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Module 6: Tips & Tricks

SubQuery TeamAlrededor de 4 min

Module 6: Tips & Tricks


In these exercises, we will learn what SubQuery Project is, how to deploy a project to SubQuery Project, and learn about SubQuery Explorer.



Block v Events v Calls

To make a SubQuery project process and index data as fast and as efficient as possible, it is necessary to understand how things work under the covers. SubQuery has three handlers to process blockchain data. They are block handlersopen in new window, event handlersopen in new window, and call handlersopen in new window.

Block handlers are very inefficient. They inspect every single block to grab data to index. This means that with over seven million blocks, if each block could be indexed in 10ms, this would take over eight (8) days to fully index the blockchain. Therefore, it is advisable to avoid using block handlers if possible.

Event and call handlers are the recommended handlers to use, in conjunction with mapping filters of course, as the performance is much better. The mapping filter allows the project to index only the blocks that satisfy the filter criteria. For example, below shows a filter indexing the staking module and the Rewarded method.

- handler: myCustomHandle
    kind: substrate/EventHandler
       module: staking
       method: Rewarded

For even more performance gains, using a dictionary is also highly recommended.

Using a dictionary

The dictionary concept was introduced in previous modules (e.g. Module 5 “Overriding endpoints) but mentioned here again due to its importance. See Understanding how a dictionary worksopen in new window

Event & Extrinsic Names

A popular question when creating SubQuery projects is how do I know what data I can extract from the Polkadot blockchain? There are several resource options:

  1. Polkadot documentation
  2. Polkadot blockchain explorer
  3. Interacting with the Polkadot API directly.

Note: These options may also apply to other blockchains.

Polkadot Documentation

The Polkadot documentation can be found at: in new window.

Polkadot Explorer

The Polkadot blockchain explorer is also a great place to become familiar with what information is available to be extracted. There are several options to choose from:

Note that not all explorers are equal and some may be easier to use and some represent the data you need in a clearer manner. You’ll end up with a few favourites over time.

The Polkadot CLI API

The previous two methods of understanding what blockchain data is available and also the type (which is just as important) is great but learning to connect directly to the Polkadot API via command line provides several advantages.

Firstly, it provides access to the most up to date API specifications because the documentationopen in new window could be slightly outdated. Secondly,it allows developers to understand the exact arguments and their types. This is especially important when there are issues and debugging is required. And finally, this is very useful when integrating with custom chains where sometimes documentation is not available.

Connecting to the API

To connect to the API, run the following:

npm install --save @polkadot/api
node --experimental-repl-await
const { ApiPromise, WsProvider } = require(`@polkadot/api`)
const provider = new

If custom chainTypes are required:

const types={}

If types are needed:

api = await ApiPromise.create({ provider, types});

Without types:

api = await ApiPromise.create({ provider });

Fetching a block

To get block hash at height h, we can get run const blockHash = await api.rpc.chain.getBlockHash(h)

const blockHash = await api.rpc.chain.getBlockHash(1234567)


const block = await api.rpc.chain.getBlock(blockHash)

Getting extrinsics within a block

To get all extrinsics:

const extrinsics = block.block.extrinsics;

For a particular extrinsic: (Change the 1 to a desired extrinsic number)

const myExtrinsic = extrinsic[1];

To check the args (input for transaction) types, enter:


You should see a Vec / array. The size of the array means how many arg this extrinsics takes, and each arg metadata info should include 'name', 'type', 'typeName'. The type is what we are looking for. eg: 'MultiAddress' is the type interface from Polkadot/api.

Getting events at a certain block height

Events can not be extracted from a block, but they can be queried. Since we already have the blockHash (from above), we can ‘lock’ the current API to this particular block height. Start with:

const apiAt = await


const events = await;

Next, specify the specific event of interest. Eg for event number 4:

const myEvent = events[4];

Finally, enter:


And you should see something like this:

> myEvent.event.meta.toJSON()
    name: 'Withdraw',
    fields: [
            { name: null, type: 0 , typeName: 'T::AccountId', docs: [] },
            { name: null, type: 6 , typeName: 'T::Balance', docs: [] }
    index: 8 ,
    docs: [
        'Some amount was withdrawn from the account (e.g. for transaction fees). \\[who, value\\]'
    args: [
        'AccountId32', 'u128'

Type Safe Properties

"Type safe" usually refers to languages that ensure that an operation is working on the right kind of data at some point before the operation is actually performed. This may be at compile time or at run time.

In Polkadot, everything has a typeopen in new window. This means that any variable created needs to have a type cast. For example:

record.blockNumber = event.block.block.header.number.toBigInt();
record.amount = (numberYes as Int).toNumber();
record.bigAmount = (data as Balance).toBigInt();
record.bool = (data as bool).valueOf();


To log data to the CLI from within the mappings functions, when a subql node is running, the command can be used:"Blockheight: " + event.block.block.header.number.toNumber());

When running a subql node locally via a command line, a log level can also be added to help troubleshoot. See Subql CLI logging referenceopen in new window


In order to debug SubQuery projects such as stepping through code, setting breakpoints, and inspecting variables, you will have to use a Node.js inspector in conjunction with Chrome developer tools. See How to debug a SubQuery project?open in new window

To debug a local subql node, the --debug flag can also be used from the command line. Eg

> subql-node -f . --debug

Changing the batch block size

Using a smaller batch size can reduce memory usage and not leave users hanging for large queries. In other words, your application can be more responsive. See How to change the batch block sizeopen in new window

Changing the starting block

Note that some events only start to occur at higher block height so one way to test a mapping function faster is to adjust the starting block height. See How to start at a different block height?open in new window