Go Functions

Writing Go functions with fission

With Go plugin mechanism, fission supports Go as one of function languages.

In this usage guide we’ll cover how to use this environment, write functions, and work with dependencies.

Before you start

We’ll assume you have Fission and Kubernetes setup. If not, head over to the install guide. Verify your Fission setup with:

fission version

Add the Go environment to your cluster

Unlike Python, Go is a compiled language that means we need to compile source code before running it. Fortunately, builder manager within fission does all this hard work automatically when a Go function/package is created. The Go builder will convert a source package into a deployable package.

Due to the Go plugin mechanism, the Go plugin can only be loaded by the server with the exact same Go version. Please use the fission release version as image tag instead of latest when adding a Go environment, so that you won’t experience the compatibility issue once we bump up the Go version. (Remember, the image tag of runtime and builder should always be the same.)

$ fission environment create --name go --image fission/go-env:<release-version> --builder fission/go-builder:<release-version> --version 3

# Example
$ fission environment create --name go --image fission/go-env-1.16 --builder fission/go-builder-1.16

You can find all images and image tags at following table.

Go environment image list

Go VersionImageBuilder Image
1.14fission/go-env-1.14fission/go-builder-1.14
1.15fission/go-env-1.15fission/go-builder-1.15
1.16fission/go-env-1.16fission/go-builder-1.16

Write a simple function in Go

Here is a hello world example (hw.go) in Go:

// Due to go plugin mechanism,
// the package of function handler must be main package
package main

import (
    "net/http"
)

// Handler is the entry point for this fission function
func Handler(w http.ResponseWriter, r *http.Request) {
    msg := "Hello, world!\n"
    w.Write([]byte(msg))
}

The entrypoint of function let go server to know how to load-in plugin file correctly. Here, our first hello world function’s entrypoint is simply the name of function: Handler.

# Create golang env with builder image to build go plugin
$ fission fn create --name helloworld --env go --src hw.go --entrypoint Handler

Before accessing function, need to ensure deploy package of function is in succeeded state.

fission pkg info --name <pkg-name>
For archive of package with size larger than 256 KB, the package is uploaded to storage service. Double check fission-storage-pvc is properly setup if pkg BUILD_STATUS is not succeeded.

Now, let’s test our first Go function with test command

fission fn test --name <function-name>
See here for how to setup different trigger for Go function.

HTTP requests and HTTP responses

From the sample above we know that go server passes HTTP Request and ResponseWriter to user function for further processing.

func Handler(w http.ResponseWriter, r *http.Request) {
    ...
}

Following we will go through steps for how to accessing/controlling the requests/responses.

Accessing HTTP Requests

Headers
package main

import (
    "fmt"
    "net/http"
    "net/textproto"
)

func Handler(w http.ResponseWriter, r *http.Request) {

    // recommend
    v1 := r.Header.Get("HEADER_KEY_1")

    // need to convert string cases manually
    key := textproto.CanonicalMIMEHeaderKey("HEADER_KEY_2")
    // v2 type is []string
    v2, _ := r.Header[key]

    w.Write([]byte(fmt.Sprintf("v1: %s, v2: %v", v1, v2)))
}

Let’s create a HTTP trigger for the function we created above.

fission httptrigger create --method GET --url "/<url>" --function <fn-namne>

$ curl -X GET http://$FISSION_ROUTER/<url> -H 'HEADER_KEY_1: foo' -H 'HEADER_KEY_2: bar'
v1: foo, v2: [bar]

One thing worth to notice is all header key will be converted to the canonical format of the MIME header key. You can access to header value by calling request.Header.Get(). However, if you prefer to access the value by map index you must convert key format with textproto.CanonicalMIMEHeaderKey().

