Debug API & Trace Module

Introduction

Geth's debug and txpool APIs and OpenEthereum's trace module provide non-standard RPC methods for getting a deeper insight into transaction processing. As part of Moonbeam's goal of providing a seamless Ethereum experience for developers, there is support for some of these non-standard RPC methods. Supporting these RPC methods is an important milestone because many projects, such as The Graph or Blockscout, rely on them to index blockchain data.

This guide will cover the supported RPC methods available on Moonbeam as well as how to invoke the methods using curl commands against a local Moonbase Alpha tracing node.

Supported RPC Methods

The following RPC methods are available:

• debug_traceTransaction
• debug_traceBlockByNumber
• debug_traceBlockByHash
• trace_filter
• txpool_content
• txpool_inspect
• txpool_status

Debug API

The debug RPC implementations follow Geth's debug API guidelines:

• debug_traceTransaction - requires the hash of the transaction to be traced
• debug_traceBlockByNumber - requires the block number of the block to be traced and an additional parameter that sets the tracer to callTracer (i.e. {"tracer": "callTracer"})
• debug_traceBlockByHash - requires the hash of the block to be traced and an additional parameter that sets the tracer to callTracer (i.e. {"tracer": "callTracer"})

As optional parameters for the supported debug methods, you can provide the following:

• disableStorage(boolean) — (default: false) setting this to true disables storage capture
• disableMemory(boolean) — (default: false) setting this to true disables memory capture
• disableStack(boolean) — (default: false) setting this to true disables stack capture

Txpool API

The txpool RPC implementations follow Geth's txpool API guidelines:

• txpool_content - no required or optional parameters
• txpool_inspect - no required or optional parameters
• txpool_status - no required or optional parameters

Trace Module

The trace_filter RPC implementation follows OpenEthereum's trace module guidelines. The RPC method requires any of the following optional parameters:

• fromBlock(uint blockNumber) — either block number (hex), earliest which is the genesis block or latest (default) best block available. Trace starting block
• toBlock(uint blockNumber) — either block number (hex), earliest which is the genesis block or latest best block available. Trace ending block
• fromAddress(array addresses) — filter transactions done from these addresses only. If an empty array is provided, no filtering is done with this field
• toAddress(array addresses) — filter transactions done from these addresses only. If an empty array is provided, no filtering is done with this field
• after(uint offset) — default offset is 0. Trace offset (or starting) number
• count(uint numberOfTraces) — number of traces to display in a batch

There are a couple default values that you should be aware of:

• The maximum number of trace entries a single request of trace_filter is allowed to return is 500. A request exceeding this limit will return an error
• Blocks processed by requests are temporarily stored on cache for 300 seconds, after which they are deleted

To change the default values you can add Additional Flags when spinning up your tracing node.

Checking Prerequisites

For this guide, you will need to have a locally running instance of a Moonbase Alpha tracing node with the debug, txpool, and tracing flags enabled for this guide. You can also adapt the instructions for Moonbeam and Moonriver.

If you haven't already done so, you can follow the guide on Running a Tracing Node. The RPC HTTP endpoint should be at http://127.0.0.1:9944.

If you have a running node, you should see a similar terminal log:

Using the Debug API

Once you have a running tracing node, you can open another tab in your terminal where you can run curl commands and start to call any of the available JSON RPC methods. For example, for the debug_traceTransaction method, you can make the following JSON RPC request in your terminal (in this case, for the transaction hash 0x04978f83e778d715eb074352091b2159c0689b5ae2da2554e8fe8e609ab463bf):

curl http://127.0.0.1:9944 -H "Content-Type:application/json;charset=utf-8" -d \
  '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"debug_traceTransaction",
    "params": ["0x04978f83e778d715eb074352091b2159c0689b5ae2da2554e8fe8e609ab463bf"]
  }'

The node responds with the step-by-step replayed transaction information (response was cropped as it is quite long):

If you're using the debug_traceBlockByNumber or debug_traceBlockByHash methods, you will need to add {"tracer": "callTracer"} to the "params". The callTracer will only return transactions and subcalls. Otherwise, the tracer will attempt to default to raw, which is not supported at this time due to the heavy nature of the call. For example, for the debug_traceBlockByHash method, you can make the following JSON RPC request in your terminal (in this case, for the block hash 0x2633b66050c99d80f65fe96de6485fd407b87f0f59b485c33ab8f119e2c6f255):

curl http://127.0.0.1:9944 -H "Content-Type:application/json;charset=utf-8" -d \
  '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"debug_traceBlockByHash",
    "params": ["0x2633b66050c99d80f65fe96de6485fd407b87f0f59b485c33ab8f119e2c6f255", {"tracer": "callTracer"}]
  }'

Using the Tracing Module

For the trace_filter call, you can make the following JSON RPC request in your terminal (in this case, the filter is from block 20000 to 25000, only for transactions where the recipient is 0x4E0078423a39EfBC1F8B5104540aC2650a756577, it will start with a zero offset and provide the first 20 traces):

curl http://127.0.0.1:9944 -H "Content-Type:application/json;charset=utf-8" -d \
  '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"trace_filter", "params":[{"fromBlock":"0x4E20","toBlock":"0x5014","toAddress":["0x4E0078423a39EfBC1F8B5104540aC2650a756577"],"after":0,"count":20}]
  }'

The node responds with the trace information corresponding to the filter (response was cropped as it is quite long).

Using the Txpool API

Since none of the currently supported txpool methods require a parameter, you can adapt the following curl command by changing the method for any of the txpool methods:

curl http://127.0.0.1:9944 -H "Content-Type:application/json;charset=utf-8" -d \
  '{
    "jsonrpc":"2.0",
    "id":1,
    "method":"txpool_status", "params":[]
  }'

For this example, the txpool_status method will return the number of transactions currently pending or queued.

Last update: March 2, 2023
| Created: April 14, 2021