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Bloblang

Bloblang, or blobl for short, is a language designed for mapping data of a wide variety of forms. It's a safe, fast, and powerful way to perform document mapping within Benthos. It also has a Go API for writing your own functions and methods as plugins.

Bloblang is available as a processor and it's also possible to use blobl queries in function interpolations.

You can also execute Bloblang mappings on the command-line with the blobl subcommand:

$ cat data.jsonl | benthos blobl 'foo.(bar | baz).buz'

This document outlines the core features of the Bloblang language, but if you're totally new to Bloblang then it's worth following the walkthrough first.

Assignment​

A Bloblang mapping expresses how to create a new document by extracting data from an existing input document. Assignments consist of a dot separated path segments on the left-hand side describing a field to be created within the new document, and a right-hand side query describing what the content of the new field should be.

The keyword root on the left-hand side refers to the root of the new document, the keyword this on the right-hand side refers to the current context of the query, which is the read-only input document when querying from the root of a mapping:

root.id = this.thing.id
root.type = "yo"

# Both `root` and `this` are optional, and will be inferred in their absence.
content = thing.doc.message

# In:  {"thing":{"id":"wat1","doc":{"title":"wut","message":"hello world"}}}
# Out: {"content":"hello world","id":"wat1","type":"yo"}

Since the document being created starts off empty it is sometimes useful to begin a mapping by copying the entire contents of the input document, which can be expressed by assigning this to root.

root = this
root.foo = "added value"

# In:  {"id":"wat1","message":"hello world"}
# Out: {"id":"wat1","message":"hello world","foo":"added value"}

If the new document root is never assigned to or otherwise mutated then the original document remains unchanged.

Special Characters in Paths​

Quotes can be used to describe sections of a field path that contain whitespace, dots or other special characters:

# Use quotes around a path segment in order to include whitespace or dots within
# the path
root."foo.bar".baz = this."buz bev".fub

# In:  {"buz bev":{"fub":"hello world"}}
# Out: {"foo.bar":{"baz":"hello world"}}

Non-structured Data​

Bloblang is able to map data that is unstructured, whether it's a log line or a binary blob, by referencing it with the content function, which returns the raw bytes of the input document:

# Parse a base64 encoded JSON document
root = content().decode("base64").parse_json()

# In:  eyJmb28iOiJiYXIifQ==
# Out: {"foo":"bar"}

And your newly mapped document can also be unstructured, simply assign a value type to the root of your document:

root = this.foo

# In:  {"foo":"hello world"}
# Out: hello world

And the resulting message payload will be the raw value you've assigned.

Deleting​

It's possible to selectively delete fields from an object by assigning the function deleted() to the field path:

root = this
root.bar = deleted()

# In:  {"id":"wat1","message":"hello world","bar":"remove me"}
# Out: {"id":"wat1","message":"hello world"}

Variables​

Another type of assignment is a let statement, which creates a variable that can be referenced elsewhere within a mapping. Variables are discarded at the end of the mapping and are mostly useful for query reuse. Variables are referenced within queries with $:

# Set a temporary variable
let foo = "yo"

root.new_doc.type = $foo

Metadata​

Benthos messages contain metadata that is separate from the main payload, in Bloblang you can modify the metadata of the resulting message with the meta assignment keyword, and you can query the metadata of the input message with the meta function:

# Delete all existing metadata
meta = deleted()

# Set a metadata value
meta bar = "hello world"

# Reference a metadata value from the input message
root.new_doc.bar = meta("kafka_topic")

The meta function returns the read-only metadata of the input message, so it will not reflect changes you've made within the same mapping. This is why it's possible to begin a mapping by removing all old metadata meta = deleted() and still be able to query the original metadata.

If you wish to set a metadata value and then refer back to it later then first set it as a variable.

Coalesce​

The pipe operator (|) used within brackets allows you to coalesce multiple candidates for a path segment. The first field that exists and has a non-null value will be selected:

root.new_doc.type = this.thing.(article | comment | this).type

# In:  {"thing":{"article":{"type":"foo"}}}
# Out: {"new_doc":{"type":"foo"}}

# In:  {"thing":{"comment":{"type":"bar"}}}
# Out: {"new_doc":{"type":"bar"}}

# In:  {"thing":{"type":"baz"}}
# Out: {"new_doc":{"type":"baz"}}

Opening brackets on a field begins a query where the context of this changes to value of the path it is opened upon, therefore in the above example this within the brackets refers to the contents of this.thing.

