[erlang-questions] Writing a Concurrent Binary Search Tree Data Structure in Erlang

Fuad Tabba <>
Thu Jun 26 00:10:03 CEST 2008


I think I understand what you're saying here; but I still can't wrap my head
around putting that into code. Is there similar code that you know of
available online? The structure doesn't have to be a BST, just anything that
demonstrates the idea of having a structure that stores lots persistent data
to be viewed/modified by many processes and is scalable...

Thanks,
/Fuad

On Wed, Jun 25, 2008 at 9:57 PM, Hynek Vychodil <>
wrote:

> You should store "persistent" state in server process. When you fall in
> performance problem you can partition this state in sub processes and server
> starts work as dispatcher. If dispatching is algorithmic you can spawn many
> dispatchers of course, but it usually performs better. You can repeat this
> fragmentation again and again on sub processes and you end up with process
> per node. For performance reasons there will be best solution some sort of
> hybrid structure. IMHO ets not performs fully scalable because I think there
> is only one "process" which can every time update it.
>
>
> On Wed, Jun 25, 2008 at 11:46 AM, Fuad Tabba <>
> wrote:
>
>> I understand that, but what if that structure (binary tree) is needed to
>> store some sort of persistent state for all processes to see - reservations
>> table for example or something like that. I know there's ETS, but what I'm
>> trying to do is understand the basics...
>>
>> Thanks Hynek,
>> /Fuad
>>
>>
>> On Wed, Jun 25, 2008 at 9:40 PM, Hynek Vychodil <>
>> wrote:
>>
>>> Because there is no shared state between processes in Erlang, I would
>>> start think about process per tree node, not process per action as usual in
>>> classic shared memory (for example C) implementations.
>>> I know, it sounds weird, but may be help you :-)
>>>
>>> 2008/6/25 Fuad Tabba <>:
>>>
>>>>  Hi,
>>>>
>>>> I'm trying to write a concurrent Binary Search Tree (BST) in Erlang with
>>>> the operations insert/delete/contains. My goal is for multiple processes to
>>>> be able to concurrently modify this structure and query it correctly.
>>>>
>>>> My problem is, I can't think of a way to take advantage of the inherent
>>>> parallelism in the BST structure. BSTs should be easy to parallelize, since
>>>> different inserts/deletes of nodes from the tree could be in completely
>>>> different branches. In C for example, it's not hard to write a nonblocking
>>>> implementation (lock-free) where adding nodes to the tree can happen
>>>> concurrently (unless there's a conflict) using Compare and Exchange (CAS).
>>>> Adding concurrency with deleting nodes is a bit more challenging but should
>>>> still be doable.
>>>>
>>>> That said, I've only managed to think of an Erlang implementation that
>>>> is capable of running contains() in parallel with an insert or delete.
>>>> However, I cannot think of a way where I could have the server thread
>>>> somehow distribute its updates if they are to nonconflicting branches. I
>>>> feel like the solution should be a simple, recursive one, and that the
>>>> answer is on the tip of my brain so to speak :) But I'm still baffled.
>>>>
>>>> My question is this; I can reason about shared memory objects and how
>>>> concurrent accesses should be, either using locks or for the simpler cases
>>>> using atomic instructions such as CAS. However, I'm finding it difficult to
>>>> do that with Erlang/functional languages in general. I did a google search,
>>>> couldn't really find any examples of what I'm looking for. Any tips or
>>>> pointers?
>>>>
>>>> Here's what my code looks like (the whole thing is attached to this
>>>> email). Apologies in advance for the quality of my code, still learning :)
>>>>
>>>> The basic idea, to be able to have contains() run in parallel with
>>>> insertions and deletions, is that whenever the server gets a request to add
>>>> something or remove something from the tree, its continue applying
>>>> contains() to the old version until it receives the modifications back.
>>>>
>>>> The code for the tree itself is pretty much the same as that in
>>>> Concurrent Programming in Erlang.
>>>>
>>>> % No changes pending for the tree
>>>> parallelServer(Tree, stable, _) ->
>>>>     receive
>>>>         {Requester, insert, Key} ->
>>>>             Self = self(),
>>>>             spawn(fun() -> insertProcessor(Self, Key, Tree) end),
>>>>             parallelServer(Tree, changing, Requester);
>>>>
>>>>         {Requester, delete, Key} ->
>>>>             Self = self(),
>>>>             spawn(fun() -> deleteProcessor(Self, Key, Tree) end),
>>>>             parallelServer(Tree, changing, Requester);
>>>>
>>>>         {Requester, contains, Key} ->
>>>>             spawn(fun() -> containsProcessor(Requester, Key, Tree) end),
>>>>             parallelServer(Tree, stable, void);
>>>>
>>>>         {Requester, dump} ->
>>>>             Requester ! Tree,
>>>>             parallelServer(Tree, stable, void)
>>>>     end;
>>>>
>>>> % Tree is being modified
>>>> parallelServer(Tree, changing, Requester) ->
>>>>     receive
>>>>         {Requester, contains, Key} ->
>>>>             spawn(fun() -> containsProcessor(Requester, Key, Tree) end),
>>>>             parallelServer(Tree, changing, Requester);
>>>>
>>>>         {Requester, dump} ->
>>>>             Requester ! Tree,
>>>>             parallelServer(Tree, changing, Requester);
>>>>
>>>>         {update, NewTree} ->
>>>>             Requester ! done,
>>>>             parallelServer(NewTree, stable, void)
>>>>     end.
>>>>
>>>>
>>>> % insertProcessor and deleteProcessor cannot notify the requester
>>>> directly to
>>>> % ensure that parallelServer knows what the new tree is like first for
>>>> % consistency.
>>>>
>>>> insertProcessor(Server, Key, Tree) ->
>>>>     Server ! {update, insert(Key, Tree)}.
>>>>
>>>>
>>>> deleteProcessor(Server, Key, Tree) ->
>>>>     Server ! {update, delete(Key, Tree)}.
>>>>
>>>>
>>>> containsProcessor(Requester, Key, Tree) ->
>>>>     Requester ! contains(Key, Tree). % Can report it directly
>>>>
>>>>
>>>>
>>>> Thanks !
>>>>
>>>> Cheers,
>>>> /Fuad
>>>> _______________________________________________
>>>> erlang-questions mailing list
>>>> 
>>>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>>>
>>>
>>>
>>>
>>> --
>>> --Hynek (Pichi) Vychodil
>>
>>
>>
>
>
> --
> --Hynek (Pichi) Vychodil
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