# 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 `fork`ed 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

    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