I had worked with both SEDA and direct, and I've also read the documentation.
But I still cannot visualize the usage of SEDA and direct. Vm is new to me.
Please explain it with an example.
There are at least four different mechanisms by which one Camel route can directly pass data to another. By "directly" I mean without using a network or some form of intermediate storage (file, database). These mechanisms can be grouped according to whether they can pass data between CamelContext instances or not, and whether they are synchronous or asynchronous.
- direct -- single CamelContext, synchronous (blocks producer)
- SEDA -- single CamelContext, asynchronous (does not block producer)
- VM -- multiple CamelContext, asynchronous (does not block producer)
- direct-VM -- multiple CamelContext, synchronous (blocks producer)
The direct and direct-VM mechanisms are synchronous, in the sense that the producing endpoint blocks until the consuming endpoint, and all the rest of its routing logic, is complete. The SEDA and VM mechanisms both use a pool of threads on the consumer, such that each request made by the producer is assigned to one of the threads in the pool. This allows the consumer endpoint and its associated routing logic to act independently of the producer.
Both the VM endpoints are required in situations where communication is between different Camel contexts. In many cases it is possible to combine routes into the same CamelContext. However, it may sometimes be inadvisable, for reasons of modularity, or impossible, because some application framework makes it so. For example, I might implement some Camel routing logic in a library (or component) with the intention that the library be used by other code. To be complete, this library will probably define a self-contained CamelContext with various routes. If I want to invoke the Camel logic in the library, I will need to use VM or direct-VM, because direct and SEDA endpoints do not contain the logic needed to route between Camel contexts.