I'm new to ZeroMQ.
Today I am trying the pub/sub pattern with NetMQ (the ZMQ library for .NET). I noticed some strange behavior (at least, to me!).
I create a subscriber socket and subscribes to topic "12345".
The publisher then publishes messages with topic "1234567890".
The subscriber can receive the messages!
That means, the filter does not compare the whole topic string, but only checks if the published topic "starts with" the subscribed topic.
To confirm that, I changed the subscribed topic to "2345". And the subscriber did not receive the messages.
If I change the publishing topic to "23456890" (while the subscribed topic is "2345"), then the messages come!
I'd like to know, is that the normal behavior of the topic filter in ZeroMQ (and NetMQ)?
Thank you very much!
" ...is that the normal behavior of the topic filter in ZeroMQ (and NetMQ)? "
Yes, this is documented property of ZeroMQ implementation of ultra-fast TOPIC-filtering. It works this way since ever ( v2.1.1+, v3.+, v4.+ and most probably it will remain so due to its ultimate performance and scaling envelopes ).
Also be informed, that a similar approach was taken by Martin SUSTRIK, the ZeroMQ co-father, in foundations of nanomsg and its ports and more recent breeds ( pynano, pynng et al ), so it could be called a de-facto industry best-practice, could it not?
Establish a new message filter. Newly created Subsriber sockets will
filtered out all incoming messages. Call this method to subscribe to
messages beginning with the given prefix. Multiple filters may be
attached to a single socket, in which case a message shall be accepted
if it matches at least one filter. Subscribing without any filters
shall subscribe to all incoming messages. const sub = new Subscriber()
// Listen to all messages beginning with 'foo'. sub.subscribe("foo")
// Listen to all incoming messages. sub.subscribe() Params: prefixes –
The prefixes of messages to subscribe to.
subscribe(...prefixes: Array<Buffer | string>): void;
Related
I have implemented the example from the RabbitMQ website:
RabbitMQ Example
I have expanded it to have an application with a button to send a message.
Now I started two consumer on two different computers.
When I send the message the first message is sent to computer1, then the second message is sent to computer2, the thrid to computer1 and so on.
Why is this, and how can I change the behavior to send each message to each consumer?
Why is this
As noted by Yazan, messages are consumed from a single queue in a round-robin manner. The behavior your are seeing is by design, making it easy to scale up the number of consumers for a given queue.
how can I change the behavior to send each message to each consumer?
To have each consumer receive the same message, you need to create a queue for each consumer and deliver the same message to each queue.
The easiest way to do this is to use a fanout exchange. This will send every message to every queue that is bound to the exchange, completely ignoring the routing key.
If you need more control over the routing, you can use a topic or direct exchange and manage the routing keys.
Whatever type of exchange you choose, though, you will need to have a queue per consumer and have each message routed to each queue.
you can't it's controlled by the server check Round-robin dispatching section
It decides which consumer turn is. i'm not sure if there is a set of algorithms you can pick from, but at the end server will control this (i think round robin algorithm is default)
unless you want to use routing keys and exchanges
I would see this more as a design question. Ideally, producers should create the exchanges and the consumers create the queues and each consumer can create its own queue and hook it up to an exchange. This makes sure every consumer gets its message with its private queue.
What youre doing is essentially 'worker queues' model which is used to distribute tasks among worker nodes. Since each task needs to be performed only once, the message is sent to only one node. If you want to send a message to all the nodes, you need a different model called 'pub-sub' where each message is broadcasted to all the subscribers. The following link shows a simple pub-sub tutorial
https://www.rabbitmq.com/tutorials/tutorial-three-python.html
Right now I have one single topic which has many different types of messages. Lets say Topic1 has messages of type A, B and C. I have three Queues which are subscribed to the topic (QueueA, QueueB, QueueC). I want each queue to consume only the message type it corresponds to but cannot find a way to do so. So far I am only able to subscribe to the topic (which will take every message that is on the topic). I can't seem to find a place on SolAdmin to set the selector for the messages.
I am using SolAdmin version 8.0.1.3 and Solace VMR version 8.0
Selectors act as egress filters on Solace queues. All messages that match the topic subscription will be spooled on the queue but only messages that match a consumer's selector will be sent to that consumer.
Alternatively, you could use a Topic Endpoint. Selectors act as ingress filters on Topic Endpoints. Only messages that match the selector will be spooled on the Topic Endpoint.
There is no way to define selectors from SolAdmin. The selector is defined in the application when the consumer binds a flow to the queue or the topic endpoint.
For more information about selectors in the Solace platform, please see here:
http://docs.solace.com/Solace-Messaging-APIs/Using-Selectors.htm
if I am sending a message onto a multicast topic using:
TibrvMsg replyMessage = TibRvdTransport.sendRequest(message,timeout)
and there are two subscribers, which one actually sends the replyMessage, and what happens to the other replyMessage ?
I can only guess the fastest one that that answers. But I cannot see this documented anywhere.
Since your components are decoupled, they are unaware of each other. Rendezvous is pub-sub, which means that all subscribers receive all messages published to subjects that they have subscribed to. Furthermore, Rendezvous uses a peer-to-peer messaging approach vis-a-vis a centralized message forwarding approach. Therefore both components will receive the message and both components will reply.
If this is not the desired behavior, with Rendezvous you can use a distributed queue (RVDQ). With that approach a "scheduler" assigns work to workers, ensuring that messages get processed only once.
I was wondering what is the difference between a JMS Queue and JMS Topic.
ActiveMQ page says
Topics
In JMS a Topic implements publish and subscribe semantics. When you publish a message it goes to all the subscribers who are
interested - so zero to many subscribers will receive a copy of the
message. Only subscribers who had an active subscription at the time
the broker receives the message will get a copy of the message.
