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Validating Transactions – From Mempool to Block

ByPK Williams Posted on Updated on
Validating Transactions - From Mempool to Block

Lesson 10: Validating Transactions – From Mempool to Block

Intent:

“Validating Transactions – From Mempool to Block” To explain the full lifecycle of a blockchain transaction -how it is created, broadcast, stored in the mempool, validated by nodes, and finally included in a block.

Introduction – What Happens After You Click “Send”?

When you send a blockchain transaction, it doesn’t go straight into a block.

Instead, it enters a waiting area, gets checked by multiple nodes, competes with other transactions, and only then becomes part of the blockchain’s permanent history.

This behind-the-scenes journey ensures that:

  • Only valid transactions are recorded
  • Double-spending is prevented
  • The network remains trustless and decentralized

In this lesson, we’ll follow a transaction step-by-step – from creation to confirmation.

Step 1: Transaction Creation

A transaction begins when a user:

  • Enters recipient address
  • Specifies amount
  • Sets transaction fee
  • Signs the transaction with their private key

This digital signature proves:

  • Ownership of funds
  • Authorization to spend them
  • Integrity of the transaction data

Once signed, the transaction becomes immutable.

Step 2: Broadcasting to the Network

The signed transaction is broadcast to:

  • A wallet’s connected node
  • Or directly to multiple peers

From there, it spreads across the network using peer-to-peer gossip propagation.

Every node that receives it performs initial checks.

Step 3: Basic Validation by Nodes

Before accepting a transaction, nodes verify:

  • Correct digital signature
  • Sufficient balance
  • Proper formatting
  • No double-spending
  • Valid fee structure

Invalid transactions are immediately rejected and never forwarded.

Valid ones move into the mempool.

What Is the Mempool?

The mempool (memory pool) is a temporary storage area where valid but unconfirmed transactions wait to be included in a block.

Key points about the mempool:

  • Each node maintains its own mempool
  • Mempools are similar but not identical
  • Transactions are prioritized by fee
  • Transactions can expire or be dropped

Think of the mempool as a waiting room for transactions.

Step 4: Transaction Selection by Miners / Validators

Block producers:

  • Miners (PoW) or
  • Validators (PoS)

select transactions from their mempool based on:

  • Highest fees
  • Validity
  • Block size limits

Higher fees usually mean faster inclusion.

Step 5: Block Construction

The selected transactions are:

  • Bundled into a block
  • Organized into a Merkle Tree
  • Assigned a block header
  • Prepared for consensus

At this stage, the block is only proposed, not final.

Step 6: Consensus & Block Proposal

Depending on the consensus mechanism:

  • PoW: Miners compete to solve a cryptographic puzzle
  • PoS: A validator is chosen to propose the block

Once a valid block is found:

  • It is broadcast to the network
  • Other nodes verify it independently

Step 7: Block Verification by the Network

Nodes check:

  • Block structure
  • All transaction validity
  • Correct previous block hash
  • Consensus rules compliance

If valid:

  • The block is added to the chain
  • Transactions are removed from the mempool

If invalid:

  • The block is rejected
  • The proposer is penalized (in PoS systems)

Confirmations – When Is a Transaction Final?

A transaction is considered more secure as more blocks are added after it.

  • 1 confirmation: Included in a block
  • 6 confirmations (Bitcoin): Highly secure
  • Finality-based chains: Near-instant finality

More confirmations = higher confidence.

Mempool Dynamics – Why Fees Matter

Mempools are dynamic:

  • They grow during high demand
  • Shrink when blocks clear transactions
  • Can become congested

This leads to:

  • Fee markets
  • Transaction prioritization
  • Replace-by-fee (RBF) mechanisms

Users can often speed up stuck transactions by increasing fees.

Real-World Analogy – Postal System

  • Creating transaction → Writing a letter
  • Broadcasting → Dropping it at the post office
  • Mempool → Sorting facility
  • Miner/Validator → Mail truck
  • Block → Delivery batch
  • Confirmations → Delivery receipts

Each step adds trust and verification.

Why This Process Secures the Network

This multi-layer validation ensures:

  • No single node can cheat
  • Invalid transactions never enter the ledger
  • Consensus protects shared history

It’s slow by design – because security beats speed.

Key Takeaway

A blockchain transaction is not a single action – it’s a journey.

From mempool to block, every transaction is:

  • Verified
  • Prioritized
  • Agreed upon
  • Permanently recorded

This lifecycle is what makes blockchain trustless, transparent, and secure.

Next Lesson Preview

👉 Lesson 11: Fees, Gas & Incentives – Why Transactions Cost Money
We’ll explore transaction fees, gas models, and how incentives keep the network running.

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