Shared Registrations¶
As a default, only a single callee may register a procedure for an URI.
Use cases for Shared Registrations¶
There are use cases where more flexibility is required. As an example, for an application component with a high computing load, several instances may run, and load balancing of calls across these may be desired. As another example, in an an application a second or third component providing a procedure may run, which are only to be called in case the primary component is no longer reachable (hot standby).
In additional to the default rule
single
which only allows a single registration per URI WAMP offers invocation rules for shared registrations which cover the above use cases:
roundrobin
random
first
last
Basics of Invocation Rules¶
When an invocation rule other than the default single (which may
also be omitted) is set, a WAMP router will accept additional
registrations for the specified URI provided that these additional
registrations have the same invocation rule.
As an example, if a first registration for the URI
com.myapp.procedure1 is made and the invocation rule last is
set, then any subsequent registration for the URI which also sets the
invocation rule last will be accepted, while any subsequent
registration attempt which sets a different invocation rule (or does not
provide one) will fail.
Load Balancing¶
Two invocation rules are provided for load balancing of calls across multiple registered components.
With roundrobin, a call is routed to the component which comes after
the component to which the previous call was routed. Order is determined
through a list of components which have registred for the procedure,
where registrations are appended to the list. Once the end of the list
has been reached, the first component in the list is called next.
With random a call is routed to a random component from the list of
components which have registered the procedure.
Here are examples of registering procedures using the above invocation rules in a JavaScript application using Autobahn|JS as its WAMP library:
session.register("com.myapp.procedure1", procedure1, { invoke: "roundrobin"});
session.register("com.myapp.procedure2", procedure2, { invoke: "random"});
High Availability¶
Two invocation rules are provided for enabling hot stand by of components. In each case, a list of components which have registered the procedure is used, where registrations are appended to the list. Components are removed from this list on de-registration or when the WAMP session on which they were registered is closed or is lost.
With first, a call is routed to the first component in the list,
while with last the call is routed to the last component in the
list.
Here are examples of registering procedures using the above invocation rules in a JavaScript application using Autobahn|JS as its WAMP library:
session.register("com.myapp.procedure3", procedure3, { invoke: "first"});
session.register("com.myapp.procedure4", procedure4, { invoke: "last"});
Default¶
It is possible to explicitly set the invocation rule for single
invocation, i.e. that only a single registration for an URI is allowed.
session.register("com.myapp.procedure5", procedure5, { invoke: "single"});
Since this is the default, it is unnecessary though, unless there is a need to make the invocation rule explicit in the code for reasons of readability.
Note: the above examples are for Autobahn|JS since we also maintain and use this WAMP client library, and JavaScript is the closest there is to a lingua franca in programming. Users of other WAMP client libraries should feel free to add code examples for these!
For an example showing shared registrations in action, see the Crossbar.io Examples.