Queues
A JMS Queue implements load balancer semantics. A single message will be received by exactly one consumer. If there are no
consumers available at the time the message is sent it will be kept
until a consumer is available that can process the message. If a
consumer receives a message and does not acknowledge it before closing
then the message will be redelivered to another consumer. A queue can
have many consumers with messages load balanced across the available
consumers.
I want to have 'something' what will send a copy of the message to each subscriber in the same sequence as that in which the message was received by the ActiveMQ broker.
Any thoughts?
That means a topic is appropriate. A queue means a message goes to one and only one possible subscriber. A topic goes to each and every subscriber.
It is simple as that:
Queues = Insert > Withdraw (send to single subscriber) 1:1
Topics = Insert > Broadcast (send to all subscribers) 1:n
Topics are for the publisher-subscriber model, while queues are for point-to-point.
A JMS topic is the type of destination in a 1-to-many model of distribution.
The same published message is received by all consuming subscribers. You can also call this the 'broadcast' model. You can think of a topic as the equivalent of a Subject in an Observer design pattern for distributed computing. Some JMS providers efficiently choose to implement this as UDP instead of TCP. For topic's the message delivery is 'fire-and-forget' - if no one listens, the message just disappears. If that's not what you want, you can use 'durable subscriptions'.
A JMS queue is a 1-to-1 destination of messages. The message is received by only one of the consuming receivers (please note: consistently using subscribers for 'topic client's and receivers for queue client's avoids confusion). Messages sent to a queue are stored on disk or memory until someone picks it up or it expires. So queues (and durable subscriptions) need some active storage management, you need to think about slow consumers.
In most environments, I would argue, topics are the better choice because you can always add additional components without having to change the architecture. Added components could be monitoring, logging, analytics, etc.
You never know at the beginning of the project what the requirements will be like in 1 year, 5 years, 10 years. Change is inevitable, embrace it :-)
Queues
Pros
Simple messaging pattern with a transparent communication flow
Messages can be recovered by putting them back on the queue
Cons
Only one consumer can get the message
Implies a coupling between producer and consumer as it’s an one-to-one relation
Topics
Pros
Multiple consumers can get a message
Decoupling between producer and consumers (publish-and-subscribe pattern)
Cons
More complicated communication flow
A message cannot be recovered for a single listener
As for the order preservation, see this ActiveMQ page. In short: order is preserved for single consumers, but with multiple consumers order of delivery is not guaranteed.
If you have N consumers then:
JMS Topics deliver messages to N of N
JMS Queues deliver messages to 1 of N
You said you are "looking to have a 'thing' that will send a copy of the message to each subscriber in the same sequence as that in which the message was received by the ActiveMQ broker."
So you want to use a Topic in order that all N subscribers get a copy of the message.
TOPIC:: topic is one to many communication... (multipoint or publish/subscribe)
EX:-imagine a publisher publishes the movie in the youtub then all its subscribers will gets notification....
QUEVE::queve is one-to-one communication ...
Ex:-When publish a request for recharge it will go to only one qreciever ...
always remember if request goto all qreceivers then multiple recharge happened so while developing analyze which is fit for a application
Queue is JMS managed object used for holding messages waiting for subscribers to consume. When all subscribers consumed the message , message will be removed from queue.
Topic is that all subscribers to a topic receive the same message when the message is published.
I have a design query regarding queues. My scenario is as follows:
I have to use a messaging system, with single producer and multiple consumers (asynchronous). The producer pushes different types of messages into the messaging system. Depending upon the message type, that particular consumer has to consume that message. (Each consumer is running on a different server). If one consumer is down and a message comes for that consumer, it will be in the messaging system only. If I use a message queue, the message in the queue will block the next messages that can be consumed by the other consumers. Are queues suitable for handling this kind of situation? Or do we need to go for a topic?
Whether you use a queue or a topic should depend on whether there an instance where multiple consumers must process the same message. If that is the case then a topic is required do generate that one-to-many pattern.
On the other hand, if any one message will only ever be consumed by one consumer, then you can use a queue or topic and the consumers specify the message type as a JMS selector. In this way, all consumers can listen on the same queue and each selects a different subset of messages. In the event one application is not there, it's messages do not "block the next messages that can be consumed by the other consumers" but rather they just stack up in the queue and other consumers still receive their messages based on selection criteria.
Please also realize that queues are lightweight constructions and you can easily have one queue per consumer. Typically, things providing a service listen on a well-known queue and each queue represents a different function of the service or a different service. Thus there may be many service input queues. Similarly, reply messages are generally uniquely addressed to the application instance that made the request and go to a unique, often dynamic, reply-to queue. Both of these implementations I have described lead to a separation of traffic across queues rather than pooling different message types into the same queue. Since JMS selectors always impart an additional processing cost, using more queues is generally more performant than selecting many types of message from the same queue.
I am responding to your question about selectors in the comment section here since I have more space and can put links in...
Section 3.8.1 of the JMS 1.1 spec states:
A JMS message selector allows a client to specify, by message header, the
messages it’s interested in. Only messages whose headers and properties
match the selector are delivered. The semantics of not delivered differ a bit
depending on the MessageConsumer being used. See Section 5.8,
“QueueReceiver,” and Section 6.11, “TopicSubscriber,” for more details.
Message selectors cannot reference message body values.
A message selector matches a message if the selector evaluates to true when
the message’s header field and property values are substituted for their
corresponding identifiers in the selector.
As noted above, selectors can be on fields that are implicit in the message such as MsgID or CorrelationID or thsey can be on fields specifically set by the message producer such as a message property. Either way, the client must specify the value of any selectors used by the message consumer.