DAML Ledger API Node.js Bindings Tutorial

Table of contents

1. Introduction
2. Prerequisites
3. Create the project
4. Compile the DAML Code
5. Run the sandbox
6. Run the skeleton app
7. Understand the skelelon
8. Retrieve the package identifiers
9. Understand the PingPong module
10. Pass the parties as parameters
11. Create a contract
12. Read the transactions
13. Exercise a choice
14. Read the active contracts

Introduction

This tutorial guides you through a series of steps to write a simple application using the Node.js bindings for DAML.

The purpose is to learn the basics of how to use them.

The task is to build an application able to send and receive "ping" messages.

The focus is not on the complexity of the model, but rather on how to use the bindings to interact with the ledger.

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Prerequisites

The DAML code in this repository makes use of the SDK 0.12.20 and the daml SDK assistant.

Make sure you have the SDK 0.12.20 (or greater) installed.

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Create the project

There is a skeleton application called ex-tutorial-nodejs that you can get from the GitHub.

To set it up, clone the repo, making sure to checkout the tag corresponding to the Node.js bindings version you are using:

git clone git@github.com:digital-asset/ex-tutorial-nodejs.git
cd ex-tutorial-nodejs
git checkout v0.6.1

The repo includes daml/PingPong.daml, which is the source for a DAML module with two templates: Ping and Pong. The app uses these.

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Compile the DAML code

Before getting started, move to the project root directory and build the DAML code with

daml build

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Run the sandbox

Use the sandbox to run and test your application:

1. open a new shell (the running sandbox will keep it busy), and

2. start the sandbox by running

   daml sandbox dist/ex-tutorial-nodejs.dar
   

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Run the skeleton app

You are now set to write your own application. The template includes a skeleton app that connects to a running ledger and quits.

1. Install the dependencies for your package (including the bindings):

   npm install
   

2. Start the application:

   npm start
   

3. Verify the output is correct

   hello from <LEDGER_ID>
   

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Understand the skeleton

The code for the script you just ran is index.js.

Let's go through the skeleton part by part to understand what's going on:

const ledger = require('@digital-asset/daml-ledger');

const daml = ledger.daml;

const uuidv4 = require('uuid/v4');

The first line loads the bindings and allows you to refer to them through the ledger object.

The second line creates a shorthand for the daml object in the daml-ledger library, that can be used to express DAML values in your code concisely.

The third line introduces a dependency that is going to be later used to generate unique identifiers; no need to worry about it now.

let [, , host, port] = process.argv;

host = host || 'localhost';
port = port || 6865;

These lines read the command-line arguments and provide some sensible defaults.

Now to the juicy part:

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;
    console.log('hello from', client.ledgerId);
});

Here the application connects to the ledger with the DamlLedgerClient.connect method.

It accepts two arguments:

• an object with the connection options
• a callback to be invoked when the connection either fails or succeeds

The connection options require you to pass the host and port of the ledger instance you are connecting to.

The callback follows the common pattern in Node.js of being invoked with two arguments: the first is an error in case of failure while the latter is the response in case of success.

In this case in particular, the response in case of success is a client object that can be used to communicate with the ledger.

The skeleton application just prints the greeting message with the ledger identifier and quits.

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Retrieve the package identifiers

Now that the sandbox is running, the PingPong.daml file has been compiled and the module loaded onto the ledger.

In order for you to refer to the templates therein you need its package identifier.

This template includes a script that connects to a running ledger instance and downloads the package identifiers for the templates.

Run it now:

npm run fetch-template-ids

If the program ran successfully, the project root now contains the template-ids.json file.

It's time to write some code to verify that you're good to go. Open the index.js file and edit it.

First of all, right after the first require statement, add a new one to load the template-ids.json file that has just been created.

const ledger = require('@digitalasset/daml-ledger');
const templateIds = require('./template-ids.json');

Right beneath that line, initialize two constants to hold the Ping and Pong template identifiers:

const PING = templateIds['PingPong:Ping'];
const PONG = templateIds['PingPong:Pong'];

Finally print the template identifiers:

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;
    console.log('hello from', client.ledgerId);
    console.log('Ping', PING);
    console.log('Pong', PONG);
});

Run the application again (npm start) to see an output like the following:

hello from sandbox-3957952d-f475-4d2f-be89-245a0799d2c0
Ping { packageId:
   '33f8c3ee29d1e358e1c5b321eab7158af0220a6b956f69528da89c3eeb06d736',
  moduleName: 'PingPong',
  entityName: 'Ping' }
Pong { packageId:
   '33f8c3ee29d1e358e1c5b321eab7158af0220a6b956f69528da89c3eeb06d736',
  moduleName: 'PingPong',
  entityName: 'Pong' }

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Understand the PingPong module

Before moving on to the implementation of the application, have a look at daml/PingPong.daml to understand the module the app uses.

