Open IMS Core does’t have standard way to define connection to MRF (Media Resource Function) on Mr interface.In IMS Mr interface is based on SIP and is similar to ISC used by Application Server (AS). Because of that we can define MRF as IMS AS and just add Wildcard PSI that has trigger on that AS. That […]
An application developed for one of my recent projects at TouK involved multiple servers. There was a requirement to ensure failover for the system’s components. Since I had already a few separate components I didn’t want to add more of that, and since there already was a Zookeeper ensemble running - required by one of the services, I’ve decided to go that way with my solution.
Just a crude distributed synchronization framework. However, it implements Paxos-style algorithms (http://en.wikipedia.org/wiki/Paxos_(computer_science)) to ensure no split-brain scenarios would occur. This is quite an important feature, since I don’t have to care about that kind of problems while using this app. You just need to create an ensemble of a couple of its instances - to ensure high availability. It is basically a virtual filesystem, with files, directories and stuff. One could ask why another filesystem? Well this one is a rather special one, especially for distributed systems. The reason why creating all the locking algorithms on top of Zookeeper is easy is its Ephemeral Nodes - which are just files that exist as long as connection for them exists. After it disconnects - such file disappears.
With such paradigms in place it’s fairly easy to create some high level algorithms for synchronization.
Having that in place, it can safely integrate multiple services ensuring loose coupling in a distributed way.
With all the base services for Zookeeper started, it seems there is nothing else, than just connect to it and start implementing necessary algorithms. Unfortunately, the API is quite basic and offers files and directories abstractions with the addition of different node type (file types) - ephemeral and sequence. It is also possible to watch a node for changes.
Creating connections is tedious - and there is lots of things to take care of. Handling an established connection is hard - when establishing connection to ensemble, it’s necessary to negotiate a session also. During the whole process a number of exceptions can occur - these are “recoverable” exceptions, that can be gracefully handled and not break the connection.
class="c8"><span>So, Zookeeper API is hard.</span></p><p class="c1"><span></span></p><p class="c8"><span>Even if one is proficient with that API, then there come recipes. The reason for using Zookeeper is to be able to implement some more sophisticated algorithms on top of it. Unfortunately those aren’t trivial and it is again quite hard to implement them without bugs.</span>
And since distributed systems are hard, why would anyone want another difficult to handle tool?
class="c8"><span>Happily, guys from Netflix implemented a nice abstraction for dealing with Zookeeper internals. They called it Curator and use it extensively in the company’s environment. Curator offers consistent API for Zookeeper’s functionality. It even implements a couple of recipes for distributed systems.</span>
class="c8"><span>The basic use of Zookeeper is as a distributed configuration repository. For this scenario I only need read/write capabilities, to be able to write and read files from the Zookeeper filesystem. This code snippet writes a sample json to a file on ZK filesystem.</span>
<a href="#"
name="0"></a>
EnsurePath ensurePath = new EnsurePath(markerPath);
ensurePath.ensure(client.getZookeeperClient());
String json = “...”;
if (client.checkExists().forPath(statusFile(core)) != null)
client.setData().forPath(statusFile(core), json.getBytes());
else
client.create().forPath(statusFile(core), json.getBytes());
Having multiple systems there may be a need of using an exclusive lock for some resource, or perhaps some big system requires it’s components to synchronize based on locks. This “recipe” is an ideal match for those situations.
ref="#"
name="b0329bbbf14b79ffaba1139881914aea887ef6a3"></a>
lock = new InterProcessSemaphoreMutex(client, lockPath);
lock.acquire(5, TimeUnit.MINUTES);
… do sth …
lock.release();
<p
class="c8"><span>This is quite an interesting use case. With many small services on different servers it is not wise to exchange ip addresses and ports between them. When some of those services may go down, while other will try to replace them - the task gets even harder. </span>
That’s why, with Zookeeper in place, it can be utilised as a registry of existing services.
If a service starts, it registers into the ServiceRegistry, offering basic information, like it’s purpose, role, address, and port.
Services that want to use a specific kind of service request an access to some instance. This way of configuring easily decouples services from their configuration.
Basically this scenario needs ? steps:
<span>1. Service starts and registers its presence (</span><span class="c5"><a class="c0"
href="https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerAdvertiser.java#L44">https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerAdvertiser.java#L44</a></span><span>)</span><span>:</span>
ServiceDiscovery discovery = getDiscovery();
discovery.start();
ServiceInstance si = getInstance();
log.info(si);
discovery.registerService(si);
2. Another service - on another host or in another JVM on the same machine tries to discover who is implementing the service (https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerFinder.java#L50):
name="3"></a>
instances = discovery.queryForInstances(serviceName);
The whole concept here is ridiculously simple - the service advertising its presence just stores a file with its whereabouts. The service that is looking for service providers just look into specific directory and read stored definitions.
