Building SaaS style multi-tenant web app with Spring Boot 2 and Spring Security 5 - Part 1

Software as a Service or SaaS has been around for quite some time now.  But most of the time, developers are building single tenant applications as per requirements.  These applications have just one database and one web server at their core.

Purpose of this blog post

I wanted a solution where multi-tenancy is achieved by having a database per tenant and all user information (username, password, etc) for authentication and authorization stored in a user table in the respective tenant databases. It meant that not only did I need a multi-tenant application, but also a secure application like any other web application secured by Spring Security.

I know how to use Spring Security to secure a web application and how to use Hibernate to connect to a database. The requirement further dictated that all users belonging to a tenant be stored in the tenant database and not a separate or central database. This would allow for complete data isolation for each tenant.

So I did what most people do - I googled :)  I found many blog posts and articles published on multi-tenancy and web application security, but could not found a single solution or example implementation which addresses database per tenant approach along with security.  Maybe it is too trivial of a topic and hence no blogs or articles ;-)

So I just built my own!


The solution I describe below might sound a little complicated but if you are familiar with the basics of Spring Security and Hibernate, then its quite simple, albeit a bit tedious.

I have broken up this blog post into two parts as the code to set up multi-tenancy and spring security is quite a lot. I don't want you to keep scrolling down to finish reading the blog post. :) 

What is used to build the example in this blog post?

Here, I am going to share how I built a SaaS style web app using the latest Spring Boot 2, Spring JPA with Hibernate as the JPA provider to connect to MySQL and Spring Security 5 to secure the web aplication.

What is Multi-Tenancy?

A multi-tenant application is where a tenant (i.e. users in a company) feels that the application has been created and deployed for them. In reality, there are many such tenants and they too are using the same application but get a feel that its built just for them. Typical examples are online applications for time-sheet management, project management, Scrum/Agile Sprint management, etc etc.

A more technical explanation of multi-tenancy

SaaS applications are multi-tenant applications which require tenant data isolation in the database layer. There are different approaches for achieving this data isolation.
The most widely used approaches are

• Schema per tenant - all tenants have their own schema but these schema live in the same database 
• Discriminator per tenant - data for all tenants is in the same schema in the database but is distinguished by a tenant discriminator column
• Database per tenant - each tenant has a separate database

Which multi-tenancy approach is better?

There are many factors to consider before choosing a data isolation approach as mentioned above. We will not go into depth of each but focus mainly on building a multi-tenant application using the latest Spring Boot 2 framework along with JPA, Hibernate as the JPA provider and Spring Security 5.

Multi-tenancy using database per tenant approach

In many applications, especially in financial and medical domains, there is a strict need for absolute data isolation, security and privacy in multi-tenant applications. As far as my experience,  knowledge and research on this topic goes, multi-tenancy using database per tenant is the best for data isolation and security. I may be wrong and your feedback is welcome.

How does the SaaS application work?

We will build a database per tenant multi-tenant application secured by Spring Security.
In most SaaS applications, there is an entry point where a user belonging to a tenant enters the

• tenant name (company name in most cases or some other identifier)
• username
• password

Step 1 - Process the login form with the extra 'tenant' field

• The login form will present the tenant name, username and password to Spring Security for authentication.  
• The Spring Security UsernamePasswordAuthenticationFilter filter intercepts the login form's request to the server. This filter needs to be extended so that the extra field 'tenant' can be extracted. 
• This tenant field is the tenant database identifier and is required by Hibernate to identify the tenant database to connect to.
• Once the tenant field is extracted by the security filter, it needs to be set in memory so that after the security filter is finished doing its job, this tenant identifier is available to the Hibernate code to connect to the correct tenant database. 
• To make this happen, the tenant field is stored in a ThreadLocal

Step 2 - Compare the login form values with existing user in the database

• Now that the username, password and tenant values are available to Spring Security, the AbstractUserDetailsAuthenticationProvider will use the retrieveUser() method to call upon the UserDetailsService to get the user details from the database. 
• If the credentials from the login form match those of the result returned by the tenant database, then the authentication is successful and Spring Security will now log the user into the application and create a secure session for the user for further access and interaction with the protected resources in the application. 

Step 3 - Select the correct tenant database

• Step 3 actually happens along side Step 2. 
• It means that Spring Security does its job of preparing user credentials for comparison with existing users in the database. 
• And Hibernate does its job of selecting the correct tenant database to provide such a user record to Spring Security for comparison. 

How does Step 3 work?