Ping and Pong are almost identical. Looking at them in detail:

• both have a sender signatory and a receiver observer
• the receiver of a Ping can exercise the ReplyPong choice, creating a Pong contract with swapped sender and receiver
• symmetrically, the receiver of a Pong contract can exercise the ReplyPing choice, creating a Ping contract with swapped parties

Note that the contracts carry a counter: when the counter reaches 3, no new contract is created and the exchange stops.

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Pass the parties as parameters

Everything's now ready to start working. Edit the index.js file.

Each contract has a sender and a receiver, so your application needs to establish it.

Read those from the command line by editing the part where the arguments are read as follows:

let [, , sender, receiver, host, port] = process.argv;
host = host || 'localhost';
port = port || 6865;
if (!sender || !receiver) {
    console.log('Missing sender and/or receiver arguments, exiting.');
    process.exit(-1);
}

Try to run it without arguments (or with just one) to see the error popping up.

Try to run it with both arguments to see the application working just as it did before, as follows:

npm start Alice Bob

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Create a contract

To kickstart the exchange between two parties you have to first make one party "send" the initial ping to the other.

To do this you need to create a Ping contract.

This requires you to submit a command to the ledger. For this, use the CommandService.

The client object returned by the DamlLedgerClient.connect method contains a reference to all services exposed by the ledger, including the CommandService.

First of all, the following is the request for the CommandService. Have a look at it:

const request = {
    commands: {
        applicationId: 'PingPongApp',
        workflowId: `Ping-${sender}`,
        commandId: uuidv4(),
        ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
        maximumRecordTime: { seconds: 5, nanoseconds: 0 },
        party: sender,
        list: [{
            commandType: 'create',
            templateId: PING,
            arguments: {
                fields: {
                    sender: daml.party(sender),
                    receiver: daml.party(receiver),
                    count: daml.int64(0)
                }
            }
        }]
    }
};

This object represents the submission of a set of commands to be applied atomically. Let's see what each bit of it means:

• applicationId: the name of your application
• workflowId: an (optional) identifier you can use to group together commands pertaining to one of your workflows
• commandId: a unique identifier for the set of submitted commands
• ledgerEffectiveTime: the time at which the set of submitted commands are applied; normally the client's current epoch time, but, since the sandbox (by default) runs with a static time fixed at epoch 0, use this value
• maximumRecordTime: the time at which the command is considered expired if it's not been applied yet; the difference with the maximumRecordTime is the time-to-live (TTL) of the command
• party: who's submitting the command

Finally, list contains all the commands to be applied. In this case, it submits a create command.

Have a look at the only command:

• templateId: the identifier of the template of the contract you wish to create (Ping)
• arguments: an object containing the fields necessary to create the contract

The keys of the fields object are the template parameter names as they appear on daml/PingPong.daml, while the values are an object indicating the type of the parameter and its actual value.

The request can now be passed to the CommandService as follows:

client.commandClient.submitAndWait(request, (error, _) => {
    if (error) throw error;
    console.log(`Created Ping contract from ${sender} to ${receiver}.`);
});

This is already a sizeable chunk of code that performs a clearly defined task. Within the body of the connect callback, wrap the code from this section in a function called createFirstPing and call it.

The code should now look like the following:

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;

    createFirstPing();

    function createFirstPing() {
        const request = {
            commands: {
                applicationId: 'PingPongApp',
                workflowId: `Ping-${sender}`,
                commandId: uuidv4(),
                ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                party: sender,
                list: [{
                    commandType: 'create',
                    templateId: PING,
                    arguments: {
                        fields: {
                            sender: daml.party(sender),
                            receiver: daml.party(receiver),
                            count: daml.int64(0)
                        }
                    }
                }]
            }
        };
        client.commandClient.submitAndWait(request, (error, _) => {
            if (error) throw error;
            console.log(`Created Ping contract from ${sender} to ${receiver}.`);
        });
    }

});

Time to test your application. Run it like this:

npm start Alice Bob

You should see the following output:

Created Ping contract from Alice to Bob.

