Basic traces

The simplest type of transaction trace that Etn-sc can generate are raw EVM opcode traces. For every VM instruction the transaction executes, a structured log entry is emitted, containing all contextual metadata deemed useful. This includes the program counter, opcode name, opcode cost, remaining gas, execution depth and any occurred error. The structured logs can optionally also contain the content of the execution stack, execution memory and contract storage.

The entire output of a raw EVM opcode trace is a JSON object having a few metadata fields: consumed gas, failure status, return value; and a list of opcode entries:

{
  "gas": 25523,
  "failed": false,
  "returnValue": "",
  "structLogs": []
}

An example log for a single opcode entry has the following format:

{
  "pc": 48,
  "op": "DIV",
  "gasCost": 5,
  "gas": 64532,
  "depth": 1,
  "error": null,
  "stack": [
    "00000000000000000000000000000000000000000000000000000000ffffffff",
    "0000000100000000000000000000000000000000000000000000000000000000",
    "2df07fbaabbe40e3244445af30759352e348ec8bebd4dd75467a9f29ec55d98d"
  ],
  "memory": [
    "0000000000000000000000000000000000000000000000000000000000000000",
    "0000000000000000000000000000000000000000000000000000000000000000",
    "0000000000000000000000000000000000000000000000000000000000000060"
  ],
  "storage": {}
}

Generating basic traces

To generate a raw EVM opcode trace, Etn-sc provides a few RPC API endpoints. The most commonly used is debug_traceTransaction.

In its simplest form, traceTransaction accepts a transaction hash as its only argument. It then traces the transaction, aggregates all the generated data and returns it as a large JSON object. A sample invocation from the Etn-sc console would be:

debug.traceTransaction('0xfc9359e49278b7ba99f59edac0e3de49956e46e530a53c15aa71226b7aa92c6f');

The same call can also be invoked from outside the node too via HTTP RPC (e.g. using Curl). In this case, the HTTP endpoint must be enabled in Etn-sc using the --http command and the debug API namespace must be exposed using --http.api=debug.

$ curl -H "Content-Type: application/json" -d '{"id": 1, "jsonrpc": "2.0", "method": "debug_traceTransaction", "params": ["0xfc9359e49278b7ba99f59edac0e3de49956e46e530a53c15aa71226b7aa92c6f"]}' localhost:8545

To follow along with this tutorial, transaction hashes can be found from a local Etn-sc node (e.g. by attaching a Javascript console and running eth.getBlock('latest') then passing a transaction hash from the returned block to debug.traceTransaction()) or from a block explorer (for Mainnet or a testnet).

It is also possible to configure the trace by passing Boolean (true/false) values for four parameters that tweak the verbosity of the trace. By default, the EVM memory and Return data are not reported but the EVM stack and EVM storage are. To report the maximum amount of data:

enableMemory: true
disableStack: false
disableStorage: false
enableReturnData: true

An example call, made in the Etn-sc Javascript console, configured to report the maximum amount of data looks as follows:

debug.traceTransaction('0xfc9359e49278b7ba99f59edac0e3de49956e46e530a53c15aa71226b7aa92c6f', {
  enableMemory: true,
  disableStack: false,
  disableStorage: false,
  enableReturnData: true
});

Alternatively, disabling EVM Stack, EVM Memory, Storage and Return data (as demonstrated in the Curl request below) results in the following, much shorter, trace dump.

$ curl -H "Content-Type: application/json" -d '{"id": 1, "jsonrpc": "2.0", "method": "debug_traceTransaction", "params": ["0xfc9359e49278b7ba99f59edac0e3de49956e46e530a53c15aa71226b7aa92c6f", {"disableStack": true, "disableStorage": true}]}' localhost:8545

Limits of basic traces

Although the raw opcode traces generated above are useful, having an individual log entry for every single opcode is too low level for most use cases, and will require developers to create additional tools to post-process the traces. Additionally, a full opcode trace can easily go into the hundreds of megabytes, making them very resource intensive to get out of the node and process externally.

To avoid those issues, Etn-sc supports running custom JavaScript tracers within the Electroneum node, which have full access to the EVM stack, memory and contract storage. This means developers only have to gather the data they actually need, and do any processing at the source.

Summary

This page described how to do basic traces in Etn-sc. Basic traces are very low level and can generate lots of data that might not all be useful. Therefore, it is also possible to use a set of built-in tracers or write custom ones in Javascript or Go.

Read more about built-in and custom traces.