Skip to main content

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
});

Preload Optimization​

Important! Preload optimization makes your handlers run twice.

Starting from envio@2.27 all new indexers are created with preload optimization pre-configured by default.

This optimization enables HyperIndex to efficiently preload entities used by handlers through batched database queries, while ensuring events are processed synchronously in their original order. When combined with the Effect API for external calls, this feature delivers performance improvements of multiple orders of magnitude compared to other indexing solutions.

Read more in the dedicated guides:

Advanced Use Cases​

HyperIndex provides many features to help you build more powerful and efficient indexers. There's definitely the one for you:

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).
tip

All addresses returned in the event object, such as event.transaction.from, event.transaction.to, and event.srcAddress, are EIP-55 checksummed. Use .toLowerCase() if you specifically need lowercase values.

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 from the database using context.Entity.get where Entity is the name of the entity you want to retrieve, which is defined in your schema.graphql file.

await context.Entity.get(entityId);

It'll return Entity object or undefined if the entity doesn't exist.

Starting from envio@2.22.0 you can use context.Entity.getOrThrow to conveniently throw an error if the entity doesn't exist:

const pool = await context.Pool.getOrThrow(poolId);
// Will throw: Entity 'Pool' with ID '...' is expected to exist.

// Or you can pass a custom message as a second argument:
const pool = await context.Pool.getOrThrow(
  poolId,
  `Pool with ID ${poolId} is expected.`
);

Or use context.Entity.getOrCreate to automatically create an entity with default values if it doesn't exist:

const pool = await context.Pool.getOrCreate({
  id: poolId,
  totalValueLockedETH: 0n,
});

// Which is equivalent to:
let pool = await context.Pool.get(poolId);
if (!pool) {
  pool = {
    id: poolId,
    totalValueLockedETH: 0n,
  };
  context.Pool.set(pool);
}

Retrieving Entities by Field​

ERC20.Approval.handler(async ({ event, context }) => {
  // Find all approvals for this specific owner
  const currentOwnerApprovals = await context.Approval.getWhere.owner_id.eq(
    event.params.owner
  );

  // Process all the owner's approvals efficiently
  for (const approval of currentOwnerApprovals) {
    // Process each approval
  }
});

You can also use context.<Entity>.getWhere.<FieldName>.gt to get all entities where the field value is greater than the given value.

Important:

  • This feature requires Preload Optimization to be enabled.

    • Either by preload_handlers: true in your config.yaml file
    • Or by using Loaders (Deprecated)

  • Works with any field that:

  • Potential Memory Issues: Very large getWhere queries might cause memory overflows.

  • Tip: Try to put the getWhere query to the top of the handler, to make sure it's being preloaded. Read more about how Preload Optimization works.

Modifying Entities​

Use context.Entity.set to create or update an entity:

context.Entity.set({
  id: entityId,
  ...otherEntityFields,
});
note

Both context.Entity.set and context.Entity.deleteUnsafe methods use the In-Memory Storage under the hood and don't require await in front of them.

Referencing Linked Entities​

When your schema defines a field that links to another entity type, set the relationship using <field>_id with the referenced entity's id. You are storing the ID, not the full entity object.

type A {
  id: ID!
  b: B!
}

type B {
  id: ID!
}
context.A.set({
  id: aId,
  b_id: bId, // ID of the linked B entity
});

HyperIndex automatically resolves A.b based on the stored b_id when querying the API.

Deleting Entities (Unsafe)​

To delete an entity:

context.Entity.deleteUnsafe(entityId);
warning

The deleteUnsafe method is experimental and unsafe. You need to 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.

context.isPreload​

If you need to skip the preload phase for CPU-intensive operations or to perform certain actions only once per event, you can use context.isPreload.

ERC20.Transfer.handler(async ({ event, context }) => {
  // Load existing data efficiently
  const [sender, receiver] = await Promise.all([
    context.Account.getOrThrow(event.params.from),
    context.Account.getOrThrow(event.params.to),
  ]);

  // Skip expensive operations during preload
  if (context.isPreload) {
    return;
  }

  // CPU-intensive calculations only happen once
  const complexCalculation = performExpensiveOperation(event.params.value); // Placeholder function for demonstration

  // Create or update sender account
  context.Account.set({
    id: event.params.from,
    balance: sender.balance - event.params.value,
    computedValue: complexCalculation,
  });
  // Create or update receiver account
  context.Account.set({
    id: event.params.to,
    balance: receiver.balance + event.params.value,
  });
});

Note: While context.isPreload can be useful for bypassing double execution, it's recommended to use the Effect API for external calls instead, as it provides automatic batching and memoization benefits.

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.

Note that with Preload Optimization all handlers run twice. But with Effect API this behavior makes your external calls run in parallel, while keeping the processing data consistent.

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

context.effect​

Define an effect and use it in your handler with context.effect:

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,
    },
    cache: true, // Optionally persist the results in the database
  },
  ({ 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.handler(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);

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

Accessing config.yaml Data in Handlers​

You can access your indexer configuration within handlers using getConfigByChainId:

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

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