WSO2 ESB Passthrough Transport Basics

When I first joined WSO2 I found it hard to get a grasp of this so-called "Passthrough Transport". All I knew was it was fast! as opposed to "nhttp transport" (I hadn't known anything about it either). However, over the past year I gradually get to understand what is this Passthrough transport and why it was so fast. So, In this blog post I'll be explaining some "good-to-know" stuff about "Passthrough Transport". Since I am no expert on this, there could be few gaps. But still better than nothing.

Passthrough Transport Vs NHTTP Transport

The main difference is, in Passthrough Transport the incoming message does not get built all the time whereas in NHTTP Transport it always gets built. What we meant by building the message is we take the message stream from the socket and transform it in to a XML representation. 

In reality, you don't always have to build the message. For instance, you maybe able to rout the message simply by looking at the headers of HTTP request. So rather than blindly building the message Passthrough Transport does this selectively which makes it smarter than its predecessor NHTTP Transport.

The main similarity between these two is they are both developed on top of the popular Apache project HTTP-core.

High level view of Passthrough Transport

OK, Now that you have an idea, let's look at high level view this transport.

It is not as simple as the diagram depicts but it is enough to get you started. In a way, Passthrough Transport is a complex implementation of Producer-Consumer pattern. Why I say so, Let me explain. Everything starts from the SourceHandler side. When a client sends a request it comes through HTTP-Core towards SourceHanlder. Then the SourceHandler starts producing data to the Pipe. As soon as SourceHanlder starts producing data to the Pipe TargetHandler starts consuming data from the Pipe. These consumed data are sent through HTTP-Core to the desired endpoint. 

As I said earlier it is not as simple as that. There are quite a lot of classes associated with the process such as ServerWorker, SourceRequest, SourceResponse, ClientWorker, TargetResponse, TargetRequest, etc. To make matters worse, the entire implementation is done in Asynchronous manner.

State Machine of Passthrough Transport

So in order to reduce the complexity of the entire process it is implemented on a state machine. SourceHanlder and TargetHanlder have their separate state machines. Following is the state machine that used by them.

The vertical split represents the SourceHandler side and the TargetHandler side. The horizontal split represents the HTTP request and HTTP response. The methods that are next to each state are the methods that get executed in each state (Forget the methods for the moment). Before I explain the state machine it is important to know that in Passthrough the HTTP message is divided into two parts and as HEADERs and BODY. 

This is how it goes, First ESB establishes the connection with the Client and sets its state to REQUEST_READY. Then it starts receiving data. First it reads the headers and goes to REQUEST_HEAD state. Afterwards, it makes itself ready to read the body of the message in the REQUEST_BODY state. Finally to finish the first quarter it reads the entire message body and moves to REQUEST_DONE state. 

The same thing continues in the next quarter but this time ESB acts as the client to some back-end server.  And in the last two quarters it happens for the response. The states of the SourceHandler and the TargetHanlder are interconnected through Pipe's buffer. So sometimes when you debug, though the error message shows in TargetHandler side the actual cause could be on the other side. 

Exact Location of Passthrough Transport

Following diagram shows the location of the Passthrough Transport in the ESB architecture.

Finally, THIS STATE MACHINE REPRESENTS AN IDEAL SCENARIO. But in real world it could deviate a bit from this. Moreover, this blog post is only covering a small bite of a complex implementation. Anyways, this knowledge is enough to get you started.