Moonbase Alpha Now Available with Pub-Sub

New in Moonbase Alpha v2: Contract Events and Pub/Sub Capabilities

With the release of Moonbase Alpha v2, which was just announced by PureStake, we are adding some new and exciting features that help Moonbeam move closer to its primary goal of providing a seamless experience for projects from Ethereum on the Polkadot ecosystem. One of the main features being added is the ability to subscribe to Ethereum smart contracts events, and other blockchain information.

Contract events are a super important part of dApps in Ethereum, as they facilitate communication between smart contracts and their user interfaces. Events can be considered asynchronous triggers with data. When a contract emits an event, this can subsequently result in an action on the front-end side.

Use Cases for Events

A simple example of an event you could track is a transfer. Let’s say a transfer is initiated by a user using the front-end of a dApp, where a transaction hash is obtained once this is submitted. But to assure the user that the payment was sent, the dApp can listen for an event (emitted by the contract) when the transaction is committed to the blockchain. This can consequently trigger a display message to the user notifying them that their action was successful.

Another powerful use case of events is cheaper storage. On average, logs cost 8 gas per byte, whereas contract storage costs 20,000 gas per 32 bytes. Therefore, events can serve as a tool to save and retrieve necessary information such as transfer logs as well. However, they can’t be used as storage for all use cases, because they can’t be accessed by other smart contracts, for example.

The Significance of Pub/Sub

Given all this context, now we are ready to talk about pub/sub.

Publish-subscribe, or pub/sub for short, is an asynchronous messaging service that acts as a middleware between the publishers of messages, and people that subscribe to them. In general terms, publishers categorize these messages into classes and publish them without really knowing who is subscribed to them. Similarly, subscribers enroll in the classes that are of interest, receiving only messages associated with that class, without knowing who their publisher is.

With the release of Moonbase Alpha v2, a pub/sub service compatible with Ethereum-style events is now available.

Tutorial: How to Use Pub/Sub on Moonbeam

Since a picture is worth a thousand words, let’s jump into some examples to showcase how pub/sub works on Moonbeam.

To follow this demo, you will need the following:

    • Have MetaMask installed and connected to Moonbase
    • Have an account with funds, which you can get from Mission Control
    • Deploy your own ERC20 token on Moonbase, which you can do following our Remix tutorial but first pointing MetaMask to Moonbase
    • Install NodeJS and the Web3 JS library. For systems on Ubuntu 18.04 or similar, you can follow the first part of this tutorial

Subscribing to Event Logs in Moonbase Alpha v2

Any contract that follows the ERC-20 token standard emits an event related to a transfer of tokens, that is, event Transfer(address indexed from, address indexed to, uint256 value). For this example, we will subscribe to the logs of such events. Using the Web3 JS library, we need the following piece of code:

const Web3 = require('web3');
const web3 = new Web3('wss://');

web3.eth.subscribe('logs', {
    address: 'ContractAddress',
    topics: ['0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef']
}, (error, result) => {
    if (error)
    .on("connected", function (subscriptionId) {
    .on("data", function (log) {

Note that we are connecting to the WebSocket endpoint of Moonbase Alpha. We use the web3.eth.subscribe(‘logs’, options [, callback]) method to subscribe to the logs, filtered by the given options. In our case, the options are the contract’s address where the events are emitted from, and the topics used to describe the event. More information about topics can be found in this Medium post. If no topics are included, you subscribe to all events emitted by the contract. But in order to filter only the Transfer event, we need to include the signature of the event, calculated as:

EventSignature = keccak256(Transfer(address,address,uint256))

The result of the previous calculation is what’s shown in the code snippet from before. We’ll go back to filtering by topics later on. The rest of the code handles the callback function. Once we execute this code, we’ll get a subscription ID, and the terminal will wait for any event through that subscription:

Next, an ERC20 token transfer will be sent with the following parameters:

  • From address: 0x6Be02d1d3665660d22FF9624b7BE0551ee1Ac91b
  • To address: 0xfcB0B397BB28046C01be6A3d66c7Eda99Fb0f344
  • Value (tokens): 10000000000000000000 – that is 10 with 18 zeros

Once we send the transaction, the log of the event emitted by the transaction will appear in the terminal:

A lot of information is provided in the logs, but you might ask yourself: where is the information in the emitted event? And the answer is: in the logs!

Our target event sends two pieces of indexed information, the “from” and “to” addresses (in that order), which are treated like topics. The other piece of data shared by our event is the number of tokens, that is not indexed. Therefore, there is a total of three topics (the maximum is four), which correspond to the opcode LOG3:

Consequently, you can see that the “from” and “to” addresses are contained inside the topics returned by the logs. Ethereum addresses are 40 hex characters long (1 hex character is 4 bits, hence 160 bits or H160 format). Thus, the extra 24 zeros are needed to fill the gap to H256, which are 64 hex characters long.

What about the number of tokens? Unindexed data is returned in the “data” field of the logs, but this is encoded in bytes32/hex. To decode it we can use for example, this online tool, and verify that the “data” is in fact 10 (plus 18 zeros).