In my example, the structure advertised by services looks like this (+ some getters and constructor - the rest is here: https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/model/WorkerMetadata.java):
public final class WorkerMetadata {
private final UUID workerId;
private final String listenAddress;
private final int listenPort;
}
<p
class="c8"><span>The above recipes are available in Curator library (</span><span class="c5"><a class="c0"
href="http://curator.incubator.apache.org/">http://curator.incubator.apache.org/</a></span><span>). Recipes’
usage examples are in my github repo at </span><span class="c5"><a class="c0"
href="https://github.com/zygm0nt/curator-playground">https://github.com/zygm0nt/curator-playground</a></span>
class="c8"><span>If you’re in need of a reliable platform for exchanging data and managing synchronization, and you need to do it in a distributed fashion - just choose Zookeeper. Then add Curator for the ease of using it. Enjoy!</span>
An application developed for one of my recent projects at TouK involved multiple servers. There was a requirement to ensure failover for the system’s components. Since I had already a few separate components I didn’t want to add more of that, and since there already was a Zookeeper ensemble running - required by one of the services, I’ve decided to go that way with my solution.
Just a crude distributed synchronization framework. However, it implements Paxos-style algorithms (http://en.wikipedia.org/wiki/Paxos_(computer_science)) to ensure no split-brain scenarios would occur. This is quite an important feature, since I don’t have to care about that kind of problems while using this app. You just need to create an ensemble of a couple of its instances - to ensure high availability. It is basically a virtual filesystem, with files, directories and stuff. One could ask why another filesystem? Well this one is a rather special one, especially for distributed systems. The reason why creating all the locking algorithms on top of Zookeeper is easy is its Ephemeral Nodes - which are just files that exist as long as connection for them exists. After it disconnects - such file disappears.
With such paradigms in place it’s fairly easy to create some high level algorithms for synchronization.
Having that in place, it can safely integrate multiple services ensuring loose coupling in a distributed way.
With all the base services for Zookeeper started, it seems there is nothing else, than just connect to it and start implementing necessary algorithms. Unfortunately, the API is quite basic and offers files and directories abstractions with the addition of different node type (file types) - ephemeral and sequence. It is also possible to watch a node for changes.
Creating connections is tedious - and there is lots of things to take care of. Handling an established connection is hard - when establishing connection to ensemble, it’s necessary to negotiate a session also. During the whole process a number of exceptions can occur - these are “recoverable” exceptions, that can be gracefully handled and not break the connection.
class="c8"><span>So, Zookeeper API is hard.</span></p><p class="c1"><span></span></p><p class="c8"><span>Even if one is proficient with that API, then there come recipes. The reason for using Zookeeper is to be able to implement some more sophisticated algorithms on top of it. Unfortunately those aren’t trivial and it is again quite hard to implement them without bugs.</span>
And since distributed systems are hard, why would anyone want another difficult to handle tool?
class="c8"><span>Happily, guys from Netflix implemented a nice abstraction for dealing with Zookeeper internals. They called it Curator and use it extensively in the company’s environment. Curator offers consistent API for Zookeeper’s functionality. It even implements a couple of recipes for distributed systems.</span>
class="c8"><span>The basic use of Zookeeper is as a distributed configuration repository. For this scenario I only need read/write capabilities, to be able to write and read files from the Zookeeper filesystem. This code snippet writes a sample json to a file on ZK filesystem.</span>
<a href="#"
name="0"></a>
EnsurePath ensurePath = new EnsurePath(markerPath);
ensurePath.ensure(client.getZookeeperClient());
String json = “...”;
if (client.checkExists().forPath(statusFile(core)) != null)
client.setData().forPath(statusFile(core), json.getBytes());
else
client.create().forPath(statusFile(core), json.getBytes());
Having multiple systems there may be a need of using an exclusive lock for some resource, or perhaps some big system requires it’s components to synchronize based on locks. This “recipe” is an ideal match for those situations.
ref="#"
name="b0329bbbf14b79ffaba1139881914aea887ef6a3"></a>
lock = new InterProcessSemaphoreMutex(client, lockPath);
lock.acquire(5, TimeUnit.MINUTES);
… do sth …
lock.release();
<p
class="c8"><span>This is quite an interesting use case. With many small services on different servers it is not wise to exchange ip addresses and ports between them. When some of those services may go down, while other will try to replace them - the task gets even harder. </span>
That’s why, with Zookeeper in place, it can be utilised as a registry of existing services.
If a service starts, it registers into the ServiceRegistry, offering basic information, like it’s purpose, role, address, and port.
Services that want to use a specific kind of service request an access to some instance. This way of configuring easily decouples services from their configuration.
