Hyperledger Composer REST服务报错及概念默认值不显示问题咨询

Hey there! Let's tackle your two connected issues one by one—these are pretty common hurdles when working with asset concepts and the Composer REST server, so let's break down possible causes and fixes:

1. REST Server Error When Starting rest-composer-server #

First, let's nail down why the server won't launch. Here are the most likely culprits:

• Invalid Asset/Concept Definition Syntax
  Even a tiny syntax mistake in your AirPollution asset or CarbonMonoxide concept can crash the server. Run the validation command first to catch issues:

  composer validate
  

  Double-check your concept's default value definitions—make sure you're using the correct default keyword (not a typo like defaults), and that the value matches the property type (e.g., string defaults are wrapped in quotes in YAML/JSON).

• Dependency Version Mismatch
  Outdated or incompatible versions of composer-rest-server or related packages can cause startup failures. Try updating the server globally:

  npm install -g composer-rest-server
  

  Also verify that your composer-cli version matches the rest server version with npm list -g composer-cli composer-rest-server.

• Database/Connection String Issues
  If your server relies on a database (like MongoDB) that's not running, or if your connection string is incorrect, it'll fail to start. Confirm:

  • The database service is active (e.g., sudo systemctl start mongod for MongoDB)
  • Your startup command uses the correct business network card, like:

    composer-rest-server -c admin@your-air-pollution-network
    

  Ensure the admin identity exists and the network name matches exactly what you deployed.

2. Missing Concept Default Values in Your Angular App #

If the server starts (or you've worked around the first issue) but your Angular app doesn't show pollutant_description/units defaults, here's what to check:

• REST Server Isn't Exposing Default Values
  By default, the composer REST server might not return default values unless explicitly configured. When starting the server, add the --expose flag to ensure all properties are included in responses:

  composer-rest-server -c admin@your-air-pollution-network --expose
  

  Test this first with a tool like Postman: send a POST request to create an AirPollution asset without providing the CarbonMonoxide fields, then check if the response includes your default values.

• Angular Data Binding Path Errors
  If the API is returning the defaults but your app isn't showing them, double-check your template bindings. For example, if your asset has a CarbonMonoxide property, you need to reference it directly in your template:

  <!-- Correct -->
  <p>Description: {{ asset.CarbonMonoxide.pollutant_description }}</p>
  <!-- Incorrect (will be undefined) -->
  <p>Description: {{ asset.pollutant_description }}</p>
  

  Also, in your component code, make sure you're properly accessing the nested CarbonMonoxide object after fetching the asset data.

• Default Values Aren't Properly Defined in the Concept
  Double-check your concept definition to ensure defaults are set correctly. A valid YAML example looks like this:

  concepts:
    org.example.AirPollution.CarbonMonoxide:
      properties:
        pollutant_description:
          type: String
          default: "Colorless, odorless toxic gas"
        units:
          type: String
          default: "ppm"
  

  Ensure indentation is correct (YAML is strict about this) and that the default key is directly under the property it applies to.

Quick Next Steps #

1. Run composer validate to fix any syntax issues in your asset/concept definitions
2. Update your composer-rest-server and verify dependencies
3. Test the REST API directly with Postman to confirm default values are returned
4. Audit your Angular template bindings and component data handling

内容的提问来源于stack exchange，提问作者kerberos17