[[common-script-uses]]
== Common scripting use cases
You can write a script to do almost anything, and sometimes, that's
the trouble. It's challenging to know what's possible with scripts,
so the following examples address common uses cases where scripts are
really helpful.

* <<scripting-field-extraction,Field extraction>>

[[scripting-field-extraction]]
=== Field extraction
The goal of field extraction is simple; you have fields in your data with a bunch of
information, but you only want to extract pieces and parts.

There are two options at your disposal:

* <<grok,Grok>> is a regular expression dialect that supports aliased
expressions that you can reuse. Because Grok sits on top of regular expressions
(regex), any regular expressions are valid in grok as well.
* <<dissect-processor,Dissect>> extracts structured fields out of text, using
delimiters to define the matching pattern. Unlike grok, dissect doesn't use regular
expressions.

Regex is incredibly powerful but can be complicated. If you don't need the
power of regular expressions, use dissect patterns, which are simple and
often faster than grok patterns. Paying special attention to the parts of the string
you want to discard will help build successful dissect patterns.

Let's start with a simple example by adding the `@timestamp` and `message`
fields to the `my-index` mapping as indexed fields. To remain flexible, use
`wildcard` as the field type for `message`:

[source,console]
----
PUT /my-index/
{
  "mappings": {
    "properties": {
      "@timestamp": {
        "format": "strict_date_optional_time||epoch_second",
        "type": "date"
      },
      "message": {
        "type": "wildcard"
      }
    }
  }
}
----

After mapping the fields you want to retrieve, index a few records from
your log data into {es}. The following request uses the <<docs-bulk,bulk API>>
to index raw log data into `my-index`. Instead of indexing all of your log
data, you can use a small sample to experiment with runtime fields.

[source,console]
----
POST /my-index/_bulk?refresh
{"index":{}}
{"timestamp":"2020-04-30T14:30:17-05:00","message":"40.135.0.0 - - [30/Apr/2020:14:30:17 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:30:53-05:00","message":"232.0.0.0 - - [30/Apr/2020:14:30:53 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:12-05:00","message":"26.1.0.0 - - [30/Apr/2020:14:31:12 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:19-05:00","message":"247.37.0.0 - - [30/Apr/2020:14:31:19 -0500] \"GET /french/splash_inet.html HTTP/1.0\" 200 3781"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:22-05:00","message":"247.37.0.0 - - [30/Apr/2020:14:31:22 -0500] \"GET /images/hm_nbg.jpg HTTP/1.0\" 304 0"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:27-05:00","message":"252.0.0.0 - - [30/Apr/2020:14:31:27 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:28-05:00","message":"not a valid apache log"}
----
// TEST[continued]

[discrete]
[[field-extraction-ip]]
==== Extract an IP address from a log message (Grok)
If you want to retrieve results that include `clientip`, you can add that
field as a runtime field in the mapping. The following runtime script defines a
grok pattern that extracts structured fields out of the `message` field. 

The script matches on the `%{COMMONAPACHELOG}` log pattern, which understands
the structure of Apache logs. If the pattern matches, the script emits the
value matching the IP address. If the pattern doesn't match
(`clientip != null`), the script just returns the field value without crashing.

[source,console]
----
PUT my-index/_mappings
{
  "runtime": {
    "http.clientip": {
      "type": "ip",
      "script": """
        String clientip=grok('%{COMMONAPACHELOG}').extract(doc["message"].value)?.clientip;
        if (clientip != null) emit(clientip); <1>
      """
    }
  }
}
----
// TEST[continued]
<1> This condition ensures that the script doesn't emit anything even if the pattern of
the message doesn't match.

You can define a simple query to run a search for a specific IP address and
return all related fields. Use the `fields` parameter of the search API to
retrieve the `http.clientip` runtime field.

[source,console]
----
GET my-index/_search
{
  "query": {
    "match": {
      "http.clientip": "40.135.0.0"
    }
  },
  "fields" : ["http.clientip"]
}
----
// TEST[continued]
// TEST[s/_search/_search\?filter_path=hits/]

The response includes documents where the value for `http.clientip` matches
`40.135.0.0`.

