Front-running awareness in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Front-running awareness

User sends transaction

↓

Transaction enters mempool

↓

Miner/Validator sees transaction

↓

Miner can reorder transactions

↓

Front-runner inserts own transaction before yours

↓

Your transaction executes after front-runner

↓

Front-runner profits from your transaction

This flow shows how a transaction sent by a user can be seen and reordered by miners or validators to insert a front-running transaction before it.

Execution Sample

Blockchain / Solidity

mempool = []
user_tx = {'type': 'buy', 'amount': 10}
front_runner_tx = {'type': 'buy', 'amount': 20}
mempool.append(user_tx)
mempool.insert(0, front_runner_tx)
# Simulate execution by clearing mempool
mempool.clear()

This code simulates a user transaction entering the mempool, then a front-runner inserts their transaction before it, and finally transactions execute in order.

Execution Table

Step	Action	Mempool State	Explanation
1	User sends transaction	[{'type': 'buy', 'amount': 10}]	User's transaction added to mempool
2	Front-runner inserts transaction at front	[{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]	Front-runner's transaction placed before user's
3	Execute transactions in order	[]	Transactions executed: front-runner first, user second
4	End	[]	No more transactions to process

💡 All transactions executed; front-runner's transaction ran before user's

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
mempool	[]	[{'type': 'buy', 'amount': 10}]	[{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]	[]	[]

Key Moments - 3 Insights

Why does the front-runner's transaction appear before the user's in the mempool?

Because miners or validators can reorder transactions in the mempool to place their own transactions first, as shown in step 2 of the execution_table.

Does the user know their transaction will be executed second?

No, the user sends the transaction without control over ordering; front-runners exploit this by inserting transactions before, as seen in the mempool state changes.

What happens if the front-runner does not insert a transaction?

Then the user's transaction executes first, as the mempool remains unchanged after step 1, shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the order of transactions in the mempool?

A []

B [{'type': 'buy', 'amount': 10}, {'type': 'buy', 'amount': 20}]

C [{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]

D [{'type': 'sell', 'amount': 5}]

💡 Hint

Check the 'Mempool State' column at step 2 in the execution_table

At which step does the mempool become empty?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for '[]' in the 'Mempool State' column in the execution_table

If the front-runner did not insert their transaction, what would the mempool look like after step 2?

A [{'type': 'buy', 'amount': 10}]

B []

C [{'type': 'buy', 'amount': 20}]

D [{'type': 'buy', 'amount': 20}, {'type': 'buy', 'amount': 10}]

💡 Hint

Refer to the mempool state after step 1 and imagine no insertion at step 2

Concept Snapshot

Front-running happens when miners reorder transactions in the mempool.
User sends transaction -> enters mempool.
Miner inserts own transaction before user's.
Transactions execute in new order.
User's transaction runs after front-runner's.
This can cause unfair profit for front-runners.

Full Transcript

Front-running awareness means understanding how transactions sent by users can be seen by miners or validators before they are confirmed. These miners can reorder transactions in the mempool, placing their own transactions ahead of users' transactions. This lets front-runners profit unfairly by acting first. The example code shows a user transaction entering the mempool, then a front-runner inserts their transaction at the front. The execution table traces each step: user transaction added, front-runner inserts before it, then transactions execute in order. The variable tracker shows how the mempool changes over time. Key moments clarify why the front-runner's transaction appears first and that users cannot control this ordering. The quiz tests understanding of mempool order and execution steps. Remember, front-running exploits transaction ordering in blockchain systems.