PREviously on LAMBDA

base data types

processes

message passing (send/receive)

process registry

gen_server

TODAY

Erlang VM (BEAM) ecosystem

Erlang language extensions

OTP

let it crash!

clustering and distributed systems

Erlang VM (BEAM)

SMP and non-SMP BEAM versions

one SMP scheduler per thread per core by default

erl> erlang:system_info (cpu_topology).

processes usually travel across schedulers

BEAM ECOSYSTEM

Erlang -> CoreErlang -> BEAM

<anyLanguage> [ -> CoreErlang ] -> BEAM

where <anyLanguage> can be:

LFE, Elixir, Joxa, Reia, ...

LuvvieScript

IDEA Object Database

Erjang (Erlang on JVM),

multiple BEAM emulators for JS

NIFs, ports, drivers, ...

Erlang language Extensions

In context of modules and data types,

we haven't mentioned yet:

namespaces

pmods

records

maps

behaviors

parse_transform, custom preprocesors, syntax tools

Open Telecom platform (OTP)

...has nothing to do with telco in fact :-)

gen_server reminder (call, cast, info)

gen_fsm (finite state machine)

gen_event (event handling)

applications, supervisors,

automatic failover

PRODUCTIVITY MATTERS

Source: Productivity Gains in Erlang

LET IT CRASH!

don't be defensive

pattern matching, tagged values

"normal" vs system messages

LIVE DEMO

process (node) monitoring

linked processes

monitor (process, node)

linked processes

process flags & trap_exit

supervisors, heartbeat

CRASH EXAMPLES

Fx1 = fun (Pid) -> monitor (process, Pid) end.
Fx2 = fun (Pid) -> link (Pid) end.
Fx3 = fun (Pid) -> link (Pid), process_flag (trap_exit, true) end.
Print = fun (Msg) -> io:format ("Received: ~p~n", [Msg]) end.

Test = fun (Fx) ->
  X = spawn (fun () ->
                 receive _ -> error ('X-crash-boom-bang!') end
             end),
  Y = spawn (fun () ->
                 Fx (X),
                 receive Msg -> Print (Msg) end
             end),
  X ! hello
end.
[ Test (F) || F <- [Fx1, Fx2, Fx3] ].

RECONSTRUCT PRE-CRASH STATE

shared nothing

processes may eventually die

process restarts with an empty state

so how to get back the state?

solution: ETS, DETS, Mnesia, ...

ETS heir and ownership transfers

state manager server

NODE MONITORING

toolbar, appmon, observer,

os_mon

CLUSTERING

JCL (^G)

epmd

  $ erl -sname foo
  $ erl -sname boo -remsh foo@gersemi

node/0, nodes/0, get_cookie/0, set_cookie/2,
~/.erlang.cookie

Cluster boot example

https://gist.github.com/ztmr/5874778

LIVE CODE UPGRADES

l (my_module)

code, code_server

weberp:upgrade/0

erlang releases

DEMO TIME: DISTRIBUTED WEBSOCKETS

DWS gen_servers:

dws_ets_table_manager

dws_service_broker

dws_session_server

Cowboy handlers:

dws_session_handler

dws_websocket_handler

Demo WebSocket API:

dws_service_example

DWS: DISTRIBUTED SESSIONS

Mnesia-powered session storage

all the rest is node-specific

(service broker per-API state [metrics or so],

websocket connection state,

etc.)

if the currently connected node dies,

the websocket client just reconnects

(to the same [restarted] or any other node)

DWS: let's CRASH!

unknown message to dws_session_server:

erl> dws_session_server ! foo.

nothing crashed, it's defensive :-)

unexpected message to dws_service_broker:

erl> dws_service_broker ! foo.
17:37:21.181 [info] dws_ets_table_manager: received ETS ownership request: {'ETS-TRANSFER',dws_service_broker_state,<0.911.0>,[]}
17:37:21.181 [info] dws_ets_table_manager: received ETS take-over request: {own,<0.20098.2>,dws_service_broker_state}
17:37:21.181 [info] dws_service_broker: received ETS ownership request: {'ETS-TRANSFER',dws_service_broker_state,<0.893.0>,reused}

...well, it crashed, gave the ETS table away, restarted and

got the ETS table back, so the state was preserved!