[source,console-result]
----
{
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index",
        "_id" : "Rq-ex3gBA_A0V6dYGLQ7",
        "_score" : 1.0,
        "_source" : {
          "timestamp" : "2020-04-30T14:30:17-05:00",
          "message" : "40.135.0.0 - - [30/Apr/2020:14:30:17 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"
        },
        "fields" : {
          "http.clientip" : [
            "40.135.0.0"
          ]
        }
      }
    ]
  }
}
----
// TESTRESPONSE[s/"_id" : "Rq-ex3gBA_A0V6dYGLQ7"/"_id": $body.hits.hits.0._id/]

[discrete]
[[field-extraction-parse]]
==== Parse a string to extract part of a field (Dissect)
Instead of matching on a log pattern like in the <<field-extraction-ip,previous example>>, you can just define a dissect pattern to include the parts of the string
that you want to discard.

For example, the log data at the start of this section includes a `message`
field. This field contains several pieces of data:

[source,js]
----
"message" : "247.37.0.0 - - [30/Apr/2020:14:31:22 -0500] \"GET /images/hm_nbg.jpg HTTP/1.0\" 304 0"
----
// NOTCONSOLE

You can define a dissect pattern in a runtime field to extract the https://developer.mozilla.org/en-US/docs/Web/HTTP/Status[HTTP response code], which is
`304` in the previous example.

[source,console]
----
PUT my-index/_mappings
{
  "runtime": {
    "http.response": {
      "type": "long",
      "script": """
        String response=dissect('%{clientip} %{ident} %{auth} [%{@timestamp}] "%{verb} %{request} HTTP/%{httpversion}" %{response} %{size}').extract(doc["message"].value)?.response;
        if (response != null) emit(Integer.parseInt(response));
      """
    }
  }
}
----
// TEST[continued]

You can then run a query to retrieve a specific HTTP response using the
`http.response` runtime field:

[source,console]
----
GET my-index/_search
{
  "query": {
    "match": {
      "http.response": "304"
    }
  },
  "fields" : ["http.response"]
}
----
// TEST[continued]
// TEST[s/_search/_search\?filter_path=hits/]

The response includes a single document where the HTTP response is `304`:

[source,console-result]
----
{
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index",
        "_id" : "Sq-ex3gBA_A0V6dYGLQ7",
        "_score" : 1.0,
        "_source" : {
          "timestamp" : "2020-04-30T14:31:22-05:00",
          "message" : "247.37.0.0 - - [30/Apr/2020:14:31:22 -0500] \"GET /images/hm_nbg.jpg HTTP/1.0\" 304 0"
        },
        "fields" : {
          "http.response" : [
            304
          ]
        }
      }
    ]
  }
}
----
// TESTRESPONSE[s/"_id" : "Sq-ex3gBA_A0V6dYGLQ7"/"_id": $body.hits.hits.0._id/]

[discrete]
[[field-extraction-split]]
==== Split values in a field by a separator (Dissect)
Let's say you want to extract part of a field like in the previous example, but you
want to split on specific values. You can use a dissect pattern to extract only the
information that you want, and also return that data in a specific format.

For example, let's say you have a bunch of garbage collection (gc) log data from {es}
in this format:

[source,txt]
----
[2021-04-27T16:16:34.699+0000][82460][gc,heap,exit]   class space    used 266K, capacity 384K, committed 384K, reserved 1048576K
----
// NOTCONSOLE

You only want to extract the `used`, `capacity`, and `committed` data, along with
the associated values. Let's index some a few documents containing log data to use as
an example:

[source,console]
----
POST /my-index/_bulk?refresh
{"index":{}}
{"gc": "[2021-04-27T16:16:34.699+0000][82460][gc,heap,exit]   class space    used 266K, capacity 384K, committed 384K, reserved 1048576K"}
{"index":{}}
{"gc": "[2021-03-24T20:27:24.184+0000][90239][gc,heap,exit]   class space    used 15255K, capacity 16726K, committed 16844K, reserved 1048576K"}
{"index":{}}
{"gc": "[2021-03-24T20:27:24.184+0000][90239][gc,heap,exit]  Metaspace       used 115409K, capacity 119541K, committed 120248K, reserved 1153024K"}
{"index":{}}
{"gc": "[2021-04-19T15:03:21.735+0000][84408][gc,heap,exit]   class space    used 14503K, capacity 15894K, committed 15948K, reserved 1048576K"}
{"index":{}}
{"gc": "[2021-04-19T15:03:21.735+0000][84408][gc,heap,exit]  Metaspace       used 107719K, capacity 111775K, committed 112724K, reserved 1146880K"}
{"index":{}}
{"gc": "[2021-04-27T16:16:34.699+0000][82460][gc,heap,exit]  class space  used 266K, capacity 367K, committed 384K, reserved 1048576K"}
----

Looking at the data again, there's a timestamp, some other data that you're not
interested in, and then the `used`, `capacity`, and `committed` data:

[source,txt]
----
[2021-04-27T16:16:34.699+0000][82460][gc,heap,exit]   class space    used 266K, capacity 384K, committed 384K, reserved 1048576K
----

You can assign variables to each part of the data in the `gc` field, and then return
only the parts that you want. Anything in curly braces `{}` is considered a variable.
For example, the variables `[%{@timestamp}][%{code}][%{desc}]` will match the first
three chunks of data, all of which are in square brackets `[]`.

