WIP backend-providers api spec readme

piker/broker/README.rst

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+provider "spec" (aka backends)
+==============================
+``piker`` abstracts and encapsulates real-time data feeds across a slew
+of providers covering many (pretty much any) instrument class.
+
+
+this is shoddy attempt as specifying what a backend must provide
+as a basic api, per functionality-feature set, in order to be
+supported in zee sytem.
+
+"providers" must offer a top plevel namespace (normally exposed as
+a python module) which offers to a certain set of (async) functions
+to deliver info through a real-time, normalized data layer.
+
+
+data feeds
+----------
+
+real-time quotes and tick streaming:
+
+.. code:: python
+
+    async def stream_quotes(
+
+        send_chan: trio.abc.SendChannel,
+        symbols: List[str],
+        shm: ShmArray,
+        feed_is_live: trio.Event,
+        loglevel: str = None,  # log level passed in from user config
+
+        # startup sync via ``trio``
+        task_status: TaskStatus[Tuple[Dict, Dict]] = trio.TASK_STATUS_IGNORED,
+
+    ) -> None:
+
+
+this routine must eventually deliver realt-time quote messages by sending them on
+the passed in ``send_chan``; these messages must have specific format.
+there is a very simple but required startup sequence:
+
+message starup sequence:
+************************
+at a minimum, and asap, a first quote message should be returned for
+each requested symbol in ``symbols``. the message should have a minimum
+format:
+
+.. code:: python
+
+    quote_msg: dict[str, Any] = {
+        'symbol': 'xbtusd',  # or wtv symbol was requested
+
+        # this field is required in the initial first quote only (though
+        # is recommended in all follow up quotes) but can be 
+        'last': <last clearing price>,  # float
+
+        # tick stream fields (see below for schema/format)
+        'ticks': list[dict[str, Any]],
+    }
+
+further streamed quote messages should be in this same format.
+``ticks`` is an optional sequence
+
+
+historical OHLCV sampling
+-------------------------
+
+.. code:: python
+
+    async def backfill_bars(
+        sym: str,
+        shm: ShmArray,  # type: ignore # noqa
+
+        count: int = 10,  # NOTE: any more and we'll overrun the underlying buffer
+
+        # startup sync via ``trio``
+        task_status: TaskStatus[trio.CancelScope] = trio.TASK_STATUS_IGNORED,
+
+    ) -> None:
+
+this routine is responsible for setting up historical data for both
+charting and any local storage requirements. it should retreive, normalize and
+push the data to shared memory. normally the format of this data is
+OHLCV sampled price and volume data but can be high reslolution
+tick/trades/book times series as well. currently charting can expects
+a max resolution of 1s (second) OHLCV sampled data (any more then this
+and you probably can't do much with it as a manual trader; this is
+obviously not true of automatic strats realized as code).
+
+OHLCV minmal schema
+********************
+ohlcv at a minimum is normally pushed to local shared memory (shm)
+numpy compatible arrays which are read by both UI components for display
+as well auto-strats and algorithmic trading engines. shm is obviously
+used for speed. we also intend to eventually support pure shm tick
+streams for ultra low latency processing by external processes/services.
+
+the provider module at a minimum must define a ``numpy`` structured
+array dtype ``ohlc_dtype = np.dtype(_ohlc_dtype)`` where the
+``_ohlc_dtype`` is normally defined  in standard list-tuple synatx as:
+
+.. code:: python
+
+    # Broker specific ohlc schema which includes a vwap field
+    _ohlc_dtype = [
+        ('index', int),
+        ('time', int),
+        ('open', float),
+        ('high', float),
+        ('low', float),
+        ('close', float),
+        ('volume', float),
+        ('count', int),
+        ('bar_wap', float),
+    ]