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![collector_loop([], NeighbourCount, Cell, Content) ->
Cell ! {self(), {next_content, next_content(Content, NeighbourCount)}};
collector_loop(WaitingOn, NeighbourCount, Cell, Content) ->
receive
{cell_content, {{{_,_},_}=XYatT, NeighbourContent}} ->
case lists:member(XYatT, WaitingOn) of
true ->
collector_loop(lists:delete(XYatT, WaitingOn),
NeighbourCount + NeighbourContent,
Cell, Content);
false -> %% ignore messages we are not waiting for
collector_loop(WaitingOn, NeighbourCount, Cell, Content)
end
end.](https://image.slidesharecdn.com/2014-12-ndc-thinking-141204083055-conversion-gate02/85/NDC-London-2014-Thinking-Like-an-Erlanger-25-320.jpg)
![Cell Mgr Code
loop(#state{cells=Cells}=State) ->
receive
{'DOWN', Ref, process, _Pid, _Info} ->
{value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells),
NewPid = await_restart(XY),
NewRef = erlang:monitor(process, NewPid),
NextState = State#state{
cells=[{NewPid, NewRef, XY}|Rest]},
spawn ( fun () -> pacer(NewPid) end ),
loop(NextState)
end.
Process down](https://image.slidesharecdn.com/2014-12-ndc-thinking-141204083055-conversion-gate02/85/NDC-London-2014-Thinking-Like-an-Erlanger-33-320.jpg)
![Cell Mgr Code
loop(#state{cells=Cells}=State) ->
receive
{'DOWN', Ref, process, _Pid, _Info} ->
{value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells),
NewPid = await_restart(XY),
NewRef = erlang:monitor(process, NewPid),
NextState = State#state{
cells=[{NewPid, NewRef, XY}|Rest]},
spawn ( fun () -> pacer(NewPid) end ),
loop(NextState)
end.
Wait for supervisor
to restart it](https://image.slidesharecdn.com/2014-12-ndc-thinking-141204083055-conversion-gate02/85/NDC-London-2014-Thinking-Like-an-Erlanger-34-320.jpg)
![Cell Mgr Code
loop(#state{cells=Cells}=State) ->
receive
{'DOWN', Ref, process, _Pid, _Info} ->
{value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells),
NewPid = await_restart(XY),
NewRef = erlang:monitor(process, NewPid),
NextState = State#state{
cells=[{NewPid, NewRef, XY}|Rest]},
spawn ( fun () -> pacer(NewPid) end ),
loop(NextState)
end.
Get the new process
up-to-date](https://image.slidesharecdn.com/2014-12-ndc-thinking-141204083055-conversion-gate02/85/NDC-London-2014-Thinking-Like-an-Erlanger-35-320.jpg)
Uploaded byTorben Hoffmann
NDC London 2014: Thinking Like an Erlanger
This document discusses Erlang concepts and how to think like an Erlanger. It provides examples of modeling systems like Game of Life and a stock exchange using Erlang's processes, message passing, and failure handling capabilities. Key points include: spawning many short-lived processes to model discrete entities, using protocols to define interactions, monitoring processes to restart them after failures, and transaction logs to ensure consistency after crashes. The document emphasizes focusing on interactions through messaging and designing systems to fail gracefully through Erlang's primitives.
NDC London 2014: Thinking Like an Erlanger
- 1. Thinking Like An Erlanger TorbenHoffmann CTO @ Erlang Solutions torben.hoffmann@erlang-solutions.com @LeHoff
- 2. Do You WantTo See The Erlang Code?
- 3. You Can’t Handle TheErlang Code!! Source: http://static7.businessinsider.com/image/4d9d0c7bcadcbbaf20120000/jack-a-few-good-men.jpg
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- 8. Paxos { Acceptors } ProposerMain Aux Learner | | | | | | -- Phase 2 -- X----------->|->|->| | | Accept!(N,I,V) | | | ! | | --- FAIL! --- |<-----------X--X--------------->| Accepted(N,I,V) | | | | | -- Failure detected (only 2 accepted) -- X----------->|->|------->| | Accept!(N,I,V) (re-transmit, include Aux) |<-----------X--X--------X------>| Accepted(N,I,V) | | | | | -- Reconfigure : Quorum = 2 -- X----------->|->| | | Accept!(N,I+1,W) (Aux not participating) |<-----------X--X--------------->| Accepted(N,I+1,W) | | | | | Source: https://en.wikipedia.org/wiki/Paxos_(computer_science)#Byzantine_Paxos
- 9. The Golden TrinityOf Erlang
- 10. Processes They are dirtcheap! Use lots of them! Lots! Focus on their interactions
- 11. Game of Life Simplecellular automaton Evolution of cells Discrete time
- 12. 2,3 neighbours: survive empty,3 neighbours: new cell all others: empty
- 19. Traditional Approach 2-d array- one per time step
- 20. Issues with Traditional Approach Doesnot scale well Imperative data structures are ugly in Erlang
- 21. Basic Erlang Idea Oneprocess per cell Talk to neighbour cells
- 22. Collect from allneighbours Update own content and go to T+1
- 23. Is this Erlangyenough?
- 24. Spawn a processto collect from all neighbours
- 25. collector_loop([], NeighbourCount, Cell,Content) -> Cell ! {self(), {next_content, next_content(Content, NeighbourCount)}}; collector_loop(WaitingOn, NeighbourCount, Cell, Content) -> receive {cell_content, {{{_,_},_}=XYatT, NeighbourContent}} -> case lists:member(XYatT, WaitingOn) of true -> collector_loop(lists:delete(XYatT, WaitingOn), NeighbourCount + NeighbourContent, Cell, Content); false -> %% ignore messages we are not waiting for collector_loop(WaitingOn, NeighbourCount, Cell, Content) end end.
- 26.
- 27. It works if… youonly take one step at a time let the cells run freely… cells get out of sync requests for old time requests for future time
- 28. Fixes Old time requests: keephistory Future time requests: queue the response
- 29.
- 30. Supervising Cells The Erlangsupervisor will restart with the original args… all state and history is lost Fix: monitor all cells step them forward to current time
- 33. Cell Mgr Code loop(#state{cells=Cells}=State)-> receive {'DOWN', Ref, process, _Pid, _Info} -> {value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells), NewPid = await_restart(XY), NewRef = erlang:monitor(process, NewPid), NextState = State#state{ cells=[{NewPid, NewRef, XY}|Rest]}, spawn ( fun () -> pacer(NewPid) end ), loop(NextState) end. Process down
- 34. Cell Mgr Code loop(#state{cells=Cells}=State)-> receive {'DOWN', Ref, process, _Pid, _Info} -> {value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells), NewPid = await_restart(XY), NewRef = erlang:monitor(process, NewPid), NextState = State#state{ cells=[{NewPid, NewRef, XY}|Rest]}, spawn ( fun () -> pacer(NewPid) end ), loop(NextState) end. Wait for supervisor to restart it
- 35. Cell Mgr Code loop(#state{cells=Cells}=State)-> receive {'DOWN', Ref, process, _Pid, _Info} -> {value, {_,_, XY}, Rest} = lists:keytake(Ref, 2, Cells), NewPid = await_restart(XY), NewRef = erlang:monitor(process, NewPid), NextState = State#state{ cells=[{NewPid, NewRef, XY}|Rest]}, spawn ( fun () -> pacer(NewPid) end ), loop(NextState) end. Get the new process up-to-date
- 36. Deadlock Remaining… N1 dieswith Cell(i,j) queued
- 37. Re-cap A process percell Short-lived processes for small tasks Focus on the protocols between processes Supervisor to restart Monitoring manager process to get restarted cells up-to-speed
- 38.
- 39. The Trigger… Erlang-Questions onusing ETS for sell and buy orders: http://erlang.org/pipermail/erlang- questions/2014-February/077969.html Painful…
- 40. An Exchange Connects buyersand sellers Buyers post buy intentions Sellers post sell intentions
- 41. Basic Erlang Idea Oneprocess per buy/sell intention Processes to negotiate deals by exchanging messages
- 42. Communication Use gproc aspub-sub mechanism to announce buy and sell intentions All buyers listen to sell intention All sellers listen to buy intentions
- 43. Can happen when Negotiationby 3-way handshake Deals priceseller pricebuyer
- 44.
- 46. Unique reference to identifythe sell offer Seller’s Pid
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- 53. What Can GoWrong? 1. Buyer dies 3. Buyer dies 2. Seller dies 1 & 2 can be fixed by timing out Danger!! Seller has closed the deal on his side Simple re-start leaves the buyer at 3@5
- 54. Monitor each other Removesthe need for timeouts Still not sure how far the other side got
- 55. Transaction Log Per Process Justreplay back to the last state Issues: Messages cannot be replayed Must ask partner about their view on the status of the deal
- 56. Ledger Create Ledger processthat tracks all completed deals Each buyer and seller get a unique OfferID when started
- 58. Thinking in Erlang Focuson protocols (MSCs) Ask “What could go wrong here?”
- 59. Tools Lots of processes!! Spawnshort-lived processes for small things Supervisors Link and monitor gproc for registering and pub/sub Timeouts Transaction logs (ledgers)
- 60. Food for Thought Whatcan I only do in Erlang? http://erlang.org/pipermail/erlang- questions/2014-November/ 081570.html You can avoid writing your own service framework. Craig Everett
- 61. Testing Async protocols arenasty Use EQC - Property Based Testing Focus on one process Mock the calls to others WIP in repo!
- 62. Code Game of life https://github.com/lehoff/egol ErlangExchange https://github.com/lehoff/ erlang_exchange Deadlock left to fix Testing incomplete No ledger
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- 65. Learning Erlang ESL trainingcourses Learn You Some Erlang http://learnyousomeerlang.com/ Use the erlang-questions mailing list Do it hands-on Give it time to sink in!!!
- 66. Elixir Built on topof the Erlang VM More Ruby-like syntax Hygienic macros - easy to do DSLs Better support for data handling But… you still have to learn the Erlang programming model
- 67. Key building blocks Sharenothing processes Message passing Fail fast approach Link/monitor concept You can deal with failures in a sensible manner because you have a language for them.