Polygon API polling method used to retrieve updates from a filter. A filter is an object used to track changes to the state of the blockchain.
Filters can be created using one of the following methods:
After creating a filter using one of the available methods, the resulting filter ID can be used to fetch changes by calling the eth_getFilterChanges
method.
Disclaimer
Note that the default interactive example in this page will not work as the filter will be expired.
To test
eth_getFilterChanges
in this page, first create a new filter using one of the following:Then use the fresh filter ID as the parameter for
eth_getFilterChanges
.
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Parameters
string
β the filter ID of the filter for which you want to retrieve the changes
Response
array
β an array that represents the changes that have occurred on the blockchain since the last time the filter was polled:- For filters created with
eth_newBlockFilter
:blockHash
β the hashes of the new blocks since the last time the filter was polled.
- For filters created with
eth_newPendingTransactionFilter
:transactionHash
β the hashes identifying new pending transactions since the last time the filter was polled.
- For filters created with
eth_newFilter
, the following event logs:address
β the contract address from which the event originated.topics
β an array of 32-byte data fields containing indexed event parameters.data
β the non-indexed data that was emitted along with the event.blocknumber
β the block number in which the event was included.null
if it is pending.transactionhash
β the hash of the transaction that triggered the event.null
if pending.transactionindex
β the integer index of the transaction within the block's list of transactions.null
if it is pending.blockhash
β the hash of the block in which the event was included.null
if it is pending.logindex
β the integer identifying the index of the event within the block's list of events.null
if pending.removed
β the boolean value indicating if the event was removed from the blockchain due to a chain reorganization.True
if the log was removed.False
if it is a valid log.
- For filters created with
eth_getFilterChanges
code examples
eth_getFilterChanges
code examplesNote that the
web3.eth.filter
methods have been deprecated and replaced with theweb3.eth.subscribe
in web3.js. See web3.js subscriptions.
const ethers = require('ethers');
const NODE_URL = "CHAINSTACK_NODE_URL";
const provider = new ethers.JsonRpcProvider(NODE_URL);
async function getFilterChanges(filter) {
try {
const changes = await provider.send('eth_getFilterChanges', [filter]);
console.log(changes); // Log the with the new data
} catch (err) {
console.error(err); // Handle errors that may occur
}
}
const filterId = '0x4e7ef166cd43f188b0f8f9e218966a8f' //'YOUR_FILTER_iD'
getFilterChanges(filterId)
from web3 import Web3
node_url = "CHAINSTACK_NODE_URL"
web3 = Web3(Web3.HTTPProvider(node_url))
def get_filtered_events(filter_id):
try:
filter_changes = web3.eth.get_filter_changes(filter_id)
print(filter_changes) # the array of events that match the filter
except Exception as e:
print(e)
filter_id = '0x0'
get_filtered_events(filter_id)
Use case
The eth_getFilterChanges
method can be used to periodically poll a filter to retrieve new data. In the following Python program, using the web3.py library creates a filter using eth_newBlockFilter to poll for new blocks, and uses eth_getFilterChanges
to poll the filter and retrieve new blocks.
Here is the implementation in Python:
from web3 import Web3
node_url = "CHAINSTACK_NODE_URL"
web3 = Web3(Web3.HTTPProvider(node_url))
def create_block_filter():
try:
# Create a new block filter
block_filter = web3.eth.filter('latest')
# Split the string at the space character
parts = str(block_filter).split(' ')
# Extract the filter value from the second part
filter_id = parts[2]
return filter_id
except Exception as e:
print(f'Error creating block filter: {e}')
def get_filtered_events(filter_id):
try:
filter_changes = web3.eth.getFilterChanges(filter_id)
return filter_changes # the array of events that match the filter
except Exception as e:
print(f'Error getting filter changes: {e}')
def main():
# Create a block filter
block_filter_id = create_block_filter()
if block_filter_id is not None:
print(f'New block filter ID: {block_filter_id}')
else:
print('Error creating block filter')
return
# Loop forever and poll the filter every 10 seconds
while True:
print('Polling for new blocks...')
new_blocks = get_filtered_events(block_filter_id)
if new_blocks is not None:
print(new_blocks)
else:
print('Error getting filter changes')
time.sleep(10)
if __name__ == '__main__':
main()
Here is the explanation of the code:
The create_block_filter
function creates a new block filter with the web3.eth.filter('latest')
method, which matches against the latest block. The resulting filter object is converted to a string, split into parts using the split
method and the filter ID is extracted from the second part using array indexing. The filter ID is then returned by the function.
The get_filtered_events
function uses the eth_getFilterChanges
method to retrieve the new blocks. The function takes a single parameter, filter_id
, which is a string that represents the ID of the filter to be polled, the filter created by the create_block_filter
function in this case.
If the filter ID is valid, get_filtered_events
calls web3.eth.getFilterChanges(filter_id)
to retrieve the array of events that match the filter criteria. This array is then returned by the function, which is then printed to the console in the main
function.
The main
function encapsulates the program logic, calling create_block_filter
to create a new block filter and print the resulting filter ID to the console. The function then enters an infinite loop, calling get_filtered_events
every 10 seconds to retrieve the changes to the blockchain that match the filter criteria. If the changes are not None
, they are printed to the console. An error message is printed to the console if an error occurs while creating the block filter or retrieving the filter changes.
Try the eth_getFilterChanges
RPC method yourself
eth_getFilterChanges
RPC method yourself