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Thu Apr 26 03:14:17 PDT 2012

strings. Adding type information would add a lot of complexity and not
have very big benefits (is it so?). The options will of course have an
implicit type to be useful. The actual users of the options
(applications and code in the server) have to validate the values
themselves. This means that we should provide parsing functions for
clients, at least for any complex types, but preferably also for simple
stuff like integers. At server side, values are validated when they are
read from the disk and when they are set by clients or by some other
code in the server than the "option owner".

Since validation is done by the option owner code, and since modules can
implement options, some of the options can be only validated when a
specific module is loaded. The way to handle the case, where an
application sets an option for a module that is not loaded, is to refuse
to set any such options. If the module isn't loaded, from the
configuration system's point of view the option doesn't exist.

== Client API ==

This is how the client API will look like:

typedef void (*pa_server_configuration_cb_t)(
                pa_context *c,
                const char *object_type,
                const char *object_id,
                const char *option,
                const char *value,
                int eol,
                void *userdata);

pa_operation *pa_server_configuration_set(
                pa_context *c,
                const char *object_type,
                const char *object_id,
                const char *option,
                const char *value,
                pa_context_success_cb_t cb,
                void *userdata);

/* object_type, object_id and option can be NULL for wildcard selection. If
 * object_type is NULL, then all other identifiers have to be NULL also. In
 * case of wildcards, the callback may get called multiple times. The last call
 * has the object_type, object_id, option and value parameters of
 * pa_server_configuration_cb_t set to NULL and eol set to 1. */
pa_operation *pa_server_configuration_get(
                pa_context *c,
                const char *object_type,
                const char *object_id,
                const char *option,
                pa_server_configuration_cb_t cb,
                void *userdata);

/* The wildcard rules described with pa_server_configuration_get apply with
 * the subscribe items too. */
typedef struct pa_server_configuration_subscribe_item {
        const char *object_type;
        const char *object_id;
        const char *option;
} pa_server_configuration_subscribe_item;

/* The items parameter is an array with n_items elements. If there are multiple
 * subscribe items given, then the subscribe callback is called when any of the
 * items match the event. If called multiple times, the subscribe items of the
 * last call will replace all the previous items. */
pa_operation *pa_server_configuration_subscribe(
                pa_context *c,
                const pa_server_configuration_subscribe_item *items,
                unsigned n_items,
                pa_context_success_cb_t cb,
                void *userdata);

/* When there are configuration changes that match the subscription, the
 * callback that is set here will be called multiple times. The last call has
 * the object_type, object_id, option and value parameters of
 * pa_server_configuration_cb_t set to NULL and eol set to 1. */
void pa_server_configuration_set_subscribe_callback(
                pa_context *c,
                pa_server_configuration_cb_t cb,
                void *userdata);

There has been some discussion about supporting also reading and
modifying client.conf. client.conf is not centrally managed, it stores
simple key-value pairs with static keys, it doesn't support change
notifications and working with it probably doesn't require an
asynchronous API. Therefore, the API is quite different, and maybe it
doesn't need to be a part of this configuration system effort. The main
point is that we '''might''' want such API now or in the future, so we
should think how the configuration system API can co-exist with the
client.conf API. That means basically that we probably don't want to use
"pa_configuration_" as the server configuration prefix, because it's too

=== Parsing functions ===


== Storage format ==


        I like ini files as described here:

== Open issues ==

The namespace concept might need some refinement. The idea of splitting
the storage into one file per module sounds nice, but it might not sense
to dedicate each object type to one module. Modules may want add options
to their own object types and to core object types, and possibly also to
other modules' types. Let's say there's an object type "Card" in
namespace "core", i.e. "core.Card". The alsa modules might want to add
an option to core.Cards for enabling and disabling timer-based
scheduling. Maybe the options should have namespacing too, i.e. the new
option would be "core.Card/foocard/alsa.tsched"? Would every option name
have a namespace part? That would be very ugly. How would this example
be handled in storage? Is splitting the storage into many files a bad
idea anyway?

Is there need for adding an API for clients for querying whether a
specific option is supported? Such API could be added also later, if we
are not certain yet.

One limitation with this proposal is that the client API supports only
modifying existing objects. For example, adding a new entry to
stream-restore is not possible in a nice way. You could set an option of
a non-existent object, but that would leave the other fields of the
entry uninitialized. Maybe that's tolerable, but if we want to support
object creation, it should probably be done in a better way.

        I'm a bit against supporting object creation through the
configuration API. Protocol extensions are still needed for that then.
And maybe exposing the stream-restore database to clients through the
configuration API isn't so good idea in the first place. The storage
part of the system could still be useful also for stream-restore. --Tanu

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