[[observability]]
=== Observability

The client does not provide a default logger, but instead it offers an event 
emitter interface to hook into internal events, such as `request` and 
`response`.

Correlating those events can be hard, especially if your applications have a 
large codebase with many events happening at the same time.

To help you with this, the client offers you a correlation id system and other 
features. Let's see them in action.


[discrete]
==== Events

The client is an event emitter, this means that you can listen for its event and 
add additional logic to your code, without need to change the client internals 
or your normal usage. You can find the events names by access the `events` key 
of the client.

[source,js]
----
const { events } = require('@elastic/elasticsearch')
console.log(events)
----


The event emitter functionality can be useful if you want to log every request, 
response and error that is happening during the use of the client.

[source,js]
----
const logger = require('my-logger')()
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' }
})

client.diagnostic.on('response', (err, result) => {
  if (err) {
    logger.error(err)
  } else {
    logger.info(result)
  }
})
----


The client emits the following events:
[cols=2*]
|===
|`serialization`
a|Emitted before starting serialization and compression. If you want to measure this phase duration, you should measure the time elapsed between this event and `request`.
[source,js]
----
client.diagnostic.on('serialization', (err, result) => {
  console.log(err, result)
})
----

|`request`
a|Emitted before sending the actual request to {es} _(emitted multiple times in case of retries)_.
[source,js]
----
client.diagnostic.on('request', (err, result) => {
  console.log(err, result)
})
----

|`deserialization`
a|Emitted before starting deserialization and decompression. If you want to measure this phase duration, you should measure the time elapsed between this event and `response`. _(This event might not be emitted in certain situations)_.
[source,js]
----
client.diagnostic.on('deserialization', (err, result) => {
  console.log(err, result)
})
----

|`response`
a|Emitted once {es} response has been received and parsed.
[source,js]
----
client.diagnostic.on('response', (err, result) => {
  console.log(err, result)
})
----

|`sniff`
a|Emitted when the client ends a sniffing request.
[source,js]
----
client.diagnostic.on('sniff', (err, result) => {
  console.log(err, result)
})
----

|`resurrect`
a|Emitted if the client is able to resurrect a dead node.
[source,js]
----
client.diagnostic.on('resurrect', (err, result) => {
  console.log(err, result)
})
----

|===

The values of `result` in `serialization`, `request`, `deserialization`, 
`response` and `sniff` are:

[source,ts]
----
body: any;
statusCode: number | null;
headers: anyObject | null;
warnings: string[] | null;
meta: {
  context: any;
  name: string;
  request: {
    params: TransportRequestParams;
    options: TransportRequestOptions;
    id: any;
  };
  connection: Connection;
  attempts: number;
  aborted: boolean;
  sniff?: {
    hosts: any[];
    reason: string;
  };
};
----


While the `result` value in `resurrect` is:

[source,ts]
----
strategy: string;
isAlive: boolean;
connection: Connection;
name: string;
request: {
  id: any;
};
----

[discrete]
===== Events order

The event order is described in the following graph, in some edge cases, the 
order is not guaranteed.
You can find in 
https://github.com/elastic/elasticsearch-js/blob/master/test/acceptance/events-order.test.js[`test/acceptance/events-order.test.js`] 
how the order changes based on the situation.

[source]
----
serialization
  │
  │ (serialization and compression happens between those two events)
  │
  └─▶ request
        │
        │ (actual time spent over the wire)
        │
        └─▶ deserialization
              │
              │ (deserialization and decompression happens between those two events)
              │
              └─▶ response
----


[discrete]
==== Correlation id

Correlating events can be hard, especially if there are many events at the same 
time. The client offers you an automatic (and configurable) system to help you 
handle this problem.

