elasticsearch-js/docs/observability.asciidoc

[[observability]]
== Observability

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

Correlating those events can be quite 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.

=== 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({ node: 'http://localhost:9200' })

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

The client emits the following events:
[cols=2*]
|===
|`request`
a|Emitted before sending the actual request to Elasticsearch _(emitted multiple times in case of retries)_.
[source,js]
----
client.on('request', (err, result) => {
  console.log(err, result)
})
----

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

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

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

|===

The values of `result` in `request`, `response` and `sniff` will be:
[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` will be:
[source,ts]
----
strategy: string;
isAlive: boolean;
connection: Connection;
name: string;
request: {
  id: any;
};
----

=== Correlation id
Correlating events can be quite 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({ node: 'http://localhost:9200' })

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

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

client.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, (err, result) => {
  if (err) console.log(err)
})
----

By default the id is an incremental integer, but you can easily configure that with the `generateRequestId` option:
[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  node: 'http://localhost:9200',
  // 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',
  body: { foo: 'bar' }
}, {
  id: 'custom-id'
}, (err, result) => {
  if (err) console.log(err)
})
----

=== 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({ node: 'http://localhost:9200' })

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

client.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',
  body: { foo: 'bar' }
}, {
  context: { winter: 'is coming' }
}, (err, result) => {
  if (err) console.log(err)
})
----

=== 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 will help you in this regard:
[source,js]
----
const { Client } = require('@elastic/elasticsearch')
const client = new Client({
  node: 'http://localhost:9200',
  name: 'parent-client' // default to 'elasticsearch-js'
})

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

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

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

client.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',
  body: { foo: 'bar' }
}, (err, result) => {
  if (err) console.log(err)
})

child.search({
  index: 'my-index',
  body: { foo: 'bar' }
}, (err, result) => {
  if (err) console.log(err)
})
----