Literals​

Bloblang supports number, boolean, string, null, array and object literals:

root = [
  7, false, "string", null, {
    "first": 11,
    "second": {"foo":"bar"},
    "third": """multiple
lines on this
string"""
  }
]

# In:  {}
# Out: [7,false,"string",null,{"first":11,"second":{"foo":"bar"},"third":"multiple\nlines on this\nstring"}]

The values within literal arrays and objects can be dynamic query expressions, as well as the keys of object literals.

Comments​

You might've already spotted, comments are started with a hash (#) and end with a line break:

root = this.some.value # And now this is a comment

Boolean Logic and Arithmetic​

Bloblang supports a range of boolean operators !, >, >=, ==, <, <=, &&, || and mathematical operators +, -, *, /, %:

root.is_big = this.number > 100
root.multiplied = this.number * 7

# In:  {"number":50}
# Out: {"is_big":false,"multiplied":350}

# In:  {"number":150}
# Out: {"is_big":true,"multiplied":1050}

For more information about these operators and how they work check out the arithmetic page.

Conditional Mapping​

Use if expressions to perform maps conditionally:

root = this
root.sorted_foo = if this.foo.type() == "array" { this.foo.sort() }

# In:  {"foo":"foobar"}
# Out: {"foo":"foobar"}

# In:  {"foo":["foo","bar"]}
# Out: {"foo":["foo","bar"],"sorted_foo":["bar","foo"]}

And add as many if else queries as you like, followed by an optional final fallback else:

root.sound = if this.type == "cat" {
  this.cat.meow
} else if this.type == "dog" {
  this.dog.woof.uppercase()
} else {
  "sweet sweet silence"
}

# In:  {"type":"cat","cat":{"meow":"meeeeooooow!"}}
# Out: {"sound":"meeeeooooow!"}

# In:  {"type":"dog","dog":{"woof":"guurrrr woof woof!"}}
# Out: {"sound":"GUURRRR WOOF WOOF!"}

# In:  {"type":"caterpillar","caterpillar":{"name":"oleg"}}
# Out: {"sound":"sweet sweet silence"}

Pattern Matching​

A match expression allows you to perform conditional mappings on a value, each case should be either a boolean expression, a literal value to compare against the target value, or an underscore (_) which captures values that have not matched a prior case:

root.new_doc = match this.doc {
  this.type == "article" => this.article
  this.type == "comment" => this.comment
  _ => this
}

# In:  {"doc":{"type":"article","article":{"id":"foo","content":"qux"}}}
# Out: {"new_doc":{"id":"foo","content":"qux"}}

# In:  {"doc":{"type":"comment","comment":{"id":"bar","content":"quz"}}}
# Out: {"new_doc":{"id":"bar","content":"quz"}}

# In:  {"doc":{"type":"neither","content":"some other stuff unchanged"}}
# Out: {"new_doc":{"type":"neither","content":"some other stuff unchanged"}}

Within a match block the context of this changes to the pattern matched expression, therefore this within the match expression above refers to this.doc.

Match cases can specify a literal value for simple comparison:

root = this
root.type = match this.type { "doc" => "document", "art" => "article", _ => this }

# In:  {"type":"doc","foo":"bar"}
# Out: {"type":"document","foo":"bar"}

The match expression can also be left unset which means the context remains unchanged, and the catch-all case can also be omitted:

root.new_doc = match {
  this.doc.type == "article" => this.doc.article
  this.doc.type == "comment" => this.doc.comment
}

# In:  {"doc":{"type":"neither","content":"some other stuff unchanged"}}
# Out: {"doc":{"type":"neither","content":"some other stuff unchanged"}}

If no case matches then the mapping is skipped entirely, hence we would end up with the original document in this case.