Your application now successfully creates a Ping contract on the ledger, congratulations!

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Read the transactions

Now that the application can create a contract to send a ping, it must also be able to listen to pongs on the ledger so that it can react to those.

The TransactionService exposes the functionality to read transactions from the ledger via the getTransactions method.

This method takes the following request:

const filtersByParty = {};
filtersByParty[sender] = { inclusive: { templateIds: [PING, PONG] } };
const request = {
    begin: { boundary: ledger.LedgerOffsetBoundaryValue.BEGIN },
    filter: { filtersByParty: filtersByParty }
};

Have a look at the request:

• begin: the offset at which you'll start reading transactions from the ledger. In this case you want to listen starting from the first one (represented by the constant daml.LedgerOffsetBoundaryValue.BEGIN)
• end: the optional offset at which you want the reads to end -- if absent (as in this case) the application keeps listening to incoming transactions
• filter: represents which contracts you want the ledger to show you: in this case you are asking for the transactions visible to sender containing contracts whose templateId matches either PING or PONG.

---

Note

Why subscribing from the beginning and not simply tailing the transactions?

Purely for educational purposes: in the end, the application will tail the transaction stream. For now, the app subscribes from the beginning to make sure it can read the events it causes, so that you can more easily observe the contract creation.

---

When the getTransactions method is invoked with this request the application listens to all transactions coming to the ledger.

The output of this method is a Node.js stream. As such, you can register callbacks on the 'data' and 'error' events.

The following code prints the incoming transaction and quits in case of 'error'.

const transactions = client.transactionClient.getTransactions(request);
console.log(`${sender} starts reading transactions.`);
transactions.on('data', response => {
    for (const transaction of response.transactions) {
        console.log('Transaction read:', transaction.transactionId);
    }
});
transactions.on('error', error => {
    console.error(`${sender} encountered an error while processing transactions!`);
    console.error(error);
    process.exit(-1);
});

---

Note

If your request specified an end, it would most probably make sense to register an 'end' event callback on the stream as well.

---

Again, this code represents a sizeable chunk of code with a clearly defined purpose.

Wrap this code in a new function called listenForTransactions, place it within the connect callback and call listenForTransactions right before you call createFirstPing.

When you are done, your code should look like the following:

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;

    listenForTransactions();
    createFirstPing();

    function createFirstPing() {
        const request = {
            commands: {
                applicationId: 'PingPongApp',
                workflowId: `Ping-${sender}`,
                commandId: uuidv4(),
                ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                party: sender,
                list: [{
                    commandType: 'create',
                    templateId: PING,
                    arguments: {
                        fields: {
                            sender: daml.party(sender),
                            receiver: daml.party(receiver),
                            count: daml.int64(0)
                        }
                    }
                }]
            }
        };
        client.commandClient.submitAndWait(request, (error, _) => {
            if (error) throw error;
            console.log(`Created Ping contract from ${sender} to ${receiver}.`);
        });
    }

    function listenForTransactions() {
        console.log(`${sender} starts reading transactions.`);
        const filtersByParty = {};
        filtersByParty[sender] = { inclusive: { templateIds: [PING, PONG] } };
        const request = {
            begin: { offsetType: 'boundary', boundary: ledger.LedgerOffsetBoundaryValue.BEGIN },
            filter: { filtersByParty: filtersByParty }
        };
        const transactions = client.transactionClient.getTransactions(request);
        transactions.on('data', response => {
            for (const transaction of response.transactions) {
                console.log('Transaction read:', transaction.transactionId);
            }
        });
        transactions.on('error', error => {
            console.error(`${sender} encountered an error while processing transactions!`);
            console.error(error);
            process.exit(-1);
        });
    }

});

Your application now should:

1. start listening to pings and pongs visible to the sender
2. create the first ping
3. receive the ping it created and print its transaction identifer

If you now run

npm start Alice Bob

You should see an output like the following:

Alice starts reading transactions.
Created Ping contract from Alice to Bob.
Transaction read: 0
Transaction read: 1

Note that the exact number of transactions read depends on whether you have already ran the application and created contracts more then once.

Your application is now able to create contracts and listen to transactions on the ledger. Very good!

You can now hit CTRL-C to quit the application.

