I am currently working on a project that uses Modbus communication protocol.I used TTL to RS485 converter module. I have one master and six slaves. The communication between the master and individual slaves works fine. But when I connect all six slaves there seems to be some random behavior observed( Getting slave response but it's not fixed) How should I proceed?
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So the documentation to the "Replicated LevelDB Store" says:
The elected master broker node starts and accepts client connections. The other nodes go into slave mode and connect the the master and synchronize their persistent state /w it. The slave nodes do not accept client connections. All persistent operations are replicated to the connected slaves. If the master dies, the slaves with the latest update gets promoted to become the master. The failed node can then be brought back online and it will go into slave mode.
So one chosen master exist, it accepts client connections and the rest are replicated slave nodes who do not accept client connections. Fine.
So if the master dies it's all working fine - the master gets reelected, clients disconnect and they eventually connect to the new master. Awesome.
Now what happens if the master isn't dead from the perspective of Zookeeper, but it's just NOT ACCESSIBLE from clients. So a master is chosen, it's considered live(as i understand zookeeper's need to be able to connect to it to be considered available), but the actual clients can't connect to it?
Sure clients CAN connect to the other slave nodes, they just can't connect to the master. But the master won't ever be changed as it's live. Is that how it works?
Not sure i understood it right.
LevelDB support in ActiveMQ is deprecated and has been for quite some time (years) so I'd suggest not bothering with it as there is no support and plenty of open bugs that will not be fixed.
I'd suggest taking a look instead at ActiveMQ Artemis.
You understand it right, and it's a reasonable design.
Clients only commuicate to master, and slaves are just used for backup. If what you described really happens, maybe caused by network problem, then you should fix the network(or any other possible reasons).
I am trying to set up cluster of brokers, which should have same feature like rabbitMQ cluster, but over WAN (my machines are in different locations), so rabbitMQ cluster does not work.
I am looking to alternatives, rabbitMQ federation is just backup the messages in the downstream, can not make sure they have exactly the same messages available at any time (downstream still keeps the old messages already consumed in the upstream)
how about ActiveMQ Master/Slave, I have found :
http://activemq.apache.org/how-do-distributed-queues-work.html
"queues and topics are all replicated between each broker in the cluster (so often to a master and maybe a single slave). So each broker in the cluster has exactly the same messages available at any time so if a master fails, clients failover to a slave and you don't loose a message."
My concern is that if it can automatically update to make sure Master/Slave always have the same messages, which means the consumed messages in Master will also disappear in Slaves.
Thanks :)
ActiveMQ has various clustering features.
First there is High Availability - "Master/Slave". The idea is that several physical servers act as a single logical ActiveMQ broker. If one goes down, another takes it place without losing data. You can do that by sharing the message store (shared file system or shared JDBC), or you could setup a replicated cluster, which replicates read/writes to the master down to all slaves (you need three+ servers). ActiveMQ is using LevelDB and Apache Zookeeper to achieve this.
The other format of cluster available in ActiveMQ is to be able to distribute load and separate security over several logical brokers. Brokers are then connected in a network of brokers. Messages are by default passed around to the broker with available consumers for that message. However, there is a rich toolbox of features in ActiveMQ to tweak a network of brokers to do things as always send a copy of a message to specific broker etc. It takes some messing with the more advanced features though (static network connectors and queue mirroring, maybe more).
Maybe there is a better way to solve your requirements, which is not really specified in the question?
Assuming I have a Master-Slave deployment of Redis (1 master, 1 slave) and a client (webapp) that will manage Publish-Subscribe.
Can I Publish messages to the slave and will they be "seen" by the master?
Or should I use only the Master for Publish and the Slave for Subscribe commands?
I've been looking around but couldn't find the answer. Anyone knows?
EDIT: As #jameshfisher pointed out, the link below is regarding Redis Cluster. The comment from #lionello seems to be the correct answer:
Publishing to a slave will not propagate to the master, only the other way around.
