Bloblang, or blobl for short (pronounced "blobble"), is a language designed specifically for mapping unstructured data. It's a safe, fast, and powerful way to perform document mapping within Benthos.

Bloblang is available as a mapping processor, as a condition, 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'


An assignment consists of a left-hand-side assignment target and a right-hand-side mapping query.

Map a new document by using dot paths: =
new_doc.type = "yo"
# Keyword `root` refers to the root of the mapped document
# Keyword `this` refers to the root of the query context
# Both are optional
root.new_doc.content = this.thing.doc.message
# In: {"thing":{"id":"wat1","doc":{"title":"wut","message":"hello world"}}}
# Out: {"new_doc":{"content":"hello world","id":"wat1","type":"yo"}}

The newly mapped document starts off empty, if you wish to begin your mapping with a full copy of the input object then start your map by assigning it to the root:

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

Variables help with value reuse:

# Set a temporary variable
let foo = "yo"
new_doc.type = $foo # Reference a variable with $

You can also use Bloblang to set and reference metadata values:

# 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")


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

new_doc.type = 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"}}

Literals and Arithmetic

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 = some.value # And now this is a comment

Boolean Logic and Arithmetic

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

is_big = number > 100
multiplied = 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
sorted_foo = if foo.type() == "array" { foo.sort() }
# 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:

foo = 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:

new_doc = match doc {
type == "article" => article
type == "comment" => 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"}}

Match cases can specify a literal value for simple comparison:

root = this
type = match type { "doc" => "document", "art" => "article", _ => this }
# In: {"type":"doc","foo":"bar"}
# Out: {"type":"document","foo":"bar"}

The match context can also be left unset, and the catch-all case can also be omitted:

new_doc = match {
doc.type == "article" => doc.article
doc.type == "comment" => 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()
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 values by chaining them: =
doc.reduced_nums = thing.nums.for_each(
match this {
this < 10 => deleted()
_ => this - 10

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


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

map things {
first = thing_one
second = thing_two
foo = value_one.apply("things")
bar = 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"}}

And even import maps from other files:

import "./common_maps.blobl"
foo = value_one.apply("things")
bar = value_two.apply("things")


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 {
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.
foo = 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.
thing = ( foo > bar && baz.contains("wut") ).catch(false)

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`.
foo = bar.index(5).or("default")