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Shannon Appelcline
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# 15.3 Receiving Bitcoind Notifications with C
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# 15.3 Receiving Bitcoind Notifications with ZMQ
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> **NOTE:** This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
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[§15.3](15_3_Receiving_Bitcoind_Notifications_with_C.md) In this chapter we will show how to create a simple listener for bitcoin blockchain to receive notifications using ZMQ-based (http://zeromq.org/) notification interface, which is a highly scalable networking library written in C that acts like a concurrency framework. Bitcoin allows connection points where clients can subscribe to get notified about blockchain events like raw transactions or raw blocks.
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[§15.2](15_2_Programming_Bitcoind_with_C.md) showed one of the advantages of accessing Bitcoin's RPC commands through a programming language: the ability to reasonably create much more complex programs. This chapter reveals another one: the ability to integrate with other libraries and to do more difficult things, such as monitoring for notifications.
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## Install ZMQ
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This section will show how to create a simple listener for the Bitcoin blockchain to receive notifications using ZMQ-based (http://zeromq.org/) notification interface. It will then link to your `bitcoind`, which supports connection points that clients can subscribe to in order to get notified about blockchain events like raw transactions or raw blocks.
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To create a blockchain listener in C you need to install ZMQ following this steps.
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> :book: ***What is ZMQ?*** ZeroMQ (ZMQ)is a high-performance asynchronous messaging library that provides a message queue. ZeroMQ supports common messaging patterns (pub/sub, request/reply, client/server and others) over a variety of transports (TCP, in-process, inter-process, multicast, WebSocket and more), making inter-process messaging as simple as inter-thread messaging. You can find more details about ZMQ notifications and others kind of messages in [this repo](https://github.com/Actinium-project/ChainTools/blob/master/docs/chainlistener.md).
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1. Install ZMQ
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2. Create C program.
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3. Compile C program.
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4. Configure bitcoind to allow ZMQ notifications
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5. Execute listener.
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## Setting Up ZMQ
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### 1. Install ZMQ
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Before you can create a blockchain listener, you will need to configure `bitcoind` to allow ZMQ notifications, and the you'll also need to install a ZMQ library to take advantage of that.
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### Configuring `bitcoind` for ZMQ
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Bitcoin Core is is ZMQ-ready, but you must specify ZMQ endpoints. ZeroMQ publish-sockets prepend each data item with an arbitrary topic
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prefix that allows subscriber clients to request only those items with a matching prefix. There are currently four topics:
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```
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$ bitcoind --help | grep zmq | grep address
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-zmqpubhashblock=<address>
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-zmqpubhashtx=<address>
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-zmqpubrawblock=<address>
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-zmqpubrawtx=<address>
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```
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You can make an endpoint accessible by adding appropriate lines to your `~/.bitcoin/bitcoin.conf` file and restarting your daemon.
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```
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sudo apt-get install libzmq3-dev
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zmqpubrawblock=tcp://127.0.0.1:28332
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zmqpubrawtx=tcp://127.0.0.1:28333
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```
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Output
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You can then test its working using the `getzmqnotifications` RPC:
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```
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Reading package lists... Done
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Building dependency tree
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Reading state information... Done
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The following NEW packages will be installed:
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libzmq3-dev
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0 upgraded, 1 newly installed, 0 to remove and 18 not upgraded.
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Need to get 400 kB of archives.
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After this operation, 2.227 kB of additional disk space will be used.
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Selecting previously unselected package libzmq3-dev:amd64.
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Preparing to unpack .../libzmq3-dev_4.2.5-1ubuntu0.2_amd64.deb ...
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Setting up libzmq3-dev:amd64 (4.2.5-1ubuntu0.2) ...
