# Contract Proxies Guide¶

## Introduction¶

A Raiden node accesses smart contracts through wrappers called proxies. The proxies are implemented in raiden/network/proxies directory. The proxies are supposed to be used with some special care, and they are implemented in a specific way.

The complication exists because there is always the possibility of a race between our node, the blockchain(s) and other parties. The Raiden node usually recognizes events on the blockchain only after some confirmation period. By the time the Raiden node recognizes an onchain event, the Raiden node has seen several blocks have been mined on top. However, when Raiden posts a transaction to the blockchain, the transaction is sent to miners who operate on the newest available block.

The proxies try to prevent the users from spending gas costs for transactions that fail anyway, and also to give a reasonable diagnosis why a transaction doesn’t (or didn’t go through).

## A Proxy’s Workflow¶

If the call is read-only, the proxy never throws a transaction to the blockchain. The proxy asks the Ethereum client to execute the call in the specified blockhash.

### State-modifying case¶

If the call modifies the state, the proxy does the following things in the successful case:

1. the proxy checks conditions on the specified block, and sees whether the call would fail (the post-condition checks)

2. the proxy then asks the Ethereum client to estimate gas, which might fail.

3. the proxy then asks the Ethereum client to submit the transaction to the blockchain.

4. the proxy sees the transaction receipt and sees if it’s successful.

When the precondition checks fail, the proxy raises BrokenPreconditionError, blaming the caller for making a call without proper checks.

When the gas estimation fails, the proxy performs additional checks on the latest available block in order to determine the cause of the failure.

When the transaction was included in a block but the execution has failed, the proxy performs additional checks on that block in order to determine the cause of the failure. The checks use the chain state at the end of the failing block.

Depending on the cause of the failure, the proxy raises different kinds of exceptions. If you use the proxies, you’ll need to make sense of these exceptions (see below).

## A Guide for Using Proxies¶

Before calling a proxy, make sure that the call is going to succeed in a confirmed block. Pick a confirmed block and query the blockchain so that the chain state allows a successful execution at that moment. And then, pass the blockhash of the confirmed block to the proxy when you call it.

The proxy checks the preconditions on the confirmed block. If any of the preconditions fail because of the chain state, it raises BrokenPreconditionError. This means there is a mistake in the Raiden codebase, and a check must be added before calling the proxy.

However, the proxy raises BrokenPreconditionError only when the error is about the chain state. There are values that are invalid regardless of the chain state. These cause RaidenValidationError instead of BrokenPreconditionError. The RaidenValidationError is not considered as a bug in the codebase.

When the proxy doesn’t raise an exception, the call was successful. A transaction was included in a block and the transaction has been executed successfully. Moreover the proxy has waited for the block to be confirmed so that the state change is visible to the Raiden node.

When the proxy raises RaidenUnrecoverableError, it means that there is a bug in the client codebase or in the smart contracts.

When the proxy raises RaidenRecoverableError, it might mean:

• somebody else posted another transaction that altered the chain state in a conflicting way.

• a third party smart contract behaved weirdly.

The proxy tries hard to determine whether the problem is in the Raiden codebase (client or smart contract), but ultimately, if the proxy is not sure, it raises RaidenRecoverableError.

## A Guide for Implementing Proxies¶

When you implement proxies, the best documentation to follow is the source of TokenNetwork proxy.

Sometimes precondition checks are impossible because the specified block is too old (pruned in the Ethereum client). In this case, the precondition check can be skipped.

Checks before the gas estimation should raise one of the following exceptions:

• BrokenPreconditionError when the specified block has an unsuitable chain state for the call.

• RaidenValidationError when the given argument is invalid regardless of the chain state.

• Instead of RaidenValidationError, you can also raise a special exception that you derive from RaidenError, but make sure you catch it. The typical use case is to provide a nice HTTP status number.

When you implement a new method of a proxy, consider refusing the string "latest" as the block identifier. Usually passing "latest" as a block identifier to a proxy method poses a possibility of BrokenPreconditionError. The method can raise a ValueError when "latest" is passed as the block identifier. Or, you might want to introduce a new type so that mypy complains against using "latest" as the block identifier.

Mind the number of RPC calls you are making. Instead of fetching a block number and then the block hash, there is usually a way to get the whole block in a single RPC call. When you post multiple RPC calls, you introduce more race conditions and performance bottlenecks.