[source,txt]
----
[%{@timestamp}][%{code}][%{desc}]  %{ident} used %{usize}, capacity %{csize}, committed %{comsize}, reserved %{rsize}
----

Your dissect pattern can include the terms `used`, `capacity`, and `committed` instead
of using variables, because you want to return those terms exactly. You also assign
variables to the values you want to return, such as `%{usize}`, `%{csize}`, and 
`%{comsize}`. The separator in the log data is a comma, so your dissect pattern also
needs to use that separator.

Now that you have a dissect pattern, you can include it in a Painless script as part
of a runtime field. The script uses your dissect pattern to split apart the `gc`
field, and then returns exactly the information that you want as defined by the
`emit` method. Because dissect uses simple syntax, you just need to tell it exactly
what you want. 

The following pattern tells dissect to return the term `used`, a blank space, the value
from `gc.usize`, and a comma. This pattern repeats for the other data that you
want to retrieve. While this pattern might not be as useful in production, it provides
a lot of flexibility to experiment with and manipulate your data. In a production
setting, you might just want to use `emit(gc.usize)` and then aggregate on that value
or use it in computations. 

[source,painless]
----
emit("used" + ' ' + gc.usize + ', ' + "capacity" + ' ' + gc.csize + ', ' + "committed" + ' ' + gc.comsize)
----

Putting it all together, you can create a runtime field named `gc_size` in a search
request. Using the <<search-fields-param,`fields` option>>, you can retrieve all values
for the `gc_size` runtime field. This query also includes a bucket aggregation to group
your data.

[source,console]
----
GET my-index/_search
{
  "runtime_mappings": {
    "gc_size": {
      "type": "keyword",
      "script": """
        Map gc=dissect('[%{@timestamp}][%{code}][%{desc}]  %{ident} used %{usize}, capacity %{csize}, committed %{comsize}, reserved %{rsize}').extract(doc["gc.keyword"].value);
        if (gc != null) emit("used" + ' ' + gc.usize + ', ' + "capacity" + ' ' + gc.csize + ', ' + "committed" + ' ' + gc.comsize);
      """
    }
  },
  "size": 1, 
  "aggs": {
    "sizes": {
      "terms": {
        "field": "gc_size",
        "size": 10
      }
    }
  }, 
  "fields" : ["gc_size"]
}
----
// TEST[continued]

The response includes the data from the `gc_size` field, formatted exactly as you
defined it in the dissect pattern!

[source,console-result]
----
{
  "took" : 2,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 6,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index",
        "_id" : "GXx3H3kBKGE42WRNlddJ",
        "_score" : 1.0,
        "_source" : {
          "gc" : "[2021-04-27T16:16:34.699+0000][82460][gc,heap,exit]   class space    used 266K, capacity 384K, committed 384K, reserved 1048576K"
        },
        "fields" : {
          "gc_size" : [
            "used 266K, capacity 384K, committed 384K"
          ]
        }
      }
    ]
  },
  "aggregations" : {
    "sizes" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "used 107719K, capacity 111775K, committed 112724K",
          "doc_count" : 1
        },
        {
          "key" : "used 115409K, capacity 119541K, committed 120248K",
          "doc_count" : 1
        },
        {
          "key" : "used 14503K, capacity 15894K, committed 15948K",
          "doc_count" : 1
        },
        {
          "key" : "used 15255K, capacity 16726K, committed 16844K",
          "doc_count" : 1
        },
        {
          "key" : "used 266K, capacity 367K, committed 384K",
          "doc_count" : 1
        },
        {
          "key" : "used 266K, capacity 384K, committed 384K",
          "doc_count" : 1
        }
      ]
    }
  }
}
----
// TESTRESPONSE[s/"took" : 2/"took": "$body.took"/]
// TESTRESPONSE[s/"_id" : "GXx3H3kBKGE42WRNlddJ"/"_id": $body.hits.hits.0._id/]