An introduction to erlang

An introduction to Erlang

Code Lovers 30-1-2012

@MirkoBonadei

Our Agenda

- Introduction

- Some bits of history

- The Erlang Way

- Inside the VM

- OTP (Open Telecom Platform)

- Who uses it?

- Conclusion

Erlang is about processes

They are the basic components to model a system.

P2P3P4P5P1

Process isolation

An O.S. Example...

ChromeWord

Process isolation

Your favourite word processor crashes but your browser is still running

ChromeWord

Erlang Processes

Processes are cheap and lightweight.We can have millions of them.Context Switch and IPC are really fast.

Some bits of History

Erlang came to the masses in 2006/07 but....

Some bits of History

Erlang is 25 years old!

So... Why 20 years of silence?

What's happened

Distributed Systems

Technology penetration

Multicore

Crystal Ball?!?

Nope, Industrial Requests

Ericsson was looking for a language to replace PLEX (their development language in the 80s)

Requests:

Time to Market

Distributed

Fault Tolerant

Concurrent

Hot Code Swapping

Declarative

Soft Realtime

Research is the key

At the Ericsson Lab they developed some pilot projects in various programming languages...

MLMirandaChillOthers...AdaSmallTalkProlog

But

There wasn't a language with all those features

Let's create our language!

Good results!

The first version was a language implemented on the top of Prolog.

After some good results, they wrote the JAM Machine in C, boosting performance of 70%.

We are in the early 90s, and the Erlang Era has just began.

But...

After lots of great results, Ericsson banned Erlang from future projects in 1998.They chose to follow competitors...

Ericsson released Erlang as Open Source

Core developers left Ericsson and founded their companies

Ericsson comes back

Understanding the error made, they come back to Erlang.

Ericsson re-hired Joe Armstrong in 2004

The OTP Team is now based in Ericsson.

Let's start....

talking about concurrency

Concurrency we are used to...

Code to solve the problem

Error Handling code

Coordination code

What this means?

Loosing Abstraction

Complexity explodes

Maintenance is hard

The Erlang Way

The best way to understand the Erlang way is to think at the real world. World is concurrent, this is the abstraction we want.

It is simple

To obtain scalability and fault tolerance is quite easy. You took 2 things, you make them share nothing and you get fault tolerance and you can scale.

Joe Armstrong (London Erlang Factory 2011)

Type System

Erlang is dynamically typed

Erlang is strongly typed

&

Type: Number

Type: Atom

Type: Binary and Bit String

Bit Syntax and Pattern Matching:

Type: Pid

Type: Port Identifier

Basic mechanism to communicate with a non Erlang program

The Port Identifier is returned by the call to the BIF open_port/2

Type: Tuple

Type: List

Type: Fun

Ehy... Erlang is a Functional language :)

Type: Reference

A term which is unique in an Erlang Runtime System

No Type: String

A String is simply a list of integer values

No Type: Boolean

true and false are nothing than atoms

No Type: Record

Pattern Matching 1/3

Referential Transparency



Ehy, we've got some recursion here!

Don't Care Variables

Really useful, but use them with care

Processes and IPC

To create a process we use the spawn functions. There are different versions of spawn, for all the possible scenarios.

Every Process has its MailBox where its messages are delivered.

To send a message to a process we use a one character operator, !

An Example

An Example

An Example

How to deal with Errors

Erlang has an Exception system with 3 classes of errors:

- error: It is a runtime error or it could be generated from the call error(Reason)

- exit: generated from the code with the call exit(Reason)

- throw: generated from the code with the call throw(Reason)

Try Catch

But Between Processes...

Exit Signals

When a process terminates it emits (with its last breath) an exit signal with the reason of its termination

The reason could be normal if things are terminated it the right way, or it could be something else, and in this case the termination is abnormal.

We can use processes to isolate errors, make that part of the system fail fast, and eventually restart it.

Links

P1P2

{'EXIT', Pid2, Reason}

We can link processes together with link/1 BIF or directly with spawn_link BIFs.

Linked processes forms a linked set, and if one of them terminates abnormally, the error propagates the the linked set, taking processes down.

Trapping Exits

P2

{'EXIT', Pid2, Reason}

We can trap exits, making a process become a system process with:process_flag(trap_exits, true).

Now it receives the message, and it can choose what to do. Stopping the propagation of the error.

Only the Reason kill is untrappable!

P1

Robustness

WWWW

S SupervisorW Worker

SS

Monitors

While links are bidirectional, monitors are unidirectional.No link set, the monitor process is like a stalker.

Distribution

Distribution is built into the language itself.

Are you kidding?

No shared memory. So, we copy data between processes on the same machine. And... We copy data between processes on different machines.

Nope, think about it...

Ok, and the location?

! operator is location transparent

A Pid also contains information on the location of the process.

Erlang Clusters

It is really simple to create an Erlang cluster.

There are apposite libraries, and when a cluster is set, you work without headache.

Hot Code Swapping

No Downtime allowed for bad guys like us!

So...

Hot Code Swapping is built into the language itself!

How does it woks?

-vsn(1.0)P

Instant 0

How does it woks?

-vsn(1.0)PInstant 1

-vsn(1.1)

How does it woks?

-vsn(1.0)PInstant 2

-vsn(1.1)

How does it woks?

-vsn(1.0)PInstant 3

-vsn(1.1)-vsn(1.2)

Warning!

Code upgrades on intra-module function calls could bite you.The function calls must be fully qualified calls such as:

If you follow OTP Principles you can get Hot Code Swapping the right way without headache

: ()

Warning!

Hot Code Swapping is a difficult task

Even if Erlang helps you...

Test Hard your solutions before taking the system down.

ERTS and the VM

No clear separation between ERTS and VM.

VM runs compiled byte-code (BEAM)

ERTS manages Processes, IPC, Memory, Load Balancing, ecc...

We always call VM both two!

The Scheduler

Fundamental part of the Erlang Runtime System.

It has been deeply changed during the last years to rule multi-core hardware the right way

In the past

The scheduler was there to manage execution of Erlang processes inside an O.S. Process called beam or werl

Scheduler

Run Queue

Then...

Symmetric Multiprocessing was released in May 2006

Scheduler #1

Run Queue

Scheduler #..Scheduler #N

Scheduler #1

Today

Bottleneck removed. One run queue for scheduler.

Migration LogicScheduler #1

RQ

Scheduler #...

RQ

Scheduler #N

Scheduler #...

RQ

Garbage Collection

Per process Garbage Collection

Small processes means quick garbage collection

Garbage Collection doesn't pause the system as happens in other languages. Soft Real-time feature is safe even with million of processes running

OTP: Open Telecom Platform

Set of tools, libraries and design principles to develop distributed applications

Do not reinvent the wheel every time, use OTP behaviours instead!

Supervisor trees, Hot Code Swapping, Packaging, abstracted away

OTP Benefits

Less code to write means, less bug and fast time to market

Easy to test your specific functionality

Common coding style between developers

OTP is battle tested

Who uses Erlang

Who uses Erlang

And many others...

Books

learnyousomeerlang.com

Next Conferences

Thanks