The answer is on the cluster-spec docs:
Publish/Subscribe
In a Redis Cluster clients can subscribe to every node, and can also publish to every other node. The cluster will make sure that published messages are forwarded as needed.
The current implementation will simply broadcast each published message to all other nodes, but at some point this will be optimized either using Bloom filters or other algorithms.
For the typical data you store in Redis, you should only write to the master.
From http://redis.io/topics/replication:
...writes [to slaves] will be discarded if the slave and the master will [sic] resynchronize, or if the slave is restarted...
In fact, starting from v2.6, you can put slaves in slave-read-only mode which would prevent the mistake of writing data to a slave.
The documentation does go on to mention a potential use case for writing data to slaves:
...often there is ephemeral data that is unimportant that can be stored
into slaves. For instance clients may take information about
reachability of master in the slave instance to coordinate a fail over
strategy.
I'm trying to set up three brokers in a network for load balancing -- clients and producers can connect to any of these brokers.
Questions:
What is the recommended topology to use to network these brokers? More specifically, what is the networkConnector configuration to use on each of these brokers? should duplex setting be enabled? (I guess duplex setting depends on the topology we choose)
A->B->C->A or A<-->B<-->C<-->A
Client should use failover protocol to connect to these brokers, right? e.g. failover://(tcp://b1:6161, tcp://b2:6161, tcp://b3:6161)
Any duplicate message handling required on the client side in case of restarts? See http://forum.springsource.org/showthread.php?108461-Failover-issue-in-ActiveMQ -- not clear why duplicate message issue exists here
Ideally we want to set up topology as shown in this post http://edelsonmedia.com/?p=143 -- not clear how to set up networkConnector on masters and slaves.
1.) I can't actually recommend a topology. This choice depends on the number of hops (between the broker where the messages enters the cluster and the broker where the consumer conects to) you can accept. In a heave traffic scenario every hop adds to the network load.
In my company we use a hypercube network (every broker knows every other broaker) and it works great.
Generaly you should make sure that your node configurations are as similar as possible. Using duplex makes sure you have less connections to configure (since the connection from B to A is already part of the duplex connection from A to B) but it introduce a large number of differences into your config file.
Personaly i created my own start script for ActiveMQ that auto-generated the connection config based on the dns names of my cluster (mycluster-01 to 06).
2.) yes. You might want to add ?randomize=false if you want to make sure the client uses the first entry in the list.
3.) Duplicate entries can happen if there are failures during message transport or as race conditions during heavy load. In general one message only is owned by one broker.
4.) dont set up network connectors between masters and slaves (REALLY DONT). Use the pure Master Slave feature of activeMQ and configure the master for each slave (you don't have to configure anything on the masters). For the all Masters configure NetworkConnections to the other Masters with failover to their slaves)
http://activemq.apache.org/pure-master-slave.html
I am wanting to setup RabbitMQ as a two (or more) node cluster with HA.
Use case: a client producer app (C#.NET) knows that the cluster has two nodes and publishes to the cluster. Various consumer apps (also C#.NET) connect to the cluster and get all messages generated by the producer. So long as at least one node is up and running the producer and consumers will all continue to work without error. Supposing nodes A and B are running and B dies for a while, then gets restarted, then a while later A dies, the clients all continue to function without receiving an error since at all times at least one node is up.
Can it be made to work like this out of the box?
Are there any other MQs that would be more appropriate (commercial ok) for a Windows/.NET application environment?
RabbitMQ v2.6.0 now supports high-availability queues using active/active clustering. Microsoft and a number of other companies have collaborated on Apache QPid which has C# bindings and which also supports active/active HA clustering.
Can it be made to work like this out of the box?
No. When a node goes down, all of its connections are closed. Since AMQP connections are stateful, there's no way around this. What you could achieve is 1) broker goes down, 2) all clients disconnect, 3) clients connect to other node (masquerading as original) and are none the wiser.
On a side note, rabbit does not support active-active HA clustering at the moment. It does support active-passive clustering and a form of logical clustering (which might be what you're looking for).