• In Step 2 above, Spring Security queries the database for a user based on the login form username and tenant field. So how does Hibernate know to get the user from the correct tenant database. In Step 1 note that the tenant field is stored in a ThreadLocal for later access. 

• Hibernate provides CurrentTenantIdentifierResolver for resolving or identifying the correct tenant id. The tenant id is extracted from the ThreadLocal storage.

• Hibernate also provides AbstractDataSourceBasedMultiTenantConnectionProviderImpl (also see DataSourceBasedMultiTenantConnectionProviderImpl) whose job is to provide the datasource corresponding to the tenant identifier.

How Spring Security and Hibernate process the login form

How Spring Security and Hibernate process the login form

Technology Stack

I am assuming that you are comfortable with Spring MVC, Spring Boot, JPA, Hibernate and Spring Security. The example application was built using -

• Spring Boot 2 (2.0.1.RELEASE)
• Spring Security 5 
• MySQL 
• Spring JPA (Spring Boot 2 provides latest Hibernate 5 as the JPA provider)
• Java 8
• Spring STS IDE

Understanding the code for the application

I will describe the significant code in this section. Rest of the code is available in the GitHub repository for this example application.  The link is in the Resources section at the end. If you want to just use the code and run it, then skip to the resources section at the end of part 2 for the GitHub link.  

Project Layout

This is how my project layout is in Spring STS

File: pom.xml

Using the Spring Initialzr create a project with Spring Web, Security, Thymeleaf, MySQL and JPA. It should generate a pom.xml as follows:

Note that I have added a few extra dependencies - 

• Cyber NekoHTML for setting ThymeLeaf in legacy mode. Why? Thymeleaf complains if there are no proper closing tags but HTML5 allows some attributes to not have closing tags. So the fix is to use this dependency. A detailed problem statement and its solution is on this stackoverflow thread. 
• Apache commons-lang-3 for StringUtils
• spring-boot-configuration-processor for helping with reading config properties. When you have annotations like @ConfigurationProperties, the STS IDE itself suggests that this dependency should be added.

This is the final pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.example</groupId>
 <artifactId>multitenancy-mysql</artifactId>
 <version>1.0.1</version>
 <packaging>jar</packaging>
 <name>multitenancy-mysql</name>
 <description>Spring Boot JPA Hibernate with Per Database Multi-Tenancy with Spring Security</description>
 <contributors>
  <contributor>
   <name>Sunit Katkar</name>
   <email>sunitkatkar@gmail.com</email>
   <url>https://sunitkatkar.blogspot.com/</url>
  </contributor>
 </contributors>

 <parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.0.1.RELEASE</version>
  <relativePath /> <!-- lookup parent from repository -->
 </parent>

 <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
  <java.version>1.8</java.version>
 </properties>

 <dependencies>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-data-jpa</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-security</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-thymeleaf</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-configuration-processor</artifactId>
   <optional>true</optional>
  </dependency>
  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <scope>runtime</scope>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>org.springframework.security</groupId>
   <artifactId>spring-security-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>net.sourceforge.nekohtml</groupId>
   <artifactId>nekohtml</artifactId>
   <version>1.9.21</version>
  </dependency>
  <!-- https://mvnrepository.com/artifact/org.apache.commons/commons-lang3 -->
  <dependency>
   <groupId>org.apache.commons</groupId>
   <artifactId>commons-lang3</artifactId>
   <version>3.7</version>
  </dependency>
 </dependencies>

 <build>
  <plugins>
   <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
   </plugin>
  </plugins>
 </build>
</project>

File: application.yml

I have worked mostly with application.properties files, but found that the YAML format is easier to use when defining multiple data sources. In this example application, I have defined two data sources multitenancy.mtapp.dataSources which are pointing to 2 MySQL databases running on a MySQL instance.

spring:
  thymeleaf:
    cache: false
    mode: LEGACYHTML5
  jpa:
    database: mysql
    show-sql: true 
    generate-ddl: false
    hibernate: 
      ddl-auto: none  

multitenancy: 
  mtapp: 
    dataSources: 
      -
        tenantId: tenant_1 
        url: jdbc:mysql://localhost:3306/dbtenant1?useSSL=false 
        username: tenant1 
        password: admin123 
        driverClassName: com.mysql.jdbc.Driver 
      -
        tenantId: tenant_2
        url: jdbc:mysql://localhost:3306/dbtenant2?useSSL=false
        username: tenant1
        password: admin123
        driverClassName: com.mysql.jdbc.Driver        

Note that spring.thymeleaf.mode is set to LEGACYHTML5. The Cyber NekoHTML dependency will then take care of not being so strict about parsing HTML5 and ThymeLeaf will not complain.

