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diff --git a/doc/zmq.md b/doc/zmq.md new file mode 100644 index 0000000000..902d1124c7 --- /dev/null +++ b/doc/zmq.md @@ -0,0 +1,101 @@ +# Block and Transaction Broadcasting With ZeroMQ + +[ZeroMQ](http://zeromq.org/) is a lightweight wrapper around TCP +connections, inter-process communication, and shared-memory, +providing various message-oriented semantics such as publish/subscribe, +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 Therefore, subscribers should validate the received data +since it may be out of date, incomplete or even invalid.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, +connections 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 requires ZeroMQ API version 4.x or +newer. Typically, it is packaged by distributions as something like +*libzmq3-dev*. The C++ wrapper for ZeroMQ is *not* needed. + +In order to run the example Python client scripts in contrib/ one must +also install *python-zmq*, though this is not necessary for daemon +operation. + +## Enabling + +By default, the ZeroMQ feature is automatically compiled in if the +necessary prerequisites are found. To disable, use --disable-zmq +during the *configure* step of building bitcoind: + + $ ./configure --disable-zmq (other options) + +To actually enable operation, one must set the appropriate options on +the commandline or in the configuration file. + +## 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](http://api.zeromq.org/4-0:_start). + +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. |