---

Note

Start from a clean slate

It's not a requirement for this tutorial, but if you want to reset the sandbox to its initial state, do the following:

1. Go to the console where the sandbox is running

2. Hit CTRL+C to stop it

3. Run it again as you did before

   daml sandbox dist/ex-tutorial-nodejs.dar
   

---

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Exercise a choice

The last piece of functionality you need consists of reacting to pings and pongs that you read from the ledger, represented by the creation of contracts.

For this, use again the submitAndWait method.

In particular, make your program exercise a choice: ReplyPing when you receive a Pong and vice versa.

You need to react to events in transactions as they are received in the listenForTransactions function.

The transaction object whose transactionId you printed so far contains an array of event objects, each representing an archived or created event on a contract.

What you want to do is loop through the events in the transaction and extract the receiver and count fields from created events.

You then want to decide which reply to give (either ReplyPing or ReplyPong) based on the contract that has been read.

For each created event, you want to send a command that reacts to it, specifying that you want to either exercise the ReplyPing choice of a Pong contract or vice versa.

The following snippet of code does precisely this.

const reactions = [];
for (const event of events) {
    const { receiver: { party: receiver }, count: { int64: count } } = event.arguments.fields;
    if (receiver === sender) {
        const templateId = event.templateId;
        const contractId = event.contractId;
        const reaction = templateId.moduleName === PING.moduleName && templateId.entityName === PING.entityName ? 'ReplyPong' : 'ReplyPing';
        console.log(`${sender} (workflow ${workflowId}): ${reaction} at count ${count}`);
        reactions.push({
            commandType: 'exercise',
            templateId: templateId,
            contractId: contractId,
            choice: reaction,
            argument: daml.record({})
        });
    }
}

You can now use the submitAndWait command to send the reactions to the ledger.

if (reactions.length > 0) {
    const request = {
        commands: {
            applicationId: 'PingPongApp',
            workflowId: workflowId,
            commandId: uuidv4(),
            ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
            maximumRecordTime: { seconds: 5, nanoseconds: 0 },
            party: sender,
            list: reactions
        }
    }
    client.commandClient.submitAndWait(request, error => {
        if (error) throw error;
    });
}

Wrap this code into a new function react that takes a workflowId and an events array with the created events. Then edit the listenForTransactions function to:

• accept one parameter called callback
• instead of printing the transaction identifier, for each transaction
  • push the created events to an array
  • pass that array to the callback (along with the workflow identifier)

Finally, pass the react function as a parameter to the only call of listenForTransactions.

The application is now ready to be tested with two instances running at once: it can now tail the transaction stream instead of subscribing to it from the beginning by replacing the occurrences of ledger.LedgerOffsetBoundaryValue.BEGIN with ledger.LedgerOffsetBoundaryValue.END.

Review the code before running the application. Your code should now look like the following:

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;

    listenForTransactions(react);
    createFirstPing();

    function createFirstPing() {
        const request = {
            commands: {
                applicationId: 'PingPongApp',
                workflowId: `Ping-${sender}`,
                commandId: uuidv4(),
                ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                party: sender,
                list: [{
                    commandType: 'create',
                    templateId: PING,
                    arguments: {
                        fields: {
                            sender: daml.party(sender),
                            receiver: daml.party(receiver),
                            count: daml.int64(0)
                        }
                    }
                }]
            }
        };
        client.commandClient.submitAndWait(request, (error, _) => {
            if (error) throw error;
            console.log(`Created Ping contract from ${sender} to ${receiver}.`);
        });
    }

    function listenForTransactions(callback) {
        console.log(`${sender} starts reading transactions.`);
        const filtersByParty = {};
        filtersByParty[sender] = { inclusive: { templateIds: [PING, PONG] } };
        const request = {
            begin: { offsetType: 'boundary', boundary: ledger.LedgerOffsetBoundaryValue.END },
            filter: { filtersByParty: filtersByParty }
        };
        const transactions = client.transactionClient.getTransactions(request);
        transactions.on('data', response => {
            for (const transaction of response.transactions) {
                const events = [];
                for (const event of transaction.events) {
                    if (event.eventType === 'created') {
                        events.push(event);
                    }
                }
                if (events.length > 0) {
                    callback(transaction.workflowId, events);
                }
            }
        });
        transactions.on('error', error => {
            console.error(`${sender} encountered an error while processing transactions!`);
            console.error(error);
            process.exit(-1);
        });
    }

