synapse/tests/storage/test_id_generators.py

# Copyright 2020 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from typing import List, Optional

from twisted.test.proto_helpers import MemoryReactor

from synapse.server import HomeServer
from synapse.storage.database import DatabasePool, LoggingTransaction
from synapse.storage.engines import IncorrectDatabaseSetup
from synapse.storage.util.id_generators import MultiWriterIdGenerator
from synapse.util import Clock

from tests.unittest import HomeserverTestCase
from tests.utils import USE_POSTGRES_FOR_TESTS


class MultiWriterIdGeneratorTestCase(HomeserverTestCase):
    if not USE_POSTGRES_FOR_TESTS:
        skip = "Requires Postgres"

    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
        self.store = hs.get_datastores().main
        self.db_pool: DatabasePool = self.store.db_pool

        self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))

    def _setup_db(self, txn: LoggingTransaction) -> None:
        txn.execute("CREATE SEQUENCE foobar_seq")
        txn.execute(
            """
            CREATE TABLE foobar (
                stream_id BIGINT NOT NULL,
                instance_name TEXT NOT NULL,
                data TEXT
            );
            """
        )

    def _create_id_generator(
        self, instance_name="master", writers: Optional[List[str]] = None
    ) -> MultiWriterIdGenerator:
        def _create(conn):
            return MultiWriterIdGenerator(
                conn,
                self.db_pool,
                stream_name="test_stream",
                instance_name=instance_name,
                tables=[("foobar", "instance_name", "stream_id")],
                sequence_name="foobar_seq",
                writers=writers or ["master"],
            )

        return self.get_success_or_raise(self.db_pool.runWithConnection(_create))

    def _insert_rows(self, instance_name: str, number: int) -> None:
        """Insert N rows as the given instance, inserting with stream IDs pulled
        from the postgres sequence.
        """

        def _insert(txn: LoggingTransaction) -> None:
            for _ in range(number):
                txn.execute(
                    "INSERT INTO foobar VALUES (nextval('foobar_seq'), ?)",
                    (instance_name,),
                )
                txn.execute(
                    """
                    INSERT INTO stream_positions VALUES ('test_stream', ?,  lastval())
                    ON CONFLICT (stream_name, instance_name) DO UPDATE SET stream_id = lastval()
                    """,
                    (instance_name,),
                )

        self.get_success(self.db_pool.runInteraction("_insert_rows", _insert))

    def _insert_row_with_id(self, instance_name: str, stream_id: int) -> None:
        """Insert one row as the given instance with given stream_id, updating
        the postgres sequence position to match.
        """

        def _insert(txn: LoggingTransaction) -> None:
            txn.execute(
                "INSERT INTO foobar VALUES (?, ?)",
                (
                    stream_id,
                    instance_name,
                ),
            )
            txn.execute("SELECT setval('foobar_seq', ?)", (stream_id,))
            txn.execute(
                """
                INSERT INTO stream_positions VALUES ('test_stream', ?, ?)
                ON CONFLICT (stream_name, instance_name) DO UPDATE SET stream_id = ?
                """,
                (instance_name, stream_id, stream_id),
            )

        self.get_success(self.db_pool.runInteraction("_insert_row_with_id", _insert))

    def test_empty(self) -> None:
        """Test an ID generator against an empty database gives sensible
        current positions.
        """

        id_gen = self._create_id_generator()

        # The table is empty so we expect an empty map for positions
        self.assertEqual(id_gen.get_positions(), {})

    def test_single_instance(self) -> None:
        """Test that reads and writes from a single process are handled
        correctly.
        """

        # Prefill table with 7 rows written by 'master'
        self._insert_rows("master", 7)

        id_gen = self._create_id_generator()

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        # Try allocating a new ID gen and check that we only see position
        # advanced after we leave the context manager.

        async def _get_next_async() -> None:
            async with id_gen.get_next() as stream_id:
                self.assertEqual(stream_id, 8)

                self.assertEqual(id_gen.get_positions(), {"master": 7})
                self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        self.get_success(_get_next_async())

        self.assertEqual(id_gen.get_positions(), {"master": 8})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 8)

    def test_out_of_order_finish(self) -> None:
        """Test that IDs persisted out of order are correctly handled"""

        # Prefill table with 7 rows written by 'master'
        self._insert_rows("master", 7)

        id_gen = self._create_id_generator()

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        ctx1 = id_gen.get_next()
        ctx2 = id_gen.get_next()
        ctx3 = id_gen.get_next()
        ctx4 = id_gen.get_next()

        s1 = self.get_success(ctx1.__aenter__())
        s2 = self.get_success(ctx2.__aenter__())
        s3 = self.get_success(ctx3.__aenter__())
        s4 = self.get_success(ctx4.__aenter__())

        self.assertEqual(s1, 8)
        self.assertEqual(s2, 9)
        self.assertEqual(s3, 10)
        self.assertEqual(s4, 11)

