Add client methods for waiting for data to land in ConsDB

python/lsst/summit/utils/consdbClient.py

@@ -21,6 +21,7 @@
 
 import logging
 import os
+import time
 from dataclasses import dataclass
 from typing import Any
 from urllib.parse import quote, urlparse
@@ -607,6 +608,130 @@ def query(self, query: str) -> Table:
             return Table(rows=[])
         return Table(rows=result["data"], names=result["columns"])
 
+    def wait_for_row_to_exist(self, query: str, timeout: float, poll_frequency_hz: float = 2) -> Table:
+        """Returns a row once it exists, or an empty table if it times out.
+
+        The supplied ``query`` must be expected to return exactly a single row,
+        (once it exists), e.g. it should be something like
+        'select * from cdb_latiss.exposure where exposure_id = 2024100200541'
+        or similar. If the query were like
+        'select * from cdb_latiss.exposure where exposure_id in (2024100200541,
+        2024100200542)', then the query would return multiple rows and an error
+        would be raised. The user is expected to check that the query meets
+        this criterion, because if 2024100200541 existed but 2024100200542 was
+        about to be created the error would not be raised, and downstream
+        beaviour would be undefined.
+
+        Parameters
+        ----------
+        query : `str`
+            A SQL query (currently) to the database.
+        timeout : `float`
+            Maximum time to wait for a non-empty result, in seconds.
+        poll_frequency_hz : `float`, optional
+            Frequency to poll the database for results, in Hz.
+
+        Returns
+        -------
+        result : `Table`
+            An ``astropy.Table`` containing the query results, or an empty
+            if the row was not inserted before the timeout.
+
+        Raises
+        ------
+        requests.RequestException
+            Raised if any kind of connection error occurs.
+        requests.HTTPError
+            Raised if a non-successful status is returned.
+        ValueError
+            Raised if the query returns more than one row.
+        """
+        sleep_duration = 1 / poll_frequency_hz
+        t0 = time.time()
+        while time.time() - t0 < timeout:
+            result = self.query(query)
+            if len(result) > 1:
+                raise ValueError(f"Query {query} returned more than one row")
+            elif len(result) == 1:
+                return result
+            time.sleep(sleep_duration)
+
+        logger.warning(f"Query {query} did not return any results within {timeout}s")
+        return Table(rows=[])
+
+    def wait_for_item_in_row(
+        self, query: str, item: str, timeout: float, poll_frequency_hz: float = 2
+    ) -> float | None:
+        """Returns the value of an item in a row once it exists, or ``None``
+        if it times out.
+
+        If the item is not in the schema of the table, an error will be raised.
+
+        The supplied ``query`` must be expected to return exactly a single row,
+        (once it exists), e.g. it should be something like
+        'select * from cdb_latiss.exposure where exposure_id = 2024100200541'
+        or similar. If the query were like
+        'select * from cdb_latiss.exposure where exposure_id in (2024100200541,
+        2024100200542)', then the query would return multiple rows and an error
+        would be raised. The user is expected to check that the query meets
+        this criterion, because if 2024100200541 existed but 2024100200542 was
+        about to be created the error would not be raised, and downstream
+        beaviour would be undefined.
+
+        Parameters
+        ----------
+        query : `str`
+            A SQL query (currently) to the database.
+        item : `str`
+            The item to check for in the query results.
+        timeout : `float`
+            Maximum time to wait for a non-empty result, in seconds.
+        poll_frequency_hz : `float`, optional
+            Frequency to poll the database for results, in Hz.
+
+        Returns
+        -------
+        value : `float` or `None`
+            The corresponding value of the item in the row in the table, or
+            ``None`` if the item was not found before the timeout.
+
+        Raises
+        ------
+        requests.RequestException
+            Raised if any kind of connection error occurs.
+        requests.HTTPError
+            Raised if a non-successful status is returned.
+        ValueError
+            Raised if the query returns more than one row, or if the requested
+            item is not in the schema of the table.
+        """
+
+        row = self.wait_for_row_to_exist(query, timeout, poll_frequency_hz)
+        if len(row) == 0:
+            # wait_for_row_to_exist already logged a warning if table is empty
+            return None
+
+        # we know the row now exists but the required item may not be there yet
+        sleep_duration = 1 / poll_frequency_hz
+        t0 = time.time()
+        while time.time() - t0 < timeout:
+            result = self.query(query)
+            if len(result) > 1:
+                raise ValueError(f"Query {query} returned more than one row")
+            assert len(result) == 1, "Somehow no rows came back, which should be impossible"
+            row = result[0]
+            if item not in row.columns:
+                raise ValueError(f"Query {query} did not return a column named {item} - check the schema")
+            value = result[0][item]
+            if value is not None:
+                return value
+            time.sleep(sleep_duration)
+
+        logger.warning(
+            f"The row returned by {query} did not end up containing a value for {item} within {timeout}s"
+        )
+        return None
+
     def schema(
         self, instrument: str | None = None, table: str | None = None
     ) -> dict[str, tuple[str, str]] | list[str]: