filediscovery: support discovering applications from file

Author: joonas-fi

pkg/erdiscovery/filediscovery/filediscovery.go

@@ -0,0 +1,38 @@
+// Discover applications from a JSON "config" file
+package filediscovery
+
+import (
+	"context"
+
+	"github.com/function61/edgerouter/pkg/erconfig"
+	"github.com/function61/edgerouter/pkg/erdiscovery"
+	"github.com/function61/gokit/jsonfile"
+)
+
+const (
+	DefaultFilename = "applications.json"
+)
+
+// file format for discovery file.
+// NOTE: not having applications at top level for extensibility + prepare for `$schema`
+type file struct {
+	Apps []erconfig.Application `json:"apps"`
+}
+
+type fileDiscovery struct {
+	file string
+}
+
+var _ erdiscovery.Reader = (*fileDiscovery)(nil)
+
+func New(file string) erdiscovery.Reader {
+	return &fileDiscovery{file}
+}
+
+func (f *fileDiscovery) ReadApplications(_ context.Context) ([]erconfig.Application, error) {
+	appsFromFile := file{}
+	if err := jsonfile.Read(f.file, &appsFromFile, true); err != nil {
+		return nil, err
+	}
+	return appsFromFile.Apps, nil
+}

pkg/erserver/serve.go

@@ -21,6 +21,7 @@ import (
 	"github.com/function61/edgerouter/pkg/erdiscovery"
 	"github.com/function61/edgerouter/pkg/erdiscovery/dockerdiscovery"
 	"github.com/function61/edgerouter/pkg/erdiscovery/ehdiscovery"
+	"github.com/function61/edgerouter/pkg/erdiscovery/filediscovery"
 	"github.com/function61/edgerouter/pkg/erdiscovery/s3discovery"
 	"github.com/function61/edgerouter/pkg/todoupgradegokit"
 	"github.com/function61/edgerouter/pkg/turbocharger"
@@ -335,6 +336,15 @@ func configureDiscovery(logger *log.Logger) (erdiscovery.Reader, error) {
 		readers = append(readers, ehDiscovery)
 	}
 
+	maybeFromFile, err := newFileDiscoveryIfFileExists(filediscovery.DefaultFilename)
+	if err != nil {
+		return nil, err
+	}
+
+	if maybeFromFile != nil {
+		readers = append(readers, maybeFromFile)
+	}
+
 	return erdiscovery.MultiDiscovery(readers), nil
 }
 

pkg/erserver/utils.go

@@ -5,6 +5,10 @@ import (
 	"net"
 	"net/http"
 	"strings"
+
+	"github.com/function61/edgerouter/pkg/erdiscovery"
+	"github.com/function61/edgerouter/pkg/erdiscovery/filediscovery"
+	"github.com/function61/gokit/fileexists"
 )
 
 // net.SplitHostPort() does not support case where port is not defined...
@@ -48,3 +52,18 @@ func cancelableServer(ctx context.Context, srv *http.Server, listener func() err
 		return err
 	}
 }
+
+// like its `New()` but don't error if file doesn't exist.
+// (still errors if existence check fails)
+func newFileDiscoveryIfFileExists(path string) (erdiscovery.Reader, error) {
+	exists, err := fileexists.Exists(path)
+	if err != nil {
+		return nil, err
+	}
+
+	if exists {
+		return filediscovery.New(path), nil
+	} else {
+		return nil, nil
+	}
+}