File: CustomUserDetails.java

Note that Spring Security provides the interface UserDetails. An implementation of this interface requires details about the user to be authenticated. For this, Spring Security provides a User class which can be used as is or extended. The example application extends this org.springframework.security.core.userdetails.User class to add the tenant attribute to the User.

package com.example.model;

import java.util.Collection;

import org.springframework.security.core.GrantedAuthority;

/**
 * CustomUserDetails class extends the Spring Security provided
 * {@link org.springframework.security.core.userdetails.User} class for
 * authentication purpose. Do not confuse this with the {@link User} class which
 * is an entity for storing application specific user details like username,
 * password, tenant, etc in the database using the JPA {@literal @}Entity
 * annotation.
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 *
 */
public class CustomUserDetails 
    extends org.springframework.security.core.userdetails.User {

    private static final long serialVersionUID = 1L;

    /**
     * The extra field in the login form is for the tenant name
     */
    private String tenant;

    /**
     * Constructor based on the spring security User class but with an extra
     * argument <code>tenant</code> to store the tenant name submitted by the end
     * user.
     * 
     * @param username
     * @param password
     * @param authorities
     * @param tenant
     */
    public CustomUserDetails(String username, String password, 
                Collection<? extends GrantedAuthority> authorities,
                String tenant) {
        super(username, password, authorities);
        this.tenant = tenant;
    }

    // Getters and Setters
    public String getTenant() {
        return tenant;
    }

    public void setTenant(String tenant) {
        this.tenant = tenant;
    }

}

File: CustomUserDetailsServiceImpl.java

Spring Security provides a interface UserDetailsService which has just one method declared in it - loadUserByUsername which returns the UserDetails.  Now this UserDetails is used for authentication. In the example application code, the CustomUserDetails already defines the attributes for the User.

package com.example.security;

import java.util.HashSet;
import java.util.Set;

import org.apache.commons.lang3.StringUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.security.core.GrantedAuthority;
import org.springframework.security.core.authority.SimpleGrantedAuthority;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.stereotype.Service;

import com.example.model.CustomUserDetails;
import com.example.model.Role;
import com.example.model.User;
import com.example.service.UserService;

/**
 * {@link CustomUserDetailsService} contract defines a single method called
 * loadUserByUsernameAndTenantname.
 * 
 * The {@link CustomUserDetailsServiceImpl} class simply implements the contract
 * and delegates to {@link UserService} to get the
 * {@link com.example.model.User} from the database so that it can be compared
 * with the {@link org.springframework.security.core.userdetails.User} for
 * authentication. Authentication occurs via the
 * {@link CustomUserDetailsAuthenticationProvider}.
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 *
 */
@Service("userDetailsService")
public class CustomUserDetailsServiceImpl implements CustomUserDetailsService {

    @Autowired
    private UserService userService;

    @Override
    public UserDetails loadUserByUsernameAndTenantname(String username, String tenant)
            throws UsernameNotFoundException {
        if (StringUtils.isAnyBlank(username, tenant)) {
            throw new UsernameNotFoundException("Username and domain must be provided");
        }
        // Look for the user based on the username and tenant by accessing the
        // UserRepository via the UserService
        User user = userService.findByUsernameAndTenantname(username, tenant);

        if (user == null) {
            throw new UsernameNotFoundException(
                    String.format("Username not found for domain, "
                            + "username=%s, tenant=%s", username, tenant));
        }

        Set<GrantedAuthority> grantedAuthorities = new HashSet<>();
        for (Role role : user.getRoles()) {
            grantedAuthorities.add(new SimpleGrantedAuthority(role.getRole()));
        }

        CustomUserDetails customUserDetails = 
                new CustomUserDetails(user.getUsername(), 
                        user.getPassword(), grantedAuthorities, tenant);
        
        return customUserDetails;
    }
}

File: CustomUserDetailsAuthenticationProvider.java

Spring Security provides class AbstractUserDetailsAuthenticationProvider which allows subclasses to override and work with UserDetails objects. The class is designed to respond to UsernamePasswordAuthenticationToken authentication requests.

