Class: Pecorino::Adapters::SqliteAdapter

Inherits:
Object
  • Object
show all
Defined in:
lib/pecorino/adapters/sqlite_adapter.rb

Instance Method Summary collapse

Constructor Details

#initialize(model_class) ⇒ SqliteAdapter

Returns a new instance of SqliteAdapter.



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 4

def initialize(model_class)
  @model_class = model_class
end

Instance Method Details

#add_tokens(key:, capacity:, leak_rate:, n_tokens:) ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 44

def add_tokens(key:, capacity:, leak_rate:, n_tokens:)
  # Take double the time it takes the bucket to empty under normal circumstances
  # until the bucket may be deleted.
  may_be_deleted_after_seconds = (capacity.to_f / leak_rate.to_f) * 2.0

  # Create the leaky bucket if it does not exist, and update
  # to the new level, taking the leak rate into account - if the bucket exists.
  query_params = {
    key: key.to_s,
    capacity: capacity.to_f,
    delete_after_s: may_be_deleted_after_seconds,
    leak_rate: leak_rate.to_f,
    now_s: Time.now.to_f, # See above as to why we are using a time value passed in
    fillup: n_tokens.to_f
  }

  sql = @model_class.sanitize_sql_array([<<~SQL, query_params])
    INSERT INTO pecorino_leaky_buckets AS t
      (key, last_touched_at, may_be_deleted_after, level)
    VALUES
      (
        :key,
        :now_s, -- Precision loss must be avoided here as it is used for calculations
        DATETIME('now', '+:delete_after_s seconds'), -- Precision loss is acceptable here
        MAX(0.0,
          MIN(
            :capacity,
            :fillup
          )
        )
      )
    ON CONFLICT (key) DO UPDATE SET
      last_touched_at = EXCLUDED.last_touched_at,
      may_be_deleted_after = EXCLUDED.may_be_deleted_after,
      level = MAX(0.0,
        MIN(
            :capacity,
            t.level + :fillup - ((:now_s - t.last_touched_at) * :leak_rate)
        )
      )
    RETURNING
      level,
      -- Compare level to the capacity inside the DB so that we won't have rounding issues
      level >= :capacity AS did_overflow
  SQL

  # Note the use of .uncached here. The AR query cache will actually see our
  # query as a repeat (since we use "select_one" for the RETURNING bit) and will not call into Postgres
  # correctly, thus the clock_timestamp() value would be frozen between calls. We don't want that here.
  # See https://stackoverflow.com/questions/73184531/why-would-postgres-clock-timestamp-freeze-inside-a-rails-unit-test
  upserted = @model_class.connection.uncached { @model_class.connection.select_one(sql) }
  capped_level_after_fillup, one_if_did_overflow = upserted.fetch("level"), upserted.fetch("did_overflow")
  [capped_level_after_fillup, one_if_did_overflow == 1]
end

#add_tokens_conditionally(key:, capacity:, leak_rate:, n_tokens:) ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 99

def add_tokens_conditionally(key:, capacity:, leak_rate:, n_tokens:)
  # Take double the time it takes the bucket to empty under normal circumstances
  # until the bucket may be deleted.
  may_be_deleted_after_seconds = (capacity.to_f / leak_rate.to_f) * 2.0

  # Create the leaky bucket if it does not exist, and update
  # to the new level, taking the leak rate into account - if the bucket exists.
  query_params = {
    key: key.to_s,
    capacity: capacity.to_f,
    delete_after_s: may_be_deleted_after_seconds,
    leak_rate: leak_rate.to_f,
    now_s: Time.now.to_f, # See above as to why we are using a time value passed in
    fillup: n_tokens.to_f
  }

  # Sadly with SQLite we need to do an INSERT first, because otherwise the inserted row is visible
  # to the WITH clause, so we cannot combine the initial fillup and the update into one statement.
  # This shuld be fine however since we will suppress the INSERT on a key conflict
  insert_sql = @model_class.sanitize_sql_array([<<~SQL, query_params])
    INSERT INTO pecorino_leaky_buckets AS t
      (key, last_touched_at, may_be_deleted_after, level)
    VALUES
      (
        :key,
        :now_s, -- Precision loss must be avoided here as it is used for calculations
        DATETIME('now', '+:delete_after_s seconds'), -- Precision loss is acceptable here
        0.0
      )
    ON CONFLICT (key) DO UPDATE SET
    -- Make sure we extend the lifetime of the row
    -- so that it can't be deleted between our INSERT and our UPDATE
      may_be_deleted_after = EXCLUDED.may_be_deleted_after
  SQL
  @model_class.connection.execute(insert_sql)

  sql = @model_class.sanitize_sql_array([<<~SQL, query_params])
    -- With SQLite MATERIALIZED has to be used so that level_post is calculated before the UPDATE takes effect
    WITH pre(level_post_with_uncapped_fillup, level_post) AS MATERIALIZED (
      SELECT
        -- Note the double clamping here. First we clamp the "current level - leak" to not go below zero,
        -- then we also clamp the above + fillup to not go below 0
        MAX(0.0, MAX(0.0, level - ((:now_s - last_touched_at) * :leak_rate)) + :fillup) AS level_post_with_uncapped_fillup,
        MAX(0.0, level - ((:now_s - last_touched_at) * :leak_rate)) AS level_post
      FROM
        pecorino_leaky_buckets
      WHERE key = :key
    ) UPDATE pecorino_leaky_buckets SET
      last_touched_at = :now_s,
      may_be_deleted_after = DATETIME('now', '+:delete_after_s seconds'),
      level = CASE WHEN (SELECT level_post_with_uncapped_fillup FROM pre) <= :capacity THEN
        (SELECT level_post_with_uncapped_fillup FROM pre)
      ELSE
        (SELECT level_post FROM pre)
      END
    RETURNING
      (SELECT level_post FROM pre) AS level_before,
      level AS level_after
  SQL

  upserted = @model_class.connection.uncached { @model_class.connection.select_one(sql) }
  level_after = upserted.fetch("level_after")
  level_before = upserted.fetch("level_before")
  [level_after, level_after >= capacity, level_after != level_before]
end

#blocked_until(key:) ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 181

def blocked_until(key:)
  now_s = Time.now.to_f
  block_check_query = @model_class.sanitize_sql_array([<<~SQL, {now_s: now_s, key: key}])
    SELECT
      blocked_until
    FROM
      pecorino_blocks
    WHERE
      key = :key AND blocked_until >= :now_s LIMIT 1
  SQL
  blocked_until_s = @model_class.connection.uncached { @model_class.connection.select_value(block_check_query) }
  blocked_until_s && Time.at(blocked_until_s)
end

#create_tables(active_record_schema) ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 201

def create_tables(active_record_schema)
  active_record_schema.create_table :pecorino_leaky_buckets do |t|
    t.string :key, null: false
    t.float :level, null: false
    t.datetime :last_touched_at, null: false
    t.datetime :may_be_deleted_after, null: false
  end
  active_record_schema.add_index :pecorino_leaky_buckets, [:key], unique: true
  active_record_schema.add_index :pecorino_leaky_buckets, [:may_be_deleted_after]

  active_record_schema.create_table :pecorino_blocks do |t|
    t.string :key, null: false
    t.datetime :blocked_until, null: false
  end
  active_record_schema.add_index :pecorino_blocks, [:key], unique: true
  active_record_schema.add_index :pecorino_blocks, [:blocked_until]
end

#prune ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 195

def prune
  now_s = Time.now.to_f
  @model_class.connection.execute("DELETE FROM pecorino_blocks WHERE blocked_until < ?", now_s)
  @model_class.connection.execute("DELETE FROM pecorino_leaky_buckets WHERE may_be_deleted_after < ?", now_s)
end

#set_block(key:, block_for:) ⇒ Object

Raises:

  • (ArgumentError) 


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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 165

def set_block(key:, block_for:)
  raise ArgumentError, "block_for must be positive" unless block_for > 0
  query_params = {key: key.to_s, block_for: block_for.to_f, now_s: Time.now.to_f}
  block_set_query = @model_class.sanitize_sql_array([<<~SQL, query_params])
    INSERT INTO pecorino_blocks AS t
      (key, blocked_until)
    VALUES
      (:key, :now_s + :block_for)
    ON CONFLICT (key) DO UPDATE SET
      blocked_until = MAX(EXCLUDED.blocked_until, t.blocked_until)
    RETURNING blocked_until;
  SQL
  blocked_until_s = @model_class.connection.uncached { @model_class.connection.select_value(block_set_query) }
  Time.at(blocked_until_s)
end

#state(key:, capacity:, leak_rate:) ⇒ Object 



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# File 'lib/pecorino/adapters/sqlite_adapter.rb', line 8

def state(key:, capacity:, leak_rate:)
  # With a server database, it is really important to use the clock of the database itself so
  # that concurrent requests will see consistent bucket level calculations. Since SQLite is
  # actually in-process, there is no point using DB functions - and besides, SQLite reduces
  # the time precision to the nearest millisecond - and the calculations with timestamps are
  # obtuse. Therefore we can use the current time inside the Ruby VM - it doesn't matter all that
  # much but saves us on writing some gnarly SQL to have SQLite produce consistent precise timestamps.
  query_params = {
    key: key.to_s,
    capacity: capacity.to_f,
    leak_rate: leak_rate.to_f,
    now_s: Time.now.to_f
  }
  # The `level` of the bucket is what got stored at `last_touched_at` time, and we can
  # extrapolate from it to see how many tokens have leaked out since `last_touched_at` -
  # we don't need to UPDATE the value in the bucket here
  sql = @model_class.sanitize_sql_array([<<~SQL, query_params])
    SELECT
      MAX(
        0.0, MIN(
          :capacity,
          t.level - ((:now_s - t.last_touched_at) * :leak_rate)
        )
      )
    FROM 
      pecorino_leaky_buckets AS t
    WHERE
      key = :key
  SQL

  # If the return value of the query is a NULL it means no such bucket exists,
  # so we assume the bucket is empty
  current_level = @model_class.connection.uncached { @model_class.connection.select_value(sql) } || 0.0
  [current_level, capacity - current_level.abs < 0.01]
end