off_chain_data

Off-chain data store sample

The off-chain data store sample demonstrates:

• Receiving block events in a client application.
• Using a checkpointer to resume event listening after a failure or application restart.
• Extracting ledger updates from block events in order to build an off-chain data store.

About the sample

This sample shows how to replicate the data in your blockchain network to an off-chain data store. Using an off-chain data store allows you to analyze the data from your network or build a dashboard without degrading the performance of your application.

This sample uses the block event listening capability of the Fabric Gateway client API for Fabric v2.4 and later.

Application

The client application provides several "commands" that can be invoked using the command-line:

• getAllAssets: Retrieve the current details of all assets recorded on the ledger. See application-typescript/src/getAllAssets.ts.
• listen: Listen for block events, and use them to replicate ledger updates in an off-chain data store. See application-typescript/src/listen.ts.
• transact: Submit a set of transactions to create, modify and delete assets. See application-typescript/src/transact.ts.

To keep the sample code concise, the listen command writes ledger updates to an output file named store.log in the current working directory. A real implementation could write ledger updates directly to an off-chain data store of choice. You can inspect the information captured in this file as you run the sample.

Note that the listen command is is restartable and will resume event listening after the last successfully processed block / transaction. This is achieved using a checkpointer to persist the current listening position. Checkpoint state is persisted to a file named checkpoint.json in the current working directory. If no checkpoint state is present, event listening begins from the start of the ledger (block number zero).

Smart Contract

The asset-transfer-basic smart contract is used to generate transactions and associated ledger updates.

Running the sample

The Fabric test network is used to deploy and run this sample. Follow these steps in order:

1. Create the test network and a channel (from the test-network folder).

   ./network.sh up createChannel -c mychannel -ca
   

2. Deploy one of the asset-transfer-basic smart contract implementations (from the test-network folder).

   # To deploy the TypeScript chaincode implementation
   ./network.sh deployCC -ccn basic -ccp ../asset-transfer-basic/chaincode-typescript/ -ccl typescript
   
   # To deploy the Go chaincode implementation
   ./network.sh deployCC -ccn basic -ccp ../asset-transfer-basic/chaincode-go/ -ccl go
   
   # To deploy the Java chaincode implementation
   ./network.sh deployCC -ccn basic -ccp ../asset-transfer-basic/chaincode-java/ -ccl java
   

3. Populate the ledger with some assets and use eventing to capture ledger updates (from the off_chain_data folder).

   # To run the TypeScript sample application
   cd application-typescript
   npm install
   npm start transact listen
   

4. Interrupt the listener process using Control-C.

5. View the current world state of the blockchain (from the off_chain_data folder). You may want to compare the results to the ledger updates captured by the listener in the store.log file.

   # To run the TypeScript sample application
   cd application-typescript
   npm --silent start getAllAssets
   

6. Make some more ledger updates, then observe listener resume capability (from the off_chain_data folder). Note from the transaction IDs recorded to the console that the listener resumes from exactly after the last successfully processed transaction.

   # To run the TypeScript sample application
   cd application-typescript
   npm start transact
   SIMULATED_FAILURE_COUNT=5 npm start listen
   npm start listen
   

7. Interrupt the listener process using Control-C.

Clean up

The persisted event checkpoint position can be removed by deleting the checkpoint.json file while the listener is stopped.

The recorded ledger updates can be removed by deleting the store.log file.

When you are finished, you can bring down the test network (from the test-network folder). The command will remove all the nodes of the test network, and delete any ledger data that you created. Be sure to remove the checkpoint.json and store.log files before attempting to run the application with a new network.

./network.sh down