The CustomUserDetailsAuthenticationProvider implemented for this application delegates to the CustomUserDetailsService (implemented by CustomUserDetailsServiceImpl) for retrieving the UserDetails for authentication

package com.example.security;

import org.springframework.security.authentication.BadCredentialsException;
import org.springframework.security.authentication.InternalAuthenticationServiceException;
import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
import org.springframework.security.authentication.dao.AbstractUserDetailsAuthenticationProvider;
import org.springframework.security.core.AuthenticationException;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.util.Assert;

/**
 * {@link CustomUserDetailsAuthenticationProvider} extends
 * {@link AbstractUserDetailsAuthenticationProvider} and delegates to the
 * {@link CustomUserDetailService} to retrieve the User. The most important
 * feature of this class is the implementation of the <code>retrieveUser</code>
 * method.
 * 
 * Note that the authentication token must be cast to CustomAuthenticationToken
 * to access the custom field - tenant
 * 
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
public class CustomUserDetailsAuthenticationProvider 
            extends AbstractUserDetailsAuthenticationProvider {

    /**
     * The plaintext password used to perform PasswordEncoder#matches(CharSequence,
     * String)} on when the user is not found to avoid SEC-2056
     * (https://github.com/spring-projects/spring-security/issues/2280).
     */
    private static final String USER_NOT_FOUND_PASSWORD = "userNotFoundPassword";

    /**
     * For encoding and/or matching the encrypted password stored in the database
     * with the user submitted password
     */
    private PasswordEncoder passwordEncoder;

    private CustomUserDetailsService userDetailsService;

    /**
     * The password used to perform
     * {@link PasswordEncoder#matches(CharSequence, String)} on when the user is not
     * found to avoid SEC-2056. This is necessary, because some
     * {@link PasswordEncoder} implementations will short circuit if the password is
     * not in a valid format.
     */
    private String userNotFoundEncodedPassword;

    public CustomUserDetailsAuthenticationProvider(PasswordEncoder passwordEncoder,
            CustomUserDetailsService userDetailsService) {
        this.passwordEncoder = passwordEncoder;
        this.userDetailsService = userDetailsService;
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.security.authentication.dao.
     * AbstractUserDetailsAuthenticationProvider#additionalAuthenticationChecks(org.
     * springframework.security.core.userdetails.UserDetails,
     * org.springframework.security.authentication.
     * UsernamePasswordAuthenticationToken)
     */
    @Override
    protected void additionalAuthenticationChecks(UserDetails userDetails,
            UsernamePasswordAuthenticationToken authentication) throws AuthenticationException {

        if (authentication.getCredentials() == null) {
            logger.debug("Authentication failed: no credentials provided");
            throw new BadCredentialsException(
                    messages.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", 
                            "Bad credentials"));
        }
        // Get the password submitted by the end user
        String presentedPassword = authentication.getCredentials().toString();

        // If the password stored in the database and the user submitted password do not
        // match, then signal a login error
        if (!passwordEncoder.matches(presentedPassword, userDetails.getPassword())) {
            logger.debug("Authentication failed: password does not match stored value");
            throw new BadCredentialsException(
                    messages.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", 
                            "Bad credentials"));
        }
    }

    @Override
    protected void doAfterPropertiesSet() throws Exception {
        Assert.notNull(this.userDetailsService, "A UserDetailsService must be set");
        this.userNotFoundEncodedPassword = this.passwordEncoder.encode(USER_NOT_FOUND_PASSWORD);
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.security.authentication.dao.
     * AbstractUserDetailsAuthenticationProvider#retrieveUser(java.lang.String,
     * org.springframework.security.authentication.
     * UsernamePasswordAuthenticationToken)
     */
    @Override
    protected UserDetails retrieveUser(String username, UsernamePasswordAuthenticationToken authentication)
            throws AuthenticationException {
        CustomAuthenticationToken auth = (CustomAuthenticationToken) authentication;
        UserDetails loadedUser;

        try {
            loadedUser = this.userDetailsService
                    .loadUserByUsernameAndTenantname(auth.getPrincipal().toString(),
                            auth.getTenant());
        } catch (UsernameNotFoundException notFound) {
            if (authentication.getCredentials() != null) {
                String presentedPassword = authentication.getCredentials().toString();
                passwordEncoder.matches(presentedPassword, userNotFoundEncodedPassword);
            }
            throw notFound;
        } catch (Exception repositoryProblem) {
            throw new InternalAuthenticationServiceException(repositoryProblem.getMessage(), 
                    repositoryProblem);
        }

        if (loadedUser == null) {
            throw new InternalAuthenticationServiceException(
                    "UserDetailsService returned null, "
                    + "which is an interface contract violation");
        }
        return loadedUser;
    }
}

Conclusion of Part 1

In this part of the blog post, I have explained how this application works, how to set up your project, how Spring Security is set up.

