Tomcat with multiple instances of the same application

Question

We're building a java web application where each customer will have an instance of it with it's own database schema.

It will be managed by my company so I would like to know what is the best approach to have several apps instances running on the same Tomcat runtime since we tried to run 3 instances on a single Tomcat and it ended up on an Out of memory exception. We considered to run multiple tomcat instances in the same server but we haven't already tested it. Also we are considering to have a separate server for each customer.

From your experience with similar scenarios, what is your opinion?

EDITED: This application can't be multi-tenant since there will be code customizations in some parts of it as well as some other business facts that require an application instance per client. So please the application architecture is not the subject here.

Thank you,

Gyo

Answer 1

My Opinion is exactly the same as Kinjal Dixit one. Your approach is wrong and will be an huge waste of resources.

If you want to be able to deploy different version of the web-app for the same server you will have to isolate the class-loading of each app and this will imply an huge memory consumption. Otherwise if all web-app will always be the same there is no interest to deploy it many times.

Having a separate server for each customer will also be a waste of resource (multiple instance of JVM, multiple classloading of the same libraries, multiplication of the number of thread and so of the cost of scheduling) and will significantly complicate the deployment, especially if you plan some clustering strategy where the load balancing will probably become a hell

Moreover if you want to have some specific feature for a given client it will also become a hell to manage / deploy / upgrade, etc...

Multi-tenant architecture does not necessary imply to share the database (you can have a DB instance per client and dispatch the request with an interceptor at low level) however sharing the web-app is an absolute necessity.

I'd also advice to provide some kind of configuration to allow to enable custom features for a given client.

I worked for a company where we encountered exactly this problem (expose a legacy web application as a SAAS one) we started by deploying one web-app per client, spent huge time in various optimization (including class loading factorization) to reach the "huge" number of 14 customer per server.

This was far from our performance expectation and we finally switched to a multi-tenant architecture, but keeping one DB instance per customer to avoid the important cost of data-model refactoring. The new deployment was able to handle more than 100 customer on the same server with incomparable performance.

EDIT (according to question update)

If you absolutely want to avoid multi-tenancy then i'd recommend to use only one servlet container (tomcat) per server. In this case you will have to let the default web-app classloading isolation (as you will have custom code in different instances) which will imply a high memory footprint. You should however put all common libraries in the common/lib tomcat directory to factorize their loading ( see http://tomcat.apache.org/tomcat-6.0-doc/class-loader-howto.html).

Answer 2

You want to use multi tenant architecture. There will be only one database and web application instance, and every record will be qualified by the 'owner' company. You can use the subdomain/domain which the client uses to access your application to differentiate between them.

Simplistically, you add a 'domain_id' column to every table and you have a 'where domain_id=?' in every query. Each user will have an associated domain_id which you will pick up on login and put in session. In reality there will be other considerations.

EDITED: Based on the edit in the question, here is additional part to the answer.

In multitenant architecture it is possible to customise every instance without maintaining separate codebases. Some of the customisations can be part of the 'profile'. This is suitable for data values and flags, like currency, date format, etc. The case where new functionality specific to a client is required, this can be achieved by supporting plugins.

Taking a one time pain to fit your solution into a multi tenant architecture will be better than the on going pain of maintaining several separate versions of your code for each client. You might want to read up on the topic of 'technical debt'.

An ERP is a complex case of a business application, and you can get inspiration from reading the OpenBravo Trial FAQ to get an idea of what we are saying. Openbravo is open source and you may get technical details by looking at their code.