Scalability is a tough nut to crack. When developing enterprise software and deploying them in mission critical environments, you need to think about the scalability aspects from day one. If you don’t, you may rest assured that a whole bunch of unpleasant surprises are heading your way. Some of the problems you may encounter are systems crashing inexplicably under heavy load, customers constantly rambling about the poor performance of the system and system administrators having to play watch dog to the deployed applications day in and day out. In addition to these possible mishaps, experience tells us that attempting to make a live production system scalable is hell of a lot more difficult and expensive. So it’s always wise to think about scalability before your solutions go live.
The crew at WSO2 have a firm grip on this reality. Therefore when designing and developing the WSO2 API Manager, we made scalability of the end product a top priority. We thought about how the overall solution is going to scale and how its individual components are going to scale. In general we thought about how the API Manager can scale under following circumstances.
- Growing number of API subscribers (growth of the user base)
- Growing number of APIs (growth of metadata and configurations)
- Growing number of API calls (growth of traffic)
When you download the WSO2 API Manager binary distribution, you get all the above components packaged as a single artifact. You can also run the entire thing in a single JVM. We call this the standalone or out-of-the-box setup. If you only have a few hundred users and a handful of APIs, then the standalone setup is probably sufficient to you. But if you have thousands and thousands of users and hundreds of APIs then you should start thinking about deploying the API Manager components in a distributed and scalable manner. Let’s go through each of the components in the above diagram and try to understand how we can make them scalable.
Databases
WSO2 API Manager uses 2 main databases - the registry database and the API management database. The registry database is used by the underlying registry components and governance components to store system and API related metadata. API management database is primarily used to store API subscriptions. In the standalone setup, these 2 databases are created in the embedded H2 server.
In a scalable setup, it will be necessary to create these databases elsewhere, ideally in a clustered and high available database engine. One may use a MySQL cluster, SQL Server cluster or an Oracle cluster for this purpose. As you may see in the next few sections of this post, in a scalable deployment we might cluster some of the internal components of the WSO2 API Manager. Therefore there will be more than one JVM involved. All these JVMs can share the same databases created in the same clustered database engine.
Settings for the registry database are configured in a file named registry.xml which resides in the repository/conf directory of the API Manager. API management database settings are configured in a file named api-manager.xml which also resides in the same directory. Additionally there’s also a master-datasources.xml file where all the different data sources can be defined and you have the option of reusing these data sources in registry.xml and api-manager.xml.
API Publisher and API Store
These 2 components are implemented as 2 web applications using Jaggery.js. However they require some of the underlying Carbon components to function – most notably the API management components, governance components and registry components. If your deployment has a large user base, then chances are both API Publisher and API Store will receive a large volume of web traffic. Therefore it’s advisable to scale these two web applications up.
One of the simplest ways to scale them up is by clustering the WSO2 API Manager. You can run multiple instances of the API Manager pointed at the same database. An external load balancer (a hardware load balancer, WSO2 Load Balancer or any HTTP load balancer) can distribute the incoming web traffic among the different API Manager nodes. Tomcat session replication can be enabled among the API Manager nodes so that the HTTP sessions established by the users are replicated across the entire cluster.
The default distribution of WSO2 API Manager has both API Publisher and API Store loaded into the same container. Therefore an out-of-the-box API Manager node plays a dual role. But you have the option of removing one of these components and making a node play a single role. That is a single node can act either as an API Publisher instance or as an API Store instance. Using this capability you can add a bit of traffic shaping into your clustered API Manager deployment. In a typical scenario there will be only a handful of people (less than 50) who create APIs but a large number of subscribers (thousands) who consume the published APIs. Therefore you can have a large cluster with many API Store nodes and a small cluster of API Publisher nodes (or even a single API Publisher node would do). Two clusters can be setup separately with their own load balancers.
Key Management
Key management component is responsible for generating and keeping track of API keys. It’s also in charge of validating API keys when APIs are invoked by subscribers. All the core functions of this component are exposed as web services. The other components such as the API Store and API Gateway communicate with the key manager via web service calls. Therefore if your system has many consumers and if it receives a large number of API calls, then it’s definitely advisable to scale this component up.
Again the easiest way to scale this component is by clustering the API Manager deployment. That way we will get multiple key management service endpoints which can be put behind a load balancer. It’s also not a bad idea to have a separate dedicated cluster of Carbon servers that run as key management servers. An API Manager node can be stripped of its API Publisher, API Store and other unnecessary components to turn it into a dedicated key management server.
User Management
This is the component against which all user authentication and permission checks are carried out. API Publisher and API Store frequently communicate with this component over a web service interface. In the standalone setup, a database in the embedded H2 server is used to store user profiles and roles. But in a real world deployment, this can be hooked up with a corporate LDAP or an Active Directory instance. To scale this component, we can again make use of simple clustering techniques. All the endpoints of the exposed user management services can be put behind a load balancer and exposed to the API Publisher and API Store.
API Gateway
This is the powerhouse where all the validating, throttling and routing of API calls take place. It mainly consists of WSO2 ESB components and hence can be easily clustered, just as how you would setup an ESB cluster. One of the gateway nodes will function as the primary node through which all API configuration changes are applied. API Publisher will communicate with the primary node via web service calls to deploy, update and undeploy APIs. Carbon’s deployment synchronizer can take care of propagating all the configuration changes from the primary node to rest of the nodes in the gateway cluster.
API Gateway also caches a lot of information related to API key validation in order to prevent having to query the key manager frequently. This information is stored in the built-in distributed cache of Carbon (based on Infinispan). Therefore in a clustered setup, information cached by a single gateway node becomes visible to other gateway nodes in the cluster. This further helps to reduce the load on the key manager and improves the response time of API invocations.
Usage Tracking
We use WSO2 BAM components to publish, analyze and display API statistics. BAM has its own scalability model. Thrift is used to publish statistics from API Gateway to a remote Cassandra cluster. Use of Thrift ensures that statistics can be published from API Gateway to the Cassandra store at a rapid rate. The BAM data publisher also employs its own queuing mechanism and thread pool so that data can be published asynchronously without having any impact on the messages routed through the API Gateway. Use of Cassandra enables fast read-write operations on enormous data sets.
Once the data has been written to the Cassandra cluster, Hadoop and Hive are used to process the collected information. Analyzed data are then stored in a separate database from which API Manager (or any other monitoring application) can pull out the numbers and display in various forms of tables and charts.
Putting It All Together
As you can see WSO2 API Manager provides many options to scale up its individual components. However it doesn’t mean you should scale up each and every piece of it for the overall solution to be scalable. You should decide which components to scale up by looking at your requirements and the expected usage patterns of the solution. For instance, if you only have a handful of subscribers you don’t have to worry about scaling up API Store and API Publisher, regardless of how much traffic they are going to send. If you have thousands of subscribers, but only a handful of them are actually sending any traffic, then the scalability of API Store will be more important than scaling up the Gateway and statistics collection components.
