Module Netplex_sharedvar

module Netplex_sharedvar: sig .. end

Netplex-wide variables

This plugin allows to have Netplex-global variables that can be read and written by all components. These variables are useful to communicate names and other small pieces of information across the whole Netplex. For instance, one component could allocate a shared memory object, and put its name into a variable to make it known to other components.

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:

String variables
Encapsulated variables (see Netplex_encap)

A string variable cannot be accessed as encapsulated variable, and vice versa.

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.

exception Sharedvar_type_mismatch of string

The (dynamically typed) variable has the wrong type (string/exn)

exception Sharedvar_no_permission of string

It is not allowed to set the value

exception Sharedvar_not_found of string

The variable does not exist. Only used by Make_var_type

exception Sharedvar_null

The initial value of a shared exception variable

val plugin : Netplex_types.plugin

To enable shared variables, call the controller's add_plugin method with this object as argument. This can e.g. be done in the post_add_hook of the processor.

The folloing functions can all be invoked in container contexts. In controller context, access is limited to 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 -> bool

Create the variable with the passed name with an empty string (or the exception Sharedvar_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 value
enc: if true, the variable stores encapsulated values, otherwise strings (defaults to false)

Variable names are global to the whole netplex system. By convention, these names are formed like "service_name.local_name", i.e. they are prefixed by the socket service to which they refer.

val delete_var : string -> bool

delete_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 -> bool

set_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 -> bool

set_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 option

get_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.

val get_enc_value : string -> Netplex_types.encap option

get_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

val wait_for_value : string -> string option

wait_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 option

Same for encapsulated variables

val get_lazily : string -> (unit -> string) -> string option

get_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 option

Same for encapsulated values

val dump : string -> Netlog.level -> unit

Dumps the access counter of this variable to Netlog. The string argument "*" dumps all variables.

module Make_var_type:

functor (T : Netplex_cenv.TYPE) -> Netplex_cenv.VAR_TYPE with type t = T.t

Creates a module with get and set functions to access variables of type T.t.

Example code:

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

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