Mar 15

How to install Homie libraries on Arduino IDE

This entry is part 3 of 3 in the series esp8266

Reading time: < 1 minute The steps that explain how to install Homie libraries and its dependencies on the official documentation are a little bit tricky and some of you asked me about how I did it. So I decided to record a clip explaining the steps:

Please tell if it was useful for you, and thanks for your interest.

Local copy of the video:

Jan 29

Ansible and Windows Playbooks

Reading time: 3 – 5 minutes

Firstly let me introduce a Windows service called: “Windows Remote Manager” or “WinRM”. This is the Windows feature that allows remote control of Windows machines and many other remote functionalities. In my case I have a Windows 7 laptop with SP1 and PowerShell v3 installed.

Secondly don’t forget that Ansible is developed using Python then a Python library have to manage the WinRM protocol. I’m talking about “pywinrm“. Using this library it’s easy to create simple scripts like that:

#!/usr/bin/env python

import winrm

s = winrm.Session('10.2.0.42', auth=('the_username', 'the_password'))
r = s.run_cmd('ipconfig', ['/all'])
print r.status_code
print r.std_out
print r.std_err

This is a remote call to the command “ipconfig /all” to see the Windows machine network configuration. The output is something like:

$ ./winrm_ipconfig.py 
0

Windows IP Configuration

   Host Name . . . . . . . . . . . . : mini7w
   Primary Dns Suffix  . . . . . . . : 
   Node Type . . . . . . . . . . . . : Hybrid
   IP Routing Enabled. . . . . . . . : No
   WINS Proxy Enabled. . . . . . . . : No
   DNS Suffix Search List. . . . . . : ymbi.net

Ethernet adapter GigaBit + HUB USB:

   Connection-specific DNS Suffix  . : ymbi.net
   Description . . . . . . . . . . . : ASIX AX88179 USB 3.0 to Gigabit Ethernet Adapter
   Physical Address. . . . . . . . . : 00-23-56-1C-XX-XX
   DHCP Enabled. . . . . . . . . . . : Yes
   Autoconfiguration Enabled . . . . : Yes
   Link-local IPv6 Address . . . . . : fe80::47e:c2c:8c25:xxxx%103(Preferred) 
   IPv4 Address. . . . . . . . . . . : 10.2.0.42(Preferred) 
   Subnet Mask . . . . . . . . . . . : 255.255.255.192
   Lease Obtained. . . . . . . . . . : mi�rcoles, 28 de enero de 2015 12:41:41
   Lease Expires . . . . . . . . . . : mi�rcoles, 28 de enero de 2015 19:17:56
   Default Gateway . . . . . . . . . : 10.2.0.1
   DHCP Server . . . . . . . . . . . : 10.2.0.1
   DHCPv6 IAID . . . . . . . . . . . : 2063606614
   DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-15-F7-BF-36-xx-C5-xx-03-xx-xx
   DNS Servers . . . . . . . . . . . : 10.2.0.27
                                       10.2.0.1
   NetBIOS over Tcpip. . . . . . . . : Enabled
...

Of course, it’s possible to run Powershell scripts like the next one which shows the system memory:

$strComputer = $Host
Clear
$RAM = WmiObject Win32_ComputerSystem
$MB = 1048576

"Installed Memory: " + [int]($RAM.TotalPhysicalMemory /$MB) + " MB"

The Python code to run that script is:

#!/usr/bin/env python

import winrm

ps_script = open('scripts/mem.ps1','r').read()
s = winrm.Session('10.2.0.42', auth=('the_username', 'the_password'))
r = s.run_ps(ps_script)
print r.status_code
print r.std_out
print r.std_err

and the output:

$ ./winrm_mem.py 
0
Installed Memory: 2217 MB

In the end it’s time to talk about how to create an Ansible Playbook to deploy anything in a Windows machine. As always the first thing that we need is a hosts file. In the next example there are several ansible variables needed to run Ansible Windows modules on WinRM, all of them are self-explanatory:

[all]
10.2.0.42

[all:vars]
ansible_ssh_user=the_username
ansible_ssh_pass=the_password
ansible_ssh_port=5985 #winrm (non-ssl) port
ansible_connection=winrm

The first basic example could be a simple playbook that runs the ‘ipconfig’ command and registers the output in an Ansible variable to be showed later like a debug information:

- name: test raw module
  hosts: all
  tasks:
    - name: run ipconfig
      raw: ipconfig
      register: ipconfig
    - debug: var=ipconfig

The command and the output to run latest example:

$ ansible-playbook -i hosts ipconfig.yml 

PLAY [test raw module] ******************************************************** 

GATHERING FACTS *************************************************************** 
ok: [10.2.0.42]

TASK: [run ipconfig] ********************************************************** 
ok: [10.2.0.42]

TASK: [debug var=ipconfig] **************************************************** 
ok: [10.2.0.42] => {
    "ipconfig": {
        "invocation": {
            "module_args": "ipconfig", 
            "module_name": "raw"
        }, 
        "rc": 0, 
        "stderr": "", 
        "stdout": "\r\nWindows IP Configuration\r\n\r\n\r\nEthernet adapter GigaBit 
...
        ]
    }
}

PLAY RECAP ******************************************************************** 
10.2.0.42                  : ok=3    changed=0    unreachable=0    failed=0 

As always Ansible have several modules, not only the ‘raw’ module. I committed two examples in my Github account using a module to download URLs and another one that runs Powershell scripts.

My examples are done using Ansible 1.8.2 installed in a Fedora 20. But main problems I’ve found are configuring Windows 7 to accept WinRM connections. Next I attach some references that helped me a lot:

If you want to use my tests code you can connect to my Github: Basic Ansible playbooks for Windows.

Jan 21

Using Ansible like library programming in Python

Reading time: 2 – 4 minutes

Ansible is a very powerful tool. Using playbooks, something like a cookbook, is very easy to automate maintenance tasks of systems. I used Puppet and other tools like that but IMHO Ansible is the best one.

In some cases you need to manage dynamic systems and take into advantage of Ansible like a Python library is a very good complement for your scripts. This is my last requirement and because of that I decided to share some simple Python snippets that help you to understand how to use Ansible as a Python library.

Firstly an example about how to call an Ansible module with just one host in the inventory (test_modules.py):

#!/usr/bin/python 
import ansible.runner
import ansible.playbook
import ansible.inventory
from ansible import callbacks
from ansible import utils
import json

# the fastest way to set up the inventory

# hosts list
hosts = ["10.11.12.66"]
# set up the inventory, if no group is defined then 'all' group is used by default
example_inventory = ansible.inventory.Inventory(hosts)

pm = ansible.runner.Runner(
    module_name = 'command',
    module_args = 'uname -a',
    timeout = 5,
    inventory = example_inventory,
    subset = 'all' # name of the hosts group 
    )

out = pm.run()

print json.dumps(out, sort_keys=True, indent=4, separators=(',', ': '))

As a second example, we’re going to use a simple Ansible Playbook with that code (test.yml):

- hosts: sample_group_name
  tasks:
    - name: just an uname
      command: uname -a

The Python code which uses that playbook is (test_playbook.py):

#!/usr/bin/python 
import ansible.runner
import ansible.playbook
import ansible.inventory
from ansible import callbacks
from ansible import utils
import json

### setting up the inventory

## first of all, set up a host (or more)
example_host = ansible.inventory.host.Host(
    name = '10.11.12.66',
    port = 22
    )
# with its variables to modify the playbook
example_host.set_variable( 'var', 'foo')

## secondly set up the group where the host(s) has to be added
example_group = ansible.inventory.group.Group(
    name = 'sample_group_name'
    )
example_group.add_host(example_host)

## the last step is set up the invetory itself
example_inventory = ansible.inventory.Inventory()
example_inventory.add_group(example_group)
example_inventory.subset('sample_group_name')

# setting callbacks
stats = callbacks.AggregateStats()
playbook_cb = callbacks.PlaybookCallbacks(verbose=utils.VERBOSITY)
runner_cb = callbacks.PlaybookRunnerCallbacks(stats, verbose=utils.VERBOSITY)

# creating the playbook instance to run, based on "test.yml" file
pb = ansible.playbook.PlayBook(
    playbook = "test.yml",
    stats = stats,
    callbacks = playbook_cb,
    runner_callbacks = runner_cb,
    inventory = example_inventory,
    check=True
    )

# running the playbook
pr = pb.run()  

# print the summary of results for each host
print json.dumps(pr, sort_keys=True, indent=4, separators=(',', ': '))

If you want to download example files you can go to my github account: github.com/oriolrius/programming-ansible-basics

I hope it was useful for you.

Sep 25

Server send push notifications to client browser without polling

Reading time: 5 – 8 minutes

Nowadays last version of browsers support websockets and it’s a good a idea to use them to connect to server a permanent channel and receive push notifications from server. In this case I’m going to use Mosquitto (MQTT) server behind lighttpd with mod_websocket as notifications server. Mosquitto is a lightweight MQTT server programmed in C and very easy to set up. The best advantage to use MQTT is the possibility to create publish/subscriber queues and it’s very useful when you want to have more than one notification channel. As is usual in pub/sub services we can subscribe the client to a well-defined topic or we can use a pattern to subscribe to more than one topic. If you’re not familiarized with MQTT now it’s the best moment to read a little bit about because that interesting protocol. It’s not the purpose of this post to explain MQTT basics.

A few weeks ago I set up the next architecture just for testing that idea:

mqtt_schema

weboscket gateway to mosquitto mqtt server with javascrit mqtt client

The browser

Now it’s time to explain this proof of concept. HTML page will contain a simple Javascript code which calls mqttws31.js library from Paho. This Javascript code will connect to the server using secure websockets. It doesn’t have any other security measure for a while may be in next posts I’ll explain some interesting ideas to authenticate the websocket. At the end of the post you can download all source code and configuration files. But now it’s time to understand the most important parts of the client code.

client = new Messaging.Client("ns.example.tld", 443, "unique_client_id");
client.onConnectionLost = onConnectionLost;
client.onMessageArrived = onMessageArrived;
client.connect({onSuccess:onConnect, onFailure:onFailure, useSSL:true});

Last part is very simple, the client connects to the server and links some callbacks to defined functions. Pay attention to ‘useSSL’ connect option is used to force SSL connection with the server.

There are two specially interesting functions linked to callbacks, the first one is:

function onConnect() {
  client.subscribe("/news/+/sport", {qos:1,onSuccess:onSubscribe,onFailure:onSubscribeFailure});
}

As you can imagine this callback will be called when the connections is established, when it happens the client subscribes to all channels called ‘/news/+/sports’, for example, ‘/news/europe/sports/’ or ‘/news/usa/sports/’, etc. We can also use, something like ‘/news/#’ and it will say we want to subscribe to all channels which starts with ‘/news/’. If only want to subscribe to one channel put the full name of the channel on that parameter. Next parameter are dictionary with quality of service which is going to use and links two more callbacks.

The second interesting function to understand is:

function onMessageArrived(message) {
  console.log("onMessageArrived:"+message.payloadString);
};

It’s called when new message is received from the server and in this example, the message is printed in console with log method.

The server

I used an Ubuntu 12.04 server with next extra repositories:

# lighttpd + mod_webserver
deb http://ppa.launchpad.net/roger.light/ppa/ubuntu precise main
deb-src http://ppa.launchpad.net/roger.light/ppa/ubuntu precise main

# mosquitto
deb http://ppa.launchpad.net/mosquitto-dev/mosquitto-ppa/ubuntu precise main
deb-src http://ppa.launchpad.net/mosquitto-dev/mosquitto-ppa/ubuntu precise main

With these new repositories you can install required packages:

apt-get install lighttpd lighttpd-mod-websocket mosquitto mosquitto-clients

After installation it’s very easy to run mosquitto in test mode, use a console for that and write the command: mosquitto, we have to see something like this:

# mosquitto
1379873664: mosquitto version 1.2.1 (build date 2013-09-19 22:18:02+0000) starting
1379873664: Using default config.
1379873664: Opening ipv4 listen socket on port 1883.
1379873664: Opening ipv6 listen socket on port 1883.

The configuration file for lighttpd in testing is:

server.modules = (
        "mod_websocket",
)

websocket.server = (
        "/mqtt" => ( 
                "host" => "127.0.0.1",
                "port" => "1883",
                "type" => "bin",
                "subproto" => "mqttv3.1"
        ),
)

server.document-root        = "/var/www"
server.upload-dirs          = ( "/var/cache/lighttpd/uploads" )
server.errorlog             = "/var/log/lighttpd/error.log"
server.pid-file             = "/var/run/lighttpd.pid"
server.username             = "www-data"
server.groupname            = "www-data"
server.port                 = 80

$SERVER["socket"] == ":443" {
    ssl.engine = "enable" 
    ssl.pemfile = "/etc/lighttpd/certs/sample-certificate.pem" 
    server.name = "ns.example.tld"
}

Remember to change ‘ssl.pemfile’ for your real certificate file and ‘server.name’ for your real server name. Then restart the lighttpd and validate SSL configuration using something like:

openssl s_client -host ns.example.tld -port 443

You have to see SSL negotiation and then you can try to send HTTP commands, for example: “GET / HTTP/1.0” or something like this. Now the server is ready.

The Test

Now you have to load the HTML test page in your browser and validate how the connections is getting the server and then how the mosquitto console says how it receives the connection. Of course, you can modify the Javascript code to print more log information and follow how the client is connected to MQTT server and how it is subscribed to the topic pattern.

If you want to publish something in MQTT server we could use the CLI, with a command mosquitto_pub:

mosquitto_pub -h ns.example.tld -t '/news/europe/sport' -m 'this is the message about european sports'

Take a look in your browser Javascript consle you have to see how the client prints the message on it. If it fails, review the steps and debug each one to solve the problem. If you need help leave me a message. Of course, you can use many different ways to publish messages, for example, you could use python code to publish messages in MQTT server. In the same way you could subscribe not only browsers to topics, for example, you could subscribe a python code:

import mosquitto

def on_connect(mosq, obj, rc):
    print("rc: "+str(rc))

def on_message(mosq, obj, msg):
    print(msg.topic+" "+str(msg.qos)+" "+str(msg.payload))

def on_publish(mosq, obj, mid):
    print("mid: "+str(mid))

def on_subscribe(mosq, obj, mid, granted_qos):
    print("Subscribed: "+str(mid)+" "+str(granted_qos))

def on_log(mosq, obj, level, string):
    print(string)

mqttc = mosquitto.Mosquitto("the_client_id")
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe

mqttc.connect("ns.example.tld", 1883, 60)
mqttc.subscribe("/news/+/sport", 0)

rc = 0
while rc == 0:
    rc = mqttc.loop()

Pay attention to server port, it isn’t the ‘https’ port (443/tcp) because now the code is using a real MQTT client. The websocket gateway isn’t needed.

The files

  • mqtt.tar.gz – inside this tar.gz you can find all referenced files
Sep 06

Celery logs through syslog

Reading time: 2 – 2 minutes

Celery logs are colorized by default, the first big idea is disable color logs. It’s as easy as setting ‘CELERYD_LOG_COLOR’ to ‘False’ in ‘celery.conf’. The code could be something like this:

celery.conf.update('CELERYD_LOG_COLOR' = False)

Secondly we need a function where we set up a new handler and other settings to celery logging system. For example, the code could be:

from __future__ import absolute_import
from logging import BASIC_FORMAT, Formatter
from logging.handlers import SysLogHandler
from celery.log import redirect_stdouts_to_logger

def setup_log(**args):
    # redirect stdout and stderr to logger
    redirect_stdouts_to_logger(args['logger'])
    # logs to local syslog
    hl = SysLogHandler('/dev/log')
    # setting log level
    hl.setLevel(args['loglevel'])
    # setting log format
    formatter = Formatter(BASIC_FORMAT)
    hl.setFormatter(formatter)
    # add new handler to logger
    args['logger'].addHandler(hl)

Pay attention to ‘redirect_stdouts_to_logger’ it’s used to send all outputs like print’s or something else to syslog.

Thirdly we want to use those settings in our celery tasks, then we have to connect ‘setup_log’ code to some celery signals. Those signals are launched when ‘task_logger’ and ‘logger’ are configured. To connect signals:

from celery.signals import after_setup_task_logger, after_setup_logger

after_setup_logger.connect(setup_log)
after_setup_task_logger.connect(setup_log)

Fourthly we have to get the ‘logger’, we can have more than one if we are interested in records with task context or without it. For example:

logger = get_logger('just_a_name_for_internal_use')
logger_with_task_context = get_task_logger('name_of_the_task_to_be_recorded_in_logs')

Finally we only have to use those loggers with common methods DEBUG, INFO, WARN, ERROR and CRITICAL:

@celery.task
def the_task():
    logger.info('this is a message without task context')
    logger_with_task_context.debug('this record will have the prefix "name_of_the_task_to_be_recorded_in_logs" in syslog')
May 28

A pair of themes for ExtJS

Reading time: 1 – 2 minutes

I’m a ExtJS JavaScript framework believer, but there other interesting and famous JavaScript frameworks like Bootstrap and jQuery. IMHO ExtJS is more focused on web applications than public web. In this post I want to share two ExtJS themes that helps to improve UI look and feel.

The first one is a bootstrap look and feel for ExtJS:

extjs-bootstrap

if you want to test it take a look to demo site. The theme is opensource and you can find the source in github.

The second and last one is Clifton theme.

clifton-theme

IMHO is a nice theme although it’s not really free. It costs around 320€, but in some professional projects it could be a really low price if you consider the effort to get a professional look and feel . You can try it in demo page.

Oct 11

Some recommendations about RESTful API design

Reading time: 4 – 6 minutes

I want to recommend to you to watch the YouTube video called RESTful API design of Brian Mulloy. In this post I make an small abstract of the most important ideas of the video, of course from my point of view:

  • Use concrete plural nouns when you are defining resources.
  • Resource URL has to be focused in access collection of elements and specific element. Example:
    • /clients – get all clients
    • /clients/23 – get the client with ID 23
  • Map HTTP methods to maintein elements (CRUD):
    • POST – CREATE
    • GET – READ
    • PUT – UPDATE
    • DELETE – DELETE
  • Workaround, if your REST client doesn’t support HTTP methods, use a parameter called ‘method’ could be a good idea. For example, when you have to use a method HTTP PUT it could be changed by method HTTP GET and the parameter ‘method=put’ in the URL.
  • Sweep complexity behind the ‘?’. Use URL parameters to filter or put some optional information to your request.
  • How to manage errors:
    • Use HTTP response codes to refer error codes. You can find a list of HTTP response codes  in Wikipedia.
    • JSON response example can be like this:
      { 'message':'problem description', 'more_info':'http://api.domain.tld/errors/12345' }
    • Workaround, if REST client doesn’t know how to capture HTTP error codes and raise up an error losing the control of the client, you can use HTTP response code 200 and put ‘response_code’ field in JSON response object. It’s a good idea use this feature as optional across URL parameter ‘supress_response_code=true’.
  • Versioning the API. Use a literal ‘v’ followed by an integer number before the resource reference in the URL. It could be the most simple and powerful solution in this case. Example: /v1/clients/
  • The selection of what information will be returned in the response can be defined in the URL parameters, like in this example: /clients/23?fields=name,address,city
  • Pagination of the response. Use the parameters ‘limit’ and ‘offset’, keep simple. Example: ?limit=10&offset=0
  • Format of the answer, in this case I’m not completely agree with Brian. I prefer to use HTTP header ‘Accept’ than his proposal. Anyway both ideas are:
    • Use HTTP header ‘Accept’ with proper format request in the answer, for example, ‘Accept: application/json’ when you want a JSON response.
    • or, use extension ‘.json’ in URL request to get the response in JSON format.
  • Use Javascript format for date and time information, when you are formatting JSON objects.
  • Sometimes APIs need to share actions. Then we can’t define an action with a noun, in this case use verb. Is common to need actions like: convert, translate, calculate, etc.
  • Searching, there are two cases:
    • Search inside a resource, in this case use parameters to apply filters.
    • Search across multiple resource, here is useful to create the resource ‘search’.
  • Count elements inside a resource, simply add ‘/count’ after the resource. Example: /clients/count
  • As far as you can use a single base URL for all API resources, something like this: ‘http://api.domain.tld’.
  • Authentication, simply use OAuth 2.0
  • To keep your API KISS usually it’s a good idea develop SDK in several languages, where you can put more high level features than in API.
  • Inside an application each resource has its own API but it’s not a good idea publish it to the world, maybe use a virtual API in a layer above it’s more secure and powerful.

 

Mar 23

Deep inside AMQP

This entry is part 3 of 4 in the series AMQP and RabbitMQ

Reading time: 5 – 8 minutes

In the next lines I’ll describe with more details the properties and features of AMQP elements. It won’t be an exhaustive description but in my opinion more than enough to start playing with AMQP queues.

Channels

When producers and consumers connects to the broker using a TCP socket after authenticating the connection they establish a channel where AMQP commands are sent. The channel is a virtual path inside a TCP connection between this is very useful because there can be multiple channels inside the TCP connection each channels is identified using an unique ID.

An interesting parameter of a channel is confirmation mode if this is set to true when messages delivered to a exchange finally gets their queues the producer receives an acknowledge message with an UID of the message. This kind of messages are asynchronous and permits to a producer send the next message when it is still waiting the ACK message. Of course if the message cannot be stored and it is lost the producer receives a NACK (not acknowledged) message.

Producers

Maybe this is the most simple part of the system. Producers only need to negotiate the authentication across a TCP connection create a channel and then publish all messages that want with its corresponding routing key. Of course, producers can create exchanges, queues and then bind them. But usually this is not a good idea is much more secure do this from consumers. Because when a producers try to send a message to a broker and doesn’t have the needed exchange then message will be lost. Usually consumers are connected all time and subscribed to queues and producers only connect to brokers when they need to send messages.

Consumers

When a consumer connects to a queue usually uses a command called basic.consume to subscribe the channel to a queue, then every time subscribed queue has a new message it is sent to consumer after last message is consumed, or rejected.

If consumer only want to receive one message without a subscription it can use the command basic.get.This is like a poll method. In fact, the consumer only gets a message each time it sends the command.

You can get the best throughput using basic.consume command because is more efficient than poll every time the consumer wants another message.

When more than one consumer was connected to a queue, messages are distributed in a round-robin. After the message is delivered to a consumer this send an acknowledge message and then queue send another message to next consumer. If the consumer sends a reject message the same message is sent to next consumer.

There are two types of acknowledgements:

  • basic.ack: this is the message that sends consumer to queue to acknowledge the reception of a message
  • auto_ack: this is a parameter we can set when consumer subscribes to a queue. The setting assumes ACK message from consumer and then queue sends next message without waiting the ACK message.

The message basic.reject is sent when the consumer wants to reject a received message. This message discards the message and it is lost. If we want to requeue the message we can set the parameter requeue=true when sent a reject message.

When the queue is created there can be a parameter called dead letter set to true, then consumer rejects a message with the parameter requeue=false the message is queued to a new queue called  dead letter. This is very useful because after all we can go tho that queue an inspect the message rejection reason.

Queues

Both consumers and producers can create a queue using queue.declare command. The most natural way is create queues from consumers and then bind it to an exchange. The consumers needs a free channel to create a queue, if a channel is subscribed to a queue, the channel is busy and cannot create new queues. When a queue is created usually we use a name to identify the queue, if the name is not specified it’s randomly generated. This is useful when create temporary and anonymous queues for RPC-over-AMQP.

Parameters we can set when create a new queue:

  • exclusive – this setting makes a queue private and is only accessible from your application. Only one consumer can connect to a queue.
  • auto-delete – when last consumer unsubscribes from queue the queue is removed.
  • passive – when create a queue that exists the server returns successfully or returns fail if parameters don’t match. If passive parameter is set and we create a queue that exists always returns success but if the queue doesn’t exist it is not created.
  • durable – the queue can persist when the services reboots.

Exchange and binding

In the first post of the serie we talked about different exchange types as you can remember these types are: direct, fanout and topic. And the most important parameter to set when producer sends a message is the routing key this is used to route the message to a queue.

Once we have declared an exchange this can be related with a queue using a binding command: queue_bind. The relation between them is made using the routing key or a pattern based in routing key. When exchange has type fanout the routing key or patterns are not needed.

