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since it may be out of date, incomplete or even invalid.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