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Block and Transaction Broadcasting With ZeroMQ
ZeroMQ is a lightweight wrapper around TCP connections, inter-process communications, and shared-memory, providing various message-oriented semantics such as publish/subcribe, request/reply, and push/pull.
The Bitcoin Core daemon can be configured to act as a trusted "border router", implementing the bitcoin wire protocol and relay, making consensus decisions, maintaining the local blockchain database, broadcasting locally generated transactions into the network, and providing a queryable RPC interface to interact on a polled basis for requesting blockchain related data. However, there exists only a limited service to notify external software of events like the arrival of new blocks or transactions.
The ZeroMQ facility implements a notification interface through a set of specific notifiers. Currently there are notifiers that publish blocks and transactions. This read-only facility requires only the connection of a corresponding ZeroMQ subscriber port in receiving software; it is not authenticated nor is there any two-way protocol involvement. Therefore, subscribers should validate the received data since it may be out of date, incomplete or even invalid.
ZeroMQ sockets are self-connecting and self-healing; that is, connects made between two endpoints will be automatically restored after an outage, and either end may be freely started or stopped in any order.
Because ZeroMQ is message oriented, subscribers receive transactions and blocks all-at-once and do not need to implement any sort of buffering or reassembly.
Prerequisites
The ZeroMQ feature in Bitcoin Core uses only a very small part of the ZeroMQ C API, and is thus compatible with any version of ZeroMQ from 2.1 onward, including all versions in the 3.x and 4.x release series. Typically, it is packaged by distributions as something like libzmq-dev.
The C++ wrapper for ZeroMQ is not needed.
Enabling
By default, the ZeroMQ port functionality is enabled. Two steps are required to enable--compiling in the ZeroMQ code, and configuring runtime operation on the command-line or configuration file.
$ ./configure --enable-zmq (other options)
This will produce a binary that is capable of providing the ZeroMQ facility, but will not do so until also configured properly.
Usage
Currently, the following notifications are supported:
-zmqpubhashtx=address
-zmqpubhashblock=address
-zmqpubrawblock=address
-zmqpubrawtx=address
The socket type is PUB and the address must be a valid ZeroMQ socket address. The same address can be used in more than one notification.
For instance:
$ bitcoind -zmqpubhashtx=tcp://127.0.0.1:28332 -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw
Each PUB notification has a topic and body, where the header
corresponds to the notification type. For instance, for the notification
-zmqpubhashtx the topic is hashtx (no null terminator) and the body is the
hexadecimal transaction hash (32 bytes).
These options can also be provided in bitcoin.conf.
ZeroMQ endpoint specifiers for TCP (and others) are documented in the ZeroMQ API.
Client side, then, the ZeroMQ subscriber socket must have the
ZMQ_SUBSCRIBE option set to one or either of these prefixes (for instance, just hash); without
doing so will result in no messages arriving. Please see contrib/zmq/zmq_sub.py
for a working example.
Remarks
From the perspective of bitcoind, the ZeroMQ socket is write-only; PUB sockets don't even have a read function. Thus, there is no state introduced into bitcoind directly. Furthermore, no information is broadcast that wasn't already received from the public P2P network.
No authentication or authorization is done on connecting clients; it is assumed that the ZeroMQ port is exposed only to trusted entities, using other means such as firewalling.
Note that when the block chain tip changes, a reorganisation may occur and just the tip will be notified. It is up to the subscriber to retrieve the chain from the last known block to the new tip.