http4s / blaze   0.23.15

Apache License 2.0 GitHub

Blazing fast NIO microframework and Http Parser

Scala versions: 3.x 2.13 2.12 2.11 2.10

blaze Continuous Integration Maven Central

blaze is a Scala library for building asynchronous pipelines, with a focus on network IO. Blaze is the original server and client backend for the http4s project.


blaze was conceived as a standalone project, but has seen the most use through http4s. This repository houses both the standalone modules and the http4s backend.

Standalone modules

These have minimal external dependencies. They represent the original vision of the blaze project and are less functional in nature.

http4s backend

These modules were housed in the http4s repository through v0.23.11. They are now maintained alongside the blaze core. These modules depend on http4s-core and blaze-http.


  • Completely asynchronous, pull-based design
  • Type safe pipeline composition
  • NIO1 and NIO2 socket engines
  • SSL integration
  • HTTP/1.x and HTTP/2 codecs
  • Basic HTTP/1.x server and client
  • Basic HTTP/2 server

blaze's goals are

  • Provides an IO framework to support the core http4s project
  • Speed is the first priority
  • Asynchronicity underpinned by Scala Future
  • Java NIO backend for fast/asynchronous network and file I/O
  • A framework for rapid development of fast, asynchronous pipelines using Scala

What blaze is not

  • Purely functional. Functional is preferred except when in conflict with the first goal.

Asynchronous network I/O

blaze was built with NIO2 features in mind. NIO2 offers a host of new features which are offered in a unified way. blaze offers TCP socket server support for both NIO1 and NIO2, depending on your preference. NIO1 tends to be the better performing technology for raw sockets.

Getting started

For a quick start, look in the examples project for some simple examples of HTTP, WebSocket, and a simple EchoServer.

Important blaze behaviors

  • By default, read and write requests should be serialized either manually, or with a SerializerStage.

Data flow

Useful asynchronous processing is done through the construction of a pipeline. A pipeline is composed of individual stages that perform operations such as decoding/encoding, SSL encryption, buffering, etc. The pipeline is constructed in a type safe manner using the PipelineBuilder types.

blaze is a pull-based framework as opposed to the reactor and actor models found in other systems. Therefore, data flow is typically controlled by the tail of the pipeline. Data is written and requested using the Future type found in the Scala standard library.

blaze doesn't make any assumptions as to whether any stage is threadsafe, so as noted above, either manually ensure a write is not performed before the previous write has completed and the same for reads, or use a SerializerStage or Serializer mixin to automatically sequence reads and writes for you.


The Contributors, as calculated by GitHub. Contributions and comments in the form of issues or pull requests are greatly appreciated.


Several concepts are inspired by the Jetty and Netty projects.


Blaze is licensed under the Apache 2 license. See the LICENSE file in the root project directory for more information.