In the next part, I will show how the JPA and Hibernate part of the code is set up. Also I will show how Spring Security is tied in with the multi-tenancy Hibernate code.

Part 2

Building SaaS style multi-tenant web app with Spring Boot 2 and Spring Security 5 - Part 2

Continuing from part 1

In part 1 of this blog post, I shared how you can build a SaaS style multi-tenant web application and how it works.

In this part of the blog post, I will continue to show how to set up multi-tenant database access to the tenant databases using Spring JPA and Hibernate. Also how Spring Security is tied into all of this.

Multi-Tenancy data access and tenant database selection

In multi-tenant setup,  Hibernate requires a datasource to be fed so that it can establish a connection/session with the database defined by the datasource.

Data access

For a multi-tenancy setup, Hibernate provides two facilities -

• CurrentTenantIdentifierResolver - an interface which should be implemented by the application code to identify the tenant
• AbstractDataSourceBasedMultiTenantConnectionProviderImpl - an abstract class which provides support for providing the correct datasource based on the tenant id to Hibernate

Tenant identification and data source selection

In part 1 of the blog I had written that the tenant identifier is provided by the end user via the login form. The application code needs to parse this tenant field in the Spring Security layer and store it for tenant id resolution.

Spring provides a UsernamePasswordAuthenticationFilter which is extended by the example application to retrieve the tenant value from the login form. This tenant value is stored in a ThreadLocal variable so that it can be used by the CurrentTenantIdentifierResolver implementation.

Understanding the code used in the application

I will explain the significant pieces of code in this part of the blog post so that you can follow how it is all related and how it all works.

File: MultitenancyProperties.java

As shown in part 1, the application.yml defines the data sources for the tenant databases. These should be available to the application code. MultitenancyProperties class has the @ConfigurationProperties annotation and is asked to parse the properties under the multitenancy.app node from application.yml file.

package com.example.multitenancy;

import java.util.List;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Configuration;

/**
 * This class reads the <code>multitenancy.mtapp</code> node from
 * <code>application.yml</code> file and populates a list of
 * {@link org.springframework.boot.autoconfigure.jdbc.DataSourceProperties}
 * objects, with each instance containing the data source details about the
 * database like url, username, password etc
 * 
 * @author Sunit Katkar
 * @version 1.0 
 * @since 1.0 (April 2018)
 */
@Configuration
@ConfigurationProperties("multitenancy.mtapp")
public class MultitenancyProperties {

    private List<DataSourceProperties> dataSourcesProps;

    public List<DataSourceProperties> getDataSources() {
        return this.dataSourcesProps;
    }

    public void setDataSources(List<DataSourceProperties> dataSourcesProps) {
        this.dataSourcesProps = dataSourcesProps;
    }

    public static class DataSourceProperties 
        extends org.springframework.boot.autoconfigure.jdbc.DataSourceProperties {

        private String tenantId;

        public String getTenantId() {
            return tenantId;
        }

        public void setTenantId(String tenantId) {
            this.tenantId = tenantId;
        }
    }
}

File: MultiTenancyJpaConfiguration.java

Hibernate needs to be fed with the datasource that it should connect to. MultiTenancyJpaConfiguration reads the data sources from MultitenancyProperties and creates a map of the data sources. This map is then used for selecting the required tenant database based on the tenant identifier and Hibernate establishes a connection with that database.

package com.example.multitenancy;

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

import javax.persistence.EntityManagerFactory;
import javax.sql.DataSource;

import org.hibernate.MultiTenancyStrategy;
import org.hibernate.cfg.Environment;
import org.hibernate.context.spi.CurrentTenantIdentifierResolver;
import org.hibernate.engine.jdbc.connections.spi.MultiTenantConnectionProvider;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.orm.jpa.JpaProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.boot.jdbc.DataSourceBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.orm.jpa.JpaTransactionManager;
import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
import org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter;
import org.springframework.transaction.PlatformTransactionManager;
import org.springframework.transaction.annotation.EnableTransactionManagement;

import com.example.model.Employee;
import com.example.multitenancy.MultitenancyProperties.DataSourceProperties;

/**
 * This class defines the data sources to be used for accessing the different
 * databases (one database per tenant). It generates the Hibernate session and
 * entity bean for database access via Spring JPA as well as the Transaction
 * manager to be used.
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
@Configuration
@EnableConfigurationProperties({ MultitenancyProperties.class, JpaProperties.class })
@EnableTransactionManagement
public class MultiTenancyJpaConfiguration {

