[sr-dev] [kamailio/kamailio] KEMI API implementation for carrierroute module (PR #3247)

[Muhammad Shahzad Shafi]

I am bit lost on how the code execution flow works for KEMI. My understanding was that all KEMI methods are exported by `sr_kemi_t` structure array, which specifies exported function names, their internal implementation function name and its arguments either as string `SR_KEMIP_STR` or integer `SR_KEMIP_INT`.

Each exported function (e.g. `cr_route`) calls internal implementation function (e.g. `ki_cr_route5`), which then checks arguments and then either do the actual implementation or calls up a wrapper function (e.g. `w_cr_do_route`) which does some formatting on input arguments (e.g. `cr_route_fixup`) and then pass it to an existing implementation (e.g. `cr_do_route`). The output arguments control flow is in the opposite direction. Also since there is no free fixup in existing implementation of `cmd_export_t`, so I didn't created or called-in any for KEMI.

```
static cmd_export_t cmds[]={
    {"cr_user_carrier",  (cmd_function)cr_load_user_carrier,  3,
        cr_load_user_carrier_fixup, 0, REQUEST_ROUTE | FAILURE_ROUTE },
    {"cr_route",         (cmd_function)cr_route5,              5,
        cr_route_fixup,             0, REQUEST_ROUTE | FAILURE_ROUTE },
    {"cr_route",         (cmd_function)cr_route,              6,
        cr_route_fixup,             0, REQUEST_ROUTE | FAILURE_ROUTE },
    {"cr_nofallback_route",(cmd_function)cr_nofallback_route5,     5,
        cr_route_fixup,             0, REQUEST_ROUTE | FAILURE_ROUTE },
    {"cr_nofallback_route",(cmd_function)cr_nofallback_route,     6,
        cr_route_fixup,             0, REQUEST_ROUTE | FAILURE_ROUTE },
    {"cr_next_domain",   (cmd_function)cr_load_next_domain,   6,
        cr_load_next_domain_fixup,  0, REQUEST_ROUTE | FAILURE_ROUTE },
    {0, 0, 0, 0, 0, 0}
};
```

On the python side, my example just shows that whatever we pass as argument (whether `$fU` or `KSR.kx.get_fuser()` or `"any-string-value"`), the module function will accept and take care of it (i.e. fetch the actual string value internally), this is already tested in my lab environment.

However, @miconda has suggested the completely different approach. Can you please point me to any existing howto or explain a bit more on how to write KEMI implementation of an existing module function? The example you provide at,

https://github.com/kamailio/kamailio/blob/master/src/modules/posops/posops_mod.c#L170-L224

is very confusing and does not give any hints on how the flow actually works.

Thank you.

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