Hyperledger Composer事务方法能否返回值？替代方案咨询

Great question! Let's break this down clearly for you:

First off, straight to the point: No, you can’t directly return a value from a Hyperledger Composer transaction function like the example you shared. Here’s why:

Hyperledger Composer transactions are built to modify the blockchain ledger (or trigger related logic) in a consistent, auditable way. When you submit a transaction, it’s processed asynchronously by blockchain peers, and any return value from the transaction function won’t be sent back to the initiating client. Your sample code:

/** 
 * @param {org.n.blockchaindemo.GetCreditScoreUser} GetCreditScoreUser - the GetCreditScoreUser transaction 
 * @transaction 
 */ 
async function getCreditScoreUser(user) { 
  return 0; 
}

That return 0 will essentially be ignored during blockchain processing—it never reaches the client that sent the transaction.

So What Are the Reliable Alternatives? #

You have two solid options to get your transaction’s result to the client:

1. Store the Result in an Asset or Participant Property #

This is the most common approach. You can update an existing asset or participant’s attribute with your calculated result, then have the client query that entity after the transaction commits.

Here’s how to implement it:

1. First, add a field for the result to your participant (or asset) definition in your CTO model:

   participant User identified by userId {
     o String userId
     o Integer creditScore optional
     // Add other existing fields here
   }
   

2. Modify your transaction function to calculate and save the result:

   /** 
    * @param {org.n.blockchaindemo.GetCreditScoreUser} tx - the transaction 
    * @transaction 
    */ 
   async function getCreditScoreUser(tx) {
     // Replace this with your actual credit score calculation logic
     const calculatedScore = 780;
     
     // Fetch the user participant from the registry
     const userRegistry = await getParticipantRegistry('org.n.blockchaindemo.User');
     const targetUser = await userRegistry.get(tx.userId); // Assume your transaction has a userId field
     
     // Update the user's credit score property
     targetUser.creditScore = calculatedScore;
     
     // Save the updated user back to the registry
     await userRegistry.update(targetUser);
   }
   

3. After the transaction is confirmed, the client can query the User participant to retrieve the creditScore value.

2. Emit an Event with the Result #

Hyperledger Composer lets you define custom events that can be triggered from transaction functions. Clients can subscribe to these events to get real-time updates when the transaction finishes processing.

Here’s the workflow:

1. Define an event in your CTO model:

   event CreditScoreCalculated {
     o String userId
     o Integer creditScore
   }
   

2. Modify your transaction function to emit this event with the result:

   /** 
    * @param {org.n.blockchaindemo.GetCreditScoreUser} tx - the transaction 
    * @transaction 
    */ 
   async function getCreditScoreUser(tx) {
     // Calculate your credit score here
     const calculatedScore = 780;
     
     // Create and emit the event
     const scoreEvent = getFactory().newEvent('org.n.blockchaindemo', 'CreditScoreCalculated');
     scoreEvent.userId = tx.userId;
     scoreEvent.creditScore = calculatedScore;
     emit(scoreEvent);
   }
   

3. On the client side, use the Composer client API to subscribe to this event. When the transaction is processed, the client will receive the event with the creditScore value in real time.

Both methods work well: the first is perfect if you need to persist the result on the ledger long-term, while the second is ideal for real-time notifications without permanent ledger storage.

内容的提问来源于stack exchange，提问作者AshT