If the event returns multiple unindexed values, these will be appended one after the other in the same order the event emits them. Therefore, each value is then obtained by deconstructing data into separate 32 bytes (or 64 hex character long) pieces.

This example showed how we could subscribe only to event logs of a specific contract. But the Web3 JS library provides other subscription types that we’ll go over in the following sections.

Subscribe to Incoming Pending Transactions

In order to subscribe to pending transactions, we can use the web3.eth.subscribe(‘pendingTransactions’, [, callback]) method, implementing the same callback function to check for the response. This is much simpler than our previous example, and it returns the transaction hash of the pending transactions.

We can verify that this transaction hash is the same as that shown in MetaMask (or Remix).

Subscribe to Incoming Block Headers

Another type available under the Web3 JS library is to subscribe to new block headers. To do so, we use the web3.eth.subscribe('newBlockHeaders' [, callback]) method, implementing the same callback function to check for the response. This subscription provides incoming block headers and can be used to track changes in the blockchain.

Note that only one block header is shown in the image. These messages are displayed for every block produced, so they can fill up the terminal quite quickly.

Check If the Node is Synchronized With the Network

With pub/sub it is also possible to check whether a particular node, which you are subscribed to, is currently synchronized with the network. For that, we can leverage the web3.eth.subscribe(‘syncing' [, callback]) method, implementing the same callback function to check for the response. This subscription will return an object when the node is synced with the network.

Current Limitations

The pub/sub implementation in Frontier is still in active development. This first version allows dApp developers (or users in general) to subscribe to specific event types, but there are still some limitations. From the previous examples, you might have noticed that some of the fields are not showing proper information, and that is because certain properties are yet to be supported by Frontier.

Another limitation is related to the logs of the event. On Ethereum, you can use wildcards and pass in multiple input addresses, for example, to filter specific logs. Let’s say we would like to subscribe to all events of a contract that have two specific addresses in the “topic_1” field (remember that topic_0 is reserved to the event signature). Then we could pass in the following topic as input:

topics: [null, [address1, address2]]

Here, by using the wildcard null in place for the event signature, we are listening to all events emitted by the contract that we subscribed to. But with this configuration, we can also use a second input field, that is topic_1, to define a filter by address as mentioned before.

The current Frontier implementation does not support these features. As an alternative, you can create multiple subscriptions for all the events of the contract and the different addresses, but this increases the number of operations to be carried out. However, this is expected to be supported in future versions of the Moonbase TestNet.

Contact Us

If you have any feedback regarding the Moonbase Alpha v2, pub/sub, or any other Moonbeam related topic, feel free to reach out through our official development Discord server.

Moonbeam Monthly Dispatch September 2020

Introducing Moonbase Alpha, the Moonbeam Public TestNet

Moonbeam released Moonbase Alpha, its first public TestNet, today. This represents a major milestone for the Moonbeam project.

The goal of Moonbase Alpha is to provide developers with a place to start experimenting and building on Moonbeam in a shared environment. Since Moonbeam will be deployed as a parachain on Kusama and Polkadot, we want our TestNet to reflect our production configuration. For this reason, we decided that it needed to be a parachain-based configuration rather than a Substrate standalone setup. It is also essential for us to have a fully automated pipeline executing from GitHub all the way to the Moonbase Alpha deployment configuration.

Developing a Parachain-Based TestNet

Deploying Substrate-based chains as parachains has just recently become possible based on Parity’s work on the Cumulus library, among other things. This functionality is still very new and is actively being developed and improved.

While working to set up the TestNet with these technologies, we encountered several challenges which helped us understand how relay chain <> parachain interactions in Polkadot work at a lower level.

The first challenge was around building the environment, which involves a fair bit of complexity. The process requires two automated pipelines, each producing different outputs (Polkadot binary, Moonbeam binary) and these outputs need to be married to the correct, generated spec files for the Relay Chain Validator and Moonbeam Collator node roles. Then a third process requires us to create the Moonbeam genesis state and WASM binary, which is then registered to the deployed relay chain. These actions need to be automated to execute in the right sequence.

A second challenge arises from the rapid change that Substrate is undergoing. Rust limits us to only one version of a library at any given time. Thus, it took some workarounds to find a version of Substrate that was compatible across Rococo (which we are using as the base for our Relay Chain build), Cumulus (which is currently based on a one-month old version of Substrate), and Moonbeam / Frontier (which we are updating with Substrate master every few days). Hopefully, this will become less difficult as parachain functionality matures and stabilizes.

Moonbase Alpha Configuration at Launch

Currently, the infrastructure supporting the Moonbase Alpha TestNet is hosted by PureStake, the team developing Moonbeam. The initial configuration consists of a parachainrelay chain setup. The parachain has one collator producing blocks, while the relay chain consists of three validators, one of which is selected to finalize each block produced at the parachain level. This setup provides room to expand to a two-parachain configuration in the future. There are a couple of RPC endpoints for both HTTPS and WebSocket calls.