Basically this scenario needs ? steps:
<span>1. Service starts and registers its presence (</span><span class="c5"><a class="c0"
href="https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerAdvertiser.java#L44">https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerAdvertiser.java#L44</a></span><span>)</span><span>:</span>
ServiceDiscovery discovery = getDiscovery();
discovery.start();
ServiceInstance si = getInstance();
log.info(si);
discovery.registerService(si);
2. Another service - on another host or in another JVM on the same machine tries to discover who is implementing the service (https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/curator/WorkerFinder.java#L50):
name="3"></a>
instances = discovery.queryForInstances(serviceName);
The whole concept here is ridiculously simple - the service advertising its presence just stores a file with its whereabouts. The service that is looking for service providers just look into specific directory and read stored definitions.
In my example, the structure advertised by services looks like this (+ some getters and constructor - the rest is here: https://github.com/zygm0nt/curator-playground/blob/master/src/main/java/pl/touk/model/WorkerMetadata.java):
public final class WorkerMetadata {
private final UUID workerId;
private final String listenAddress;
private final int listenPort;
}
<p
class="c8"><span>The above recipes are available in Curator library (</span><span class="c5"><a class="c0"
href="http://curator.incubator.apache.org/">http://curator.incubator.apache.org/</a></span><span>). Recipes’
usage examples are in my github repo at </span><span class="c5"><a class="c0"
href="https://github.com/zygm0nt/curator-playground">https://github.com/zygm0nt/curator-playground</a></span>
class="c8"><span>If you’re in need of a reliable platform for exchanging data and managing synchronization, and you need to do it in a distributed fashion - just choose Zookeeper. Then add Curator for the ease of using it. Enjoy!</span>
Gradle promises to hit the sweet spot between Ant and Maven. It uses Ivy's approach for dependency resolution. It allows for convention over configuration but also includes Ant tasks as first class citizens. It also wisely allows you to use existing Maven/Ivy repositories.So why would one use yet another JVM build tool such as Gradle? The answer is simple: to avoid frustration involved by Ant or Maven.
<project root>Have I just referred myself for the 1st time? Achievement unlocked! ;)
│
├── build.gradle
└── src
├── main
│ ├── java
│ └── groovy
│
└── test
├── java
└── groovy
the-appthe-app itself has no src sub-folder as its purpose is only to contain sub-projects and build configuration. If needed it could've been provided with own src though.
│
├── dao-layer
│ └── src
│
├── domain-model
│ └── src
│
├── web-frontend
│ └── src
│
├── build.gradle
└── settings.gradle
include 'dao-layer', 'domain-model', 'web-frontend'Now the gradle projects command can be executed to obtain such a result:
:projects...so we know that Gradle noticed the modules. However gradle build command won't run successful yet because build.gradle file is still empty.
------------------------------------------------------------
Root project
------------------------------------------------------------
Root project 'the-app'
+--- Project ':dao-layer'
+--- Project ':domain-model'
\--- Project ':web-frontend'
apply plugin: 'java'This single line of config for any of modules is enough to execute gradle build command under the-app directory with following result:
:dao-layer:compileJavaTo use Groovy plugin slightly more configuration is needed:
:dao-layer:processResources UP-TO-DATE
:dao-layer:classes
:dao-layer:jar
:dao-layer:assemble
:dao-layer:compileTestJava UP-TO-DATE
:dao-layer:processTestResources UP-TO-DATE
:dao-layer:testClasses UP-TO-DATE
:dao-layer:test
:dao-layer:check
:dao-layer:build
BUILD SUCCESSFUL
Total time: 3.256 secs
apply plugin: 'groovy'At lines 3 to 6 Maven repositories are set. At line 9 dependency with groovy library version is specified. Of course plugin as 'java', 'groovy' and many more can be mixed each other.
repositories {
mavenLocal()
mavenCentral()
}
dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.5'
}
def langLevel = 1.7At the beginning simple variable langLevel is declared. It's worth knowing that we can use almost any Groovy code inside build.gradle file, statements like for example if conditions, for/while loops, closures, switch-case, etc... Quite an advantage over inflexible XML, isn't it?
allprojects {
apply plugin: 'idea'
group = 'com.tamashumi'
version = '0.1'
}
subprojects {
apply plugin: 'groovy'
sourceCompatibility = langLevel
targetCompatibility = langLevel
repositories {
mavenLocal()
mavenCentral()
}
dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.5'
testCompile 'org.spockframework:spock-core:0.7-groovy-2.0'
}
}
project(':dao-layer') {
dependencies {
compile 'org.hibernate:hibernate-core:4.1.7.Final'
}
}
project(':domain-model') {
dependencies {
compile project(':dao-layer')
}
}
project(':web-frontend') {
apply plugin: 'war'
dependencies {
compile project(':domain-model')
compile 'org.springframework:spring-webmvc:3.1.2.RELEASE'
}
}
idea {
project {
jdkName = langLevel
languageLevel = langLevel
}
}
