Tracing allows users to examine precisely what was executed by the EVM during some specific transaction or set of transactions. There are two different types of [transactions](https://ethereum.org/en/developers/docs/transactions) in Ethereum: value transfers and contract executions. A value transfer just moves ETH from one account to another. A contract interaction executes some code stored at a contract address which can include altering stored data and transacting multiple times with other contracts and externally-owned accounts. A contract execution transaction can therefore be a complicated web of interactions that can be difficult to unpick. The transaction receipt contains a status code that shows whether the transaction succeeded or failed, but more detailed information is not readily available, meaning it is very difficult to know what a contract execution actually did, what data was modified and which addresses were touched. This is the problem that EVM tracing solves. Geth traces transactions by re-running them locally and collecting data about precisely what was executed by the EVM.
In its simplest form, tracing a transaction entails requesting the Ethereum node to reexecute the desired transaction with varying degrees of data collection and have it return an aggregated summary. In order for a Geth node to reexecute a transaction, all historical state accessed by the transaction must be available. This includes:
This means there are limits on the transactions that can be traced imposed by the synchronization and pruning configuration of a node:
- An **archive** node retains **all historical data** back to genesis. It can therefore trace arbitrary transactions at any point in the history of the chain. Tracing a single transaction requires reexecuting all preceding transactions in the same block.
- A **node synced from genesis** node only retains the most recent 128 block states in memory. Older states are represented by a sequence of occasional checkpoints that intermediate states can be regenerated from. This means that states within the most recent 128 blocks are immediately available, older states have to be regenerated from snapshots "on-the-fly". If the distance between the requested transaction and the most recent checkpoint is large, rebuilding the state can take a long time. Tracing a single transaction requires reexecuting all preceding transactions in the same block **and** all preceding blocks until the previous stored snapshot.
- A **snap synced** node holds the most recent 128 blocks in memory, so transactions in that range are always accessible. However, snap-sync only starts processing from a relatively recent block (as opposed to genesis for a full node). Between the initial sync block and the 128 most recent blocks, the node stores occasional checkpoints that can be used to rebuild the state on-the-fly. This means transactions can be traced back as far as the block that was used for the initial sync. Tracing a single transaction requires reexecuting all preceding transactions in the same block, **and** all preceding blocks until the previous stored snapshot.
- A **light synced** node retrieving data **on demand** can in theory trace transactions for which all required historical state is readily available in the network. This is because the data required to generate the trace is requested from an les-serving full node. In practice, data availability **cannot** be reasonably assumed.
When a trace of a specific transaction is executed, the state is prepared by fetching the state of the parent block from the database. If it is not available, Geth will crawl backwards in time to find the next available state but only up to a limit defined in the `reexec` parameter which defaults to 128 blocks. If no state is available within the `reexec` window then the trace fails with `Error: required historical state unavailable` and the `reexec` parameter must be increased. If a valid state *is* found in the `reexec` window, then Geth sequentially re-executes the transcations in each block between the last available state and the target block. The greater the value of `reexec` the longer the tracing will take because more blocks have to be re-executed to regenerate the target state.
The `debug_getAccessibleStates` endpoint is a useful tool for estimating a suitable value for `reexec`. Passing the number of the block that contains the target transaction and a search distance to this endpoint will return the number of blocks behind the current head where the most recent available state exists. This value can be passed to the tracer as `re-exec`.
It is also possible to force Geth to store the state for specific sequences of block by stopping Geth, running again with `--gcmode archive` for some period - this prevents state pruning for blocks that arrive while Geth is running with `--gcmode archive`.
The simplest type of transaction trace that Geth can generate are raw EVM opcode traces. For every EVM 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*.
Read more about Geth's basic traces on the [basic traces page](/docs/evm-tracing/basic-traces).
The tracing API accepts an optional `tracer` parameter that defines how the data returned to the API call should be processed. If this parameter is ommitted the default tracer is used. The default is the struct (or 'opcode') logger. These raw opcode traces are sometimes useful, but the returned data is very low level and can be too extensive and awkward to read for many use-cases. 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. For these reasons, there are a set of non-default built-in tracers that can be named in the API call to return different data from the method. Under the hood, these tracers are Go or Javascript
In addition to built-in tracers, it is possible to provide custom code that hooks to events in the EVM to process and return data in a consumable format. Custom tracers can be written either in Javascript or Go. JS tracers are good for quick prototyping and experimentation as well as for less intensive applications. Go tracers are performant but require the tracer to be compiled together with the Geth source code. This means developers only have to gather the data they actually need, and do any processing at the source.
This page gave an introduction to the concept of tracing and explained issues around state availability. More detailed information on Geth's built-in and custom tracers can be found on their dedicated pages.
