[erlang-questions] Guidance to a cache mechanism

dmitry kolesnikov dmkolesnikov@REDACTED
Thu Nov 1 07:47:03 CET 2012

Hello Richard,

Somehow, I have missed your point here. It is obvious that two indexes
are required: hash and b-tree. Unless we have a double linked list but
we do not :-(

we can implement those indexes as ETS tables set and ordered_set, isn't IT?

Best Regards,
Dmitry >-|-|-*>

On 1.11.2012, at 2.28, Richard O'Keefe <ok@REDACTED> wrote:

> On 31/10/2012, at 6:55 PM, Maruthavanan Subbarayan wrote:
>> I thought of keeping the time stamp as the key in the ets table and maintain that in state when a timeout event occurs. I hope this would help me to delete the entries in ets table with less burden.
> Like I said, "Time tells your program when to delete a message."
> But the question was "How is your program asked to give a message back?"
> Because if your program is never asked to give a message back, why keep
> it at all?
> Suppose you are running Brekekek.example.com, a site where users
> exchange 'croaks' (which must be at most 140 characters of Attic
> Greek).  You receive
>    {add,User,Croak}
>        Here is User's latest Croak, which you add to
>        the cache with erlang:now() -- ignoring the
>        microsecond part -- as its timestamp.
>    {ask,User,Sender,Time}
>        The Sender process wants to be told all of
>        User's Croaks since Time.
> messages, amongst others.  In order to delete Croaks easily,
> you want to store them keyed by time.  But in order to answer
> 'ask' requests, you want to store them keyed by User.  And
> ETS allows a table only one key.  Oh dear...
> Think of it in relational data base terms.
>    Cache(User, Time, Croak)
>    fd User x Time -> Croak
>    hash bag index on User
>    b-tree index on Time
>    {add,User,Croak} => into Cache insert (User, now(), Croak)
>    {ask,User,Sender,Time} =>
>        Sender ! select Croak from Cache
>               where Cache.User = User and Cache.Time >= Time
>    tick => delete Cache where Cache.Time < now() - Expiry_Time
> Basically what I am saying is that if your cache is to be any use,
> there _have_ to be operations other than adding and deleting, and
> you need to consider _all_ the operations.
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