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        self.get_success(ctx2.__aexit__(None, None, None))

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        self.get_success(ctx1.__aexit__(None, None, None))

        self.assertEqual(id_gen.get_positions(), {"master": 9})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 9)

        self.get_success(ctx4.__aexit__(None, None, None))

        self.assertEqual(id_gen.get_positions(), {"master": 9})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 9)

        self.get_success(ctx3.__aexit__(None, None, None))

        self.assertEqual(id_gen.get_positions(), {"master": 11})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 11)

    def test_multi_instance(self) -> None:
        """Test that reads and writes from multiple processes are handled
        correctly.
        """
        self._insert_rows("first", 3)
        self._insert_rows("second", 4)

        first_id_gen = self._create_id_generator("first", writers=["first", "second"])
        second_id_gen = self._create_id_generator("second", writers=["first", "second"])

        # The first ID gen will notice that it can advance its token to 7 as it
        # has no in progress writes...
        self.assertEqual(first_id_gen.get_positions(), {"first": 7, "second": 7})
        self.assertEqual(first_id_gen.get_current_token_for_writer("first"), 7)
        self.assertEqual(first_id_gen.get_current_token_for_writer("second"), 7)

        # ... but the second ID gen doesn't know that.
        self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 7})
        self.assertEqual(second_id_gen.get_current_token_for_writer("first"), 3)
        self.assertEqual(second_id_gen.get_current_token_for_writer("second"), 7)

        # Try allocating a new ID gen and check that we only see position
        # advanced after we leave the context manager.

        async def _get_next_async() -> None:
            async with first_id_gen.get_next() as stream_id:
                self.assertEqual(stream_id, 8)

                self.assertEqual(
                    first_id_gen.get_positions(), {"first": 7, "second": 7}
                )

        self.get_success(_get_next_async())

        self.assertEqual(first_id_gen.get_positions(), {"first": 8, "second": 7})

        # However the ID gen on the second instance won't have seen the update
        self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 7})

        # ... but calling `get_next` on the second instance should give a unique
        # stream ID

        async def _get_next_async2() -> None:
            async with second_id_gen.get_next() as stream_id:
                self.assertEqual(stream_id, 9)

                self.assertEqual(
                    second_id_gen.get_positions(), {"first": 3, "second": 7}
                )

        self.get_success(_get_next_async2())

        self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 9})

        # If the second ID gen gets told about the first, it correctly updates
        second_id_gen.advance("first", 8)
        self.assertEqual(second_id_gen.get_positions(), {"first": 8, "second": 9})

    def test_get_next_txn(self) -> None:
        """Test that the `get_next_txn` function works correctly."""

        # Prefill table with 7 rows written by 'master'
        self._insert_rows("master", 7)

        id_gen = self._create_id_generator()

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        # Try allocating a new ID gen and check that we only see position
        # advanced after we leave the context manager.

        def _get_next_txn(txn: LoggingTransaction) -> None:
            stream_id = id_gen.get_next_txn(txn)
            self.assertEqual(stream_id, 8)

            self.assertEqual(id_gen.get_positions(), {"master": 7})
            self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        self.get_success(self.db_pool.runInteraction("test", _get_next_txn))

        self.assertEqual(id_gen.get_positions(), {"master": 8})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 8)

    def test_get_persisted_upto_position(self) -> None:
        """Test that `get_persisted_upto_position` correctly tracks updates to
        positions.
        """

        # The following tests are a bit cheeky in that we notify about new
        # positions via `advance` without *actually* advancing the postgres
        # sequence.

        self._insert_row_with_id("first", 3)
        self._insert_row_with_id("second", 5)

        id_gen = self._create_id_generator("worker", writers=["first", "second"])

        self.assertEqual(id_gen.get_positions(), {"first": 3, "second": 5})

        # Min is 3 and there is a gap between 5, so we expect it to be 3.
        self.assertEqual(id_gen.get_persisted_upto_position(), 3)

        # We advance "first" straight to 6. Min is now 5 but there is no gap so
        # we expect it to be 6
        id_gen.advance("first", 6)
        self.assertEqual(id_gen.get_persisted_upto_position(), 6)

        # No gap, so we expect 7.
        id_gen.advance("second", 7)
        self.assertEqual(id_gen.get_persisted_upto_position(), 7)

        # We haven't seen 8 yet, so we expect 7 still.
        id_gen.advance("second", 9)
        self.assertEqual(id_gen.get_persisted_upto_position(), 7)

        # Now that we've seen 7, 8 and 9 we can got straight to 9.
        id_gen.advance("first", 8)
        self.assertEqual(id_gen.get_persisted_upto_position(), 9)

        # Jump forward with gaps. The minimum is 11, even though we haven't seen
        # 10 we know that everything before 11 must be persisted.
        id_gen.advance("first", 11)
        id_gen.advance("second", 15)
        self.assertEqual(id_gen.get_persisted_upto_position(), 11)

    def test_get_persisted_upto_position_get_next(self) -> None:
        """Test that `get_persisted_upto_position` correctly tracks updates to
        positions when `get_next` is called.
        """

        self._insert_row_with_id("first", 3)
        self._insert_row_with_id("second", 5)

        id_gen = self._create_id_generator("first", writers=["first", "second"])

        self.assertEqual(id_gen.get_positions(), {"first": 5, "second": 5})

        self.assertEqual(id_gen.get_persisted_upto_position(), 5)

        async def _get_next_async() -> None:
            async with id_gen.get_next() as stream_id:
                self.assertEqual(stream_id, 6)
                self.assertEqual(id_gen.get_persisted_upto_position(), 5)

        self.get_success(_get_next_async())

        self.assertEqual(id_gen.get_persisted_upto_position(), 6)

        # We assume that so long as `get_next` does correctly advance the
        # `persisted_upto_position` in this case, then it will be correct in the
        # other cases that are tested above (since they'll hit the same code).

    def test_restart_during_out_of_order_persistence(self) -> None:
        """Test that restarting a process while another process is writing out
        of order updates are handled correctly.
        """

        # Prefill table with 7 rows written by 'master'
        self._insert_rows("master", 7)

        id_gen = self._create_id_generator()

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        # Persist two rows at once
        ctx1 = id_gen.get_next()
        ctx2 = id_gen.get_next()

        s1 = self.get_success(ctx1.__aenter__())
        s2 = self.get_success(ctx2.__aenter__())

        self.assertEqual(s1, 8)
        self.assertEqual(s2, 9)

        self.assertEqual(id_gen.get_positions(), {"master": 7})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), 7)

        # We finish persisting the second row before restart
        self.get_success(ctx2.__aexit__(None, None, None))

        # We simulate a restart of another worker by just creating a new ID gen.
        id_gen_worker = self._create_id_generator("worker")

        # Restarted worker should not see the second persisted row
        self.assertEqual(id_gen_worker.get_positions(), {"master": 7})
        self.assertEqual(id_gen_worker.get_current_token_for_writer("master"), 7)

        # Now if we persist the first row then both instances should jump ahead
        # correctly.
        self.get_success(ctx1.__aexit__(None, None, None))

        self.assertEqual(id_gen.get_positions(), {"master": 9})
        id_gen_worker.advance("master", 9)
        self.assertEqual(id_gen_worker.get_positions(), {"master": 9})

    def test_writer_config_change(self) -> None:
        """Test that changing the writer config correctly works."""

        self._insert_row_with_id("first", 3)
        self._insert_row_with_id("second", 5)

        # Initial config has two writers
        id_gen = self._create_id_generator("worker", writers=["first", "second"])
        self.assertEqual(id_gen.get_persisted_upto_position(), 3)
        self.assertEqual(id_gen.get_current_token_for_writer("first"), 3)
        self.assertEqual(id_gen.get_current_token_for_writer("second"), 5)

        # New config removes one of the configs. Note that if the writer is
        # removed from config we assume that it has been shut down and has
        # finished persisting, hence why the persisted upto position is 5.
        id_gen_2 = self._create_id_generator("second", writers=["second"])
        self.assertEqual(id_gen_2.get_persisted_upto_position(), 5)
        self.assertEqual(id_gen_2.get_current_token_for_writer("second"), 5)

        # This config points to a single, previously unused writer.
        id_gen_3 = self._create_id_generator("third", writers=["third"])
        self.assertEqual(id_gen_3.get_persisted_upto_position(), 5)

        # For new writers we assume their initial position to be the current
        # persisted up to position. This stops Synapse from doing a full table
        # scan when a new writer comes along.
        self.assertEqual(id_gen_3.get_current_token_for_writer("third"), 5)

        id_gen_4 = self._create_id_generator("fourth", writers=["third"])
        self.assertEqual(id_gen_4.get_current_token_for_writer("third"), 5)

        # Check that we get a sane next stream ID with this new config.

        async def _get_next_async() -> None:
            async with id_gen_3.get_next() as stream_id:
                self.assertEqual(stream_id, 6)

        self.get_success(_get_next_async())
        self.assertEqual(id_gen_3.get_persisted_upto_position(), 6)

        # If we add back the old "first" then we shouldn't see the persisted up
        # to position revert back to 3.
        id_gen_5 = self._create_id_generator("five", writers=["first", "third"])
        self.assertEqual(id_gen_5.get_persisted_upto_position(), 6)
        self.assertEqual(id_gen_5.get_current_token_for_writer("first"), 6)
        self.assertEqual(id_gen_5.get_current_token_for_writer("third"), 6)

    def test_sequence_consistency(self) -> None:
        """Test that we error out if the table and sequence diverges."""

        # Prefill with some rows
        self._insert_row_with_id("master", 3)

        # Now we add a row *without* updating the stream ID
        def _insert(txn):
            txn.execute("INSERT INTO foobar VALUES (26, 'master')")

        self.get_success(self.db_pool.runInteraction("_insert", _insert))

        # Creating the ID gen should error
        with self.assertRaises(IncorrectDatabaseSetup):
            self._create_id_generator("first")


class BackwardsMultiWriterIdGeneratorTestCase(HomeserverTestCase):
    """Tests MultiWriterIdGenerator that produce *negative* stream IDs."""

    if not USE_POSTGRES_FOR_TESTS:
        skip = "Requires Postgres"

    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
        self.store = hs.get_datastores().main
        self.db_pool: DatabasePool = self.store.db_pool

        self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))

    def _setup_db(self, txn: LoggingTransaction) -> None:
        txn.execute("CREATE SEQUENCE foobar_seq")
        txn.execute(
            """
            CREATE TABLE foobar (
                stream_id BIGINT NOT NULL,
                instance_name TEXT NOT NULL,
                data TEXT
            );
            """
        )

    def _create_id_generator(
        self, instance_name="master", writers: Optional[List[str]] = None
    ) -> MultiWriterIdGenerator:
        def _create(conn):
            return MultiWriterIdGenerator(
                conn,
                self.db_pool,
                stream_name="test_stream",
                instance_name=instance_name,
                tables=[("foobar", "instance_name", "stream_id")],
                sequence_name="foobar_seq",
                writers=writers or ["master"],
                positive=False,
            )

        return self.get_success(self.db_pool.runWithConnection(_create))

    def _insert_row(self, instance_name: str, stream_id: int) -> None:
        """Insert one row as the given instance with given stream_id."""

        def _insert(txn: LoggingTransaction) -> None:
            txn.execute(
                "INSERT INTO foobar VALUES (?, ?)",
                (
                    stream_id,
                    instance_name,
                ),
            )
            txn.execute(
                """
                INSERT INTO stream_positions VALUES ('test_stream', ?, ?)
                ON CONFLICT (stream_name, instance_name) DO UPDATE SET stream_id = ?
                """,
                (instance_name, -stream_id, -stream_id),
            )

        self.get_success(self.db_pool.runInteraction("_insert_row", _insert))

    def test_single_instance(self) -> None:
        """Test that reads and writes from a single process are handled
        correctly.
        """
        id_gen = self._create_id_generator()

        async def _get_next_async() -> None:
            async with id_gen.get_next() as stream_id:
                self._insert_row("master", stream_id)

        self.get_success(_get_next_async())

        self.assertEqual(id_gen.get_positions(), {"master": -1})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), -1)
        self.assertEqual(id_gen.get_persisted_upto_position(), -1)

        async def _get_next_async2() -> None:
            async with id_gen.get_next_mult(3) as stream_ids:
                for stream_id in stream_ids:
                    self._insert_row("master", stream_id)

        self.get_success(_get_next_async2())

        self.assertEqual(id_gen.get_positions(), {"master": -4})
        self.assertEqual(id_gen.get_current_token_for_writer("master"), -4)
        self.assertEqual(id_gen.get_persisted_upto_position(), -4)

        # Test loading from DB by creating a second ID gen
        second_id_gen = self._create_id_generator()

        self.assertEqual(second_id_gen.get_positions(), {"master": -4})
        self.assertEqual(second_id_gen.get_current_token_for_writer("master"), -4)
        self.assertEqual(second_id_gen.get_persisted_upto_position(), -4)

    def test_multiple_instance(self) -> None:
        """Tests that having multiple instances that get advanced over
        federation works corretly.
        """
        id_gen_1 = self._create_id_generator("first", writers=["first", "second"])
        id_gen_2 = self._create_id_generator("second", writers=["first", "second"])

        async def _get_next_async() -> None:
            async with id_gen_1.get_next() as stream_id:
                self._insert_row("first", stream_id)
                id_gen_2.advance("first", stream_id)

        self.get_success(_get_next_async())

        self.assertEqual(id_gen_1.get_positions(), {"first": -1})
        self.assertEqual(id_gen_2.get_positions(), {"first": -1})
        self.assertEqual(id_gen_1.get_persisted_upto_position(), -1)
        self.assertEqual(id_gen_2.get_persisted_upto_position(), -1)

        async def _get_next_async2() -> None:
            async with id_gen_2.get_next() as stream_id:
                self._insert_row("second", stream_id)
                id_gen_1.advance("second", stream_id)

        self.get_success(_get_next_async2())

        self.assertEqual(id_gen_1.get_positions(), {"first": -2, "second": -2})
        self.assertEqual(id_gen_2.get_positions(), {"first": -1, "second": -2})
        self.assertEqual(id_gen_1.get_persisted_upto_position(), -2)
        self.assertEqual(id_gen_2.get_persisted_upto_position(), -2)


class MultiTableMultiWriterIdGeneratorTestCase(HomeserverTestCase):
    if not USE_POSTGRES_FOR_TESTS:
        skip = "Requires Postgres"

    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
        self.store = hs.get_datastores().main
        self.db_pool: DatabasePool = self.store.db_pool

        self.get_success(self.db_pool.runInteraction("_setup_db", self._setup_db))

    def _setup_db(self, txn: LoggingTransaction) -> None:
        txn.execute("CREATE SEQUENCE foobar_seq")
        txn.execute(
            """
            CREATE TABLE foobar1 (
                stream_id BIGINT NOT NULL,
                instance_name TEXT NOT NULL,
                data TEXT
            );
            """
        )

        txn.execute(
            """
            CREATE TABLE foobar2 (
                stream_id BIGINT NOT NULL,
                instance_name TEXT NOT NULL,
                data TEXT
            );
            """
        )

    def _create_id_generator(
        self, instance_name="master", writers: Optional[List[str]] = None
    ) -> MultiWriterIdGenerator:
        def _create(conn):
            return MultiWriterIdGenerator(
                conn,
                self.db_pool,
                stream_name="test_stream",
                instance_name=instance_name,
                tables=[
                    ("foobar1", "instance_name", "stream_id"),
                    ("foobar2", "instance_name", "stream_id"),
                ],
                sequence_name="foobar_seq",
                writers=writers or ["master"],
            )

        return self.get_success_or_raise(self.db_pool.runWithConnection(_create))

    def _insert_rows(
        self,
        table: str,
        instance_name: str,
        number: int,
        update_stream_table: bool = True,
    ):
        """Insert N rows as the given instance, inserting with stream IDs pulled
        from the postgres sequence.
        """

        def _insert(txn: LoggingTransaction) -> None:
            for _ in range(number):
                txn.execute(
                    "INSERT INTO %s VALUES (nextval('foobar_seq'), ?)" % (table,),
                    (instance_name,),
                )
                if update_stream_table:
                    txn.execute(
                        """
                        INSERT INTO stream_positions VALUES ('test_stream', ?,  lastval())
                        ON CONFLICT (stream_name, instance_name) DO UPDATE SET stream_id = lastval()
                        """,
                        (instance_name,),
                    )

        self.get_success(self.db_pool.runInteraction("_insert_rows", _insert))

    def test_load_existing_stream(self) -> None:
        """Test creating ID gens with multiple tables that have rows from after
        the position in `stream_positions` table.
        """
        self._insert_rows("foobar1", "first", 3)
        self._insert_rows("foobar2", "second", 3)
        self._insert_rows("foobar2", "second", 1, update_stream_table=False)

        first_id_gen = self._create_id_generator("first", writers=["first", "second"])
        second_id_gen = self._create_id_generator("second", writers=["first", "second"])

        # The first ID gen will notice that it can advance its token to 7 as it
        # has no in progress writes...
        self.assertEqual(first_id_gen.get_positions(), {"first": 7, "second": 6})
        self.assertEqual(first_id_gen.get_current_token_for_writer("first"), 7)
        self.assertEqual(first_id_gen.get_current_token_for_writer("second"), 6)
        self.assertEqual(first_id_gen.get_persisted_upto_position(), 7)

        # ... but the second ID gen doesn't know that.
        self.assertEqual(second_id_gen.get_positions(), {"first": 3, "second": 7})
        self.assertEqual(second_id_gen.get_current_token_for_writer("first"), 3)
        self.assertEqual(second_id_gen.get_current_token_for_writer("second"), 7)
        self.assertEqual(first_id_gen.get_persisted_upto_position(), 7)