    @Autowired
    private JpaProperties jpaProperties;

    @Autowired
    private MultitenancyProperties multitenancyProperties;

    /**
     * Builds a map of all data sources defined the application.yml file
     * 
     * @return
     */
    @Primary
    @Bean(name = "dataSourcesMtApp")
    public Map<String, DataSource> dataSourcesMtApp() {
        Map<String, DataSource> result = new HashMap<>();
        for (DataSourceProperties dsProperties : this.multitenancyProperties.getDataSources()) {
            
            DataSourceBuilder factory = DataSourceBuilder.create()
                    .url(dsProperties.getUrl())
                    .username(dsProperties.getUsername())
                    .password(dsProperties.getPassword())
                    .driverClassName(dsProperties.getDriverClassName());
            
            result.put(dsProperties.getTenantId(), factory.build());
        }
        return result;
    }

    /**
     * Autowires the data sources so that they can be used by the Spring JPA to
     * access the database
     * 
     * @return
     */
    @Bean
    public MultiTenantConnectionProvider multiTenantConnectionProvider() {
        // Autowires dataSourcesMtApp
        return new DataSourceBasedMultiTenantConnectionProviderImpl();
    }

    /**
     * Since this is a multi-tenant application, Hibernate requires that the current
     * tenant identifier is resolved for use with
     * {@link org.hibernate.context.spi.CurrentSessionContext} and
     * {@link org.hibernate.SessionFactory#getCurrentSession()}
     * 
     * @return
     */
    @Bean
    public CurrentTenantIdentifierResolver currentTenantIdentifierResolver() {
        return new CurrentTenantIdentifierResolverImpl();
    }

    /**
     * org.springframework.beans.factory.FactoryBean that creates a JPA
     * {@link javax.persistence.EntityManagerFactory} according to JPA's standard
     * container bootstrap contract. This is the most powerful way to set up a
     * shared JPA EntityManagerFactory in a Spring application context; the
     * EntityManagerFactory can then be passed to JPA-based DAOs via dependency
     * injection. Note that switching to a JNDI lookup or to a
     * {@link org.springframework.orm.jpa.LocalEntityManagerFactoryBean} definition
     * is just a matter of configuration!
     * 
     * @param multiTenantConnectionProvider
     * @param currentTenantIdentifierResolver
     * @return
     */
    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactoryBean(
            MultiTenantConnectionProvider multiTenantConnectionProvider,
            CurrentTenantIdentifierResolver currentTenantIdentifierResolver) {

        Map<String, Object> hibernateProps = new LinkedHashMap<>();
        hibernateProps.putAll(this.jpaProperties.getProperties());
        hibernateProps.put(Environment.MULTI_TENANT, MultiTenancyStrategy.DATABASE);
        hibernateProps.put(Environment.MULTI_TENANT_CONNECTION_PROVIDER, multiTenantConnectionProvider);

        hibernateProps.put(Environment.MULTI_TENANT_IDENTIFIER_RESOLVER, currentTenantIdentifierResolver);

        // No dataSource is set to resulting entityManagerFactoryBean
        LocalContainerEntityManagerFactoryBean result = new LocalContainerEntityManagerFactoryBean();
        result.setPackagesToScan(new String[] { Employee.class.getPackage().getName() });
        result.setJpaVendorAdapter(new HibernateJpaVendorAdapter());
        result.setJpaPropertyMap(hibernateProps);

        return result;
    }

    /**
     * Interface used to interact with the entity manager factory for the
     * persistence unit.
     * 
     * @param entityManagerFactoryBean
     * @return
     */
    @Bean
    public EntityManagerFactory entityManagerFactory(LocalContainerEntityManagerFactoryBean 
            entityManagerFactoryBean) {
        return entityManagerFactoryBean.getObject();
    }

    /**
     * Creates a new
     * {@link org.springframework.orm.jpa.JpaTransactionManager#JpaTransactionManager(EntityManagerFactory emf)}
     * instance.
     * 
     * {@link org.springframework.transaction.PlatformTransactionManager} is the
     * central interface in Spring's transaction infrastructure. Applications can
     * use this directly, but it is not primarily meant as API: Typically,
     * applications will work with either TransactionTemplate or declarative
     * transaction demarcation through AOP.
     * 
     * @param entityManagerFactory
     * @return
     */
    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory entityManagerFactory) {
        return new JpaTransactionManager(entityManagerFactory);
    }
}

File: DataSourceBasedMultiTenantConnectionProviderImpl.java

Hibernate provides AbstractDataSourceBasedMultiTenantConnectionProviderImpl which provides support for connections in a multi-tenant setup like the example application.

