> For the complete documentation index, see [llms.txt](https://developer.electroneum.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.electroneum.com/etn-sc-client/interacting-with-etn-sc/json-rpc-server/batch-requests.md).

# Batch requests

The JSON-RPC [specification](https://www.jsonrpc.org/specification#batch) outlines how clients can send multiple requests at the same time by filling the request objects in an array. This feature is implemented by Etn-sc's API and can be used to cut network delays. Batching offers visible speed-ups specially when used for fetching larger amounts of mostly independent data objects.

Below is an example for fetching a list of blocks in JS:

```javascript
import fetch from 'node-fetch';

async function main() {
  const endpoint = 'http://127.0.0.1:8545';
  const from = parseInt(process.argv[2]);
  const to = parseInt(process.argv[3]);

  const reqs = [];
  for (let i = from; i < to; i++) {
    reqs.push({
      method: 'eth_getBlockByNumber',
      params: [`0x${i.toString(16)}`, false],
      id: i - from,
      jsonrpc: '2.0'
    });
  }

  const res = await fetch(endpoint, {
    method: 'POST',
    body: JSON.stringify(reqs),
    headers: { 'Content-Type': 'application/json' }
  });
  const data = await res.json();
}

main()
  .then()
  .catch(err => console.log(err));
```

In this case there's no dependency between the requests. Often the retrieved data from one request is needed to issue a second one. Let's take the example of fetching all the receipts for a range of blocks. The JSON-RPC API provides `eth_getTransactionReceipt` which takes in a transaction hash and returns the corresponding receipt object, but no method to fetch receipt objects for a whole block. We need to get the list of transactions in a block and then call `eth_getTransactionReceipt` for each of them.

We can break this into 2 batch requests:

* First to download the list of transaction hashes for all of the blocks in our desired range
* And then to download the list of receipts objects for all of the transaction hashes

For use-cases which depend on several JSON-RPC endpoints the batching approach can get easily complicated. In that case Etn-sc offers a [GraphQL API](/etn-sc-client/interacting-with-etn-sc/graphql-server.md) which is more suitable.


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