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Event Handlers

Registration​

A handler is a function that receives blockchain data, processes it, and inserts it into the database. You can register handlers in the file defined in the handler field in your config.yaml file. By default this is src/EventHandlers.* file.

import { <CONTRACT_NAME> } from "generated";

<CONTRACT_NAME>.<EVENT_NAME>.handler(async ({ event, context }) => {
  // Your logic here
});
note

The generated module contains code and types based on config.yaml and schema.graphql files. Update it by running pnpm codegen command whenever you change these files.

Basic Example​

Here's a handler example for the NewGreeting event. It belongs to the Greeter contract from our beginners Greeter Tutorial:

import { Greeter, User } from "generated";

// Handler for the NewGreeting event
Greeter.NewGreeting.handler(async ({ event, context }) => {
  const userId = event.params.user; // The id for the User entity
  const latestGreeting = event.params.greeting; // The greeting string that was added
  const currentUserEntity = await context.User.get(userId); // Optional user entity that may already exist

  // Update or create a new User entity
  const userEntity: User = currentUserEntity
    ? {
        id: userId,
        latestGreeting,
        numberOfGreetings: currentUserEntity.numberOfGreetings + 1,
        greetings: [...currentUserEntity.greetings, latestGreeting],
      }
    : {
        id: userId,
        latestGreeting,
        numberOfGreetings: 1,
        greetings: [latestGreeting],
      };

  context.User.set(userEntity); // Set the User entity in the DB
});

Advanced Use Cases​

HyperIndex provides many features to help you build more powerful and efficient indexers. Read more about these on separate pages:

Event Object​

Each handler receives an event object containing details about the emitted event, including parameters and blockchain metadata.

Accessing Event Parameters​

Event parameters are accessed via:

event.params.<PARAMETER_NAME>

Example usage:

const sender = event.params.sender;
const amount = event.params.amount;

Additional Event Information​

The event object also contains additional metadata:

  • event.chainId – Chain ID of the network emitting the event.
  • event.srcAddress – Contract address emitting the event.
  • event.logIndex – Index of the log within the block.
  • event.block – Block fields (By default: number, timestamp, hash).
  • event.transaction – Transaction fields (eg hash, gasUsed, etc. Empty by default).
note

Configure block and transaction fields with field_selection in your config.yaml file.

Example event type definition:

type Event<Params, TransactionFields, BlockFields> = {
  params: Params;
  chainId: 1 | 137;
  srcAddress: string;
  logIndex: number;
  transaction: TransactionFields;
  block: BlockFields;
};

Context Object​

The handler context provides methods to interact with entities stored in the database.

Retrieving Entities​

Retrieve entities asynchronously using get:

await context.<ENTITY_NAME>.get(entityId);

Modifying Entities​

Use set to create or update an entity:

context.<ENTITY_NAME>.set(entityObject);

Deleting Entities (Unsafe)​

To delete an entity:

context.<ENTITY_NAME>.deleteUnsafe(entityId);
warning

The deleteUnsafe method is experimental and unsafe. Manually handle all entity references after deletion to maintain database consistency.

Updating Specific Entity Fields​

Use the following approach to update specific fields in an existing entity:

const pool = await context.Pool.get(poolId);
if (pool) {
  context.Pool.set({
    ...pool,
    totalValueLockedETH: pool.totalValueLockedETH.plus(newDeposit),
  });
}

context.log​

The context object also provides a logger that you can use to log messages to the console. Compared to console.log calls, these logs will be displayed on our Hosted Service runtime logs page.

Read more in the Logging Guide.

External Calls​

Envio indexer runs using Node.js runtime. This means that you can use fetch or any other library like viem to perform external calls from your handlers.

Check out our IPFS Integration and Accessing Contract State guides for more information.

context.effect (Experimental)​

To ensure consistent and reliable data, all handlers are executed synchronously in the on-chain order. This means that external calls might easily blow up the processing time.

To avoid this, you can use Loaders together with Effect API to parallelize external calls and make the indexing process more efficient.

import { experimental_createEffect, S } from "envio";
import { ERC20 } from "generated";

// Define an effect that will be called from the handler.
const getMetadata = experimental_createEffect(
  {
    name: "getMetadata",
    input: S.string,
    output: {
      description: S.string,
      value: S.bigint,
    },
  },
  ({ input }) => {
    const response = await fetch(`https://api.example.com/metadata/${input}`);
    const data = await response.json();
    return {
      description: data.description,
      value: data.value,
    };
  }
);

ERC20.Transfer.handlerWithLoader({
  loader: async ({ event, context }) => {
    // Load metadata for the token.
    // This will be executed in parallel for all events in the batch.
    // The call is automatically memoized, so you don't need to worry about duplicate requests.
    const sender = await context.effect(getMetadata, event.params.from);

    // Return the loaded data to the handler
    return {
      sender,
    };
  },
  handler: async ({ event, context, loaderReturn }) => {
    const { sender } = loaderReturn;

    // Process the transfer with the pre-loaded data
  },
});

Read more about the Effect API and Loaders in the dedicated guides.

Accessing config.yaml Data in Handlers​

You can access your indexer configuration within handlers using getConfigByChainId:

import { getConfigByChainId } from "../generated/src/ConfigYAML.gen";

Greeter.NewGreeting.handler(async ({ event, context }) => {
  const config = getConfigByChainId(event.chainId);
});

This exposes configuration data such as:

  • syncSource, startBlock, confirmedBlockThreshold
  • Contract-specific data (abi, addresses, events)

Performance Considerations​

For performance optimization and best practices, refer to:

These guides offer detailed recommendations on optimizing entity loading and indexing performance.