From 9c51ac3f8de2a4a38503fcee7eb4e20bd051d1b2 Mon Sep 17 00:00:00 2001
From: Trisha Anand <trisha@appsmith.com>
Date: Tue, 28 Jul 2020 20:06:12 +0530
Subject: [PATCH] =?UTF-8?q?=F0=9F=94=A5=20&=20forget=20asynchronous=20emai?=
 =?UTF-8?q?l=20sending=20(#187)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* Email sending is now an asynchronous function. It follows a 🔥 & forget strategy to make it run asynchronously.
---
 .../server/notifications/EmailSender.java     | 23 ++++++++++++-------
 .../server/services/UserServiceImpl.java      |  2 +-
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/app/server/appsmith-server/src/main/java/com/appsmith/server/notifications/EmailSender.java b/app/server/appsmith-server/src/main/java/com/appsmith/server/notifications/EmailSender.java
index 5bac64dab0..a76503497e 100644
--- a/app/server/appsmith-server/src/main/java/com/appsmith/server/notifications/EmailSender.java
+++ b/app/server/appsmith-server/src/main/java/com/appsmith/server/notifications/EmailSender.java
@@ -9,9 +9,9 @@ import org.springframework.mail.MailException;
 import org.springframework.mail.javamail.JavaMailSender;
 import org.springframework.mail.javamail.MimeMessageHelper;
 import org.springframework.stereotype.Component;
-import org.springframework.util.StringUtils;
 import reactor.core.Exceptions;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import javax.mail.MessagingException;
 import javax.mail.internet.InternetAddress;
@@ -51,7 +51,7 @@ public class EmailSender {
         return matcher.find();
     }
 
-    public Mono<Void> sendMail(String to, String subject, String text, Map<String, String> params) {
+    public Mono<String> sendMail(String to, String subject, String text, Map<String, String> params) {
         return Mono
                 .fromSupplier(() -> {
                     try {
@@ -60,15 +60,22 @@ public class EmailSender {
                         throw Exceptions.propagate(e);
                     }
                 })
-                .flatMap(emailBody -> sendMail(to, subject, emailBody));
-    }
-
-    public Mono<Void> sendMail(String to, String subject, String text) {
-        return Mono.fromRunnable(() -> sendMailSync(to, subject, text));
+                // Sending email is a high cost I/O operation. Schedule the same on non-netty threads
+                // to implement a fire-and-forget strategy.
+                // CAUTION : We may run into scenarios where too many tasks have been created and queued and the master tasks have already exited with success.
+                .doOnNext(emailBody ->
+                        Mono.fromRunnable(() -> {
+                            sendMailSync(to, subject, emailBody);
+                        })
+                        // Scheduling using boundedElastic because the number of active tasks are capped
+                        // and hence not allowing the background threads to grow indefinitely
+                        .subscribeOn(Schedulers.boundedElastic())
+                        .subscribe()
+                );
     }
 
     /**
-     * This function sends an HTML email to the user from the default email address
+     * [Synchronous]This function sends an HTML email to the user from the default email address
      *
      * @param to      Single valid string email address to send to. Multiple addresses doesn't work.
      * @param subject Subject string.
diff --git a/app/server/appsmith-server/src/main/java/com/appsmith/server/services/UserServiceImpl.java b/app/server/appsmith-server/src/main/java/com/appsmith/server/services/UserServiceImpl.java
index 58ab45e1e6..c30a7de09c 100644
--- a/app/server/appsmith-server/src/main/java/com/appsmith/server/services/UserServiceImpl.java
+++ b/app/server/appsmith-server/src/main/java/com/appsmith/server/services/UserServiceImpl.java
@@ -641,7 +641,7 @@ public class UserServiceImpl extends BaseService<UserRepository, User, String> i
                     }
                     params.put("inviter_org_name", updatedOrg.getName());
 
-                    Mono<Void> emailMono;
+                    Mono<String> emailMono;
                     if (userExisted.get()) {
                         // If the user already existed, just send an email informing that the user has been added
                         // to a new organization