On install, the plugins are loaded and started again. This ensures that we don't have to restart the server every time a new plugin is installed. Earlier the plugins were loaded and started only during the boot

app/server/appsmith-server/pom.xml

@@ -116,6 +116,11 @@
             <artifactId>guava</artifactId>
             <version>28.1-jre</version>
         </dependency>
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>2.6</version>
+        </dependency>
         <!-- Plugin dependencies -->
         <dependency>
             <groupId>org.pf4j</groupId>

app/server/appsmith-server/src/main/java/com/appsmith/server/exceptions/AppsmithError.java

@@ -16,7 +16,8 @@ public enum AppsmithError {
     PAGE_DOESNT_BELONG_TO_USER_ORGANIZATION(400, 4006, "Page {0} does not belong to the current user {1} organization."),
     UNAUTHORIZED_DOMAIN(401, 4001, "Invalid email domain provided. Please sign in with a valid work email ID"),
     INTERNAL_SERVER_ERROR(500, 5000, "Internal server error while processing request"),
-    REPOSITORY_SAVE_FAILED(500, 5001, "Repository save failed.");
+    REPOSITORY_SAVE_FAILED(500, 5001, "Repository save failed."),
+    PLUGIN_INSTALLATION_FAILED_DOWNLOAD_ERROR(500, 5002, "Due to error in downloading the plugin from remote repository, plugin installation has failed. Check the jar location and try again.");
 
     private Integer httpErrorCode;
     private Integer appErrorCode;

app/server/appsmith-server/src/main/java/com/appsmith/server/services/PluginServiceImpl.java

@@ -12,6 +12,8 @@ import com.appsmith.server.exceptions.AppsmithException;
 import com.appsmith.server.repositories.PluginRepository;
 import com.segment.analytics.Analytics;
 import lombok.extern.slf4j.Slf4j;
+import org.apache.commons.io.FileUtils;
+import org.pf4j.PluginManager;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.context.ApplicationContext;
 import org.springframework.data.mongodb.core.ReactiveMongoTemplate;
@@ -21,6 +23,9 @@ import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
 
 import javax.validation.Validator;
+import java.io.File;
+import java.net.URL;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
 
@@ -31,6 +36,10 @@ public class PluginServiceImpl extends BaseService<PluginRepository, Plugin, Str
     private final PluginRepository pluginRepository;
     private final ApplicationContext applicationContext;
     private final OrganizationService organizationService;
+    private final PluginManager pluginManager;
+
+    private static final int CONNECTION_TIMEOUT = 1000;
+    private static final int READ_TIMEOUT = 1000;
 
     @Autowired
     public PluginServiceImpl(Scheduler scheduler,
@@ -41,11 +50,12 @@ public class PluginServiceImpl extends BaseService<PluginRepository, Plugin, Str
                              ApplicationContext applicationContext,
                              OrganizationService organizationService,
                              Analytics analytics,
-                             SessionUserService sessionUserService) {
+                             SessionUserService sessionUserService, PluginManager pluginManager) {
         super(scheduler, validator, mongoConverter, reactiveMongoTemplate, repository, analytics, sessionUserService);
         this.applicationContext = applicationContext;
         pluginRepository = repository;
         this.organizationService = organizationService;
+        this.pluginManager = pluginManager;
     }
 
     public OldPluginExecutor getPluginExecutor(PluginType pluginType, String className) {
@@ -113,17 +123,51 @@ public class PluginServiceImpl extends BaseService<PluginRepository, Plugin, Str
 
         //Find the organization using id and plugin id -> This is to find if the organization already has the plugin installed
         Mono<User> userMono = super.sessionUserService.getCurrentUser();
-        Mono<Organization> organizationMono = userMono.flatMap(user ->
+        Mono<Organization> pluginInOrganizationMono = userMono.flatMap(user ->
                 organizationService.findByIdAndPluginsPluginId(user.getOrganizationId(), pluginDTO.getPluginId()));
 
 
         //If plugin is already present for the organization, just return the organization, else install and return organization
-        return organizationMono
+        return pluginInOrganizationMono
                 .switchIfEmpty(Mono.defer(() -> {
                     //If the plugin is not found in the organization, its not installed already. Install now.
-                    return userMono
+                    return pluginRepository
+                            .findById(pluginDTO.getPluginId())
+                            .map(plugin -> {
+                                if (plugin.getJarLocation() == null) {
+                                    // Plugin jar location not set. Must be local
+                                    /** TODO
+                                     * In future throw an error if jar location is not set
+                                     */
+                                    return plugin;
+                                }
+
+                                String baseUrl = "../dist/plugins/";
+                                String pluginJar = plugin.getName() + ".jar";
+
+                                // Else download the plugin jar to the local
+                                try {
+                                    FileUtils.copyURLToFile(
+                                            new URL(plugin.getJarLocation()),
+                                            new File(baseUrl, pluginJar),
+                                            CONNECTION_TIMEOUT,
+                                            READ_TIMEOUT);
+                                } catch (Exception e) {
+                                    log.error("",e);
+                                    return Mono.error(new AppsmithException(AppsmithError.PLUGIN_INSTALLATION_FAILED_DOWNLOAD_ERROR));
+                                }
+
+                                //Now that the plugin has been downloaded, load and restart the plugin
+                                pluginManager.loadPlugin(Path.of(baseUrl + pluginJar));
+                                pluginManager.startPlugins();
+
+                                return plugin;
+                            })
+                            //Now that the plugin jar has been successfully downloaded, go on and add the plugin to the organization
+                            .then(userMono)
                             .flatMap(user -> organizationService.findById(user.getOrganizationId()))
                             .map(organization -> {
+
                                 List<OrganizationPlugin> organizationPluginList = organization.getPlugins();
                                 if (organizationPluginList == null) {
                                     organizationPluginList = new ArrayList<OrganizationPlugin>();
@@ -134,6 +178,8 @@ public class PluginServiceImpl extends BaseService<PluginRepository, Plugin, Str
                                 organizationPlugin.setStatus(status);
                                 organizationPluginList.add(organizationPlugin);
                                 organization.setPlugins(organizationPluginList);
+
+                                //return the organization
                                 return organization;
                             })
                             .flatMap(organizationService::save);