Develop Reference

Hazelcast client subscriber not receiving message

We are running Hazelcast Java client app in one docker container & hazelcast server with single member in other container on same machine.
Our client app is able to connect to hz server container & play around with data but pub/sub is not working.
Both pub/sub threads are running in client app itself. Data is getting published as it can be seen in Hazelcast management center but it's not getting received.
Does anyone have any idea about this?
PS: We are using hazelcast v5.1.3 with default server & client config.

We resolved this issue by replacing hazelcast-5.3.1 jar with hazelcast-client-4.x jar in our client app docker image.
Posting an answer here just in case if someone faces similar issue.

How RabbitMQ connection works between 2+ endpoints?

I've looked at the tutorial for RabbitMQ RPC. The client and server basicly do the same. Assumed i've 2 or more computer who want to consume from a queue. I wonder how the know from each other if i just pass the own network hostname/ip to the ConnectionFactory. Does this example work on to different machines? (Can not test because of config issues).

Yes, it works on different machines. If you want to connect from a different computer just pass a different Host to your ConnectionFactory.
If you can't connect from a different machine due to configuration problems, perhaps you are using the guest:guest user, which can only connect from localhost? https://www.rabbitmq.com/access-control.html

GAE: MQTT broker

So I was wondering if it's possible to run a MQTT broker on the Google App Engine platform?
Couldn't find any information about it (or maybe I might be using the wrong keywords).
I've got my GAE running on Java so I'd like to go into direction of running the MQTT broker on GAE using a backend.
EDIT:
Did some further research and it seems Moquette is running on Java. Does someone have experience running Moquette on the GAE?
EDIT2:
Ok, it seems the examples of Moquette are running using an OSGi container, which is unavailble in GAE. Looking for a script to start this server on GAE.

MQTT is protocol on top of TCP. In order to run MQTT server, one needs to be able to open a listening socket. Those are still not supported on normal AppEngine instances.
Note: GAE backends have been replaced: now you just have automatic scaled (aka frontend) instances and manual scaled (aka backend) instances.
Back to your problem: Managed VMs have most of the benefits of GAE (access to services), but run a full JVM, which allows listening sockets.

An alternative to Moquette would also be the HiveMQ broker, it also runs on Java and can be easily installed. All the documentation is available here.
We haven't tested it on GAE yet, but if you have any problems running it, you could ask in the support forum.
Update: If Peter Knego is right, then HiveMQ or any other MQTT broker won't work on GAE.
Full disclose: I'm working for the company, who develops HiveMQ.
Cheers,
Christian

#Peter Knego is definitely right, and all i would add to his answer is that,
If you manage to configure you application to use a custom Runtime on the Managed Vms of Appengine and Compute Engine,
then you will be able to run you MQTT brooker perfectly sound and well.
As long as you define a fire wall to allow a tcp connection at the port which your broker is listening from.
By default the ports are blocked for security reasons.

Manage Topics on Mosquitto

I want to develop a simple push notification system for various android devices.
After two days of documentation i've found very interesting to use MQTT protocol.
System needs:
- an MQTT client on Android: eclipse Paho seems to be a good solution
- any MQTT broker you like for the server part: my attention is on Mosquitto cause is Open-Source
I've read documentation and the configuration file and seemed clear. The only thing that I couldn't understand is if i can manage topics (where clients subscribe to). In particular if i can create topics and how to.
Thanks.

It's not clear whether you mean on the client side or the broker side...
Assuming you mean the broker side, you don't need to create topics, they are created automatically as needed. You can control what clients have access to what topics using the built in authentication and ACL code, or you could use an authentication and access plugin. Writing your own is quite easy, or you could use the excellent https://github.com/jpmens/mosquitto-auth-plug
On the client side, simply subscribing or publishing to a topic is sufficient to create it on the broker, assuming you have permissions for that topic. This may not be the case in all MQTT implementations.
As an example, try subscribing to the # topic on test.mosquitto.org or iot.eclipse.org. These show a huge range of available topics and there is no admin to support them in the background.
This basically means that the only administrative load you have is restricting access on the broker where required.

Using code stole from somewhere, I made a pretty simple "MQTTMessenger" app for Android. https://bitbucket.org/mjgasner/mqttmessenger
From what I remember, you'll need to go into src/com/gasner/mqttmessenger/MessengerService.java
and change the host ip to the ip of your MQTT server.
For the MQTT server, I used Mossquitto server.
You'll also have to make sure you port forward port 1883 to the server.
I used this to send stock numbers to my phone. It's been almost 2 years ago, otherwise I could tell you more.
The C2DM directory is unrelated. It is Google's push notification service that I was also playing with at the time.
In the other directories at Bitbucket https://bitbucket.org/mjgasner are the .NET service and winforms app I wrote to communicate with the MQTT server. The service would accept the message from the android app through the server and reply with the stock amount it pulled from google's api.
If you have any more questions, comment below.

Cannot Connect to HBase Server from Java Code

I have installed Hadoop and Hbase on a VirtualBox VM running Ubuntu; both Hadoop and Hbase are running successfully in pseudo-distributed mode. I have disabled IPv6 on Ubuntu and changed the localhost to 127.0.0.1 in the hosts file on the VM.
I am trying to write some basic Java code on a Windows machine in Eclipse to connect to the Hbase instance, create a table, insert and retrieve data, etc. The code fails with an error that it cannot connect to the master. However, it makes the Zookeeper connection to the VM just fine.
On the Windows machine, I am able to connect to the Hbase instance info via the web browser via the same IP address and port that I specify in the Java code.
I have searched everywhere and tried everything that I could find, but it is still failing to connect to the master after it makes the zookeeper connection.
I have read that others have had this problem too, but no one has posted a solution.
Please help! Thanks!

The IP and Port used to view information are not the one used to read/write from/into HBase. To do so you need to use either the REST API (included in HBase) or Apache Thrift (2 thrift servers are included in HBase - thrift & thrift2)
I would recommend you to use Apache Thrift (thrift2)
To start REST use :
$HBASE-INSTALL-DIR/bin/hbase-deamon.sh start rest
To start Thrift use :
$HBASE-INSTALL-DIR/bin/hbase-deamon.sh start thrift
To start Thrift (v2) use :
$HBASE-INSTALL-DIR/bin/hbase-deamon.sh start thrift2
To use the Thrift client from Java for example you will need to install thrift on the server and then generate the Java Classes using the hbase thrift file included with HBase.
By default Thrift will be listening on the 9090 port and REST on the 8080
Usefull Links :
HBase Thrift
HBase REST

Ok -- Someone gave me some 1-1 help that fixed the problem and I wanted to pass it along. It turned out to be an IP addressing issue with the VM and with my Windows machine. First, in the etc/hosts file on the VM, I had to take out '127.0.0.1 locahost' and instead insert ' localhost'. Second, on my Windows hosts file, I had to add ' '. Thankfully, that fixed the problem. Please let me know this is unclear since I have seen this problem posted quite a few times without suitable resolution. Also, since I am writing Java code to access the HBase instance in the VM, there was no need to use Thrift or REST -- the Java API was sufficient.