Discovery
The Akka Discovery API enables service discovery to be provided by different technologies. It allows to delegate endpoint lookup so that services can be configured depending on the environment by other means than configuration files.
Implementations provided by the Akka Discovery module are
- Configuration (HOCON)
- DNS (SRV records)
- Aggregate multiple discovery methods
In addition the Akka Management toolbox contains Akka Discovery implementations for
Discovery used to be part of Akka Management but has become an Akka module as of 2.5.19
of Akka and version 1.0.0
of Akka Management. If you’re also using Akka Management for other service discovery methods or bootstrap make sure you are using at least version 1.0.0
of Akka Management.
See Migration hints
Module info
- sbt
val AkkaVersion = "2.7.1" libraryDependencies += "com.typesafe.akka" %% "akka-discovery" % AkkaVersion
- Gradle
def versions = [ ScalaBinary: "2.13" ] dependencies { implementation platform("com.typesafe.akka:akka-bom_${versions.ScalaBinary}:2.7.1") implementation "com.typesafe.akka:akka-discovery_${versions.ScalaBinary}" }
- Maven
<properties> <scala.binary.version>2.13</scala.binary.version> </properties> <dependencyManagement> <dependencies> <dependency> <groupId>com.typesafe.akka</groupId> <artifactId>akka-bom_${scala.binary.version}</artifactId> <version>2.7.1</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement> <dependencies> <dependency> <groupId>com.typesafe.akka</groupId> <artifactId>akka-discovery_${scala.binary.version}</artifactId> </dependency> </dependencies>
Project Info: Akka Discovery | |
---|---|
Artifact | com.typesafe.akka
akka-discovery
2.7.1
|
JDK versions | Adopt OpenJDK 8 Adopt OpenJDK 11 |
Scala versions | 2.13.10, 2.12.17, 3.1.3 |
JPMS module name | akka.discovery |
License | |
Readiness level | Supported, Lightbend Subscription provides support
Since 2.5.19, 2018-12-07
|
Home page | https://akka.io/ |
API documentation | |
Forums | |
Release notes | akka.io blog |
Issues | Github issues |
Sources | https://github.com/akka/akka |
How it works
Loading the extension:
- Scala
-
source
import akka.discovery.Discovery val system = ActorSystem() val serviceDiscovery = Discovery(system).discovery
- Java
-
source
ActorSystem as = ActorSystem.create(); ServiceDiscovery serviceDiscovery = Discovery.get(as).discovery();
A Lookup
contains a mandatory serviceName
and an optional portName
and protocol
. How these are interpreted is discovery method dependent e.g.DNS does an A/AAAA record query if any of the fields are missing and an SRV query for a full look up:
- Scala
-
source
import akka.discovery.Lookup serviceDiscovery.lookup(Lookup("akka.io"), 1.second) // Convenience for a Lookup with only a serviceName serviceDiscovery.lookup("akka.io", 1.second)
- Java
-
source
serviceDiscovery.lookup(Lookup.create("akka.io"), Duration.ofSeconds(1)); // convenience for a Lookup with only a serviceName serviceDiscovery.lookup("akka.io", Duration.ofSeconds(1));
portName
and protocol
are optional and their meaning is interpreted by the method.
- Scala
-
source
import akka.discovery.Lookup import akka.discovery.ServiceDiscovery.Resolved val lookup: Future[Resolved] = serviceDiscovery.lookup(Lookup("akka.io").withPortName("remoting").withProtocol("tcp"), 1.second)
- Java
-
source
CompletionStage<ServiceDiscovery.Resolved> lookup = serviceDiscovery.lookup( Lookup.create("akka.io").withPortName("remoting").withProtocol("tcp"), Duration.ofSeconds(1));
Port can be used when a service opens multiple ports e.g. a HTTP port and an Akka remoting port.
Discovery Method: DNS
Akka Discovery with DNS does always use the Akka-native “async-dns” implementation (it is independent of the akka.io.dns.resolver
setting).