DataSourceBasedMultiTenantConnectionProviderImpl extends the abstract class and implements the selectDataSource() and selectAnyDataSource() methods.  This class basically does the job of selecting the correct database based on the tenant id found by the tenant id resolver, which I will show next.

package com.example.multitenancy;

import java.util.Map;

import javax.sql.DataSource;

import org.hibernate.engine.jdbc.connections.spi.AbstractDataSourceBasedMultiTenantConnectionProviderImpl;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

/**
 * This class does the job of selecting the correct database based on the tenant
 * id found by the {@link CurrentTenantIdentifierResolverImpl}
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
@Component
public class DataSourceBasedMultiTenantConnectionProviderImpl
        extends AbstractDataSourceBasedMultiTenantConnectionProviderImpl {

    private static final long serialVersionUID = 1L;

    @Autowired
    private Map<String, DataSource> dataSourcesMtApp;

    /*
     * (non-Javadoc)
     * 
     * @see org.hibernate.engine.jdbc.connections.spi.
     * AbstractDataSourceBasedMultiTenantConnectionProviderImpl#selectAnyDataSource(
     * )
     */
    @Override
    protected DataSource selectAnyDataSource() {
        return this.dataSourcesMtApp.values().iterator().next();
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.hibernate.engine.jdbc.connections.spi.
     * AbstractDataSourceBasedMultiTenantConnectionProviderImpl#selectDataSource(
     * java.lang.String)
     */
    @Override
    protected DataSource selectDataSource(String tenantIdentifier) {
        return this.dataSourcesMtApp.get(tenantIdentifier);
    }
}

File:  CurrentTenantIdentifierResolverImpl.java

This class does the job of identifying the tenant. In the example application the tenant id is stored in a ThreadLocal variable which is accessed in this class and provided to Hibernate

package com.example.multitenancy;

import org.apache.commons.lang3.StringUtils;
import org.hibernate.context.spi.CurrentTenantIdentifierResolver;
import org.springframework.stereotype.Component;

import com.example.util.TenantContextHolder;

/**
 * Hibernate needs to know which database to use i.e. which tenant to connect
 * to. This class provides a mechanism to provide the correct datasource at run
 * time.
 * 
 * @see {@link com.example.util.TenantContextHolder}
 * @see {@link com.example.security.CustomAuthenticationFilter}
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
@Component
public class CurrentTenantIdentifierResolverImpl implements CurrentTenantIdentifierResolver {

    private static final String DEFAULT_TENANT_ID = "tenant_1";

    /*
     * (non-Javadoc)
     * 
     * @see org.hibernate.context.spi.CurrentTenantIdentifierResolver#
     * resolveCurrentTenantIdentifier()
     */
    @Override
    public String resolveCurrentTenantIdentifier() {
        // The tenant is stored in a ThreadLocal before the end user's login information
        // is submitted for spring security authentication mechanism. Refer to
        // CustomAuthenticationFilter
        String tenant = TenantContextHolder.getTenant();
        return StringUtils.isNotBlank(tenant) ? tenant : DEFAULT_TENANT_ID;
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.hibernate.context.spi.CurrentTenantIdentifierResolver#
     * validateExistingCurrentSessions()
     */
    @Override
    public boolean validateExistingCurrentSessions() {
        return true;
    }

}

Please note the yellow highlighted line in the code above which retrieves the tenant identifier. I will explain how it was stored and retrieved in the next file description.

File: TenantContextHolder.java

When Spring Security parses the login form which has the extra 'tenant' field, the field value is important. This value is the tenant identifier which will Hibernate which data source to use to connect to the desired tenant database.

ThreadLocal is a good solution to storing this tenant identifier. Please refer to the javadocs for a detailed explanation and some example code to understand the concept if you are new to it.

The following diagram shows how the tenant id is retrieved from the login form and stored in a ThreadLocal variable and finally retrieved from this variable to identify the tenant database.

ThreadLocal storage to store and retrieve tenant identifier

The example application uses the following code for a ThreadLocal variable

package com.example.util;


/**
 * When the end user submits the login form, the tenant id is required to
 * determine which database to connect to. This needs to be captured in the
 * spring security authentication mechanism, specifically in the
 * {@link UsernamePasswordAuthenticationFilter} implemented by
 * {@link CustomAuthenticationFilter}. This tenant id is then required by the
 * {@link CurrentTenantIdentifierResolver} implemeted by the
 * {@link CurrentTenantIdentifierResolverImpl}
 * 
 * <br/>
 * <br/>
 * <b>Explanation:</b> Thread Local can be considered as a scope of access, like
 * a request scope or session scope. It’s a thread scope. You can set any object
 * in Thread Local and this object will be global and local to the specific
 * thread which is accessing this object. Global and local at the same time? :
 * 
 * <ul>
 * <li>Values stored in Thread Local are global to the thread, meaning that they
 * can be accessed from anywhere inside that thread. If a thread calls methods
 * from several classes, then all the methods can see the Thread Local variable
 * set by other methods (because they are executing in same thread). The value
 * need not be passed explicitly. It’s like how you use global variables.</li>
 * <li>Values stored in Thread Local are local to the thread, meaning that each
 * thread will have it’s own Thread Local variable. One thread can not
 * access/modify other thread’s Thread Local variables.</li>
 * </ul>
 * 
 * @see https://dzone.com/articles/painless-introduction-javas-threadlocal-storage
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
public class TenantContextHolder {

    private static final ThreadLocal<String> CONTEXT = new ThreadLocal<>();

    public static void setTenantId(String tenant) {
        CONTEXT.set(tenant);
    }

    public static String getTenant() {
        return CONTEXT.get();
    }

    public static void clear() {
        CONTEXT.remove();
    }
}

These are the significant files for the application. The application described in this blog post is checked into GitHub where you can see other code not explained here.

Modifying the Spring Boot main application class

Typically, the Spring Boot main application class is nothing special as it has a main method which starts the application. But there is a lot of Spring Boot magic going on behind the scenes. A lot of defaults are assumed, a lot of configurations are done automatically by detecting artifacts in the classpath, application properties file and pom.xml analysis.

If you are familiar with Spring Boot and JPA or even JDBC, then you know that any mention of spring jpa properties in the application properties file will cause Spring Boot to automatically configure the datasource. In this web application, we dont want Spring Boot to configure the data sources because we want to define multiple data sources and there is no default data source available when the application starts. Only when a user logs in with tenant information, the correct datasource needs to be used to connect to the tenant database.

So, using a simple exclude attribute to exclude the DataSourceAutoConfiguration class in the main @SpringBootApplication annotation, we can tell Spring Boot to leave data source configuration aside.

Since we have asked Spring Boot to not do some auto configuration related to data sources, it is a good idea to specifically ask Spring Boot to enable JPA Repositories using the @EnableJpaRepositories annotation.

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.data.jpa.repository.config.EnableJpaRepositories;

/**
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })
@EnableJpaRepositories("com.example.repository")
public class MultitenancyMySqlApplication {

    public static void main(String[] args) {
        SpringApplication.run(MultitenancyMySqlApplication.class, args);
    }
}

Sample SQL to create a tenant database on MySQL 

Had this example been set up as a single database application and the property spring.jpa.hibernate.ddl-auto set to create or update, then the following three tables would be generated automatically by Hibernate based on the JPA entity definitions. Read more about it here.

Table: User

CREATE TABLE `user` (
 `user_id` INT(11) NOT NULL,
 `password` VARCHAR(255) NOT NULL,
 `username` VARCHAR(255) NOT NULL,
 `active` BIT(1) NULL DEFAULT NULL,
 `tenant` VARCHAR(255) NULL DEFAULT NULL,
 PRIMARY KEY (`user_id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=MyISAM
;

Table: Role

CREATE TABLE `role` (
 `role_id` INT(11) NOT NULL,
 `role` VARCHAR(255) NULL DEFAULT NULL,
 PRIMARY KEY (`role_id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=MyISAM
;

Table: User_Role

This is the mapping table where users and roles are recordeed

CREATE TABLE `user_role` (
 `user_id` INT(11) NOT NULL,
 `role_id` INT(11) NOT NULL,
 PRIMARY KEY (`user_id`, `role_id`),
 INDEX `FKa68196081fvovjhkek5m97n3y` (`role_id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=MyISAM
;

Resources 

The complete source code is checked into GitHub. Its a standard Maven project which you can import into your IDE. 

Conclusion

I have explained multi-tenancy and shown you with example code how you can build a SaaS style 'tenant per database' multi-tenant web application with Spring Boot; and how to secure it with Spring Security. I will write about adapting this code to use Microsoft SQL Server in a future blog post as there were some challenges and how I had to change some code to overcome them.

Please do let me know your feedback by adding it in the comments section below. I am sure I will learn something new.

Happy coding :)