Add an initial implementation of insert subscriptions.

This implementation also includes an example bytewax adapter
which can be used to process inserts with bytewax.

An example bytewax flow is shown below:

```
import uuid
import btrdb
from btrdb.experimental.bytewax_connectors import InsertSubscription
from bytewax.dataflow import Dataflow
from bytewax.connectors.stdio import StdOutput

def selector(db):
	# Selector can be anything that returns a list of uuids.
	rows = db.query('select uuid from streams')
	uuids = [uuid.UUID(row['uuid']) for row in rows]
	return uuids

flow = Dataflow()
flow.input("realtime_sub", InsertSubscription(selector, selector_refresh_interval=30))
flow.output("print_output", StdOutput())
```

btrdb/endpoint.py

@@ -386,3 +386,25 @@ def sql_query(self, stmt, params: typing.List):
         for page in self.stub.SQLQuery(request):
             check_proto_stat(page.stat)
             yield page.SQLQueryRow
+
+    @error_handler
+    def subscribe(self, update_queue):
+        def updates():
+            while True:
+                update = update_queue.get()
+                if update is None:
+                    return
+                (to_add, to_remove) = update
+                if len(to_add) != 0:
+                    yield btrdb_pb2.SubscriptionUpdate(
+                        op=0, uuid=[uu.bytes for uu in to_add]
+                    )
+                if len(to_remove) != 0:
+                    yield btrdb_pb2.SubscriptionUpdate(
+                        op=1, uuid=[uu.bytes for uu in to_remove]
+                    )
+
+        for response in self.stub.Subscribe(updates()):
+            check_proto_stat(response.stat)
+            with pa.ipc.open_stream(response.arrowBytes) as reader:
+                yield uuid.UUID(bytes=response.uuid), reader.read_all()

btrdb/experimental/bytewax_connectors.py

@@ -0,0 +1,168 @@
+import time
+import queue
+import threading
+import weakref
+import pyarrow as pa
+
+from bytewax.inputs import DynamicInput, StatelessSource
+
+import btrdb
+
+_empty_values = pa.Table.from_arrays(
+    [pa.array([]), pa.array([])],
+    schema=pa.schema(
+        [
+            pa.field("time", pa.timestamp("ns", tz="UTC"), nullable=False),
+            pa.field("value", pa.float64(), nullable=False),
+        ]
+    ),
+)
+
+
+class InsertSubscription(DynamicInput):
+    def __init__(
+        self,
+        selector_fn,
+        selector_refresh_interval=60 * 60 * 6,
+        heartbeat_interval=None,
+        profile=None,
+        conn_str=None,
+        apikey=None,
+    ):
+        self._selector_fn = selector_fn
+        self._conn_str = conn_str
+        self._apikey = apikey
+        self._profile = profile
+        self._selector_refresh_interval = selector_refresh_interval
+        self._heartbeat_interval = heartbeat_interval
+
+    class Source(StatelessSource):
+        def __init__(
+            self,
+            db,
+            selector_fn,
+            selector_refresh_interval,
+            heartbeat_interval,
+            worker_index,
+            worker_count,
+        ):
+            self._db = db
+            self._selector_fn = selector_fn
+            self._worker_index = worker_index
+            self._worker_count = worker_count
+            self._selector_refresh_interval = selector_refresh_interval
+            self._heartbeat_interval = heartbeat_interval
+            self._read_worker_exception = None
+            self._background_worker_exception = None
+            self._del_event = threading.Event()
+            self._update_queue = queue.Queue(1)
+            self._data_queue = queue.Queue(512)
+
+            # self is wrapped in a weakref with the worker threads so
+            # that the worker threads keep self alive.
+            def read_worker(self, data):
+                try:
+                    # Avoid exessive weakref lookups
+                    # by doing the lookup upfront initially.
+                    del_event = self._del_event
+                    data_queue = self._data_queue
+                    for dat in data:
+                        if del_event.is_set():
+                            return
+                        data_queue.put(dat)
+                except Exception as e:
+                    self._read_worker_exception = e
+
+            # Self is a weakref, same as above.
+            def background_worker(self):
+                try:
+                    # Avoid exessive weakref lookups
+                    # by doing the lookup upfront initially.
+                    db = self._db
+                    del_event = self._del_event
+                    selector_fn = self._selector_fn
+                    worker_index = self._worker_index
+                    worker_count = self._worker_count
+                    data_queue = self._data_queue
+                    heartbeat_interval = self._heartbeat_interval
+                    last_heartbeat = time.monotonic()
+                    selector_refresh_interval = self._selector_refresh_interval
+                    last_selector_refresh = time.monotonic() - selector_refresh_interval
+                    current_uuids = set()
+                    while True:
+                        now = time.monotonic()
+                        if (now - last_selector_refresh) >= selector_refresh_interval:
+                            last_selector_refresh = now
+                            next_uuids = {
+                                uuid
+                                for uuid in selector_fn(db)
+                                if uuid.int % worker_count == worker_index
+                            }
+                            added_uuids = next_uuids - current_uuids
+                            removed_uuids = current_uuids - next_uuids
+                            if len(added_uuids) != 0 or len(removed_uuids) != 0:
+                                self._update_queue.put([added_uuids, removed_uuids])
+                            current_uuids = next_uuids
+                        if (
+                            heartbeat_interval is not None
+                            and (now - last_heartbeat) >= heartbeat_interval
+                        ):
+                            last_heartbeat = now
+                            for uuid in current_uuids:
+                                while not del_event.is_set():
+                                    try:
+                                        data_queue.put((uuid, _empty_values), 0.1)
+                                        break
+                                    except queue.Full:
+                                        pass
+                        if del_event.wait(1.0):
+                            return
+                except Exception as e:
+                    self._background_worker_exception = e
+
+            weakself = weakref.proxy(self)
+            data = db.ep.subscribe(self._update_queue)
+            self._background_worker = threading.Thread(
+                target=background_worker, args=[weakself], daemon=True
+            )
+            self._read_worker = threading.Thread(
+                target=read_worker, args=[weakself, data], daemon=True
+            )
+            self._background_worker.start()
+            self._read_worker.start()
+
+        def next(self):
+            # Check if the selector thread has died.
+            background_worker_exception = self._background_worker_exception
+            if background_worker_exception is not None:
+                raise background_worker_exception
+            try:
+                return self._data_queue.get_nowait()
+            except queue.Empty:
+                # Check if the reason no data arrived is because
+                # the reader thead has died.
+                read_worker_exception = self._read_worker_exception
+                if read_worker_exception is not None:
+                    raise read_worker_exception
+                return None
+
+        def __del__(self):
+            # Signal workers to exit.
+            self._del_event.set()
+            # Signal the end of the subscription.
+            self._update_queue.put(None)
+
+    def build(self, worker_index, worker_count):
+        db = btrdb.connect(
+            profile=self._profile,
+            conn_str=self._conn_str,
+            apikey=self._apikey,
+        )
+        return InsertSubscription.Source(
+            db,
+            self._selector_fn,
+            self._selector_refresh_interval,
+            self._heartbeat_interval,
+            worker_index,
+            worker_count,
+        )

btrdb/grpcinterface/btrdb.proto

@@ -28,6 +28,7 @@ service BTrDB {
   rpc GetMetadataUsage(MetadataUsageParams) returns (MetadataUsageResponse);
   rpc GenerateCSV(GenerateCSVParams) returns (stream GenerateCSVResponse);
   rpc SQLQuery(SQLQueryParams) returns (stream SQLQueryResponse);
+  rpc Subscribe(stream SubscriptionUpdate) returns (stream SubscriptionResp);
   //rpc SetCompactionConfig(SetCompactionConfigParams) returns (SetCompactionConfigResponse);
   //rpc GetCompactionConfig(GetCompactionConfigParams) returns (GetCompactionConfigResponse);
 }
@@ -426,3 +427,19 @@ message ReducedResolutionRange {
   int64 End = 2;
   uint32 Resolution = 3;
 }
+
+enum SubscriptionUpdateOp {
+  ADD_UUIDS = 0;
+  REMOVE_UUIDS = 1;
+}
+
+message SubscriptionUpdate {
+  SubscriptionUpdateOp op = 1;
+  repeated bytes uuid = 2;
+}
+
+message SubscriptionResp {
+  Status stat = 1;
+  bytes uuid = 2;
+  bytes arrowBytes = 3;
+}