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' }
})

client.diagnostic.on('request', (err, result) => {
  const { id } = result.meta.request
  if (err) {
    console.log({ error: err, reqId: id })
  }
})

client.diagnostic.on('response', (err, result) => {
  const { id } = result.meta.request
  if (err) {
    console.log({ error: err, reqId: id })
  }
})

client.search({
  index: 'my-index',
  query: { match_all: {} }
}).then(console.log, console.log)
----


By default the id is an incremental integer, but you can configure it with the 
`generateRequestId` option:

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' },
  // it takes two parameters, the request parameters and options
  generateRequestId: function (params, options) {
    // your id generation logic
    // must be syncronous
    return 'id'
  }
})
----


You can also specify a custom id per request:

[source,js]
----
client.search({
  index: 'my-index',
  query: { match_all: {} }
}, {
  id: 'custom-id'
}).then(console.log, console.log)
----


[discrete]
==== Context object

Sometimes, you might need to make some custom data available in your events, you 
can do that via the `context` option of a request: 

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' }
})

client.diagnostic.on('request', (err, result) => {
  const { id } = result.meta.request
  const { context } = result.meta
  if (err) {
    console.log({ error: err, reqId: id, context })
  }
})

client.diagnostic.on('response', (err, result) => {
  const { id } = result.meta.request
  const { winter } = result.meta.context
  if (err) {
    console.log({ error: err, reqId: id, winter })
  }
})

client.search({
  index: 'my-index',
  query: { match_all: {} }
}, {
  context: { winter: 'is coming' }
}).then(console.log, console.log)
----

The context object can also be configured as a global option in the client
configuration. If you provide both, the two context objects will be shallow 
merged, and the API level object will take precedence.

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' },
  context: { winter: 'is coming' }
})

client.diagnostic.on('request', (err, result) => {
  const { id } = result.meta.request
  const { context } = result.meta
  if (err) {
    console.log({ error: err, reqId: id, context })
  }
})

client.diagnostic.on('response', (err, result) => {
  const { id } = result.meta.request
  const { winter } = result.meta.context
  if (err) {
    console.log({ error: err, reqId: id, winter })
  }
})

client.search({
  index: 'my-index',
  query: { match_all: {} }
}, {
  context: { winter: 'has come' }
}).then(console.log, console.log)
----


[discrete]
==== Client name

If you are using multiple instances of the client or if you are using multiple 
child clients _(which is the recommended way to have multiple instances of the 
client)_, you might need to recognize which client you are using. The `name` 
options help you in this regard.

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' },
  name: 'parent-client' // default to 'elasticsearch-js'
})

const child = client.child({
  name: 'child-client'
})

console.log(client.name, child.name)

client.diagnostic.on('request', (err, result) => {
  const { id } = result.meta.request
  const { name } = result.meta
  if (err) {
    console.log({ error: err, reqId: id, name })
  }
})

client.diagnostic.on('response', (err, result) => {
  const { id } = result.meta.request
  const { name } = result.meta
  if (err) {
    console.log({ error: err, reqId: id, name })
  }
})

client.search({
  index: 'my-index',
  query: { match_all: {} }
}).then(console.log, console.log)

child.search({
  index: 'my-index',
  query: { match_all: {} }
}).then(console.log, console.log)
----


[discrete]
==== X-Opaque-Id support

To improve observability, the client offers an easy way to configure the 
`X-Opaque-Id` header. If you set the `X-Opaque-Id` in a specific request, this 
allows you to discover this identifier in the 
https://www.elastic.co/guide/en/elasticsearch/reference/8.2/logging.html#deprecation-logging[deprecation logs], 
helps you with https://www.elastic.co/guide/en/elasticsearch/reference/8.2/index-modules-slowlog.html#_identifying_search_slow_log_origin[identifying search slow log origin] 
as well as https://www.elastic.co/guide/en/elasticsearch/reference/8.2/tasks.html#_identifying_running_tasks[identifying running tasks].

The `X-Opaque-Id` should be configured in each request, for doing that you can 
use the `opaqueId` option, as you can see in the following example. The 
resulting header will be `{ 'X-Opaque-Id': 'my-search' }`.

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' }
})

client.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, {
  opaqueId: 'my-search'
}).then(console.log, console.log)
----

Sometimes it may be useful to prefix all the `X-Opaque-Id` headers with a 
specific string, in case you need to identify a specific client or server. For 
doing this, the client offers a top-level configuration option: 
`opaqueIdPrefix`. In the following example, the resulting header will be 
`{ 'X-Opaque-Id': 'proxy-client::my-search' }`.

[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  cloud: { id: '<cloud-id>' },
  auth: { apiKey: 'base64EncodedKey' },
  opaqueIdPrefix: 'proxy-client::'
})

client.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, {
  opaqueId: 'my-search'
}).then(console.log, console.log)
----

