context: Partial<TContext>,
+  ...rest: TProviderParameters
+) => TContext[TContextName];
+```
+
+### 3. Core should not expose API's as observables
+All Core API's should be reactive, when internal state changes their behaviour
+should change accordingly. But, exposing these internal state changes as part
+of the API contract leaks internal implementation details consumers can't do
+anything useful with and don't care about.
+
+For example: Core currently exposes `core.elasticsearch.adminClient$`, an
+Observable which emits a pre-configured elasticsearch client every time there's
+a configuration change. This includes changes such as the elasticsearch
+cluster `hosts` that alter the behaviour of the elasticsearch client. As a
+plugin author who wants to make search requests against elasticsearch I
+shouldn't have to care about, react to, or keep track of, how many times the
+underlying configuration has changed. I want to use the `callAsInternalUser`
+method and I expect Core to use the most up to date configuration to send this
+request to the correct `hosts`.
+
+This does not mean we should remove all observables from Core's API's. When an
+API consumer is interested in the *state changes itself* it absolutely makes
+sense to expose this as an Observable. Good examples of this is exposing
+plugin config as this is state that changes over time to which a plugin should
+directly react to.
+
+This is important in the context of synchronous lifecycle methods and context 
+handlers since exposing convenient API's become very ugly:
+
+*(3.1): exposing Observable-based API's through the route handler context:*
+```ts
+// Before: Using an asynchronous context provider
+coreSetup.http.registerRouteHandlerContext(coreId, 'core', async (context, req) => {
+  const adminClient = await coreSetup.elasticsearch.adminClient$.pipe(take(1)).toPromise();
+  const dataClient = await coreSetup.elasticsearch.dataClient$.pipe(take(1)).toPromise();
+  return {
+    elasticsearch: {
+      adminClient: adminClient.asScoped(req),
+      dataClient: dataClient.asScoped(req),
+    },
+  };
+});
+
+// After: Using a synchronous context provider
+coreSetup.http.registerRouteHandlerContext(coreId, 'core', async (context, req) => {
+  return {
+    elasticsearch: {
+      // (3.1.1) We can expose a convenient API by doing a lot of work
+      adminClient: () => {
+        callAsInternalUser: async (...args) => {
+          adminClient = await coreSetup.elasticsearch.adminClient$.pipe(take(1)).toPromise();
+          return adminClient.asScoped(req).callAsinternalUser(args);
+        },
+        callAsCurrentUser: async (...args) => {
+          adminClient = await coreSetup.elasticsearch.adminClient$.pipe(take(1)).toPromise();
+          return adminClient.asScoped(req).callAsCurrentUser(args);
+        }
+      },
+      // (3.1.2) Or a lazy approach which perpetuates the problem to consumers:
+      dataClient: async () => {
+        const dataClient = await coreSetup.elasticsearch.dataClient$.pipe(take(1)).toPromise();
+        dataClient.asScoped(req),
+      },
+    },
+  };
+});
+```
+
+### 4. Complete example code
+*(4.1) Doing async operations in a plugin's setup lifecycle*
+```ts
+export class Plugin {
+  public setup(core: CoreSetup) {
+    // Async setup is possible and any operations involving asynchronous API's
+    // will still block until these API's are ready, (savedObjects find only 
+    // resolves once the elasticsearch client has established a connection to 
+    // the cluster). The difference is that these details are now internal to
+    // the API.
+    (async () => {
+      const docs = await context.core.savedObjects.client.find({...});
+      ...
+      await context.core.savedObjects.client.update(...);
+    })();
+  }
+}
+```
+
+*(4.2) Exposing an API from a plugin's setup lifecycle*
+```ts
+export class Plugin {
+  public async setup(core: CoreSetup) {
+    return {
+      ping: async () => {
+        // async & await isn't necessary here, but makes example a bit clearer.
+        // Note that the elasticsearch client no longer exposes an adminClient$
+        // observable.
+        const result = await core.elasticsearch.adminClient.callAsInternalUser('ping', ...);
+        return result;
+      }
+    };
+  }
+}
+```
+
+*(4.3) Exposing an observable free Elasticsearch API from the route context*
+```ts
+coreSetup.http.registerRouteHandlerContext(coreId, 'core', async (context, req) => {
+  return {
+    elasticsearch: {
+      adminClient: coreSetup.elasticsearch.adminClient.asScoped(req),
+      dataClient: coreSetup.elasticsearch.adminClient.asScoped(req),
+    },
+  };
+});
+```
+
+# Drawbacks
+Not being able to block on a lifecycle method also means plugins can no longer
+be certain that all setup is complete before they reach the start lifecycle.
+Plugins will have to manage such state internally. Core will still expose
+special API's that are able block the setup lifecycle such as registering
+Saved Object migrations, but this will be limited to operations where the risk
+of blocking all of kibana starting up is limited.
+
+# Alternatives
+## 1. Introduce a lifecycle/context provider timeout
+Lifecycle methods and context providers would timeout after X seconds and any
+API's they expose would not be available if the timeout had been reached.
+
+Drawbacks:
+1. For lifecycle methods: there would be no way to recover from a timeout,
+   once a timeout had been reached the API will remain unavailable.
+
+   Context providers have the benefit of being re-created for each handler
+   call, so a single timeout would not permanently disable the API.
+
+3. Plugins have less control over their behaviour. When an upstream server
+   becomes unavailable, a plugin might prefer to keep retrying the request
+   indefinitely or only timeout after more than X seconds. It also isn't able
+   to expose detailed error information to downstream consumers such as
+   specifying which host or service is unavailable.
+
+5. (minor) Introduces an additional failure condition that needs to be handled.
+   Consumers should handle the API not being available in setup, as well as,
+   error responses from the API itself. Since remote hosts like Elasticsearch
+   could go down even after a successful setup, this effectively means API
+   consumers have to handle the same error condition in two places.
+
+## 2. Treat anything that blocks Kibana from starting up as a bug
+Effectively do what we've always been doing. We can improve on the status quo
+by logging detailed diagnostic info on any plugins that appear to be blocking
+startup.
+
+Drawbacks:
+1. Since plugins load serially, even if they don't block startup, all the
+   delays could add up and potentially make startup very slow.
+2. This opens up the potential for a third-party plugin to effectively "break"
+   kibana which creates a bad user experience.
+
+# Adoption strategy (WIP)
+
+Making context provider functions synchronous (2) and not exposing core API's
+as observables (3) would require the least amount of change from plugins since
+adoption on these API's are still fairly low.
+
+Having synchronous lifecycle methods (1) would have a bigger impact on plugins
+since most NP shims were built with asynchronous methods in mind.
+
+# How we teach this (TBD)
+
+What names and terminology work best for these concepts and why? How is this
+idea best presented? As a continuation of existing Kibana patterns?
+
+Would the acceptance of this proposal mean the Kibana documentation must be
+re-organized or altered? Does it change how Kibana is taught to new developers
+at any level?
+
+How should this feature be taught to existing Kibana developers?
+
+# Unresolved questions
+Are the drawbacks worth the benefits or can we live with Kibana potentially
+being blocked for the sake of convenient asynchronous lifecycle stages?