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Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
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$ bitcoin-cli getzmqnotifications
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[
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{
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"type": "pubrawblock",
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"address": "tcp://127.0.0.1:28332",
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"hwm": 1000
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},
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{
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"type": "pubrawtx",
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"address": "tcp://127.0.0.1:28333",
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"hwm": 1000
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}
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]
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```
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Then
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Your `bitcoind` will now issue ZMQ notifications
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### Installing ZMQ
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To take advantage of those notifications, you need a ZMQ library to go with your C; we'll be using that instead of the `libbitcoinrpc` library in this section, to show the variable power of C. FOrtunately, this one is available as a standard Debian package:
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```
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sudo apt-get install libczmq-dev
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$ sudo apt-get install libzmq3-dev
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$ sudo apt-get install libczmq-dev
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```
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Output:
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You're now ready to code!
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```
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Reading package lists... Done
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Building dependency tree
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Reading state information... Done
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The following additional packages will be installed:
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libczmq4
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The following NEW packages will be installed:
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libczmq-dev libczmq4
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0 upgraded, 2 newly installed, 0 to remove and 18 not upgraded.
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Need to get 496 kB of archives.
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After this operation, 2.148 kB of additional disk space will be used.
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Do you want to continue? [Y/n] y
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Get:1 http://es.archive.ubuntu.com/ubuntu bionic/universe amd64 libczmq4 amd64 4.1.0-2 [145 kB]
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Fetched 496 kB in 0s (1.287 kB/s)
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Selecting previously unselected package libczmq4:amd64.
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(Reading database ... 314055 files and directories currently installed.)
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Preparing to unpack .../libczmq4_4.1.0-2_amd64.deb ...
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Unpacking libczmq4:amd64 (4.1.0-2) ...
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Selecting previously unselected package libczmq-dev:amd64.
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Preparing to unpack .../libczmq-dev_4.1.0-2_amd64.deb ...
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Unpacking libczmq-dev:amd64 (4.1.0-2) ...
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Setting up libczmq4:amd64 (4.1.0-2) ...
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Setting up libczmq-dev:amd64 (4.1.0-2) ...
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Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
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Processing triggers for libc-bin (2.27-3ubuntu1) ...
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```
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## Writing Your Notification Program
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### 2. Create C Program
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Now we've installed ZMQ and we can compile our C program using it's notifications. This C program it's a simple client that subscribes to a connection point served by bitcoind and ZMQ interface and reads incoming messages.
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The program use czmq.h library and receives two parameters as follows, first param is the point exposed by bitcoind that should be tcp connection point depending second param that could be one of these:
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zmqpubrawblock
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zmqpubrawtx
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zmqpubhashtx
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zmqpubhashblock
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The following C program is a simple client that subscribes to a ZMQ connection point served by `bitcoind` and reads incoming messages.
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The program receives two parameters: the first parameter is the "server", the TCP connection point exposed by `bitcoind` that should be tcp connection point; and the second is the "topic", which is currently `zmqpubhashblock`, `zmqpubhashtx`, `zmqpubrawblock`, or `zmqpubrawtx`. The topic must be supported through the `bitcoin.conf` and the server's IP address and port must match what's defined there.
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``` c
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#include <czmq.h>
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@ -99,10 +77,14 @@ int main(int argc, char ** argv) {
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zmqserver = argv[1];
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topic = argv[2];
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}
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```
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You will open a ZMQ socket to the defined server for the defined topic:
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``` c
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zsock_t *socket = zsock_new_sub(zmqserver, topic);
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assert(socket);
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```
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After that, you wait:
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```
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while(1) {
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zmsg_t *msg;
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int rc = zsock_recv(socket, "m", &msg);
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@ -125,69 +107,25 @@ int main(int argc, char ** argv) {
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zmsg_destroy(&msg);
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sleep(1);
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}
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```
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While, waiting, you watch for messages on the ZMQ socket. Whenever you receive a message, you will `pop` it off the stack and report out its number, its length, and most importantly the data.
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That's it!
