Mempools & How Transactions Queue

Intent: Explain what a mempool is, why transactions wait before confirmation, and how fees and priority determine inclusion in blocks.

Introduction – Why Your Transaction Doesn’t Confirm Instantly

You send a crypto transaction.
The wallet says “pending”.
Minutes or sometimes hours – go by.

Did it fail?
Is the network broken?

No. Your transaction is waiting in the mempool.

Understanding mempools explains:

• Why confirmations take time
• Why fees change
• Why higher fees get processed first

What Is a Mempool?

Mempool stands for memory pool.

It is a temporary holding area where:

• Unconfirmed transactions wait
• Nodes store pending transactions
• Validators or miners choose what to include next

Every node has its own mempool, but they are largely synchronized across the network.

Transaction Lifecycle – Big Picture

A transaction doesn’t go straight into a block.

Here’s the full path:

Wallet → Network → Mempool → Block → Confirmed

The mempool is the waiting room.

Step-by-Step: What Happens After You Click “Send”

1. Transaction Is Created

Your wallet:

• Signs the transaction with your private key
• Sets a fee
• Broadcasts it to the network

2. Nodes Verify Basic Rules

Before accepting it into the mempool, nodes check:

• Signature validity
• Sufficient balance
• Correct format
• Nonce or sequence order

Invalid transactions are rejected immediately.

3. Transaction Enters the Mempool

If valid, the transaction:

• Sits in the mempool
• Is shared with other nodes
• Waits to be picked up by a block producer

This is where competition begins.

Why Transactions Wait

Blocks have limited space.

Example:

• Bitcoin blocks ≈ every 10 minutes
• Ethereum blocks ≈ every 12 seconds

When demand exceeds capacity:

• Mempool fills up
• Transactions compete for inclusion

Fees — The Priority Signal

Fees are not arbitrary – they are incentives.

Block producers prioritize transactions that:

• Pay higher fees
• Consume less space per fee unit

Higher fee = higher priority.

Fee Markets – Supply and Demand

Fees rise when:

• Network usage spikes
• NFT mints launch
• Market volatility increases

Fees fall when:

• Network demand drops
• Fewer users are transacting

The mempool acts as a real-time auction.

Gas Fees vs Simple Fees

Bitcoin

• Flat fee per transaction
• Priority by satoshis per byte

Ethereum

• Gas-based fee model
• Base fee + priority tip
• Dynamic fee adjustment

Different chains, same concept:
👉 Pay more to go faster.

Mempool Congestion

When mempools are congested:

• Low-fee transactions get stuck
• Confirmation times increase
• Some transactions expire or get dropped

Wallets often show:

• “Slow”
• “Average”
• “Fast” fee options

These are mempool-based estimates.

Replace-By-Fee (RBF) & Fee Bumping

Some blockchains allow:

• Increasing the fee of a pending transaction
• Replacing the old transaction in the mempool

This helps if you:

• Underpriced your fee
• Need urgent confirmation

Not all chains support this.

Transaction Dropping & Expiry

If a transaction:

• Sits too long
• Has too low a fee

Nodes may:

• Drop it from the mempool
• Forget it entirely

Funds are not lost—just unconfirmed.

Why Mempools Matter

Mempools:

• Control transaction ordering
• Shape fee markets
• Affect user experience
• Enable priority mechanisms

They are the bridge between users and blocks.

Common Misunderstandings

“The blockchain is slow”
The mempool is congested

“My transaction failed”
It’s waiting for a higher priority

“Fees are random”
Fees are market-driven

Key Takeaway

Mempools are:

• Temporary
• Competitive
• Fee-driven

Your transaction confirms when:

A block producer decides it’s worth including.

Next Lesson Preview

👉 Transaction Fees & Gas – Why Blockchains Aren’t Free
We’ll break down gas, fee calculation, and why computation costs money.