Querying Best Practices

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The Graph provides a decentralized way to query data from blockchains. Its data is exposed through a GraphQL API, making it easier to query with the GraphQL language.

Learn the essential GraphQL language rules and best practices to optimize your subgraph.

A differenza del REST API, un GraphQL API si basa su uno schema che definisce le query che possono essere eseguite.

Ad esempio, una query per ottenere un token utilizzando la query token avrà il seguente aspetto:

query GetToken($id: ID!) {
  token(id: $id) {
    id
    owner
  }
}

che restituirà la seguente risposta JSON prevedibile (quando si passa il valore corretto della variabile $id):

{
  "token": {
    "id": "...",
    "owner": "..."
  }
}

Le query GraphQL utilizzano il linguaggio GraphQL, definito da una specificazione.

La query GetToken di cui sopra è composta da più parti linguistiche (sostituite di seguito con i segnaposto [...]):

query [operationName]([variableName]: [variableType]) {
  [queryName]([argumentName]: [variableName]) {
    # "{ ... }" express a Selection-Set, we are querying fields from `queryName`.
    [field]
    [field]
  }
}

• Ogni queryName deve essere usato una sola volta per ogni operazione.
• Ogni field deve essere utilizzato una sola volta in una selezione (non si può effettuare query di id due volte sotto token)
• Some fields or queries (like tokens) return complex types that require a selection of sub-field. Not providing a selection when expected (or providing one when not expected - for example, on id) will raise an error. To know a field type, please refer to Graph Explorer.
• Qualsiasi variabile assegnata a un argomento deve corrispondere al suo tipo.
• In un determinato elenco di variabili, ciascuna di esse deve essere unica.
• Tutte le variabili definite devono essere utilizzate.

For a complete list of rules with code examples, check out GraphQL Validations guide.

GraphQL is a language and set of conventions that transport over HTTP.

It means that you can query a GraphQL API using standard fetch (natively or via @whatwg-node/fetch or isomorphic-fetch).

However, as mentioned in "Querying from an Application", it's recommended to use graph-client, which supports the following unique features:

• Gestione dei subgraph a cross-chain: effettuare query di più subgraph in un'unica query
• Tracciamento automatico dei blocchi
• Paginazione automatica
• Risultato completamente tipizzato

Here's how to query The Graph with graph-client:

import { execute } from '../.graphclient'

const query = `
query GetToken($id: ID!) {
  token(id: $id) {
    id
    owner
  }
}
`
const variables = { id: '1' }

async function main() {
  const result = await execute(query, variables)
  // `result` is fully typed!
  console.log(result)
}

main()

More GraphQL client alternatives are covered in "Querying from an Application".

A common (bad) practice is to dynamically build query strings as follows:

const id = params.id
const fields = ['id', 'owner']
const query = `
query GetToken {
  token(id: ${id}) {
    ${fields.join('\n')}
  }
}
`

// Execute query...

While the above snippet produces a valid GraphQL query, it has many drawbacks:

• rende più difficile la comprensione della query nel suo insieme
• gli sviluppatori sono responsabili della sanificazione sicura dell'interpolazione delle stringhe
• non inviare i valori delle variabili come parte dei parametri della richiesta previene eventuali cache sul lato server
• impedisce agli strumenti di analizzare staticamente la query (ad esempio, Linter o strumenti di generazione dei tipi)

For this reason, it is recommended to always write queries as static strings:

import { execute } from 'your-favorite-graphql-client'

const id = params.id
const query = `
query GetToken($id: ID!) {
  token(id: $id) {
    id
    owner
  }
}
`

const result = await execute(query, {
  variables: {
    id,
  },
})

Doing so brings many advantages:

• Query facili da leggere e da mantenere
• Il server GraphQL gestisce la sanificazione delle variabili
• Le variabili possono essere messe in cache a livello di server
• Le query possono essere analizzate staticamente dagli strumenti (maggiori informazioni nelle sezioni successive)

You might want to include the owner field only on a particular condition.

For this, you can leverage the @include(if:...) directive as follows:

import { execute } from 'your-favorite-graphql-client'

const id = params.id
const query = `
query GetToken($id: ID!, $includeOwner: Boolean) {
  token(id: $id) {
    id
    owner @include(if: $includeOwner)
  }
}
`

const result = await execute(query, {
  variables: {
    id,
    includeOwner: true,
  },
})

GraphQL became famous for its "Ask for what you want" tagline.

For this reason, there is no way, in GraphQL, to get all available fields without having to list them individually.

• Quando si interrogano le GraphQL API, si deve sempre pensare di effettuare query di solo i campi che verranno effettivamente utilizzati.
• Make sure queries only fetch as many entities as you actually need. By default, queries will fetch 100 entities in a collection, which is usually much more than what will actually be used, e.g., for display to the user. This applies not just to top-level collections in a query, but even more so to nested collections of entities.

For example, in the following query:

query listTokens {
  tokens {
    # will fetch up to 100 tokens
    id
    transactions {
      # will fetch up to 100 transactions
      id
    }
  }
}

The response could contain 100 transactions for each of the 100 tokens.

If the application only needs 10 transactions, the query should explicitly set first: 10 on the transactions field.