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Of course when you're done, you should clean up:
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```
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zsock_destroy(&socket);
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return 0;
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}
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```
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### 3. Compile C program
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To compile this C program you have to do it with clang or gcc compiler and test zmq and czmq libraries.
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### Testing the Notification Code
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The source code is in the [src directory](src/15_3_chainlistener.c) as usual. You should compile it:
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```
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gcc -o chainlistener chainlistener.c -I/usr/local/include -L/usr/local/lib -lzmq -lczmq
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$ cc -o chainlistener chainlistener.c -I/usr/local/include -L/usr/local/lib -lzmq -lczmq
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```
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### 4. Configure ZMQ on bitcoind
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#### ZMQ
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ZeroMQ is a high-performance asynchronous messaging library that provides a message queue. ZeroMQ supports common messaging patterns (pub/sub, request/reply, client/server and others) over a variety of transports (TCP, in-process, inter-process, multicast, WebSocket and more), making inter-process messaging as simple as inter-thread messaging. As the purpose of this chapter is to show how to receive bitcoind notifications in the C language, ZMQ will be used for it.
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Currently, the ZeroMQ facility only needs to have the ZeroMQ endpoint specified. ZeroMQ publish sockets prepend each data item with an arbitrary topic
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prefix that allows subscriber clients to request only those items with a matching prefix.
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Topics.
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```
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zmqpubrawblock=tcp://127.0.0.1:28332
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zmqpubrawtx=tcp://127.0.0.1:28333
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zmqpubhashtx=tcp://127.0.0.1:28334
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zmqpubhashblock=tcp://127.0.0.1:28335
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```
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In this example we'll use raw tx that is the topic that notifies about new transactions in raw format.
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Add this lines to bitcoin.conf file and restart daemon.
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```
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zmqpubrawblock=tcp://127.0.0.1:28332
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zmqpubrawtx=tcp://127.0.0.1:28333
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```
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Then test it's working using this command
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```
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$ bitcoin-cli --testnet getzmqnotifications
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```
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Output:
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```
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[
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{
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"type": "pubrawblock",
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"address": "tcp://127.0.0.1:28332",
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"hwm": 1000
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},
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{
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"type": "pubrawtx",
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"address": "tcp://127.0.0.1:28333",
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"hwm": 1000
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}
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]
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```
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### 5. Execute listener:
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When you execute chainlistener and passes these params you'll get an output like this:
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Afterward, you can run it with the topics and addresses you defined in your `bitcoin.conf`:
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```
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$ ./chainlistener tcp://127.0.0.1:28333 rawtx
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Size: 250
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@ -204,19 +142,16 @@ Data: 0200000000010137527957C9AD6CFF0C9A74597E6EFCD7E1EBD53E942AB2FA34A831046CA1
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No: 70
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.......
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```
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The first param it's bitcoind connection point where ZMQ will notifies all about transactions in raw format. It corresponds to this line in bitcoin.conf file zmqpubrawtx=tcp://127.0.0.1:28333
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The second param is the topic selected rawtx explained above.
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### For More Information
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In this repository you can browse find more details about ZMQ notifications and others kind of messages. (https://github.com/Actinium-project/ChainTools/blob/master/docs/chainlistener.md)
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### Summary Receiving Bitcoind Notifications with C.md
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By using ZMQ framework, you can easily receive notifications by subscribing to a connection point exposed by bitcoind changing configuration file.
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> :fire: ***What is the Power of Notifications?*** With notifications, you're no longer entirely dependent upon users to issue commands. Instead, you can create programs that monitor the Bitcoin blockchain and take appropriate actions when certain things occur. This in turn could be merged with the RPC commands that you programmed in previous sections. This is also a big step beyond what you could do with shell scripts: certainly, you can create infinite-loop listener shell scripts, but programming languages tend to be a better tool for that task.
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## What's Next?
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Learn more about "Talking to Bitcoind with Libwally" in [Chapter 16: Talking to Bitcoind with Libwally](16_0_Talking_to_Libwally.md).
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