# Routing

Routing is the process of searching for the actor that will handle a sent message. In order to append the message to his mailbox, we need to discover the actor's address.

elfo offers a two-level routing system. What does it mean? Messages can pass through up to two steps of routing, depending on used methods, as shown on the following diagram:

1. If we don't know any address (ctx.(try_)send(msg) and ctx.request(msg)), the inter-group router is called to determine which groups are interested in the message. Then, the corresponding inner-group router is called for each interested group to decide which shards should receive the message.
2. Only the inner-group router is called if we already know a group's address (ctx.(try_)send_to(group_addr, msg) and ctx.request_to(group_addr, msg)).
3. If we already know an actor's address (ctx.(try_)send_to(actor_addr, msg) and ctx.request_to(actor_addr, msg)), nothing additional is done because we already know which actor should handle the message.

Note that if several actors are interested in the message, each receives a copy of the message. It's ok for messages without heap-allocated fields or for rare messages, but for the big ones consider wrapping into Arc to reduce the clone() overhead.

## Inter-group routing

Inter-group routing is responsible for connecting actor groups among themselves.

Let's consider the following architecture scheme:

Actor groups and connections between them are defined in so-called "topology":


#![allow(unused)]
fn main() {
fn topology(config_path: &str) -> elfo::Topology {
let topology = elfo::Topology::empty();
let logger = elfo::logger::init();

// Define system groups.
let loggers = topology.local("system.loggers");
let telemeters = topology.local("system.telemeters");
let dumpers = topology.local("system.dumpers");
let pingers = topology.local("system.pingers");
let configurers = topology.local("system.configurers").entrypoint();

// Define user groups.
let group_a = topology.local("group_a");
let group_b = topology.local("group_b");
let group_c = topology.local("group_c");

// Define connections between user actor groups.
group_a.route_to(&group_b, |e| { // "e" means "Envelope"
msg!(match e {
MessageX | MessageY => true,
_ => false,
})
});
group_a.route_to(&group_c, |e| {
msg!(match e {
MessageX => true,
_ => false,
})
});
group_b.route_to(&group_c, |e| {
msg!(match e {
MessageZ => true,
_ => false,
})
});
group_c.route_to(&group_a, |e| {
msg!(match e {
MessageW => true,
_ => false,
})
});

// Mount specific implementations.
loggers.mount(logger);
telemeters.mount(elfo::telemeter::new());
dumpers.mount(elfo::dumper::new());
pingers.mount(elfo::pinger::new(&topology));

// Actors can use topology as a service locator.
// Usually it should be used for utilities only.
configurers.mount(elfo::configurer::from_path(&topology, config_path));

group_a.mount(group_a::new());
group_b.mount(group_b::new());
group_c.mount(group_c::new());

topology
}
}


Not all messages should be specified at this level. Usually, only requests and some multicast events are specified here, but not messages that will be passed directly, such as responses or events of subscriptions.

## Inner-group routing

Inner-group routing is responsible for choosing which shards should handle incoming messages.

The inner-group router is defined next to the actor implementation, in the group declaration.

### Stateless router


#![allow(unused)]
fn main() {
use elfo::routers::{Outcome, MapRouter};

ActorGroup::new()
.router(MapRouter::new(|e| {
msg!(match e {
MessageX { key, .. } => Outcome::Unicast(key),
MessageY { keys, .. } => Outcome::Multicast(keys.to_vec()),
_ => Outcome::Default,
})
}))
.exec(exec);
}

• Outcome::Unicast sends the message only to the actor with a specified key. If there is no active or restarting actor for the key, the new one will be started.
• Outcome::Multicast sends to several actors. New actors will be started.
• Outcome::Broadcast sends to all active actors.
• Outcome::Discard drops the message (that leads to an error on sending side).
• Outcome::Default works as Outcome::Discard for user-defined messages, but as Outcome::Broadcast for the system ones, such as UpdateConfig, ValidateConfig, Terminate and so on.

### Stateful router

Sometimes we need to use a router with rarely changing state. If the state should be changed often, consider using a dedicated actor as a router.

To create a stateful router use the MapRouter::with_state constructor. Note that the state type should implement Default, Send and Sync.


#![allow(unused)]
fn main() {
ActorGroup::new()
.router(MapRouter::with_state(
// Called if the config is changed.
|config: &Config, state| make_new_state(..),
// The routing function.
|e, state| {
msg!(match e { .. })
}
))
.exec(exec);
}


The state is recreated every time when the config is changed. Useful when all needed information (e.g. a list of actors) can be extracted from the config. Note that recreation doesn't block execution; the state is atomically replaced only once make_new_state(..) is finished.