# Structure and Functionality of the Ledger
Understanding the structure and functionality of the BSV Ledger is crucial for grasping how it operates. The ledger is characterized as a **WORM database**—**Write Once, Read Many**—which means that once data is recorded, it cannot be altered without leaving evidence of tampering. This property ensures that the ledger is **tamper-evident**, as any attempts to modify the data will be visible and traceable.
**Nodes in the BSV network** maintain the authoritative ledger. They receive transactions and write them to blocks.
[**Overlay services** ](https://bsvblockchain.360learning.com/course/play/6900be15b988fc5078a74c1c)now handle most transaction submission and validation. These specialized services sit between users and nodes. They **validate transactions using SPV** before forwarding them to the node network.
**Traditional mempools** stored all pending transactions globally. This model doesn't scale at high volumes.
**Overlay architecture replaces global mempools** with distributed, specialized transaction pools. Each overlay maintains its own pool of **relevant transactions** for its specific use case.
**Users submit transactions directly to overlays**, not to a global mempool. The overlay validates the transaction. Then it **submits valid transactions to nodes** for block inclusion.
**Overlays track transaction propagation status**. They provide feedback to users about confirmation. This replaces the mempool's monitoring function.
**Different overlays may hold different transactions**. They serve different applications and business needs. This **distributes the computational burden** across many services.
**Nodes must still reach consensus** on the ledger state. The blockchain itself remains the single source of truth. **SPV ensures data integrity** between overlays and the blockchain.
This **shift from global listening to targeted services** enables massive scalability. Businesses select and pay for only the overlay services they need.
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