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Off Chain data

This sample demonstrates how you can use Peer channel-based event services to replicate the data on your blockchain network to an off chain database. Using an off chain database allows you to analyze the data from your network or build a dashboard without degrading the performance of your application.

This sample uses the Fabric network event listener from the Node.JS Fabric SDK to write data to local instance of CouchDB.

Getting started

This sample uses Node Fabric SDK application code to connect to a running instance of the Fabric test network. Make sure that you are running the following commands from the off_chain_data directory.

Starting the Network

Use the following command to start the sample network:

./startFabric.sh

This command will deploy an instance of the Fabric test network. The network consists of an ordering service, two peer organizations with one peers each, and a CA for each org. The command also creates a channel named mychannel. The asset-transfer-basic chaincode will be installed on both peers and deployed to the channel.

Configuration

The configuration for the listener is stored in the config.json file:

{
    "peer_name": "peer0.org1.example.com",
    "channelid": "mychannel",
    "use_couchdb":true,
    "create_history_log":true,
    "couchdb_address": "http://localhost:5990"
}

peer_name: is the target peer for the listener. channelid: is the channel name for block events. use_couchdb: If set to true, events will be stored in a local instance of CouchDB. If set to false, only a local log of events will be stored. create_history_log: If true, a local log file will be created with all of the block changes. couchdb_address: is the local address for an off chain CouchDB database.

Create an instance of CouchDB

If you set the "use_couchdb" option to true in config.json, you can run the following command start a local instance of CouchDB using docker:

docker run --publish 5990:5984 --detach --name offchaindb couchdb:2.3.1
docker start offchaindb

Install dependencies

You need to install Node.js version 8.9.x to use the sample application code. Execute the following commands to install the required dependencies:

npm install

Starting the Channel Event Listener

After we have installed the application dependencies, we can use the Node.js SDK to create the identity our listener application will use to interact with the network. Run the following command to enroll the admin user:

node enrollAdmin.js

You can then run the following command to register and enroll an application user:

node registerUser.js

We can then use our application user to start the block event listener:

node blockEventListener.js

If the command is successful, you should see the output of the listener reading the configuration blocks of mychannel in addition to the blocks that recorded the approval and commitment of the assets chaincode definition.

blockEventListener.js creates a listener named "offchain-listener" on the channel mychannel. The listener writes each block added to the channel to a processing map called BlockMap for temporary storage and ordering purposes. blockEventListener.js uses nextblock.txt to keep track of the latest block that was retrieved by the listener. The block number in nextblock.txt may be set to a previous block number in order to replay previous blocks. The file may also be deleted and all blocks will be replayed when the block listener is started.

BlockProcessing.js runs as a daemon and pulls each block in order from the BlockMap. It then uses the read-write set of that block to extract the latest key value data and store it in the database. The configuration blocks of mychannel did not any data to the database because the blocks did not contain a read-write set.

The channel event listener also writes metadata from each block to a log file defined as channelid_chaincodeid.log. In this example, events will be written to a file named mychannel_basic.log. This allows you to record a history of changes made by each block for each key in addition to storing the latest value of the world state.

Note: Leave the blockEventListener.js running in a terminal window. Open a new window to execute the next parts of the demo.

Generate data on the blockchain

Now that our listener is setup, we can generate data using the assets chaincode and use our application to replicate the data to our database. Open a new terminal and navigate to the fabric-samples/off_chain_data directory.

You can use the addAssets.js file to add random sample data to blockchain. The file uses the configuration information stored in addAssets.json to create a series of assets. This file will be created during the first execution of addAssets.js if it does not exist. This program can be run multiple times without changing the properties. The nextAssetNumber will be incremented and stored in the addAssets.json file.

    {
        "nextAssetNumber": 100,
        "numberAssetsToAdd": 20
    }

Open a new window and run the following command to add 20 assets to the blockchain:

node addAssets.js

After the assets have been added to the ledger, use the following command to transfer one of the assets to a new owner:

node transferAsset.js asset110 james

Now run the following command to delete the asset that was transferred:

node deleteAsset.js asset110

Offchain CouchDB storage:

If you followed the instructions above and set use_couchdb to true, blockEventListener.js will create two tables in the local instance of CouchDB. blockEventListener.js is written to create two tables for each channel and for each chaincode.