Functions​

Functions can be placed anywhere and allow you to extract information from your environment, generate values, or access data from the underlying message being mapped:

root.doc.id = uuid_v4()
root.doc.received_at = now()
root.doc.host = hostname()

Functions support both named and nameless style arguments:

root.values_one = range(start: 0, stop: this.max, step: 2)
root.values_two = range(0, this.max, 2)

You can find a full list of functions and their parameters in the functions page.

Methods​

Methods are similar to functions but enact upon a target value, these provide most of the power in Bloblang as they allow you to augment query values and can be added to any expression (including other methods):

root.doc.id = this.thing.id.string().catch(uuid_v4())
root.doc.reduced_nums = this.thing.nums.map_each(num -> if num < 10 {
  deleted()
} else {
  num - 10
})
root.has_good_taste = ["pikachu","mewtwo","magmar"].contains(this.user.fav_pokemon)

Methods also support both named and nameless style arguments:

root.foo_one = this.(bar | baz).trim().replace_all(old: "dog", new: "cat")
root.foo_two = this.(bar | baz).trim().replace_all("dog", "cat")

You can find a full list of methods and their parameters in the methods page.

Maps​

Defining named maps allows you to reuse common mappings on values with the apply method:

map things {
  root.first  = this.thing_one
  root.second = this.thing_two
}

root.foo = this.value_one.apply("things")
root.bar = this.value_two.apply("things")

# In:  {"value_one":{"thing_one":"hey","thing_two":"yo"},"value_two":{"thing_one":"sup","thing_two":"waddup"}}
# Out: {"foo":{"first":"hey","second":"yo"},"bar":{"first":"sup","second":"waddup"}}

Within a map the keyword root refers to a newly created document that will replace the target of the map, and this refers to the original value of the target. The argument of apply is a string, which allows you to dynamically resolve the mapping to apply.

Import Maps​

It's possible to import maps defined in a file with an import statement:

import "./common_maps.blobl"

root.foo = this.value_one.apply("things")
root.bar = this.value_two.apply("things")

Imports from a Bloblang mapping within a Benthos config are relative to the process running the config. Imports from an imported file are relative to the file that is importing it.

Filtering​

By assigning the root of a mapped document to the deleted() function you can delete a message entirely:

# Filter all messages that have fewer than 10 URLs.
root = if this.doc.urls.length() < 10 { deleted() }

Error Handling​

Functions and methods can fail under certain circumstances, such as when they receive types they aren't able to act upon. These failures, when not caught, will cause the entire mapping to fail. However, the method catch can be used in order to return a value when a failure occurs instead:

# Map an empty array to `foo` if the field `bar` is not a string.
root.foo = this.bar.split(",").catch([])

Since catch is a method it can also be attached to bracketed map expressions:

# Map `false` if any of the operations in this boolean query fail.
root.thing = ( this.foo > this.bar && this.baz.contains("wut") ).catch(false)

And one of the more powerful features of Bloblang is that a single catch method at the end of a chain of methods can recover errors from any method in the chain:

# Catch errors caused by:
# - foo not existing
# - foo not being a string
# - an element from split foo not being a valid JSON string
root.things = this.foo.split(",").map_each( ele -> ele.parse_json() ).catch([])

# Specifically catch a JSON parse error
root.things = this.foo.split(",").map_each( ele -> ele.parse_json().catch({}) )

However, the catch method only acts on errors, sometimes it's also useful to set a fall back value when a query returns null in which case the method or can be used the same way:

# Map "default" if either the element index 5 does not exist, or the underlying
# element is `null`.
root.foo = this.bar.index(5).or("default")

Unit Testing​

It's possible to execute unit tests for your Bloblang mappings using the standard Benthos unit test capabilities outlined in this document.

Trouble Shooting​

  1. I'm seeing unable to reference message as structured (with 'this') when I try to run mappings with benthos blobl.

That particular error message means the mapping is failing to parse what's being fed in as a JSON document. Make sure that the data you are feeding in is valid JSON, and also that the documents do not contain line breaks as benthos blobl will parse each line individually.

Why? That's a good question. Bloblang supports non-JSON formats too, so it can't delimit documents with a streaming JSON parser like tools such as jq, so instead it uses line breaks to determine the boundaries of each message.