GenBatcher

GenBatcher is a simple and lightweight batching utility for Elixir.

Installation

This package can be installed by adding :gen_batcher to your list of dependencies in mix.exs:

def deps do
  [
    {:gen_batcher "~> 1.0.0"}
  ]
end

Documentation

For additional documentation, see HexDocs.

Usage

GenBatcher processes are easy to start and have a number of powerful configuration options:

Child Spec Example

A GenBatcher can be started with a simple child spec:

opts = [
  name: :my_gen_batcher,
  flush_trigger: {:size, 3},
  batch_timeout: 30_000,
  handle_flush:
    fn items, _ ->
      items
      |> Enum.join(",")
      |> IO.puts()
    end
]

children = [
  {GenBatcher, opts}
]

Supervisor.start_link(children, strategy: :one_for_one)

Once started, items can be inserted into the GenBatcher:

GenBatcher.insert(:my_gen_batcher, "foo")
GenBatcher.insert(:my_gen_batcher, "bar")

And, once a flush condition has been met, a flush operation will be triggered:

GenBatcher.insert(:my_gen_batcher, "baz")
# Flush operation outputs "foo,bar,baz"

GenBatcher.insert(:my_gen_batcher, "foo")
# After 30 seconds pass...
# Flush operation outputs "foo"

Behaviour Example

A GenBatcher can also be started with any module that implements the GenBatcher behaviour:

defmodule MyGenBatcher do
  use GenBatcher

  def start_link(opts \\ []) do
    GenBatcher.start_link(__MODULE__, opts)
  end

  @impl GenBatcher
  def handle_flush(items, _) do
    items
    |> Enum.join(",")
    |> IO.puts()
  end

  @impl GenBatcher
  def handle_insert(item, acc) do
    size = acc + byte_size(item)
    if size >= 9, do: :flush, else: {:cont, size}
  end

  @impl GenBatcher
  def initial_acc, do: 0
end

Again, items can be inserted into the GenBatcher once it starts:

GenBatcher.insert(MyGenBatcher, "foo")
GenBatcher.insert(MyGenBatcher, "bar")

And, again, a flush operation will be triggered once a flush condition is met:

GenBatcher.insert(MyGenBatcher, "baz")
# Flush operation outputs "foo,bar,baz"

Flushing

By default, flush operations are asynchronous to both the caller and the GenBatcher partition. In practice, this is achieved by making use of the Task.Supervisor module. However, this can be configured with the :blocking_flush? option for GenBatcher.start_link/2. If set to true, the partition process will perform the flush operation instead of delegating to a Task. This can be useful for applying backpressure and ensuring the system isn't completely flooded.

A GenBatcher with an extremely cheap flush operation might see a higher throughput when utilizing blocking flushes but this is generally not the case.

Complex Flush Triggers

In general, a size condition and/or timeout condition is sufficient for most use cases. However, GenBatcher also supports defining custom item-based flush triggers. For example, these callbacks can be used to trigger a flush based on byte size.

In cases where an item-based flush trigger is temporarily delayed (ie GenBatcher.insert_all/3), the c:GenBatcher.handle_insert/2 callback will not be called again until after a flush operation is triggered. This means that the accumulator term is guaranteed to be in a valid state whenever this callback is invoked.

Partitioning

GenBatcher leverages Elixir's PartitionSupervisor in order to support partitioning. All of a GenBatcher's partitions collect items and flush independently.

By default, GenBatcher uses a round-robin partitioner when inserting items. However, the partitioner can be overridden with the :partition_key option for GenBatcher.insert/3 and GenBatcher.insert_all/3, allowing for custom partitioning strategies.

All of a GenBatcher's partitions utilize the same flush conditions. This can occasionally lead to bursts of flush operations being triggered at around the same time. The c:GenBatcher.initial_acc/0 callback can be leveraged to "jitter" item-based flush triggers in order to "desync" flush operations and mitigate this issue. For example, the GenBatcher below enforces an absolute maximum size of 1,000 items but randomly assigns each partition a maximum size between 901 and 1,000 items:

defmodule MyJitteredGenBatcher do
  use GenBatcher

  def start_link(opts \\ []) do
    opts = Keyword.put(opts, :partitions, 5)
    GenBatcher.start_link(__MODULE__, opts)
  end

  @impl GenBatcher
  def handle_flush(items, _) do
    items
    |> Enum.join(",")
    |> IO.puts()
  end

  @impl GenBatcher
  def handle_insert(_, 1), do: :flush
  def handle_insert(_, acc), do: {:cont, acc - 1}

  @impl GenBatcher
  def initial_acc, do: 900 + :rand.uniform(100)
end

Shutdown

As long as a GenBatcher is shutdown gracefully, it's guaranteed to flush all inserted items. The last flush operation for each partition is always performed by the partition itself, regardless of the :blocking_flush? option provided to GenBatcher.start_link/2.

By default, non-blocking flushes are given unlimited time to complete during shutdown to ensure that data is not lost. However, if this behavior is not desirable and data loss is acceptable, this can be managed with the :shutdown option for GenBatcher.start_link/2.

ExBuffer

GenBatcher was created as a conceptual fork of the now-archived ExBuffer package and is intended to supersede it.