alerts-server: Create AvailabilityFilter

Now we can reject incoming requests when the application is shutting down.

alerts-server/app/com/alexitc/coinalerts/filters/AvailabilityFilter.scala

@@ -0,0 +1,32 @@
+package com.alexitc.coinalerts.filters
+
+import javax.inject.Inject
+
+import akka.stream.Materializer
+import com.alexitc.coinalerts.core.ShutdownHandler
+import org.slf4j.LoggerFactory
+import play.api.mvc.{Filter, RequestHeader, Result, Results}
+
+import scala.concurrent.Future
+
+/**
+ * Allow us to reject incoming requests when the application is shutting down.
+ */
+class AvailabilityFilter @Inject() (
+    shutdownHandler: ShutdownHandler,
+    implicit val mat: Materializer)
+    extends Filter {
+
+  private val logger = LoggerFactory.getLogger(this.getClass)
+
+  def apply(nextFilter: RequestHeader => Future[Result])
+      (requestHeader: RequestHeader): Future[Result] = {
+
+    if (shutdownHandler.isShuttingDown) {
+      logger.info("Rejecting request because we are shutting down")
+      Future.successful(Results.ServiceUnavailable)
+    } else {
+      nextFilter(requestHeader)
+    }
+  }
+}