The first table is an offline representation of the current world state of the blockchain ledger. This table was created using the read-write set data from the blocks. If the listener is running, this table should be the same as the latest values in the state database running on your peer. The table is named after the channelid and chaincodeid, and is named mychannel_basic in this example. You can navigate to this table using your browser: http://127.0.0.1:5990/mychannel_basic/_all_docs

A second table records each block as a historical record entry, and was created using the block data that was recorded in the log file. The table name appends history to the name of the first table, and is named mychannel_basic_history in this example. You can also navigate to this table using your browser: http://127.0.0.1:5990/mychannel_basic_history/_all_docs

Configure a map/reduce view for summarizing counts of assets by color:

Now that we have state and history data replicated to tables in CouchDB, we can use the following commands query our off-chain data. We will also add an index to support a more complex query. Note that if the blockEventListener.js is not running, the database commands below may fail since the database is only created when events are received.

Open a new terminal window and execute the following:

curl -X PUT http://127.0.0.1:5990/mychannel_basic/_design/colorviewdesign -d '{"views":{"colorview":{"map":"function (doc) { emit(doc.color, 1);}","reduce":"function ( keys , values , combine ) {return sum( values )}"}}}' -H 'Content-Type:application/json'

Execute a query to retrieve the total number of assets (reduce function):

curl -X GET http://127.0.0.1:5990/mychannel_basic/_design/colorviewdesign/_view/colorview?reduce=true

If successful, this command will return the number of assets in the blockchain world state, without having to query the blockchain ledger:

{"rows":[
  {"key":null,"value":19}
  ]}

Execute a new query to retrieve the number of assets by color (map function):

curl -X GET http://127.0.0.1:5990/mychannel_basic/_design/colorviewdesign/_view/colorview?group=true

The command will return a list of assets by color from the CouchDB database.

{"rows":[
  {"key":"blue","value":2},
  {"key":"green","value":2},
  {"key":"purple","value":3},
  {"key":"red","value":4},
  {"key":"white","value":6},
  {"key":"yellow","value":2}
  ]}

To run a more complex command that reads through the block history database, we will create an index of the blocknumber, sequence, and key fields. This index will support a query that traces the history of each asset. Execute the following command to create the index:

curl -X POST http://127.0.0.1:5990/mychannel_basic_history/_index -d '{"index":{"fields":["blocknumber", "sequence", "key"]},"name":"asset_history"}'  -H 'Content-Type:application/json'

Now execute a query to retrieve the history for the asset we transferred and then deleted:

curl -X POST http://127.0.0.1:5990/mychannel_basic_history/_find -d '{"selector":{"key":{"$eq":"asset110"}}, "fields":["blocknumber","is_delete","value"],"sort":[{"blocknumber":"asc"}, {"sequence":"asc"}]}'  -H 'Content-Type:application/json'

You should see the transaction history of the asset that was created, transferred, and then removed from the ledger.

{"docs":[
{"blocknumber":12,"is_delete":false,"value":"{\"docType\":\"asset\",\"name\":\"asset110\",\"color\":\"blue\",\"size\":60,\"owner\":\"debra\"}"},
{"blocknumber":22,"is_delete":false,"value":"{\"docType\":\"asset\",\"name\":\"asset110\",\"color\":\"blue\",\"size\":60,\"owner\":\"james\"}"},
{"blocknumber":23,"is_delete":true,"value":""}
  ]}

Getting historical data from the network

You can also use the blockEventListener.js program to retrieve historical data from your network. This allows you to create a database that is up to date with the latest data from the network or recover any blocks that the program may have missed.

If you ran through the example steps above, navigate back to the terminal window where blockEventListener.js is running and close it. Once the listener is no longer running, use the following command to add 20 more assets to the ledger:

node addAssets.js

The listener will not be able to add the new assets to your CouchDB database. If you check the current state table using the reduce command, you will only be able to see the original assets in your database.

curl -X GET http://127.0.0.1:5990/mychannel_basic/_design/colorviewdesign/_view/colorview?reduce=true

To add the new data to your off-chain database, remove the nextblock.txt file that kept track of the latest block read by blockEventListener.js:

rm nextblock.txt

You can new re-run the channel listener to read every block from the channel:

node blockEventListener.js

This will rebuild the CouchDB tables and include the 20 assets that have been added to the ledger. If you run the reduce command against your database one more time,

curl -X GET http://127.0.0.1:5990/mychannel_basic/_design/colorviewdesign/_view/colorview?reduce=true

you will be able to see that all of the assets have been added to your database:

{"rows":[
{"key":null,"value":39}
]}

Clean up

If you are finished using the sample application, you can bring down the network and any accompanying artifacts by running the following command:

./network-clean.sh

Running the script will complete the following actions:

  • Bring down the Fabric test network.
  • Takes down the local CouchDB database.
  • Remove the certificates you generated by deleting the wallet folder.
  • Delete nextblock.txt so you can start with the first block next time you operate the listener.
  • Removes addAssets.json.