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.

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'


A Bloblang mapping expresses how to create a new document by extracting data from an existing input document. Assignments consist of a dot path argument 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.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 = "added value"
# In: {"id":"wat1","message":"hello world"}
# Out: {"id":"wat1","message":"hello world","foo":"added value"}

Non-structured Results

Your newly mapped document doesn't need to be a structured object, simply assign a value type to the root of your document:

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

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


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

root = this = deleted()
# In: {"id":"wat1","message":"hello world","bar":"remove me"}
# Out: {"id":"wat1","message":"hello world"}


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


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

# Delete all existing metadata
meta = deleted()
# Set a metadata value
meta bar = "hello world"
# Reference a metadata value from the input message = meta("kafka_topic")

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."".baz = this."buz bev".fub
# In: {"buz bev":{"fub":"hello world"}}
# Out: {"":{"baz":"hello world"}}


The pipe operator (|) used within brackets allows you to coalesce values within a path:

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"}}

This is a called a bracketed mapping and within it the context changes to the path value it is added to, therefore in the above example this within the brackets refers to the contents of this.thing.


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


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 arithmetic 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}

Conditional Mapping

Use if expressions to perform maps conditionally:

root = this
root.sorted_foo = if == "array" { }
# In: {"foo":"foobar"}
# Out: {"foo":"foobar"}
# In: {"foo":["foo","bar"]}
# Out: {"foo":["foo","bar"],"sorted_foo":["bar","foo"]}

And add an else for alternative maps: = if this.exists("foo") {
} else {
# In: {"foo":"foobar"}
# Out: {"foo":"foobar"}
# In: {}
# Out: {"foo":"default"}

Pattern Matching

A match expression allows you to perform conditional mappings on a value using boolean logic:

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 can be placed anywhere and allow you to extract information from your environment, generate values, or access data from the underlying message being mapped: = uuid_v4()
root.doc.received_at = timestamp_unix() = hostname()

You can find a full list of functions in this doc.


Methods provide most of the power in Bloblang as they allow you to augment query values and can be added to any expression: =
root.doc.reduced_nums = this.thing.nums.for_each(
match this {
this < 10 => deleted()
_ => this - 10
root.has_good_taste = ["pikachu","mewtwo","magmar"].contains(this.user.fav_pokemon)

You can find a full list of methods in this doc.


It's possible to declare reusable maps for common operations:

map things {
root.first = this.thing_one
root.second = this.thing_two
} = this.value_one.apply("things") = 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, and this refers to whatever the map is applied to.

Import Maps

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

import "./common_maps.blobl" = this.value_one.apply("things") = 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.


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 = match {
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. =",").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.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 =",").map_each( this.parse_json() ).catch([])
# Specifically catch a JSON parse error
root.things =",").map_each( this.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`. ="default")