By default, subgraphs have a singular entity for one record. For multiple records, use the plural entities and filter: where: {id_in:[X,Y,Z]} or where: {volume_gt:100000}

Example of inefficient querying:

query SingleRecord {
  entity(id: X) {
    id
    name
  }
}
query SingleRecord {
  entity(id: Y) {
    id
    name
  }
}

Example of optimized querying:

query ManyRecords {
  entities(where: { id_in: [X, Y] }) {
    id
    name
  }
}

Your application might require querying multiple types of data as follows:

import { execute } from "your-favorite-graphql-client"

const tokensQuery = `
query GetTokens {
  tokens(first: 50) {
    id
    owner
  }
}
`
const countersQuery = `
query GetCounters {
  counters {
    id
    value
  }
}
`

const [tokens, counters] = Promise.all(
  [
    tokensQuery,
    countersQuery,
  ].map(execute)
)

While this implementation is totally valid, it will require two round trips with the GraphQL API.

Fortunately, it is also valid to send multiple queries in the same GraphQL request as follows:

import { execute } from "your-favorite-graphql-client"

const query = `
query GetTokensandCounters {
  tokens(first: 50) {
    id
    owner
  }
  counters {
    id
    value
  }
}
`

const  { result: { tokens, counters } } = execute(query)

This approach will improve the overall performance by reducing the time spent on the network (saves you a round trip to the API) and will provide a more concise implementation.

A helpful feature to write GraphQL queries is GraphQL Fragment.

Looking at the following query, you will notice that some fields are repeated across multiple Selection-Sets ({ ... }):

query {
  bondEvents {
    id
    newDelegate {
      id
      active
      status
    }
    oldDelegate {
      id
      active
      status
    }
  }
}

Such repeated fields (id, active, status) bring many issues:

• More extensive queries become harder to read.
• When using tools that generate TypeScript types based on queries (more on that in the last section), newDelegate and oldDelegate will result in two distinct inline interfaces.

A refactored version of the query would be the following:

query {
  bondEvents {
    id
    newDelegate {
      ...DelegateItem
    }
    oldDelegate {
      ...DelegateItem
    }
  }
}

# we define a fragment (subtype) on Transcoder
# to factorize repeated fields in the query
fragment DelegateItem on Transcoder {
  id
  active
  status
}

Using GraphQL fragment will improve readability (especially at scale) and result in better TypeScript types generation.

When using the types generation tool, the above query will generate a proper DelegateItemFragment type (see last "Tools" section).

A Fragment cannot be based on a non-applicable type, in short, on type not having fields:

fragment MyFragment on BigInt {
  # ...
}

BigInt is a scalar (native "plain" type) that cannot be used as a fragment's base.

Fragments are defined on specific types and should be used accordingly in queries.

Example:

query {
  bondEvents {
    id
    newDelegate {
      ...VoteItem # Error! `VoteItem` cannot be spread on `Transcoder` type
    }
    oldDelegate {
      ...VoteItem
    }
  }
}

fragment VoteItem on Vote {
  id
  voter
}

newDelegate and oldDelegate are of type Transcoder.

It is not possible to spread a fragment of type Vote here.

GraphQL Fragments must be defined based on their usage.

For most use-case, defining one fragment per type (in the case of repeated fields usage or type generation) is sufficient.

Here is a rule of thumb for using fragments:

• When fields of the same type are repeated in a query, group them in a Fragment.
• When similar but different fields are repeated, create multiple fragments, for instance:

# base fragment (mostly used in listing)
fragment Voter on Vote {
  id
  voter
}

# extended fragment (when querying a detailed view of a vote)
fragment VoteWithPoll on Vote {
  id
  voter
  choiceID
  poll {
    id
    proposal
  }
}

Iterating over queries by running them in your application can be cumbersome. For this reason, don't hesitate to use Graph Explorer to test your queries before adding them to your application. Graph Explorer will provide you a preconfigured GraphQL playground to test your queries.

If you are looking for a more flexible way to debug/test your queries, other similar web-based tools are available such as Altair and GraphiQL.

In order to keep up with the mentioned above best practices and syntactic rules, it is highly recommended to use the following workflow and IDE tools.

GraphQL ESLint

GraphQL ESLint will help you stay on top of GraphQL best practices with zero effort.

Setup the "operations-recommended" config will enforce essential rules such as:

• @graphql-eslint/fields-on-correct-type: un campo è utilizzato su un tipo corretto?
• @graphql-eslint/no-unused variables: una determinata variabile deve rimanere inutilizzata?
• e altro ancora!

This will allow you to catch errors without even testing queries on the playground or running them in production!

VSCode and GraphQL

The GraphQL VSCode extension is an excellent addition to your development workflow to get:

• Syntax highlighting
• Autocomplete suggestions
• Validation against schema
• Snippets
• Go to definition for fragments and input types

If you are using graphql-eslint, the ESLint VSCode extension is a must-have to visualize errors and warnings inlined in your code correctly.

WebStorm/Intellij and GraphQL

The JS GraphQL plugin will significantly improve your experience while working with GraphQL by providing:

• Syntax highlighting
• Autocomplete suggestions
• Validation against schema
• Snippets

For more information on this topic, check out the WebStorm article which showcases all the plugin's main features.