BA-09 · LESSON 06

RPC and ZMQ

KEY DEFINITIONS

Terms used in this lesson

RPC: Remote procedure call: the command interface applications use to ask a Bitcoin node to perform operations or return state.
ZMQ: ZeroMQ: a messaging interface Bitcoin Core can use to notify applications about new blocks and transactions.

RPC controls a node while event interfaces support applications that need chain and mempool updates.

45 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · p2p propagationIndependent peers relay claims, then verify locallySelect any state to inspect its exact role.

RELATIONSHIP MAPIndependent peer relationships

01
ORIGINWallet or peer
A node receives a transaction, header, block, or request from an untrusted connection.
02
PARSEBounded message
Length, command, encoding, and resource limits are checked before deeper work.
03
VALIDATERPC and ZMQ
The RPC and ZMQ stage applies local policy or consensus rules as appropriate.
04
STOREMempool or chainstate
Accepted data enters a local view that can differ temporarily from another node's view.
05
RELAYSelected peers
Relay spreads a valid candidate; it does not make the candidate final or universally known.

Rule shown: Network messages are invitations to verify, never authority to accept state.

STAGE 1 OF 5ORIGIN: Wallet or peer

A node receives a transaction, header, block, or request from an untrusted connection.

Governing relationship: Network messages are invitations to verify, never authority to accept state. Complete path: 1. ORIGIN, Wallet or peer. A node receives a transaction, header, block, or request from an untrusted connection. 2. PARSE, Bounded message. Length, command, encoding, and resource limits are checked before deeper work. 3. VALIDATE, RPC and ZMQ. The RPC and ZMQ stage applies local policy or consensus rules as appropriate. 4. STORE, Mempool or chainstate. Accepted data enters a local view that can differ temporarily from another node's view. 5. RELAY, Selected peers. Relay spreads a valid candidate; it does not make the candidate final or universally known.

1. State transitionWhat exists, what changes, and what results

BEFORE

An indexer last processed block hash A at height 500. Its ZMQ connection drops, then reconnects when node tip is C at height 502.

MECHANISM

Applications connect to a controlled local or protected endpoint with least-privilege credentials and avoid exposing administrative methods to untrusted networks.

AFTER

Notifications improve responsiveness; checkpointed RPC reconciliation provides correctness across gaps and reorgs.

Text equivalent: Starting from An indexer last processed block hash A at height 500. Its ZMQ connection drops, then reconnects when node tip is C at height 502. The mechanism begins when Applications connect to a controlled local or protected endpoint with least-privilege credentials and avoid exposing administrative methods to untrusted networks. The resulting lesson is Notifications improve responsiveness; checkpointed RPC reconciliation provides correctness across gaps and reorgs.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Authenticate and scope RPC

Applications connect to a controlled local or protected endpoint with least-privilege credentials and avoid exposing administrative methods to untrusted networks.

Complete path: 1. Authenticate and scope RPC: Applications connect to a controlled local or protected endpoint with least-privilege credentials and avoid exposing administrative methods to untrusted networks. 2. Issue a method request: The caller supplies a method and typed parameters. The node returns a result or structured error from its local view. 3. Subscribe to notifications: A ZMQ subscriber listens to configured topics. Delivery signals that an event occurred but does not provide durable queue semantics by itself. 4. Reconcile missed events: Applications record processed chain position and use RPC to recover authoritative state after restart, disconnect, sequence gap, or reorg. 5. Handle reorg and mempool differences: A new-block event can later be disconnected. Applications track block hashes and confirmations instead of treating one notification as final settlement.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Authenticated RPC supplies authoritative request responses, ZMQ supplies transient notifications, and a durable hash checkpoint drives common-ancestor rollback and ordered replay.

CHANGED CONDITION

A sequence gap cannot be repaired by incrementing height, and a reorg requires reversing the old suffix before applying the new branch.

Important distinction: Receiving a ZMQ transaction notification proves the transaction is confirmed and final. The notification may describe mempool acceptance or a newly observed block. The application must inspect local node state and continue tracking confirmations and reorgs.

Text equivalent: The accepted path is enforced by Authenticated RPC supplies authoritative request responses, ZMQ supplies transient notifications, and a durable hash checkpoint drives common-ancestor rollback and ordered replay. If the required condition changes, A sequence gap cannot be repaired by incrementing height, and a reorg requires reversing the old suffix before applying the new branch.

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Learn the idea

RPC, or remote procedure call, is Bitcoin Core's authenticated command interface for requesting state or actions such as wallet transaction creation. ZMQ, or ZeroMQ, is a publish-subscribe notification interface for events such as new block or transaction hashes. RPC answers a request; ZMQ pushes a signal, and applications must query or validate details as needed.

GUIDED EXPLANATION1/5 ideas inspected

Authenticate and scope RPC

Applications connect to a controlled local or protected endpoint with least-privilege credentials and avoid exposing administrative methods to untrusted networks.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Receiving a ZMQ transaction notification proves the transaction is confirmed and final.”

The notification may describe mempool acceptance or a newly observed block. The application must inspect local node state and continue tracking confirmations and reorgs.

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