Asyncapi-runtime-expression NPM

asyncapi-runtime-expression

Runtime Expressions A runtime expression allows values to be defined based on information that will be available within the message. This mechanism is used by Correlation ID Object of AsyncAPI specification.

asyncapi-runtime-expression is a parser and validator for AsyncAPI Runtime Expressions. It supports Runtime Expressions defined in following AsyncAPI specification versions:

Getting started

Installation

You can install asyncapi-runtime-expression using npm:

 $ npm install asyncapi-runtime-expression

Given that asyncapi-runtime-expression is a pure ESM package you can also install it directly from GitHub.

 $ npm install github:char0n/asyncapi-runtime-expression

Usage

asyncapi-runtime-expression currently supports parsing and validation. Both parser and validator are based on a superset of ABNF (SABNF) and use apg-js parser generator.

Parsing

Parsing a Runtime Expression is as simple as importing the parse function and calling it.

import { parse } from 'asyncapi-runtime-expression';

const parseResult = parse('$message.payload#/user/id');

parseResult variable has the following shape:

{
  result: {
    success: true,
    state: 101,
    length: 22,
    matched: 22,
    maxMatched: 22,
    maxTreeDepth: 14,
    nodeHits: 152,
    inputLength: 22,
    subBegin: 0,
    subEnd: 22,
    subLength: 22
  },
  ast: exportsAst {
    callbacks: [
      expression: [Function: expression],
      source: [Function: source],
      'payload-reference': [Function: payloadReference],
      fragment': [Function: fragment],
      'reference-token': [Function: referenceToken],
    ],
    astObject: 'astObject',
    init: [Function: init],
    ruleDefined: [Function: ruleDefined],
    udtDefined: [Function: udtDefined],
    down: [Function: down],
    up: [Function: up],
    translate: [Function: translate],
    setLength: [Function: setLength],
    getLength: [Function: getLength],
    toXml: [Function: toSml],
    phrases: [Function: phrases]
  }
}

Interpreting AST as list of entries

import { parse } from 'asyncapi-runtime-expression';

const parseResult = parse('$message.payload#/user/id');
const parts = [];

parseResult.ast.translate(parts);

After running the above code, parts variable has the following shape:

[
  ['expression', '$message.payload#/user/id'],
  ['source', 'payload#/user/id'],
  ['payload-reference', 'payload#/user/id'],
  ['fragment', '/user/id'],
  ['reference-token', 'user'],
  ['reference-token', 'id']
]

Interpreting AST as XML

import { parse } from 'asyncapi-runtime-expression';

const parseResult = parse('$message.payload#/user/id');
const xml = parseResult.ast.toXml();

After running the above code, xml variable has the following content:

<?xml version="1.0" encoding="utf-8"?>
<root nodes="6" characters="25">
  <!-- input string, decimal integer character codes -->
  36,109,101,115,115,97,103,101,46,112,97,121,108,111,97,100,35,47,117,115,101,114,47,105,100
  <node name="expression" index="0" length="25">
    36,109,101,115,115,97,103,101,46,112,97,121,108,111,97,100,35,47,117,115,101,114,47,105,100
    <node name="source" index="9" length="16">
      112,97,121,108,111,97,100,35,47,117,115,101,114,47,105,100
      <node name="payload-reference" index="9" length="16">
        112,97,121,108,111,97,100,35,47,117,115,101,114,47,105,100
        <node name="fragment" index="17" length="8">
          47,117,115,101,114,47,105,100
          <node name="reference-token" index="18" length="4">
            117,115,101,114
          </node><!-- name="reference-token" -->
          <node name="reference-token" index="23" length="2">
            105,100
          </node><!-- name="reference-token" -->
        </node><!-- name="fragment" -->
      </node><!-- name="payload-reference" -->
    </node><!-- name="source" -->
  </node><!-- name="expression" -->
</root>

NOTE: AST can also be traversed in classical way using depth first traversal. For more information about this option please refer to apg-js and apg-js-examples.

Validation

Validating a Runtime Expression is as simple as importing the test function and calling it.

import { test } from 'asyncapi-runtime-expression';

test('$message.payload#/user/id'); // => true
test('nonsensical string'); // => false

Grammar

New grammar instance can be created in following way:

import { Grammar } from 'asyncapi-runtime-expression';

const grammar = new Grammar();

To obtain original ABNF (SABNF) grammar as a string:

import { Grammar } from 'asyncapi-runtime-expression';

const grammar = new Grammar();

grammar.toString();
// or
String(grammar);

More about AsyncAPI runtime expressions

The runtime expression is defined by the following ABNF syntax

expression = ( "$message" "." source )
source = ( header-reference | payload-reference )
header-reference = "header" ["#" fragment]
payload-reference = "payload" ["#" fragment]
fragment = a JSON Pointer [RFC 6901](https://tools.ietf.org/html/rfc6901)

The table below provides examples of runtime expressions and examples of their use in a value:

Examples

Source Location	Example expression	Notes
Message Header Property	`$message.header#/MQMD/CorrelId`	Correlation ID is set using the `CorrelId` value from the `MQMD` header.
Message Payload Property	`$message.payload#/messageId`	Correlation ID is set using the `messageId` value from the message payload.

Runtime expressions preserve the type of the referenced value.

License

asyncapi-runtime-expression is licensed under Apache 2.0 license. asyncapi-runtime-expression comes with an explicit NOTICE file containing additional legal notices and information.