Erlang Pragmatic Studio with Joe Armstrong & Dave Thomas – Chicago, Feb 13-25, 2008.
Day 3 – Feb 14,2008.
Here are my notes from Day 3 of the Pragmatic Studio Erlang course. Note that these notes are only vaguely edited. Be aware that these are notes so there are certainly typos and errors.
Beginning of the Day
Always good to develop from the specific to the general. e.g. make a simple app, then parameterize it with functions. Universal server that evaluates F is a very powerful generalization of a specific application (e.g. messaging).
can send a message to your self:
self ! a. % Could tie into recursion
a lot of users
Erlang guru’s (tailf, Kredita) don’t use OTP, as they participated in the OTP creation. But if you build up the Gen server from incremental steps, you may find it not so difficult. The question is, does OTP really fit the problem at hand.
If you do spawn_link from the shell, report errors back to the shell.
erl -bootstart sasle % spelling system application support library. Get error logger and a few other items.
sasle libraries were added into Erlang at a later date for historical reasons.
Bit Syntax
In ruby or other languages would be fairly tedious (ands, ors, bitshifts, etc). Erlang has a high level syntax that lets you specify the structure of binary data.
Red = 2, Green=61, Blue=20.
=> 20
Bin = <<Red:5,Green:6,Blue:5>>
=> <<23,180>>
io:format("~8.2.0B~8.2.0b~n", binary_to_list(Bin)).
=> 00010111 10110100
Bit Twiddling History:
Erlang had lots of protocols internally in Erickson. Decided to implement every RFC – but was too difficult. Lots of bit twiddling. Because of this they made a sub-language for doing this bit-twiddling so that this was implement any protocol. Optimal and efficient unpacking of datastructures.
1/3rd of details were in the Erlang book. The rest needs to be found in the man pages.
There are also bit comprehensions – generators of bit stream. Good for huffman encodings, MIPS processing, etc. Very efficient. This is in the experimental phase but will be integrated into the system. Undocumented items are not official and may be changed. Once the docs are written it’s official.
Erlang mailing list erlang.org > FAQs and mailing lists.
Book page 86 great shoutcast server for distributed audio. There is an excercise for bit processing, but we are skipping it in the interest of time.
file at a time I/O
Reading file at at time is about 10-20 times faster than reading line at a time. Erlang is not as efficient as some other languages for string matching. Use [H|T] to parse. Only multi-gig pattern are difficult for this. binary input and output is definitely fastest.
file:consult(File) -> {ok, Term} | {error, Why}
Term Access – read configuration file
-module(test4).
-
read_config() ->
{ok,[{host,Host,port,Port}]} = file:consult(...)
...
Term IO
io:read('enter a term').
enter a term > {hello, "joe"}
io:get_line(...) %read from a shell
erl_scan:string("abc,123,{hello,joe}"). % turn into erlang tokens - very efficient
% written with explicit recursion that is more efficient than regex
file:pread ...
map onto standard file manipulations:
file:list_file(Dir)
file:delete()
...
filename:split(FileName) => [Component]
DNS servers are actually fairly easy. In a real DNS server there is tree walking for the sub-domain, domain, and tld xxx.xxxx.xxx
lots of DNS servers have interesting traffic patterns… tie ins with advertising sites, etc.
Erlang doesn’t link code into it’s Kernal. Need to establish a socket for C libs. Send messages using binary.
Orientation:
- nodes – when you say erl it starts a node. Doesn’t register this node by default.
- distributed erlang consists of a number of nodes that know about eachother.
- SMP takes advantage of multi-core – this is not distributed Erlang
- Socket distribution – is also not distributed Erlang
erl -sname %short name
erl -name % different names
There is a magic cookie that they must share. Simple challenge request, traffic is not encrypted. Can communicate over SSH. DNS and name recognition systems are often poorly configured and this causes some difficulties for setting up distributed erlang apps. There are som FAQs on debugging distributed Erlang.
Three new primitives:
- spawn(Node, Mod, Func, Args) – links work the same
- alive(Node) – tell the system you are alive
Main libs:
Dave: mnesias cool be cause you can query it using list comprehensions.
Test on local nodes, then over a cluster.
erl -sname dave
rpc:call('joe@Daves-Powerbook', erlang, node, []).
rpc:call('joe@Daves-Powerbook', erlang, exit, [kill]).
rpc:call('joe@Daves-Powerbook', erlang, halt, []).
Shell commands for interacting with erl nodes:
- ^g gets you into the shell
- j – jobs
- r ‘dave@Daves-Powerbook’ % open a remote shell
- c 2 – change to the second process
- now at (dave@Daves-Powerbook)>
- toolbar:start(). % show current processes – queries OTP structure
Security:
- Security on distributed Erlang is very course grain. It’s all access or nothing.
- Perfect for a fire-walled corporate cluster.
Need to shore cookie to other machines. Check your home directory – .erlang.cookie
OTP – Open Telecomms Platform
This is the platform / framework for building Erlang applications.
- Error logs
- Hot swapping modules
- Common tasks that everyone needs
- etc.
- Mnesia
- SASL
- SNMP Agents
- Web Server
Lots of undocumented corners. Programming Erlang book is very helpful. It’s Open Source in the sense that you can see the source. But commit rights are controlled. It is product quality. There is no difference between Erickson’s products released and the Open Source distribution. There are 15 people working full time to maintain this – stable, battle tested.
OTP Principles
- Joe Armstron PHD Thesis
- erlang.org/doc/design_principles/part_frame.html
- client-server gen_server
- finite state machines gen_fsm
- event handling gen_event
- supervisor gen_sup
- Applications – e.g. mnesia, standard libraries, collections of processes shipped in it’s own right. start and stop etc.
- Releases – 4-5 applications combined form a product
- Application upgrade – upgrade a running system
Finite state machines
- State x Event -> State1 x Action
- Way of writing pattern matching on state and event.
gen_event
- no reply back (error log)
- event to new state, no reply
Order of reading:
- Joe’s thesis – best intro to OTP: sics.se/~joe/thesis
- erlang.org/doc/design_principles/part_frame.html
- forthcoming O’Reilly book
Behaviors:
- OTP name for “design patterns”
- Callback module
Case studies:
- AXD301 – 1.6 million lines of Erlang, built using OTP.
- Nortel Networks
Factoids:
- gen_server is the most used behavior.
- supervisor bridge – allows a C module to be fault tolerant in use with Erlang.
- gen_server – A generic client-server model
Types of Supervision:
- 1:1 vs 1:N supervision
- 1:1 – if child dies only 1 is restarted
- 1:N – if challed dies all child sibling processes restarted
trapexit.org is a good site.
Distributed applications:
- Must have the same beam code on all machines. There are checksums.
- Must have the same version of Erlang.
One way of solving this is to have the share the same backend NFS system.
Explicit code distribution. Send the code in a message:
{ok, Bin} = file:read_file("xxx.beam"),
Term = {apply, erlang, load_module, [Mod, Bin]}),
gen_tcp:send(Socket, term_to_binary(Term))
run it:
{apply, xxx, start, [...]}
dynamic code upgrade:
- Mod:Func(Args, ...) the latest version of Mod is called
- If you reload Mod then you can run two versions of the code at the same time
- You can only have two versions of the code running at the same time, an old and a new version
- To load a third version you must call erlang:purge_module(Mod) before reloading the module (think of this as a two place shift register)
- to load code call erlang:load_module(Mod, BeamCodeBin)
One process per whole Erlang instance on the OS.
This would be very dangerous. But GC and as many things as possible should be written in Erlang.
Hierarchy of tests:
- Unit test framework that comes from France (used by ejabber).
- Erickson has a test server that is distributed with Erlang. Massive regression and unit tests.
- Credita has modified to run test any time anything is checked into their repositories.
- Quick check written by John Hues. Was written in Haskell has been ported to Erling. Generates random tests that satisfy certain properties. Does some automatic reduction of code execution to minimize error conditions. This is an expensive use.
- E Unit – Dave found this somewhat cumbersom. Straight pattern matching was easier. Didn’t seem to have a huge value add. Would like to see how he can use differen’t naming conventions to automate and have some sort of runner. Find *.erl run anything named test and report if it doesn’t return a pass. Gen server seems easily testible by calling handler methods.
Not currently.
Petri Nets or activity diagrams?
Message sequence diagrams are used frequently. Joe is not a fan of drawing programs. Although, message sequence diagrams are extremely useful (Y = Time. X = Process). Joe is writing a program for animating Erlang.
erlang:trace(Pid, [Msgs, Call, list of things to trace]).
This process get’s lots of messages. Joe is writing a program that stores these in a file and then animates the activities of these processes over time. Working with a games developer to animate this.
Erlyweb?
Joe – hasn’t spent much time with this and YAWS (Yet Another Web Server).
Erlang graphic user interface?
Lots written. Tend to be hard to use – wswidgets being used for 3D process modeling. Interfaces to SDL, GTK, Cairo, etc. TCL library is the mainly supported one that works everywhere. Lots of people have been doing GUIs in web browsers – action script 3 and flex. Can also use flex2 as a device driver for Erlang – e.g. video stream. Makes it possible to make these applications more portable. Therefore can use Flash as a device driver. Drop dead beautiful – flash with AIR may be the way to go.
Redeployable packaging?
Erlang is packaged for win, linux, and mac. CEAN (Comprehensive Erlang Application …) ... Wings. Martin Logan, faxian. Martin Logan also has generator applications for Erlang (Erlware?).
Good frameworks for doing ontological modeling and reasoning?
Expert systems shell. University of Corona. Reported at the Erlang user conferences. Multi agent listener but not generalized to owl.
List of reference projects?
No canonical reference. But check trapexit.org.
Where is Erlang center of gravity?
Attracting lots of interest in Financials – especially in London. Finance industry targeted conference coming up in London. Much bank interest – extreme real time demands. Number of pure erlang based trading companies. Debt buying company – purchases debt and collect. Buys these debts a few moments after the transaction. Mnesia founder involved.
Future changes – where is it going?
Not many changes to the language. Millions of legacy lines of code needed for compatibility – syntax unlikely to change. But, implementation speed optimization and multi-processor support is likely to improve.
Embedded Erlang?
4MB probably the minimum size. Used to be 640k but may be a better domain for C.
String localization – UTF8?
In a sense it is solved. In another sense it is not solved. In Erlang there is no string type – it is a list of integers. UTF8 is an integer and can be stored. However, the interpreter from text to binary and back is not written. However, it is not difficult. It is in fact a library change.
What is a good candidate for Erlang?
Telecomm applications, lots of small processes. Individual processes have low ammount of short computations. Good at coordination, concurrency, switching. Not good at matrix multiplication, gif encoding etc. Financial applications fit the erlang profile.
Libraries for interfacing to SQL databases?
ODBC libraries. Could just open a socket to the DB. Java has a good arbitrary support for sockets. Check Yariv’s Erly web implementation.
Summary
Fault tolerance drives a lot of the design decisions behind Erlang.
Simple functional language.
Function selection is by pattern matching
Variables are immutable
Concurrent Erlang
- 3 primitives: Spawn, Send, Receive – very simplified from Object Oriented model.
- register / unregister can be used to associate a name with a process
Fault Tolerant
- catch .. throw, try … catch … end
- link, process_flag(trap_exit, true)
Distributed Erlang
- spawn(Node, Mod, Func, Args)
- or explicit term passing
Benefits of Erlang:
- Technologies becoming more concurrent and people are looking for solutions to that probelm. Cooperative applications, etc. Erlang is a working solution. Not operating system dependent. 20 – 30 years of experience with fault tolerance measured in years.
- Multi-Core ready
- Processes in the language
Some projects to research:
- AXD301 – biggest Erlang and Functional programming ever made 60 programmers x 3 years. Runs backbone of british telecom. Market leading in that sector. Peaked at 9×9s of reliability. This wasn’t a commercial project on 2-3% of the market.
- Kreditor (Kreditor.se) – buys debt. #2 IP startup in sweden. $150 year. Self funded. Founded by alumni of Blue Tail.
- SimpleDB (Amazon)
- CouchDB (text db)
- MociWeb (Mochimedia)
- Ejabberd (jabber server – by extension Twitter?)
- ErlyWeb (Erlang web framework inspired by Rails)