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6688 :: How to use global vars with Jester?

• jasonfi (orginal) [2020-08-17T14:57:09+02:00] view original

  When I look at the Nim Forums source, the globals don't have any special pragmas: https://github.com/nim-lang/nimforum/blob/master/src/forum.nim

  However when I compile my own Jester web app in release mode, the globals fail if they are not {.threadvar.}. But even when I add that pragma the global vars are nil despite being initialized in a function.

  What am I doing wrong here?

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• JohnAD (orginal) [2020-08-17T15:59:26+02:00] view original

  The answer is one you are probably not wanting: don't use global vars in a web page unless that structure is specifically designed to be thread-safe. Some work arounds:

  1. If the var is not changing, make it a const. Global constants are perfectly safe in a thread.
  2. You can use a template to automatically setup the thread-local vars for each page. That way you are only adding a single line to the top of each page route. I generally do this. I also establish any connections to any SQL databases in this template; since I know of no SQL drivers that support pooling. (If pooling is supported, then your thread can simply call the pooled drivers equivalent of "getNextConnection()".)

  I'm doing an experiment with jester called jesterwithplugins that allows you to do certains tasks and setups at the start of each thread. So, if ambitious, you could also write a plugin. But that is very much beta right now.

  I'll be making a fourth video in my jester series that deals with databases and addresses this issue as well since you cannot share a common database connection safely in a thread unless it is pooled (or has some other mechanism to handle conflict.)

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• jasonfi (orginal) [2020-08-17T18:01:00+02:00] view original

  Thanks for the advice. I'll watch for your next video.

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• dom96 (orginal) [2020-08-18T17:51:10+02:00] view original

  You can use global vars, you just cannot use them with --threads:on. If you disable threads via --threads:off it should work, if not you can just mark the vars with {.threadvar.} and treat it as a normal variable (and possibly complain on the GitHub issue tracker that the Nim compiler consider that an error).

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• jasonfi (orginal) [2020-08-18T19:22:04+02:00] view original

  Thanks, but is this a viable option for a production server? If not threads, then what is the model? Async?

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• ThomasTJdev (orginal) [2020-08-18T21:52:55+02:00] view original

  Hi @jasonfi

  The global vars marked with {.threadvar.} needs to be initialized in each thread at creation: Every thread local variable needs to be replicated at thread creation

  If you are using Jester with httpbeast, you need to find a method to catch that, since httpbeast doesn't have a callback or similar, when the threads are created (as far as I know?). I use a combination of treeforms pg library with a hacky table, where I store the database pool with the threadID. In each route I use something like the Nim forums createTFD() which checks if the user is logged in. In that template, I also check if my global database pool table has an instance with the threadID - if it does not exist, it is created. By doing that, I create the database pool for each thread the first time it is "activated". But yeah, it does have some overhead.. any one else has some ideas?

  If you include treeforms pg library and include the following:

  import anotheFileWithVars
  
  # treeform pg: AsyncPool, newAsyncPool, getFreeConnIdx, returnConn
  
  var dbt {.threadvar.}: Table[int, AsyncPool]
  
  proc avaiDbNr(threadId: int): int =
    ## Check if the thread has a DbConn available
    if not dbt.hasKey(threadId):
      initVarsDynamic() # This init other global vars with {.threadlocal.}, which cannot be const due to being loaded form a config file.
      dbt[threadId] = newAsyncPool(cfgDbHost, cfgDbUser, cfgDbPass, cfgDbName, parseInt(cfgDbPoolConnNr))
    
    return dbt[threadId].getFreeConnIdx()
  
  template currThreadId(): int =
    when compileOption("threads"):
      getThreadId()
    else:
      0
  
  proc checkDbPoolIsInitiated*() =
    # Init DB connection and vars for thread. Checked in routes.
    let threadID = currThreadId()
    if not dbt.hasKey(threadId):
      initVarsDynamic()
      dbt[threadId] = newAsyncPool(cfgDbHost, cfgDbUser, cfgDbPass, cfgDbName, parseInt(cfgDbPoolConnNr))
  
  proc getValueCustom*(query: SqlQuery, args: varargs[string, `$`]): string =
    let threadId = currThreadId()
    let conIdx   = avaiDbNr(threadId)
    result = getValue(dbt[threadId].conns[conIdx], query, args)
    dbt[threadId].returnConn(conIdx)
  
  Run

  anotherFileWithVars.nim

  var var1* {.threadvar.}: string
  var var2* {.threadvar.}: string
  var var3* {.threadvar.}: string
  
  proc initVarsDynamic() =
    var1 = "valueFromConfig"
    var2 = "valueFromConfig"
    var3 = "valueFromConfig"
  Run

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