[erlang-questions] "actor database" - architectural strategy question
Mon Feb 17 22:14:44 CET 2014
You then have the choice of trying to tweak the GC with parameters to try and avoid consuming too much memory, but that only works if your throughput stays roughly the same (within what you expect), otherwise you then have to play with the GC settings again, based on a new throughput max... or, you can use a short-lived process to manipulate memory, such that the result is returned upon the short-lived process' death (the third option would be manually making garbage collection happen, which is dirty, but possible with http://www.erlang.org/doc/man/erlang.html#garbage_collect-0). So, using short-lived processes to facilitate the work of longer-lived processes is generally the solution to this problem. Making an Erlang process is cheap, and using a short-lived process to help the GC know what memory is old, is a simple way of handling the problem. If you use CloudI, the cloudi_service behaviour does this for you by default, when you receive service requests, with the
request_pid_uses service configuration option (http://cloudi.org/api.html#2_services_add_config_opts)... so it is part of CloudI's service abstraction.
On 02/17/2014 12:42 PM, Mahesh Paolini-Subramanya wrote:
> "Large number of processes with very long persistence"
> You *will* run into GC issues here, and of all kinds
> - design artifacts ("hmm, the number of lists that I manipulate increases relentlessly...")
> - misunderstanding ("But I passed the binary on, without manipulating it at all!")
> - Bugs (Fred has a great writeup on this somewhere)
> Just keep in mind that in the end, you will almost certainly end up doing some form of manual GC activities. Again, the Heroku gang can probably provide a whole bunch of pointers on this...
> *Mahesh Paolini-Subramanya <http://www.gravatar.com/avatar/204a87f81a0d9764c1f3364f53e8facf.png>*
> That tall bald Indian guy..
> Google+ <https://plus.google.com/u/0/108074935470209044442/posts> | Blog <http://dieswaytoofast.blogspot.com/> | Twitter <https://twitter.com/dieswaytoofast>| LinkedIn <http://www.linkedin.com/in/dieswaytoofast>
> On February 17, 2014 at 3:22:22 PM, Miles Fidelman ( <mailto://email@example.com>) wrote:
>> Joe Armstrong wrote:
>> > This sounds interesting. To start wit, I think swapping processes to
>> > disk is just an optimization.
>> > In theory you could just keep everything in RAM forever. I guess
>> > processes could keep their state in dictionaries (so you could roll
>> > them back) or ets tables (if you didn't want to roll them back).
>> > You would need some form of crash recovery so processes should write
>> > some state information
>> > to disk at suitable points in the program.
>> Joe... can you offer any insight into the dynamics of Erlang, when
>> running with large number of processes that have very long persistence?
>> Somehow, it strikes me that 100,000 processes with 1MB of state, each
>> running for years at a time, have a different dynamic than 100,000
>> processes, each representing a short-lived protocol transaction (say a
>> web query).
>> Coupled with a communications paradigm for identifying a group of
>> processes and sending each of them the same message (e.g., 5000 people
>> have a copy of a book, send all 5000 of them a set of errata; or send a
>> message asking 'who has updates for section 3.2).
>> In some sense, the conceptual model is:
>> 1. I send you an empty notebook.
>> 2. The notebook has an address and a bunch of message handling routines
>> 3. I can send a page to the notebook, and the notebook inserts the page.
>> 4. You can interact with the notebook - read it, annotate it, edit
>> certain sections - if you make updates, the notebook can distribute
>> updates to other copies - either through a P2P mechanism or a
>> publish-subscribe mechanism.
>> At a basic level, this maps really well onto the Actor formalism - every
>> notebook is an actor, with it's own address. Updates, interactions,
>> queries, etc. are simply messages.
>> Since Erlang is about the only serious implementation of the Actor
>> formalism, I'm trying to poke at the edge cases - particularly around
>> long-lived actors. And who better to ask than you :-)
>> In passing: Early versions of Smalltalk were actor-like, encapsulating
>> state, methods, and process - but process kind of got dropped along the
>> way. By contrast, it strikes me that Erlang focuses on everything being
>> a process, and long-term persistence of state has taken a back seat.
>> I'm trying to probe the edge cases. (I guess another way of looking at
>> this is: to what extent is Erlang workable for writing systems based
>> around the mobile agent paradigm?)
>> > What I think is a more serious problem is getting data into the system
>> > in the first place.
>> > I have done some experiments with document commenting and annotation
>> > systems and
>> > found it very difficult to convert things like word documents into a
>> > form that looks half
>> > decent in a user interface.
>> Haven't actually thought a lot about that part of the problem. I'm
>> thinking of documents that are more form-like in nature, or at least
>> built up from smaller components - so it's not so much going from Word
>> to an internal format, as much as starting with XML or JSON (or tuples),
>> building up structure, and then adding presentation at the final step.
>> XML -> Word is a lot easier than the reverse :-)
>> On the other hand, I do have a bunch of applications in mind where
>> parsing Word and/or PDF would be very helpful - notably stripping
>> requirements out of specifications. (I can't tell you how much of my
>> time I spend manually cutting and pasting from specifications into
>> spreadsheets - for requirements tracking and such.) Again, presentation
>> isn't that much of an issue - structural and semantic analysis is. But,
>> while important, that's a separate set of problems - and there are some
>> commercial products that do a reasonably good job.
>> > I want to parse Microsoft word files and PDF etc. and display them in
>> > a format that is
>> > recognisable and not too abhorrent to the user. I also want to allow
>> > on-screen manipulation of
>> > documents (in a browser) - all of this seems to require a mess of
>> > Before we can manipulate documents we must parse them and turn them
>> > into a format
>> > that can be manipulated. I think this is more difficult that the
>> > storing and manipulating documents
>> > problem. You'd also need support for full-text indexing, foreign
>> > language and multiple character sets and so
>> > on. Just a load of horrible messy small problems, but a significant
>> > barrier to importing large amounts
>> > of content into the system.
>> > You'd also need some quality control of the documents as they enter
>> > the system (to avoid rubbish in rubbish out), also to maintain the
>> > integrity of the documents.
>> Again, for this problem space, it's more about building up complex
>> documents from small pieces, than carving up pre-existing documents.
>> More like the combination of an IDE and a distributed CVS - where fully
>> "compiled" documents are the final output.
>> > If you have any ideas of now to get large volumes of data into the
>> > system from proprietary formats
>> > (like ms word) I'd like to hear about it.
>> Me too :-) Though, I go looking for such things every once in a while, and:
>> - there are quite a few PDF to XML parsers, but mostly commercial ones
>> - there are a few PDF and Word "RFP stripping" products floating around,
>> that are smart enough to actually analyze the content of structured
>> documents (check out Meridian)
>> - later versions of Word export XML, albeit poor XML
>> - there are quite a few document analysis packages floating around,
>> including ones that start from OCR images - but they generally focus on
>> content (lexical analyis) and ignore structure (it's easier to scan a
>> document and extract some measure of what it's about - e.g. for indexing
>> purposes; it's a lot harder to find something that will extract the
>> outline structure of a document)
>> In theory, there is no difference between theory and practice.
>> In practice, there is. .... Yogi Berra
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