DNS discovery maps Lookup
queries as follows:
serviceName
,portName
andprotocol
set: SRV query in the form:_port._protocol.name
Where the_
s are added.- Any query missing any of the fields is mapped to a A/AAAA query for the
serviceName
The mapping between Akka service discovery terminology and SRV terminology:
- SRV service = port
- SRV name = serviceName
- SRV protocol = protocol
Configure akka-dns
to be used as the discovery implementation in your application.conf
:
sourceakka {
discovery {
method = akka-dns
}
}
From there on, you can use the generic API that hides the fact which discovery method is being used by calling:
- Scala
-
source
import akka.discovery.Discovery import akka.discovery.ServiceDiscovery val discovery: ServiceDiscovery = Discovery(system).discovery // ... val result: Future[ServiceDiscovery.Resolved] = discovery.lookup("akka.io", resolveTimeout = 3.seconds)
- Java
-
source
import akka.discovery.Discovery; import akka.discovery.ServiceDiscovery; ServiceDiscovery discovery = Discovery.get(system).discovery(); // ... CompletionStage<ServiceDiscovery.Resolved> result = discovery.lookup("foo", Duration.ofSeconds(3));
DNS records used
DNS discovery will use either A/AAAA records or SRV records depending on whether a Simple
or Full
lookup is issued. The advantage of SRV records is that they can include a port.
SRV records
Lookups with all the fields set become SRV queries. For example:
dig srv _service._tcp.akka.test
; <<>> DiG 9.11.3-RedHat-9.11.3-6.fc28 <<>> srv service.tcp.akka.test
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60023
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADDITIONAL: 5
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 5ab8dd4622e632f6190f54de5b28bb8fb1b930a5333c3862 (good)
;; QUESTION SECTION:
;service.tcp.akka.test. IN SRV
;; ANSWER SECTION:
_service._tcp.akka.test. 86400 IN SRV 10 60 5060 a-single.akka.test.
_service._tcp.akka.test. 86400 IN SRV 10 40 5070 a-double.akka.test.
In this case service.tcp.akka.test
resolves to a-single.akka.test
on port 5060
and a-double.akka.test
on port 5070
. Currently discovery does not support the weightings.
A/AAAA records
Lookups with any fields missing become A/AAAA record queries. For example:
dig a-double.akka.test
; <<>> DiG 9.11.3-RedHat-9.11.3-6.fc28 <<>> a-double.akka.test
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11983
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 1, ADDITIONAL: 2
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 16e9815d9ca2514d2f3879265b28bad05ff7b4a82721edd0 (good)
;; QUESTION SECTION:
;a-double.akka.test. IN A
;; ANSWER SECTION:
a-double.akka.test. 86400 IN A 192.168.1.21
a-double.akka.test. 86400 IN A 192.168.1.22
In this case a-double.akka.test
would resolve to 192.168.1.21
and 192.168.1.22
.
Discovery Method: Configuration
Configuration currently ignores all fields apart from service name.
For simple use cases configuration can be used for service discovery. The advantage of using Akka Discovery with configuration rather than your own configuration values is that applications can be migrated to a more sophisticated discovery method without any code changes.
Configure it to be used as discovery method in your application.conf
akka {
discovery.method = config
}
By default the services discoverable are defined in akka.discovery.config.services
and have the following format:
akka.discovery.config.services = {
service1 = {
endpoints = [
{
host = "cat"
port = 1233
},
{
host = "dog"
port = 1234
}
]
},
service2 = {
endpoints = []
}
}
Where the above block defines two services, service1
and service2
. Each service can have multiple endpoints.
Discovery Method: Aggregate multiple discovery methods
Aggregate discovery allows multiple discovery methods to be aggregated e.g. try and resolve via DNS and fall back to configuration.
To use aggregate discovery add its dependency as well as all of the discovery that you want to aggregate.
Configure aggregate
as akka.discovery.method
and which discovery methods are tried and in which order.
akka {
discovery {
method = aggregate
aggregate {
discovery-methods = ["akka-dns", "config"]
}
config {
services {
service1 {
endpoints = [
{
host = "host1"
port = 1233
},
{
host = "host2"
port = 1234
}
]
}
}
}
}
}
The above configuration will result in akka-dns
first being checked and if it fails or returns no targets for the given service name then config
is queried which i configured with one service called service1
which two hosts host1
and host2
.
Migrating from Akka Management Discovery (before 1.0.0)
Akka Discovery started out as a submodule of Akka Management, before 1.0.0 of Akka Management. Akka Discovery is not compatible with those versions of Akka Management Discovery.
At least version 1.0.0
of any Akka Management module should be used if also using Akka Discovery.
Migration steps:
- Any custom discovery method should now implement
akka.discovery.ServiceDiscovery
discovery-method
now has to be a configuration location underakka.discovery
with at minimum a propertyclass
specifying the fully qualified name of the implementation ofakka.discovery.ServiceDiscovery
. Previous versions allowed this to be a class name or a fully qualified config location e.g.akka.discovery.kubernetes-api
rather than justkubernetes-api