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.
You can also execute Bloblang mappings on the command-line with the
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.
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:
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
Your newly mapped document doesn't need to be a structured object, simply assign a value type to the
root of your document:
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:
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
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:
Quotes can be used to describe sections of a field path that contain whitespace, dots or other special characters:
The pipe operator (
|) used within brackets allows you to coalesce values within a path:
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
Bloblang supports number, boolean, string, null, array and object literals:
You might've already spotted, comments are started with a hash (
#) and end with a line break:
Bloblang supports a range of boolean operators
|| and arithmetic operators
if expressions to perform maps conditionally:
And add an
else for alternative maps:
match expression allows you to perform conditional mappings on a value using boolean logic:
Within a match block the context of
this changes to the pattern matched expression, therefore
this within the match expression above refers to
Match cases can specify a literal value for simple comparison:
The match expression can also be left unset which means the context remains unchanged, and the catch-all case can also be omitted:
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:
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:
You can find a full list of methods in this doc.
It's possible to declare reusable maps for common operations:
Within a map the keyword
root refers to a newly created document, and
this refers to whatever the map is applied to.
It's possible to import maps defined in a file with an
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:
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:
catch is a method it can also be attached to bracketed map expressions:
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 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: