Getting started with Akka

Akka is a library designed for building applications using the actor model. From their site:

Akka is a toolkit and runtime for building highly concurrent, distributed, and resilient message-driven applications on the JVM.

In today’s post, I’m going to start with some of the primitives to using this framework.

Messages

Actors process messages that you’ll define in your modules. For today’s example, I’m going to implement a very basic logging application. Messages sent into this system are expected to be logged out to the console. To start off, we define the messages for this system:

case object Log
case class LogMessage(when: Date, level: String, text: String)
case class LogString(message: String)
case class LogException(e: Exception)

Using scala’s case classes we can clean up the definition of these log messages. We have a message that will do general logging LogMessage, one that will log a string in LogString and one that will dissect and log out an exception object LogException.

Actor Logic

We now focus on the logic required to log information out from our actor. This is really quite simple; we’re just going to push everything out to the console:

class LogActor extends Actor {

  def receive = {
    case LogMessage(when, level, text) => println(String.format("%s [%s] %s", when.toString(), level, text))
    case LogString(message) => self ! LogMessage(new Date, "info", message)
    case LogException(e) => self ! LogMessage(new Date, "error", e.toString())
  }

}

The receive method is just a big pattern matching statement. Each of the message types are handled in here. Note how LogString and LogException send messages to self. self is a built-in, given to us representing this actor. All we’re doing is just on-forwarding the message in the string and exception cases.

Creating a system

We have actors; we have messages to pass between the actors; we now need a system that the actors will participate in.

// create the system
val system = ActorSystem("myLoggingSystem")

// create an actor
val logger = system.actorOf(Props[LogActor], "logger")

Using the tell and ask methods, we can send and send/receive messages to/from this actor. We also can create a logic-less actor that just acts as a message sender/receiver:

val inbox = Inbox.create(system)

Mailboxes are an important abstraction; they hold messages for actors. Each actor has its own mailbox, but we’ve created one above attached to a system that we can pipe messages into:

inbox.send(logger, LogString("This is the first line of log"))
inbox.send(logger, LogException(new Exception("DOH!")))

Lots of Actors

A slightly more complex topic is to create a pool of actors. In this next snippet, we’ll create a RoundRobinPool.

val actors = system.actorOf(Props[LogActor].withRouter(RoundRobinPool(5)), name = "LoggingActors")

Now that we’ve created a pool, it’s time to smash!

Range(1, 1000000).map(i => actors ! LogString(String.format("Message number %s", i.toString())))

Scheduled

Finally, we can schedule these messages to be sent . . as if they were sent from no where using the actor system that we’d created earlier:

system.scheduler.schedule(0.seconds, 1.second, actors, LogString("Yerrr!"))(system.dispatcher, Actor.noSender)

This will send a LogString message to the actor system actors after zero seconds and then another message every second there after.

Bash Gems

The Bourne Again SHell is one of the most widely deployed shell for Linux that I use all the time. In today’s post, I’m going to collate a lot of the gems that I’d discovered in my travels of using this software.

Finding Help

Nothing can substitute the reference manual materials distributed with this software when it’s installed. At the console, you can read documentation in info format on bash using the following:

info bash

You’re able to deduce executing this command by doing some research at the console, by yourself. Using apropos (which searches the manual pages) you can look for key words.

If you wanted to find any command that begins with the characters ‘ls’ in an attempt to find the command ls, you can perform the following search:

apropos ls | grep '^ls.*'

On my system here, this function emits the following result:

ls (1)               - list directory contents
lsattr (1)           - list file attributes on a Linux second extended file s...
lsb_release (1)      - print distribution-specific information
lsblk (8)            - list block devices
lscpu (1)            - display information about the CPU architecture
lsdiff (1)           - show which files are modified by a patch
lsearch (3)          - linear search of an array
lseek (2)            - reposition read/write file offset
lseek64 (3)          - reposition 64-bit read/write file offset
lshw (1)             - list hardware
lsinitramfs (8)      - list content of an initramfs image
lslocks (8)          - list local system locks
lsmod (8)            - Show the status of modules in the Linux Kernel
lsof (8)             - list open files
lspci (8)            - list all PCI devices
lspcmcia (8)         - display extended PCMCIA debugging information
lspgpot (1)          - extracts the ownertrust values from PGP keyrings and l...
lstat (2)            - get file status
lstat64 (2)          - get file status
lsusb (8)            - list USB devices

We’re only interested in the first item there, but we’re given all of the options. We can now display the manual page with the following:

man ls 1

Variables

Variable creation is fairly straight forward:

# stores the string "John" in var1
var1="John"
# stores the text output of the command 'ls' into var2
var2=`ls -al`

# simple string replacement
var3=${var1/h/a}
# sub-string (turns "John" into "Jo")
var4=${var1:0:2}
# default string substitution (where null)
var6=${var5:-"Value for var5 was not supplied"}

# string interpolation is achieved with $
echo "His name is $var1"

Special variables exist to tell the developer a little bit about their environment:

Functions

# define a function
function syntax() {
  echo "usage: prog.sh [options]"
  return 0
}

function print_name() {
  echo "Hello $1"
  return 0
}

# call the function
syntax
print_name "John"

Control Flow Constructs

# Conditionals
if [ var1 == 10 ] then
  echo "It was 10"
else
  echo "It was not 10"
fi

case "$var1" in
  0) echo "Value was zero";;
  1) echo "Value was one";;
  *) echo "Anything but null";;
esac 

# Repetition
for var1 in {1..10} do
done

for ((x=1; x <= 10; x++)) do
done

while [ var1 == 10 ] do
done

Redirection

Special file descriptors of 0 as /dev/stdin, 1 as /dev/stdout and 2 as /dev/stderr.

From the redirections section in the bash manual:

Note that the order of redirections is significant. For example, the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.

JDBC

JDBC (Java Database Connectivity) is a general purpose data access library baked into the Java development and runtime. This library’s purpose is to lower the level of complexity in connecting to different database vendors providing a consistent interface no matter what database you’re connecting to.

In today’s post, I’ll go through the basics of using this library. This blog post will be in context of connecting to a PostgreSQL database.

Drivers

JDBC is based on the premise of drivers. The driver code itself is what fills in the architecture with an implementation that your applications will use. To enumerate all of the drivers, currently in context of your application you can use the following:

Enumeration drivers = DriverManager.getDrivers();

while (drivers.hasMoreElements()) {
  Driver driver = (Driver) drivers.nextElement();
  System.out.println(driver.getClass());
}

I use the term “in context” because whilst you may have the required JAR installed on your system which will be a particular database vendor’s connection library for JDBC, you’ll need to make sure that it’s available on your class path.

For my example, I only have Postgres available to me:

class org.postgresql.Driver

The driver string that you saw in the section above plays an important role in establishing a connection to your database. Before you can start to work with Connection, Statement and ResultSet objects you first need to load in your vendor’s library implementation.

Class.forName("org.postgresql.Driver");

This will reflect your driver into your application ready for use.

Making a connection

To establish a connection with a database, you’ll need to specify a connection string with all of the attributes required to direct your application to the database.

JDBC has a uniform format for specifying its connections with each vendor. Postgres conncetions are no different.

A connection is established using the DriverManager class like so.

Connection connection = DriverManager.getConnection("jdbc:postgresql://localhost:5432/crumbs?user=postgres&password=password");

Running queries

Running retrieves on your database normally comprises of three processes:

• Preparing a statement to run
• Executing the statement
• Enumerating the results

The preparation of the statement is fairly straight forward. The createStatement method on the Connection object will allow you to create an empty statement, whereas prepareStatement will allow you to provide some SQL directly.

// prepare the statement 
Statement retrieveStatement = connection.createStatement();

// execute the statement
ResultSet streetTypes = retrieveStatement.executeQuery("SELECT * FROM \"StreetType\"");

// enumerate the result
while (streetTypes.next()) {
  int id = streetTypes.getInt(streetTypes.findColumn("ID"));
  String name = streetTypes.getString(streetTypes.findColumn("Name"));
  
  System.out.println(String.format("ID: %d, Name: %s\n", id, name));
}

A slightly more complex example where you’d pass in some parameters into your statement might look like this:

PreparedStatement retrieveStatement = 
  connection.prepareStatement("SELECT * FROM \"StreetType\" WHERE \"ID\" > ?");

retrieveStatement.setInt(1, 10);
ResultSet streetTypes = retrieveStatement.executeQuery();

Enumerating a ResultSet object can be achieved with a simple while loop:

while (streetTypes.next()) {
  int id = streetTypes.getInt(streetTypes.findColumn("ID"));
  String name = streetTypes.getString(streetTypes.findColumn("Name"));
  
  System.out.println(String.format("ID: %d, Name: %s\n", id, name));
}

Cleaning up

Finally, all objects should be cleaned up afterwards by using the close functions provided.

streetTypes.close();
retrieveStatement.close();
connection.close();

Other topics

This blog post is just enough to get up and running. There are plenty more complex topics inside of JDBC to be learned:

• Stored Procedures
• Batches
• Transactions

H2

H2 is a relational database written entirely in Java. It has an extremely small footprint and has an in-memory mode making it an excellent choice for embedded applications.

In today’s post, I’ll take you through using the H2 shell.

Shell

Once you’ve downloaded H2 from their site, you can get a database created and running using the shell. You can invoke the shell with the following command:

java -cp h2-1.4.190.jar org.h2.tools.Shell -url jdbc:h2:~/testdb

I’m using version 1.4.190 here. The -url command line directs us to the file of the database that we’ll create/open.

Once the shell is running, you’re presented with a sql> prompt. You can start creating your table definitions. The documentation on the website is quite extensive with the supported sql grammar, functions and data types.

Further development

Now that you’ve created a database, you can write java applications using JDBC to run queries against your H2 database.

External modules in Python

You can extend Python relatively easily with the development libraries. Once installed, you can write a module in C, build it and start using it in your Python code.

In today’s post, I’ll create a Hello world module and use it from python.

Environment

In order to get started, you’ll need to prepare your environment with the right tools. It’s also and idea to create a bit of a project structure.

Create a directory that your code will go into. My source structure looks like this:

.
├── Dockerfile
└── pymod
    ├── README
    ├── setup.py
    ├── src
    │   ├── hello.c
    │   └── hello.h
    └── test.py

My Dockerfile looks as follows. This should describe how to setup your environment:

FROM python:2

RUN apt-get update && \
    apt-get install -y build-essential python-dev && \
    apt-get clean && \
    rm -Rf /tmp/* /var/tmp/*

The module

The module itself really consists of a c header and source file and a setup.py file to build/install the module.

The header file looks as follows:

#ifndef __hello_h_
#define __hello_h_

#include <Python.h>

static PyObject *hello_say_hello(PyObject *self, PyObject *args);

#endif

Note the Python.h header as well as the PyObject types being used. These are a part of the python-dev library that we installed before. This header file then gets implemented pretty simply. Here I’ve cheated using printf to do the printing for us:

#include "hello.h"

static char module_doc[] = "This is a simple, useless, hello module";
static char say_hello_doc[] = "This function will say hello";

static PyMethodDef module_methods[] = {
  { "say_hello", hello_say_hello, METH_VARARGS, say_hello_doc },
  { NULL, NULL, 0, NULL }
};

PyMODINIT_FUNC init_hello(void) {
    PyObject *m = Py_InitModule3("_hello", module_methods, module_doc);

    if (m == NULL)
        return;
}

PyObject *hello_say_hello(PyObject *self, PyObject *args) {
  printf("I'm here");
  return Py_None;
}

A brief analysis of this code sees us building a PyMethodDef array. We expose it out using Py_InitModule3' from within the initialization function (typed with PyMODINIT_FUNC`).

To out actual function itself, we’re printing “I’m here” to the console and then bailing out with a return value of Py_None, which is equivalent to None.

Building

To build our module, we’ll use setup.py. It’ll read as follows:

from distutils.core import setup, Extension

setup(
  ext_modules=[Extension("_hello", ["src/hello.c"])]
)

To invoke the build, we issue the following:

python setup.py build_ext --inplace

Testing it out

Now that our module is built, we can give it a test. Easiest to use the python console to do this for us:

Python 2.7.10 (default, Sep  9 2015, 20:21:51) 
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import _hello
>>> _hello.say_hello()
I'm here>>> 

Finishing up

I couldn’t pick a simpler example. More complex examples to come, but this is how we establish the bridge between C and python.