As part of this release, the Moonbase Alpha features implementations of the EVM pallet and the Web3 RPC in Substrate. Moonbeam leverages the Frontier project (for which PureStake’s contributions earned a Web3 grant) to support native Ethereum RPCs. Compatibility with existing Ethereum developer tools such as Truffle, Remix, and MetaMask have been tested and examples of using these can be found on our docs site.

TestNet Feature Roadmap

For the next TestNet update, PureStake is planning to add two key features.

First is the unification of Substrate and Ethereum accounts under the H160 format, an effort that we refer to as Unified Accounts. This is an important milestone because, in absence of this functionality, there are two completely different sets of states: one for Substrate accounts, and one contained within the EVM for Ethereum-based accounts. By unifying them, there will be only one kind of account in the system represented by a single address, instead of two.

The second feature is the inclusion of event subscription support (Pub/Sub). This component is missing on the Web3 RPC side, so we plan to add it since it is commonly used by dApps.

The alpha TestNet version that was released today does not support third-party collators. We will add support in a future update to enable interested parties to test their setups. This may also include the implementation of the rewards system, as well as the token economic model.

Other features such as on-chain governance and the treasury might be added in future updates as well.

Because the system is hosted by PureStake, Moonbase Alpha will run similar to how Polkadot MainNet ran in early times: with Proof of Authority instead of Proof of Stake. This means that block finalization is carried out by a known identity, in this case, the PureStake validators.

Getting Started with the New TestNet

As the first Moonbeam TestNet, there will be some limitations. Tokens on Moonbase Alpha, named DEV, will be issued on-demand. To request tokens, we’ve created a Discord bot (named Mission Control 😎) that will automatically send a maximum of 10 DEV tokens per hour (per Discord user). You can check it out on our Discord channel.

In addition, users only have access to Moonbeam (the parachain). In the future, we might open access to the relay chain, so users can test transferring tokens between Moonbeam and the relay chain.

Regarding smart contracts, this early iteration has no gas limit per block in order to provide an easy on-ramp for developers. This configuration will change in the future.

Getting started is fairly simple. Point your provider to the following RPC DNS for connections via HTTPS:

For the Web3 library, create a local Web3 instance and set the provider to connect to the Moonbeam TestNet:

const Web3 = require('web3'); //Load Web3 library
//Create local Web3 instance - set the Moonbeam TestNet as provider
const web3 = new Web3('; 

Any Ethereum wallet should be able to generate a valid address or Moonbeam (for example, MetaMask).

For connections via WebSocket, you can use the following RPC DNS:


If you have any feedback regarding the Moonbeam TestNet, feel free to reach out through our official development Discord channel.

Moonbeam TestNet Announcement

Moonbase Alpha, the Moonbeam Network TestNet, is Now Available

This First TestNet Release Enables DApp Developers to Start Building
DApps and Solidity Smart Contracts in a Parachain Environment

Boston, MA – September 21, 2020 – PureStake, the company developing the Moonbeam smart contract platform, announced the availability of Moonbase Alpha, its first public TestNet. Now developers can test the Ethereum compatibility features of the Moonbeam network with their decentralized applications without needing to run their own nodes or host their own network.

Moonbase Alpha makes it easier for developers to deploy smart contracts to the platform. Developers can:

  • Use the existing Ethereum-based tools they know and love, like MetaMask, Truffle, and Remix
  • Re-deploy dApp front-ends and Solidity smart contract backends to Moonbeam with little or no change necessary
  • Collaborate with other teams in a shared environment that is monitored, managed, and will be updated with the latest Moonbeam code on an ongoing basis

“This is a big first step toward our KusamaNet and Polkadot-based deployments, and we’re eager to see what developers build on top of the platform,” said Derek Yoo, CEO of PureStake. “Understanding the real world use cases for some of the more complex smart contracts that we’ll support is an important part of both testing and vetting our code so we can prioritize the features we work on. The feedback we gather over the next several months will have a significant impact on the future of the network.”

To get started on Moonbase Alpha, join our Discord and visit our website:

This first Moonbeam TestNet is purely for development purposes — it continues to undergo significant development and enhancements from our engineering team, and the nodes are solely operated by PureStake. The team plans to release TestNet upgrades in the future that will add additional features, including the ability for external collators to start producing blocks on the network. More details are available on the Moonbeam documentation website:

About the Moonbeam Network

Moonbeam is an Ethereum-compatible smart contract platform on the Polkadot network that makes it easy to build natively interoperable applications. This Ethereum compatibility allows developers to deploy existing Solidity smart contracts and DApp frontends to Moonbeam with minimal changes. As a parachain on the Polkadot network, Moonbeam will benefit from the shared security of the Polkadot relay chain and integrations with other chains that are connected to Polkadot. Currently in active development by PureStake, Moonbeam is expected to reach MainNet by early 2021. Learn more:

About PureStake

PureStake’s team has extensive experience building technology companies and complex software platforms. Led by Derek Yoo, former Fuze Founder and CTO, PureStake provides protocol implementation services and creates developer tools for next-generation blockchain networks. Learn more: