Class: Pecorino::Adapters::PostgresAdapter

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

Instance Method Summary collapse

Constructor Details

#initialize(model_class) ⇒ PostgresAdapter

Returns a new instance of PostgresAdapter.



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# File 'lib/pecorino/adapters/postgres_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/postgres_adapter.rb', line 37

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,
    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,
        clock_timestamp(),
        clock_timestamp() + ':delete_after_s second'::interval,
        GREATEST(0.0,
          LEAST(
            :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 = GREATEST(0.0,
        LEAST(
            :capacity,
            t.level + :fillup - (EXTRACT(EPOCH FROM (EXCLUDED.last_touched_at - 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 at_capacity
  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, at_capacity = upserted.fetch("level"), upserted.fetch("at_capacity")
  [capped_level_after_fillup, at_capacity]
end

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



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# File 'lib/pecorino/adapters/postgres_adapter.rb', line 91

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,
    fillup: n_tokens.to_f
  }

  sql = @model_class.sanitize_sql_array([<<~SQL, query_params])
    WITH pre 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
        GREATEST(0.0, 
            GREATEST(0.0, level - (EXTRACT(EPOCH FROM (clock_timestamp() - last_touched_at)) * :leak_rate)) + :fillup
        ) AS level_post_with_uncapped_fillup,
        GREATEST(0.0,
            level - (EXTRACT(EPOCH FROM (clock_timestamp() - last_touched_at)) * :leak_rate)
        ) AS level_post
      FROM pecorino_leaky_buckets
      WHERE key = :key
    )
    INSERT INTO pecorino_leaky_buckets AS t
      (key, last_touched_at, may_be_deleted_after, level)
    VALUES
      (
        :key,
        clock_timestamp(),
        clock_timestamp() + ':delete_after_s second'::interval,
        GREATEST(0.0,
          (CASE WHEN :fillup > :capacity THEN 0.0 ELSE :fillup END)
        )
      )
    ON CONFLICT (key) DO UPDATE SET
      last_touched_at = EXCLUDED.last_touched_at,
      may_be_deleted_after = EXCLUDED.may_be_deleted_after,
      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
      COALESCE((SELECT level_post FROM pre), 0.0) 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/postgres_adapter.rb', line 165

def blocked_until(key:)
  block_check_query = @model_class.sanitize_sql_array([<<~SQL, key])
    SELECT blocked_until FROM pecorino_blocks WHERE key = ? AND blocked_until >= clock_timestamp() LIMIT 1
  SQL
  @model_class.connection.uncached { @model_class.connection.select_value(block_check_query) }
end

#create_tables(active_record_schema) ⇒ Object 



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# File 'lib/pecorino/adapters/postgres_adapter.rb', line 177

def create_tables(active_record_schema)
  active_record_schema.create_table :pecorino_leaky_buckets, id: :uuid 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, id: :uuid 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/postgres_adapter.rb', line 172

def prune
  @model_class.connection.execute("DELETE FROM pecorino_blocks WHERE blocked_until < NOW()")
  @model_class.connection.execute("DELETE FROM pecorino_leaky_buckets WHERE may_be_deleted_after < NOW()")
end

#set_block(key:, block_for:) ⇒ Object

Raises:

  • (ArgumentError) 


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# File 'lib/pecorino/adapters/postgres_adapter.rb', line 150

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}
  block_set_query = @model_class.sanitize_sql_array([<<~SQL, query_params])
    INSERT INTO pecorino_blocks AS t
      (key, blocked_until)
    VALUES
      (:key, clock_timestamp() + ':block_for seconds'::interval)
    ON CONFLICT (key) DO UPDATE SET
      blocked_until = GREATEST(EXCLUDED.blocked_until, t.blocked_until)
    RETURNING blocked_until
  SQL
  @model_class.connection.uncached { @model_class.connection.select_value(block_set_query) }
end

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



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

def state(key:, capacity:, leak_rate:)
  query_params = {
    key: key.to_s,
    capacity: capacity.to_f,
    leak_rate: leak_rate.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
      GREATEST(
        0.0, LEAST(
          :capacity,
          t.level - (EXTRACT(EPOCH FROM (clock_timestamp() - 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