    function react(workflowId, events) {
        const reactions = [];
        for (const event of events) {
            const { receiver: { party: receiver }, count: { int64: count } } = event.arguments.fields;
            if (receiver === sender) {
                const templateId = event.templateId;
                const contractId = event.contractId;
                const reaction = templateId.moduleName === PING.moduleName && templateId.entityName === PING.entityName ? 'ReplyPong' : 'ReplyPing';
                console.log(`${sender} (workflow ${workflowId}): ${reaction} at count ${count}`);
                reactions.push({
                    commandType: 'exercise',
                    templateId: templateId,
                    contractId: contractId,
                    choice: reaction,
                    argument: daml.record({})
                });
            }
        }
        if (reactions.length > 0) {
            const request = {
                commands: {
                    applicationId: 'PingPongApp',
                    workflowId: workflowId,
                    commandId: uuidv4(),
                    ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                    maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                    party: sender,
                    list: reactions
                }
            }
            client.commandClient.submitAndWait(request, error => {
                if (error) throw error;
            });
        }
    }

});

To test your code you need to run two different commands in two different terminals.

First, run:

npm start Alice Bob

After starting this, the application creates a ping contract on the ledger and waits for replies.

Alice starts reading transactions.
Created Ping contract from Alice to Bob.

Keep this command running, open a new shell and run the following command:

npm start Bob Alice

You should now see the exchange happening on both terminals.

npm start Alice Bob

Alice starts reading transactions.
Created Ping contract from Alice to Bob.
Alice (workflow Ping-Bob): Pong at count 0
Alice (workflow Ping-Bob): Pong at count 2

npm start Bob Alice

Bob starts reading transactions.
Created Ping contract from Bob to Alice.
Bob (workflow Ping-Bob): Ping at count 1
Bob (workflow Ping-Bob): Ping at count 3

You can now close both applications.

Your application is now able to complete the full exchange. Very well done!

Back to the table of contents

Read the active contracts

You may have noticed that the program has a flaw: by subscribing to transactions from the end of the ledger the application is going to see transactions only after that, missing contracts created earlier.

You can see the error as only the workflow initiated by Alice is executed, due to the fact that the process that does so starts earlier.

You could address this problem by listening for transactions from the beginning, but you'd have to take care of handling contracts that could already have been processed by your application.

To handle this use case the API has a service which returns the set of active contracts on the ledger and the absolute office from which start to listen for live transactions.

In this new example the application first processes the current active contracts. Since that process is asynchronous the rest of the program should be passed in as a callback.

function processActiveContracts(transactionFilter, callback, onComplete) {
    const request = { filter: transactionFilter };
    const activeContracts = client.activeContractsClient.getActiveContracts(request);
    let offset = undefined;
    activeContracts.on('data', response => {
        if (response.activeContracts) {
            const events = [];
            for (const activeContract of response.activeContracts) {
                events.push(activeContract);
            }

            if (events.length > 0) {
                callback(response.workflowId, events);
            }
        }          
       
        if (response.offset) {
            offset = response.offset;
        }
    });    

    activeContracts.on('error', error => {
        console.error(`${sender} encountered an error while processing active contracts!`);
        console.error(error);
        process.exit(-1);
    });    

    activeContracts.on('end', () => onComplete(offset));
}

Note that the transaction filter was factored out as it can be shared. The final code would look like this:

const ledger = require('@digitalasset/daml-ledger');
const templateIds = require('./template-ids.json');

const PING = templateIds['PingPong:Ping'];
const PONG = templateIds['PingPong:Pong'];

const daml = ledger.daml;

const uuidv4 = require('uuid/v4');

let [, , sender, receiver, host, port] = process.argv;
host = host || 'localhost';
port = port || 6865;
if (!sender || !receiver) {
    console.log('Missing sender and/or receiver arguments, exiting.');
    process.exit(-1);
}

ledger.DamlLedgerClient.connect({ host: host, port: port }, (error, client) => {
    if (error) throw error;

    const filtersByParty = {};
    filtersByParty[sender] = { inclusive: { templateIds: [PING, PONG] } };
    const transactionFilter = { filtersByParty: filtersByParty };

    processActiveContracts(transactionFilter, react, offset => {
        listenForTransactions(offset, transactionFilter, react);
        createFirstPing();
    });

    function createFirstPing() {
        const request = {
            commands: {
                applicationId: 'PingPongApp',
                workflowId: `Ping-${sender}`,
                commandId: uuidv4(),
                ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                party: sender,
                list: [{
                    commandType: 'create',
                    templateId: PING,
                    arguments: {
                        fields: {
                            sender: daml.party(sender),
                            receiver: daml.party(receiver),
                            count: daml.int64(0)
                        }
                    }
                }]
            }
        };
        client.commandClient.submitAndWait(request, (error, _) => {
            if (error) throw error;
            console.log(`Created Ping contract from ${sender} to ${receiver}.`);
        });
    }

    function listenForTransactions(offset, transactionFilter, callback) {
        console.log(`${sender} starts reading transactions from offset: ${offset}.`);
        const request = {
            begin: { offsetType: 'boundary', boundary: ledger.LedgerOffsetBoundaryValue.END },
            filter: transactionFilter
        };
        const transactions = client.transactionClient.getTransactions(request);
        transactions.on('data', response => {
            for (const transaction of response.transactions) {
                const events = [];
                for (const event of transaction.events) {
                    if (event.eventType === 'created') {
                        events.push(event);
                    }
                }
                if (events.length > 0) {
                    callback(transaction.workflowId, events);
                }
            }
        });
        transactions.on('error', error => {
            console.error(`${sender} encountered an error while processing transactions!`);
            console.error(error);
            process.exit(-1);
        });
    }

    function processActiveContracts(transactionFilter, callback, onComplete) {
        console.log(`processing active contracts for ${sender}`);
        const request = { filter: transactionFilter };
        const activeContracts = client.activeContractsClient.getActiveContracts(request);
        let offset = undefined;
        activeContracts.on('data', response => {
            if (response.activeContracts) {
                const events = [];
                for (const activeContract of response.activeContracts) {
                    events.push(activeContract);
                }
                if (events.length > 0) {
                    callback(response.workflowId, events);
                }
            }

            if (response.offset) {
                offset = response.offset;
            }
        });

        activeContracts.on('error', error => {
            console.error(`${sender} encountered an error while processing active contracts!`);
            console.error(error);
            process.exit(-1);
        });

        activeContracts.on('end', () => onComplete(offset));
    }

    function react(workflowId, events) {
        const reactions = [];
        for (const event of events) {
            const { receiver: { party: receiver }, count: { int64: count } } = event.arguments.fields;
            if (receiver === sender) {
                const templateId = event.templateId;
                const contractId = event.contractId;
                const reaction = templateId.moduleName === PING.moduleName && templateId.entityName === PING.entityName ? 'ReplyPong' : 'ReplyPing';
                console.log(`${sender} (workflow ${workflowId}): ${reaction} at count ${count}`);
                reactions.push({
                    commandType: 'exercise',
                    templateId: templateId,
                    contractId: contractId,
                    choice: reaction,
                    argument: { valueType: 'record', fields: {} }
                });
            }
        }
        if (reactions.length > 0) {
            const request = {
                commands: {
                    applicationId: 'PingPongApp',
                    workflowId: workflowId,
                    commandId: uuidv4(),
                    ledgerEffectiveTime: { seconds: 0, nanoseconds: 0 },
                    maximumRecordTime: { seconds: 5, nanoseconds: 0 },
                    party: sender,
                    list: reactions
                }
            }
            client.commandClient.submitAndWait(request, error => {
                if (error) throw error;
            });
        }
    }

});

Before running the application, start with a clean ledger to avoid receiving unprocessed contracts from previous examples:

1. go to the shell where you are running the sandbox

2. hit CTRL+C to shut it down and wait for your shell prompt

3. restart the sandbox

   daml sandbox dist/ex-tutorial-nodejs.dar
   

Then run:

npm start Alice Bob

and finally run in another shell:

npm start Bob Alice

You should see the following outputs respectively:

processing active contracts for Alice
Alice starts reading transactions from offset: 0.
Created Ping contract from Alice to Bob.
Alice (workflow Ping-Bob): Pong at count 0
Alice (workflow Ping-Alice): Ping at count 1
Alice (workflow Ping-Bob): Pong at count 2
Alice (workflow Ping-Alice): Ping at count 3

---

processing active contracts for Bob
Bob (workflow Ping-Alice): Pong at count 0
Bob starts reading transactions from offset: 1.
Created Ping contract from Bob to Alice.
Bob (workflow Ping-Bob): Ping at count 1
Bob (workflow Ping-Alice): Pong at count 2
Bob (workflow Ping-Bob): Ping at count 3

Alice joining an empty ledger has no active contracts to process. Bob however, who joins later, will see Alice's Ping contract and process it. Afterwards he will continue listening to transactions from offset 1.

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