Some pattern examples can be: log.*, message.* and #.

The most important exchange parameters are:

  • type: direct, fanout and topic.
  • durable: makes an exchange persistent to reboots.

Broker and virtual hosts

A broker is a container where exhanges, bindings and queues are created. Usually we can define more than one virtual brokers in the same server. Virtual brokers are also called virtual hosts. The users, permissions and something else related to a Broker cannot be used from another one. This is very useful because we can create multiple brokers in the same physical server like multi-domain web server and when some of this virtual hosts is too big it can be migrated to another physical server and it can be clustered if it is required.

Messages

An AMQP message is a binary without a fixed size and format. Each application can set it’s own messages. The AMQP broker only will add small headers to be routed among different queues as fast as possible.

Messages are not persistent inside a broker unless the producer sets the parameter persistent=true. In the other way the messages needs to be stored in durable exchanges and durable queues to persist in the broker when it is restarted. Of course when the messages are persistent these must be wrote to disk and the throughput will fall down. Then maybe sometimes create persistent messages is not a good idea.

 

 

Mar 15

What is AMQP? and the architecture

This entry is part 2 of 4 in the series AMQP and RabbitMQ

Reading time: 3 – 4 minutes

What is AMQP? (Advanced Message Queuing Protocol)

When two applications need to communicate there are a lot of solutions like IPC, if these applications are remote we can use RPC. When two or more applications communicate with each other we can use ESB. And there are many more solutions. But when more than two applications communicate and the systems need to be scalable the problem is a bit more complicated. In fact, when we need to send a call to a remote process or distribute object processing among different servers we start to think about queues.

Typical examples are rendering farms, massive mail sending, publish/subscriptions solutions like news systems. At that time we start to consider a queue-based solution. In my case the first approach to these types of solutions was Gearman; that is a very simple queue system where workers connect to a central service where producers have to call the methods published by workers; the messages are queued and delivered to workers in a simple queue.

Another interesting solution can be use Redis like a queue service using their features like publish/subscribe. Anyway always you can develop your own queue system. Maybe there a lot of solutions like that but when you are interested in develop in standard way and want a long-run solution with scalability and high availability then you need to think in use AMQP-based solutions.

The most simple definition of AMQP is: “message-oriented middleware”. Behind this simple definition there are a lot of features available. Before AMQP there was some message-oriented middlewares, for example, JMS. But AMQP is the standard protocol to keep when you choice a queue-based solution.

AMQP have features like queuing, routing, reliability and security. And most of the implementations of AMQP have a really scalable architectures and high availability solutions.

The architecture

The basic architecture is simple, there are a client applications called producers that create messages and deliver it to a AMQP server also called broker. Inside the broker the messages are routed and filtered until arrive to queues where another applications called consumers are connected and get the messages to be processed.

When we have understood this maybe is the time to deep inside the broker where there are AMQP magic. The broker has three parts:

  1. Exchange: where the producer applications delivers the messages,  messages have a routing key and exchange uses it to route messages.
  2. Queues: where messages are stored and then consumers get the messages from queues.
  3. Bindings: makes relations between exchanges and queues.

When exchange have a message uses their routing key and three different exchange methods to choose where the message goes:

    1. Direct Exchange:  routing key matches the queue name.
    2. Fanout Exchange: the message is cloned and sent to all queues connected to this exchange.
    3. Topic Exchange: using wildcards the message can be routed to some of connected queues.

This is the internal schema of a broker:

Mar 09

AMQP and RabbitMQ [TOC]

This entry is part 1 of 4 in the series AMQP and RabbitMQ

Reading time: 1 – 2 minutes

After reading the book ‘RabbitMQ in action‘ I’m working on series of posts  that will include the following subjects:

  1. What is AMQP? and the architecure
  2. Deep inside AMQP
  3. RabbitMQ CLI quick reference
  4. Hello World using ‘kombu’ library and python
  5. Parallel programming
  6. Events example
  7. RPC
  8. Clustering fundamentals
  9. Managing RabbitMQ from administration web interface
  10. Managing RabbitMQ from REST API

Please let me know if you are interested in this series of posts. Because in my opinion this is very interesting and it always comes in handy to know if someone has been working on those subjects.