module Netplex_sharedvar:sig..end
This implementation works in both multi-processing and multi-threading netplex environments. It is, however, not very fast, because the variables live in the controller, and the access operations are realized by RPC's. It is good enough when these operations are only infrequently called, e.g. in the post-start and pre-finish processor callbacks.
    Furthermore, note that it is unwise to put large values into
    variables when using them in multi-processing contexts. The controller
    process is also the parent process of all forked children, and
    when a lot of memory is allocated in the controller, all
    this memory needs to be copied when the fork is done. As workaround,
    put such values into temporary files, and only pass the names of the
    files around via variables.
Variables come in two flavors:
Netplex_encap)The latter kind is useful to safely store structured ocaml values in Netplex variables.
More documentation can also be found here: Storing global state
    Thread safety: Full. The functions can be called from any thread.
 of string
 of string
 of string
Make_var_typeval plugin : Netplex_types.pluginadd_plugin method
      with this object as argument. This can e.g. be done in the
      post_add_hook of the processor.get_value.
    If called from the wrong context the exception
    Netplex_cenv.Not_in_container_thread is raised.
val create_var : ?own:bool -> ?ro:bool -> ?enc:bool -> string -> boolSharedvar_null) as
      initial value. If the creation is possible (i.e. the variable did
      not exist already), the function returns true, otherwise 
      the already existing variable is left modified, and false is
      passed back. By default, the variable can be modified and deleted
      by any other container. Two options allow you to change that:
own: If true, the created variable is owned by the calling
        socket service. Only the caller can delete it, and when the 
        last component of the socket service terminates, the variable is
        automatically deleted. The deletion happens after the
        post_finish_hook is executed, so the variable is still accessible
        from this hook.ro: if true, only the owner can set the valueenc: if true, the variable stores encapsulated values, otherwise
        strings
        (defaults to false)"service_name.local_name", i.e. they
      are prefixed by the socket service to which they refer.val delete_var : string -> booldelete_var name: Deletes the variable name. Returns true if
      the deletion could be carried out, and false when the variable
      does not exist, or the container does not have permission to delete
      the variable.val set_value : string -> string -> boolset_value name value: Sets the variable name to value. This
      is only possible when the variable exists, and is writable.
      Returns true if the function is successful, and false when
      the variable does not exist.
      Raises Sharedvar_no_permission if the variable cannot be modified.
      Raises Sharedvar_type_mismatch if the variable is not a string
      variable.
val set_enc_value : string -> Netplex_types.encap -> boolset_enc_value name value: Sets the variable name to value.
      Return value as for set_value.
      Raises Sharedvar_no_permission if the variable cannot be modified.
      Raises Sharedvar_type_mismatch if the variable is not encapsulated
val get_value : string -> string optionget_value name: Gets the value of the variable name. If the
      variable does not exist, None is returned.
      Raises Sharedvar_type_mismatch if the variable is not a string
      variable.
      As an exception of the general rules, this function can also be
      called from the controller, and not only from a container.
val get_enc_value : string -> Netplex_types.encap optionget_enc_value name: Gets the value of the variable name. If the
      variable does not exist, None is returned.
      Raises Sharedvar_type_mismatch if the variable is not encapsulated
      As an exception of the general rules, this function can also be
      called from the controller, and not only from a container.
val wait_for_value : string -> string optionwait_for_value name: If the variable exists and set_value has
      already been called at least once, the current value is returned. 
      If the variable exists, but set_value has not yet been called at all,
      the function waits until set_value is called, and returns the value
      set then. If the variable does not exist, the function immediately
      returns None.
      An ongoing wait is interrupted when the variable is deleted. In this
      case None is returned.
val wait_for_enc_value : string -> Netplex_types.encap optionval get_lazily : string -> (unit -> string) -> string optionget_lazily name f: Uses the variable name to ensure that f
      is only invoked when get_lazily is called for the first time,
      and that the value stored in the variable is returned the
      next times. This works from whatever component get_lazily
      is called.
      If f() raises an exception, the exception is suppressed, and
      None is returned as result of get_lazily. Exceptions are not
      stored in the variable, so the next time get_lazily is called
      it is again tried to compute the value of f(). If you want to
      catch the exception this must done in the body of f.
      No provisions are taken to delete the variable. If delete_var
      is called by user code (which is allowed at any time), and
      get_lazily is called again, the lazy value will again be computed.
val get_enc_lazily : string -> (unit -> Netplex_types.encap) -> Netplex_types.encap optionval dump : string -> Netlog.level -> unitNetlog. The
      string argument "*" dumps all variables.module Make_var_type:
get and set functions to access variables
      of type T.t.
    Here, one randomly chosen container computes precious_value, and
    makes it available to all others, so the other container can simply
    grab the value. This is similar to what get_lazily does internally:
      let get_precious_value() =
        let container = Netplex_cenv.self_cont() in
        let var_name = "my_service.precious" in
        if Netplex_sharedvar.create_var var_name then (
          let precious_value = 
            try ...    (* some costly computation *)
            with exn ->
              ignore(Netplex_sharedvar.delete_var var_name);
              raise exn in
          let b = Netplex_sharedvar.set_value var_name precious_value in
          assert b;
          precious_value
        )
        else (
          match Netplex_sharedvar.wait_for_value var_name with
           | Some v -> v
           | None -> failwith "get_precious_value"
                       (* or do plan B, e.g. compute the value *)
        )
    
    We don't do anything here for deleting the value when it is no longer
    needed. Finding a criterion for that is very application-specific. 
    If the variable can be thought as being another service endpoint
    of a socket service, it is a good idea to acquire the ownership
    (by passing ~own:true to create_var), so the variable is automatically
    deleted when the socket service stops.
    Of course, the plugin must be enabled, e.g. by overriding the 
    post_add_hook processor hook:
 
    method post_add_hook sockserv ctrl =
      ctrl # add_plugin Netplex_sharedvar.plugin