Query string
package main

import (
    "net/http"
)

func Handler(w http.ResponseWriter, r *http.Request) {
    v := r.URL.Query().Get("key-name")
    w.Write([]byte(v))
}

$ curl -X GET http://$FISSION_ROUTER/<url>?key-name=123
123
Request Body
PLAIN TEXT
package main

import (
    "io/ioutil"
    "net/http"
)

func Handler(w http.ResponseWriter, r *http.Request) {
    reqBody, err := ioutil.ReadAll(r.Body)
    if err != nil {
        w.WriteHeader(http.StatusBadRequest)
        w.Write([]byte(err.Error()))
    }

    body := string(reqBody)

    w.Write([]byte(body))
}

$ curl -X POST http://$FISSION_ROUTER/<url> -d foobar
foobar
JSON
package main

import (
    "encoding/json"
    "net/http"
    "io/ioutil"
)

type msg struct {
    Content string `json:"content"`
}

func Handler(w http.ResponseWriter, r *http.Request) {
    reqBody, err := ioutil.ReadAll(r.Body)
    if err != nil {
        w.WriteHeader(http.StatusBadRequest)
        w.Write([]byte(err.Error()))
    }

    m := msg{}
    err = json.Unmarshal(reqBody, &m)
    if err != nil {
        w.WriteHeader(http.StatusBadRequest)
        w.Write([]byte(err.Error()))
    }

    respBody, err := json.Marshal(m)
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        w.Write([]byte(err.Error()))
    }

    w.Write([]byte(respBody))
}

curl -X POST http://$FISSION_ROUTER/<url> -d '{"content": "foobar"}'
{"content":"foobar"}

Controlling HTTP Responses

Setting Response Headers
package main

import (
    "net/http"
)

func setCORS(w http.ResponseWriter) http.ResponseWriter {
    w.Header().Set("Access-Control-Allow-Origin", "*")
    w.Header().Set("Access-Control-Allow-Headers", "*")
    w.Header().Set("Access-Control-Allow-Credentials", "true")
    w.Header().Set("Access-Control-Allow-Methods", "*")
    w.Header().Set("Access-Control-Expose-Headers", "*")
    return w
}

func Handler(w http.ResponseWriter, r *http.Request) {
    w = setCORS(w)
    w.WriteHeader(http.StatusOK)
}

$ curl -i -X GET "http://$FISSION_ROUTER/<url>"
HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: *
Access-Control-Allow-Methods: *
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: *
Content-Length: 0
Content-Type: text/plain; charset=utf-8
Date: Sat, 27 Oct 2018 15:00:14 GMT
Setting Status Codes
Always write response status code before writing response body! Otherwise, the client may receive unexpected status code.

You can set response status code by calling function writer.WriteHeader(). However, one thing wroth to notice is if a function writes response body before status code, the client will receive HTTP 200 OK no matter what actual status code is.

package main

import (
    "net/http"
)

func Handler(w http.ResponseWriter, r *http.Request) {
    // DONT
    // w.Write([]byte("foobar"))
    // w.WriteHeader(http.StatusBadRequest)

    // DO
    http.Error(w, "dummy error", http.StatusBadRequest)

    // or DO
    // w.WriteHeader(http.StatusBadRequest)
    // w.Write([]byte("foobar"))
}

Working with dependencies

See full image list here

Go module support

Please use image version equal or after fission/go-env-1.16.

Initialize your project

$ go mod init "<module>"

# Example
$ go mod init "github.com/fission/fission/examples/go/go-module-example"

Follow the official guide to add project dependencies. To clean and verify the dependencies you can:

go mod tidy
go mod verify

Archive and create package as usual

$ zip -r go.zip .
    adding: go.mod (deflated 26%)
    adding: go.sum (deflated 1%)
    adding: README.md (deflated 37%)
    adding: main.go (deflated 30%)

$ fission pkg create --env go --src go.zip

Add dependencies to vendor directory

This part is for the old Go environment that doesn't support Go Moudle

Unlike pip for Python has been widely adopted by community, there are various dependency management tools for Go like gb, dep and glide. Hence fission Go builder image doesn’t contain a default tool for downloading dependencies during build processes.

In order to support 3rd party dependencies, users need to put all necessary packages to vendor directory and archive it into source archive.

Following I will use glide to demonstrate how to add dependencies to source archive.

$ mkdir example
$ pushd example
$ glide init --non-interactive
$ glide get "github.com/golang/example/stringutil"
$ curl https://raw.githubusercontent.com/fission/examples/main/go/vendor-example/main.go > main.go
$ zip -r example.zip .
$ fission fn create --name foobar --src example.zip --entrypoint Handler
# wait for package build process
$ fission fn test --name foobar

Custom builds

If no custom build script included in source archive, by default builder manager will execute the build.sh in go builder image. In this part, we will go through how to run custom build script during the build process.

