Adds extension points for runner pools in load-balanced mode (Patch 1/2) (#851)

* adds extension points for runner pools in load-balanced mode

* adds error to return values in RunnerPool and Runner interfaces

api/models/runner_pool.go

@@ -0,0 +1,31 @@
+package models
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"time"
+)
+
+// RunnerPool is the abstraction for getting an ordered list of runners to try for a call
+type RunnerPool interface {
+	Runners(call RunnerCall) ([]Runner, error)
+	Shutdown() error
+}
+
+// Runner is the interface to invoke the execution of a function call on a specific runner
+type Runner interface {
+	TryExec(ctx context.Context, call RunnerCall) (bool, error)
+	Close() error
+	Address() string
+}
+
+// RunnerCall provides access to the necessary details of request in order for it to be
+// processed by a RunnerPool
+type RunnerCall interface {
+	SlotDeadline() time.Time
+	Request() *http.Request
+	ResponseWriter() io.Writer
+	StdErr() io.ReadWriteCloser
+	Model() *Call
+}

api/server/server.go

@@ -113,6 +113,7 @@ type Server struct {
 	rootMiddlewares []fnext.Middleware
 	apiMiddlewares  []fnext.Middleware
 	promExporter    *prometheus.Exporter
+	runnerPool      models.RunnerPool
 	// Extensions can append to this list of contexts so that cancellations are properly handled.
 	extraCtxs []context.Context
 }
@@ -842,6 +843,12 @@ func (s *Server) Datastore() models.Datastore {
 	return s.datastore
 }
 
+// WithRunnerPool provides an extension point for overriding
+// the default runner pool implementation when running in load-balanced mode
+func (s *Server) WithRunnerPool(runnerPool models.RunnerPool) {
+	s.runnerPool = runnerPool
+}
+
 // returns the unescaped ?cursor and ?perPage values
 // pageParams clamps 0 < ?perPage <= 100 and defaults to 30 if 0
 // ignores parsing errors and falls back to defaults.

fnext/setup.go

@@ -38,6 +38,9 @@ type ExtServer interface {
 	// AddRouteEndpoint adds an endpoints to /v1/apps/:app/routes/:route/x
 	AddRouteEndpointFunc(method, path string, handler func(w http.ResponseWriter, r *http.Request, app *models.App, route *models.Route))
 
+	// WithRunnerPool overrides the default runner pool implementation when running in load-balanced mode
+	WithRunnerPool(runnerPool models.RunnerPool)
+
 	// Datastore returns the Datastore Fn is using
 	Datastore() models.Datastore
 }