Hyperledger Composer中能否创建只读事务？新手实操问询

Hey there! I totally get where you're coming from as a Hyperledger Composer newbie—this was a confusing spot for me too when I started out. It's easy to think transactions are only for modifying data, but you can absolutely build read-only transactions that fetch, process, and return custom results. Let me walk you through exactly how to do this.

Step 1: Define Your Assets, Result Concept, and Transaction in the CTO File #

First, you need to formalize your asset types, a custom concept to hold your processed results, and the read-only transaction itself in your model file (.cto):

namespace org.example.myapp

// Example Asset Type 1
asset AssetA identified by assetAId {
  o String assetAId
  o Integer value
}

// Example Asset Type 2
asset AssetB identified by assetBId {
  o String assetBId
  o Integer quantity
}

// A custom concept to structure your processed output
concept ProcessedResult {
  o Integer totalCalculatedValue
  o String summaryMessage
  o AssetA[] matchingAssetsA
  o AssetB[] matchingAssetsB
}

// The read-only transaction (add parameters here if you need filters)
transaction FetchAndProcessAssets {
  // Optional: Add filter parameters like o String assetCategory if needed
}

Step 2: Write the Transaction Processor Logic #

Next, create a JavaScript transaction processor that fetches both asset types, runs your custom calculations, and returns the processed result. This is where all the heavy lifting happens:

/**
 * Fetch, process, and return combined data from AssetA and AssetB
 * @param {org.example.myapp.FetchAndProcessAssets} tx
 * @transaction
 */
async function fetchAndProcessAssets(tx) {
  // Fetch all AssetA records (add a WHERE clause to buildQuery if you need filtering)
  const assetAQuery = buildQuery('SELECT org.example.myapp.AssetA');
  const assetARecords = await query(assetAQuery);
  
  // Fetch all AssetB records
  const assetBQuery = buildQuery('SELECT org.example.myapp.AssetB');
  const assetBRecords = await query(assetBQuery);
  
  // Run your custom math/processing logic here
  let totalValue = 0;
  assetARecords.forEach(asset => {
    totalValue += asset.value; // Sum values from AssetA
  });
  assetBRecords.forEach(asset => {
    totalValue += asset.quantity * 15; // Example: Multiply AssetB quantity by 15 and add to total
  });
  
  // Build your custom result object
  const factory = getFactory();
  const processedResult = factory.newConcept('org.example.myapp', 'ProcessedResult');
  
  processedResult.totalCalculatedValue = totalValue;
  processedResult.summaryMessage = `Processed ${assetARecords.length} AssetA and ${assetBRecords.length} AssetB records. Total calculated value: ${totalValue}`;
  processedResult.matchingAssetsA = assetARecords;
  processedResult.matchingAssetsB = assetBRecords;
  
  // Return the custom result to the caller
  return processedResult;
}

Step 3: Call the Transaction and Get Your Result #

Once you've deployed your updated business network, you can call this transaction just like any other:

• Via Composer REST API: Send a POST request to the /transactions endpoint with the transaction type org.example.myapp.FetchAndProcessAssets (and any parameters if you added them). The response will be your ProcessedResult object.
• Via CLI: Use composer transaction submit -n your-network-name -c admin@your-network-name -d '{"$class":"org.example.myapp.FetchAndProcessAssets"}'—the output will show your processed result.

Why This Works (And Why Queries Aren't Enough) #

You're right that the built-in query system (queries.qry) is limited to fetching data without custom processing. Transactions, however, are designed to run arbitrary logic—including read-only operations. The key here is defining a custom Concept to shape your return data, since Composer needs a formal structure to serialize the response back to the caller.

内容的提问来源于stack exchange，提问作者parth.patil