What function to use to get and cache the latest blockhash with the @solana/web3.js library?

Question

I found the following YouTube short on the official Solana channel.

At 0:20 min you can see a part of his code where he uses and caches the result from the BlockHash function.

Unfortunately, I'm not able to find anything about this function. Neither in the docs nor when I search the repository on GitHub.

Does someone know where I can find more information about this function?

Answer 1

You can find the getLatestBlockhash method on the Connection class that's exported by @solana/web3.js

You can find the reference in the documentation here.

Basic usage

To use it, you need to initialize the Connection class and you can call it, like so:

import { Connection } from '@solana/web3.js'
const conn = new Connection('your-rpc-endpoint', 'confirmed')
const result = await conn.getLatestBlockhash()
console.log(result)

This will output the blockhash and the lastValidBlockHeight:

{
  blockhash: 'G4oTG19N9mkWfNv6VUJzTtGCTFx2xw4JgWF1MhKA4hdG',
  lastValidBlockHeight: 225956979
}

Adding a cache

The @solana/web3.js package does not come with cache out of the box, but it's pretty easy to add it yourself.

In the example below, I'm using the library lru-cache but you can use other libraries.

To get this working, we first install the package and add it to our imports:

import { Connection } from '@solana/web3.js'
import { LRUCache } from 'lru-cache'

Next up, we define a cache that lives for 30 seconds:

// We need our connection
const conn = new Connection('your-rpc-endpoint', 'confirmed')

const cache = new LRUCache<string, string>({
  max: 1000,
  // 30 seconds
  ttl: 1000 * 30,
  // Define the fetch method for this cache
  fetchMethod: async () => {
    const result = await conn.getLatestBlockhash()
    console.log(`[CACHE WRITE] blockhash ${result.blockhash}`)
    return result.blockhash
  }
})

With this in place, we can call the fetch() function on the cache instance. It will retrieve the blockhash from the cache; if not found there, it will obtain it from Solana and store it in the cache.

You can see an example here:

async function main() {
  // Show the cached blockhash for 60 seconds
  const attempts = 60
  for (let i = 0; i < attempts; i++) {
    const start = Date.now()
    // Get the cached blockhash
    const blockhash = await cache.fetch('blockhash')
    // If the blockhash is not cached, throw an error
    if (!blockhash) {
      throw new Error('Expected blockhash')
    }
    // Show the blockhash
    console.log(`[CACHE READ] blockhash ${blockhash} took ${Date.now() - start}ms`)
    // Wait 1 second
    await new Promise((resolve) => setTimeout(resolve, 1000))
  }
}

main()

The output looks something like this:

[CACHE WRITE] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 544ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 0ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 0ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 0ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 3ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 0ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 0ms
[CACHE READ] blockhash DbFB5dTNgUAfshKQVva7hCiXJbUnUeWq8PHfUoFjJKF9 took 1ms
........

From looking at the results, there is a huge benefit in doing this and might lead to an enhanced user experience depending on the app.