Before we write our own script, we need to dive into the original build script to see what’s useful inside.

#!/bin/sh

set -eux

srcDir=${GOPATH}/src/$(basename ${SRC_PKG})

trap "rm -rf ${srcDir}" EXIT

if [ -d ${SRC_PKG} ]
then
    echo "Building in directory ${srcDir}"
    ln -sf ${SRC_PKG} ${srcDir}
elif [ -f ${SRC_PKG} ]
then
    echo "Building file ${SRC_PKG} in ${srcDir}"
    mkdir -p ${srcDir}
    cp ${SRC_PKG} ${srcDir}/function.go
fi

cd ${srcDir}
go build -buildmode=plugin -i -o ${DEPLOY_PKG} .

There are environment variables as shown above:

  1. $SRC_PKG: The path to the source code directory/file.
  2. $DEPLOY_PKG: The path to the deploy directory/executable binary.

The steps of build script are:

  1. Check the $SRC_PKG is a file or directory
  2. Copy/Link source code to $GOPATH
  3. Change location to $SRC_PKG, build source code into plugin file then output to $DEPLOY_PKG

For example, we want to add git commit SHA or build timestamp to go binaries like following.

package main

import (
    "fmt"
    "net/http"
)

var (
    Timestamp string
)

// Handler is the entry point for this fission function
func Handler(w http.ResponseWriter, r *http.Request) {
    msg := fmt.Sprintf("Build timestamp: %v", Timestamp)
    w.Write([]byte(msg))
}

To set build timestamp we can set it through ldflags during build process. But it’s not supported with original build script.

Since we understand how a build script (customBuild.sh) works, let’s try to modify it a little bit.

#!/bin/sh

set -eux

srcDir=${GOPATH}/src/$(basename ${SRC_PKG})

trap "rm -rf ${srcDir}" EXIT

if [ -d ${SRC_PKG} ]
then
    echo "Building in directory ${srcDir}"
    ln -sf ${SRC_PKG} ${srcDir}
elif [ -f ${SRC_PKG} ]
then
    echo "Building file ${SRC_PKG} in ${srcDir}"
    mkdir -p ${srcDir}
    cp ${SRC_PKG} ${srcDir}/function.go
fi


pkgpath=$(basename ${SRC_PKG})
timestamp=$(date +%s)

cd ${srcDir}
go build -ldflags "-X ${pkgpath}.Timestamp=$timestamp" -buildmode=plugin -i -o ${DEPLOY_PKG} .

At this moment, the directory structure would be like:

.
├── customBuild.sh
└── main.go

Archive source package.

chmod +x customBuild.sh
zip -r example.zip .

After editing custom build script, next step is to assign a build command for builder to know what command to run with.

$ fission fn create --name foobar --env go --src example.zip --entrypoint Handler --buildcmd "./customBuild.sh"
$ fission fn test --name foobar
Build timestamp: 1540566887

Modifying/Rebuild the environment images

Refer to our Go environment guide for Fission to learn about rebuilding environment images.

Resource usage

By default, a function can use all resource on node it runs on, however, other functions will be affected once a function takes too many resources. To prevent this, Fission allows users to specify a resource limitation for function.

Following are flags to manage function resource usage limitation.

--mincpu value                         Minimum CPU to be assigned to pod (In millicore, minimum 1)
--maxcpu value                         Maximum CPU to be assigned to pod (In millicore, minimum 1)
--minmemory value                      Minimum memory to be assigned to pod (In megabyte)
--maxmemory value                      Maximum memory to be assigned to pod (In megabyte)

And you can specify them when creating/updating a function.

$ fission fn create --name g1 --env go --src example.zip --entrypoint Handler \
                    --mincpu 20 --maxcpu 100 --minmemory 128 --maxmemory 256

So what’s the reasonable resource setting for a function? It really depends on what type of your function is.

Here’s a tip, use kubectl top to get actual resource consumption of pod when doing benchmarking. Then you will know what’s the best setting for a Go function.

$ kubectl -n fission-function top pod -l functionName=g1
NAME                                                              CPU(cores)   MEMORY(bytes)
go-1ef65ed0-d5eb-11e8-91e8-080027114863-9x7y4xmh-569bfdd9bhmlrf   112m           6Mi
Last modified April 21, 2022: Examples link update (#174) (14997b6)