February 15, 2019

752 words 4 mins read



Fast event-loop networking for Go

repo name tidwall/evio
repo link https://github.com/tidwall/evio
language Go
size (curr.) 440 kB
stars (curr.) 3981
created 2017-07-04
license MIT License

evio is an event loop networking framework that is fast and small. It makes direct epoll and kqueue syscalls rather than using the standard Go net package, and works in a similar manner as libuv and libevent.

The goal of this project is to create a server framework for Go that performs on par with Redis and Haproxy for packet handling. It was built to be the foundation for Tile38 and a future L7 proxy for Go.

Please note: Evio should not be considered as a drop-in replacement for the standard Go net or net/http packages.


Getting Started


To start using evio, install Go and run go get:

$ go get -u github.com/tidwall/evio

This will retrieve the library.


Starting a server is easy with evio. Just set up your events and pass them to the Serve function along with the binding address(es). Each connections is represented as an evio.Conn object that is passed to various events to differentiate the clients. At any point you can close a client or shutdown the server by return a Close or Shutdown action from an event.

Example echo server that binds to port 5000:

package main

import "github.com/tidwall/evio"

func main() {
	var events evio.Events
	events.Data = func(c evio.Conn, in []byte) (out []byte, action evio.Action) {
		out = in
	if err := evio.Serve(events, "tcp://localhost:5000"); err != nil {

Here the only event being used is Data, which fires when the server receives input data from a client. The exact same input data is then passed through the output return value, which is then sent back to the client.

Connect to the echo server:

$ telnet localhost 5000


The event type has a bunch of handy events:

  • Serving fires when the server is ready to accept new connections.
  • Opened fires when a connection has opened.
  • Closed fires when a connection has closed.
  • Detach fires when a connection has been detached using the Detach return action.
  • Data fires when the server receives new data from a connection.
  • Tick fires immediately after the server starts and will fire again after a specified interval.

Multiple addresses

A server can bind to multiple addresses and share the same event loop.

evio.Serve(events, "tcp://", "unix://socket")


The Tick event fires ticks at a specified interval. The first tick fires immediately after the Serving events.

events.Tick = func() (delay time.Duration, action Action){
	delay = time.Second


The Serve function can bind to UDP addresses.

  • All incoming and outgoing packets are not buffered and sent individually.
  • The Opened and Closed events are not availble for UDP sockets, only the Data event.


The events.NumLoops options sets the number of loops to use for the server. A value greater than 1 will effectively make the server multithreaded for multi-core machines. Which means you must take care when synchonizing memory between event callbacks. Setting to 0 or 1 will run the server as single-threaded. Setting to -1 will automatically assign this value equal to runtime.NumProcs().

Load balancing

The events.LoadBalance options sets the load balancing method. Load balancing is always a best effort to attempt to distribute the incoming connections between multiple loops. This option is only available when events.NumLoops is set.

  • Random requests that connections are randomly distributed.
  • RoundRobin requests that connections are distributed to a loop in a round-robin fashion.
  • LeastConnections assigns the next accepted connection to the loop with the least number of active connections.


Servers can utilize the SO_REUSEPORT option which allows multiple sockets on the same host to bind to the same port.

Just provide reuseport=true to an address:

evio.Serve(events, "tcp://"))

More examples

Please check out the examples subdirectory for a simplified redis clone, an echo server, and a very basic http server.

To run an example:

$ go run examples/http-server/main.go
$ go run examples/redis-server/main.go
$ go run examples/echo-server/main.go



These benchmarks were run on an ec2 c4.xlarge instance in single-threaded mode (GOMAXPROC=1) over Ipv4 localhost. Check out benchmarks for more info.


Josh Baker @tidwall


evio source code is available under the MIT License.

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