Messaging middleware solutions (JMS, Tibco, etc.) allow publish/subscribe with "topic" filtering using wildcards to subscribe to all messages of a certain "topic", e.g. SUBSCRIBE("ACCOUNT.*") topic allows you to subscribe to both "ACCOUNT.WITHDRAW" message and "ACCOUNT.CHECKBALANCE" message.
The problem is that such subscription also receives my own published messages.
I'm looking for a mechanism, similar to, say, UDP multicast loopback which can be turned ON or OFF by the transport layer without messing with the data being sent.
Is there a common, declarative (no custom code, configuration only) way to configure the middleware not to receive messages which that very same service instance has published? Ideally, this should also be able to filter out everything published by ALL servers (nodes) of the same "kind".
Thanks in advance.
The JMS API contains this option for TopicSubscribers, e.g. TIBCO EMS let's you create a consumer with the "noLocal" property. That means no messages published over the same connection, get consumed by clients on the same connection.
e.g. take a look here how to create a topic subscriber with the "noLocal" option:
https://docs.tibco.com/pub/enterprise_message_service/7.0.1-march-2013/doc/html/tib_ems_api_reference/api/javadoc/javax/jms/TopicSession.html
No one is answering, so I'll chime in (in a hand-wavey way).
I believe there's nothing in the JMS spec around controlling whether you get your own sent messages back on a topic receiver. So any capability like this would be a non-portable vendor feature. Especially for your second requirement (based on "kind" of JMS client versus some control based on the same connection doing the sending/receiving).
If you've got no flexibility to modify code or message content (properties), I think you've got no portable solutions. And likely no solution at all for that second "kind" requirement.
If you want to investigate vendor-specific options, you'll need to tell us which vendor you're interested in. You may get nothing, but there's no way to know without asking.
Related
Recently I am trying to use MassTransit in our microservice ecosystem.
According to MassTransit vocabulary and from documents my understanding is :
Publish: Sends a message to 1 or many subscribers (Pub/Sub Pattern) to propagate the message.
Send: Used to send messages in fire and forget fashion like publish, but instead It is just used for one receiver. The main difference with Publish is that in Send if your destination didn't receive a message, it would return an exception.
Requests: uses request/reply pattern to just send a message and get a response in a different channel to be able to get response value from the receiver.
Now, my question is according to the Microservice concept, to follow the event-driven design, we use Publish to propagate messages(Events) to the entire ecosystem. but what is exactly the usage (use case) of Send here? Just to get an exception if the receiver doesn't exist?
My next question is that is it a good approach to use Publish, Send and Requests in a Microservices ecosystem at the same time? like publish for propagation events, Send for command (fire and forget), and Requests for getting responses from the destination.
----- Update
I also found here which Chris Patterson clear lots of things. It also helps me a lot.
Your question is not related to MassTransit. MassTransit implements well-known messaging patterns thoughtfully described on popular resources such as Enterprise Integration Patterns
As Eben wrote in his answer, the decision of what pattern to use is driven by intent. There are also technical differences in the message delivery mechanics for each pattern.
Send is for commands, you tell some other service to do something. You do not wait for a reply (fire and forget), although you might get a confirmation of the action success or failure by other means (an event, for example).
It is an implementation of the point-to-point channel, where you also can implement competing consumers to scale the processing, but those will be instances of the same service.
With MassTransit using RabbitMQ it's done by publishing messages to the endpoint exchange rather than to the message type exchange, so no other endpoints will get the message even though they can consume it.
Publish is for events. It's a broadcast type of delivery or fan-out. You might be publishing events to which no one is listening, so you don't really know who will be consuming them. You also don't expect any response.
It is an implementation of the publish-subscribe channel.
MassTransit with RabbitMQ creates exchanges for each message type published and publishes messages to those exchanges. Consumers create bindings between their endpoint exchanges and message exchanges, so each consumer service (different apps) will get those in their independent queues.
Request-response can be used for both commands that need to be confirmed, or for queries.
It is an implementation of the request-reply message pattern.
MassTransit has nice diagrams in the docs explaining the mechanics for RabbitMQ.
Those messaging patterns are frequently used in a complex distributed system in different combinations and variations.
The difference between Send and Publish has to do with intent.
As you stated, Send is for commands and Publish is for events. I worked on a large enterprise system once running on webMethods as the integration engine/service bus and only events were used. I can tell you that it was less than ideal. If the distinction had been there between commands and events it would've made a lot more sense to more people. Anyway, technically one needs a message enqueued and on that level it doesn't matter, which is why a queueing mechanism typically would not care about such semantics.
To illustrate this with a silly example: Facebook places and Event on my timeline that one of my friends is having a birthday on a particular day. I can respond directly (send a message) or I could publish a message on my timeline and hope my friend sees it. Another silly example: You send an e-mail to PersonA and CC 4 others asking "Please produce report ABC". PersonA would be expected to produce the report or arrange for it to be done. If that same e-mail went to all five people as the recipient (no CC) then who gets to do it? I know, even for Publish one could have a 1-1 recipient/topic but what if another endpoint subscribed? What would that mean?
So the sender is responsible, still configurable as subscriptions are, to determine where to Send the message to. For my own service bus I use an implementation of an IMessageRouteProvider interface. A practical example in a system I once developed was where e-mails received had to have their body converted to an image for a content store (IBM FileNet P8 if memory serves). For reasons I will not go into the systems were stopped each night at 20h00 and restarted at 6h00 in the morning. This led to a backlog of usually around 8000 e-mails that had to be converted. The conversion endpoint would process a conversion in about 2 seconds but that still takes a while to work through. In the meantime the web front-end folks could request PDF files for conversion to paged TIFF files. Now, these ended up at the end of the queue and they would have to wait hours for that to come back. The solution was to implement another conversion endpoint, with its own queue, and have the web front-end configured to send the same message type, e.g. ConvertDocumentCommand to that "priority" queue for processing. Pretty easy to do. Now, if that had been a publish how would I do that split? The same event going to 2 different endpoints under different circumstances? Well, you could have another subscription store for your system but now you'd need to maintain both. There could be another answer such as coding this logic into the send bit but that is a design choice and would require coding changes.
In my own Shuttle.Esb service bus I only have Send and Publish. For request/response both the sender and receiver have an inbox and a request would be sent (Send) to the receiver and it in turn could reply (also a Send but uses the sender's URI).
We have a requirement to copy messages from one ActiveMQ broker to another. Here the message has to just copy and the message should exist in both broker.
I can think of a custom application that subscribes to a certain destination and read that message and re-post the messages to the destination in multiple brokers.
I do not have access to make changes in the Broker so I couldn't think of Network of Brokers option.
Is there any best practice or tools available to copy A-MQ messages from one broker to another?
Without having access to the target broker, as far as I know and I have read, I believe there is not shortcut to avoid the custom application that re-post those messages.
However, depending on the messages you want to re-post, there might be some functionalities offered by ActiveMQ that could facilitate your implementation (but they would not be for free, regarding the computational costs).
For example, in the case you want to copy ALL the messages sent through that broker to the other, then you might consider using Mirrored Queues, with a specific prefix (e.g. "copy"), that would allow you to just have a single consumer using a wildcard after that prefix (e.g. "copy.>"). That consumer would get ALL the messages sent to the broker, and it would simplify your implementation since you would just have to care about that single consumer and re-post from it. However this has costs, since as it is described in the documentation, enabling the mirrored queues will make a duplicate of each queue/topic in the system, and will post each message twice. You need to consider if this is an important inconvenient in your case, depending on the amount of messages and the available memory that your broker disposes.
In case you just wanted to copy SOME of the messages and not all, then I believe the most elegant way to handle it is by creating an abstraction of your Consumer class (or specific implementation), and use that special implementation for those queues you want to re-post. That class would be responsible of re-posting the messages to the other broker, in a way that would be transparent from the other Consumer class when using it.
I have talked above about consumers, but the same concept could apply to topics and subscribers. Hope these ideas help :)
I'm attempting to do a pub/sub architecture where multiple publishers and multiple subscribers exist on the same bus. According to what I've read on the internet, only one socket should ever call bind(), and all others (whether pub or sub) should call connect().
The problem is, with this approach I'm finding that only the publisher that actually calls bind() on the socket ever publishes messages. All of my publishers that call connect() seem to fail silently and don't actually publish any messages to the bus. I've confirmed this isn't a subscriber key issue, as I've written a simple "sniffer" app that subscribes to all messages on the bus, and it is only showing the publisher that called bind().
If I attempt multiple binds with the publisher, the "expected" zmq behavior of silently stealing the bus occurs with ipc, and a port in use error is thrown with tcp.
I've verified this behavior with ipc and tcp endpoints, but ultimately the full system will be using epgm. I assume (though of course may be wrong) that in this situation I wouldn't need a broker since there's no dynamic discovery occurring (endpoints are known, whether ipc, tcp, or epgm multicast).
Is there something I'm missing, perhaps a socket setting, that would be causing the connecting publishers to not actually send their data? According to the literature I've seen on the internet, I'm doing things the "correct" way but it still doesn't work.
For reference, my publisher class has the following methods for setting up the endpoint:
ZmqPublisher::ZmqPublisher()
: m_zmqContext(1), m_zmqSocket(m_zmqContext, ZMQ_PUB)
{}
void ZmqPublisher::bindEndpoint(std::string ep)
{
m_zmqSocket.bind(ep.c_str());
}
void ZmqPublisher::connect(std::string ep)
{
m_zmqSocket.connect(ep.c_str());
}
So ultimately, my question is this: What is the proper way to handle multiple publishers on the same endpoint, and why am I not seeing messages from more than one publisher?
It may or may not be relevant, but The 0MQ Guide has the following slightly enigmatic remark:
In theory with ØMQ sockets, it does not matter which end connects and which end binds. However, in practice there are undocumented differences that I'll come to later. For now, bind the PUB and connect the SUB, unless your network design makes that impossible.
I've not yet discovered where the "come to later" actually happens, but I don't use pub/sub so much, and haven't read the "Advanced Pub-Sub Patterns" part of the guide in great detail.
However, the idea of multiple publishers on a single end-point, to me, suggests the need for an XPUB/XSUB style broker; it's not about dynamic discovery, it's about single point of contact and routing. Ultimately, I think a broker-based topology would simplify your application, and make it easier to identify problems.
Your mistake was that you call a single publisher with bind and others with connect. This is not supported with plain PUB-SUB pattern.
Plain PUB-SUB in ZeroMQ supports only two scenarios (see the image below):
single publisher, multiple subscribers
single subscriber, multiple publishers
In both cases, the party that is "single" must bind and the party that is "multiple" must connect. Otherwise, if you want many-to-many, you can use XPUB-XSUB or some other pattern.
I need some help with topics and selectors.
I have a scenario with a topic having multiple durable subscribers (each with a selector)
not all messages going into the topic aren't read by the consumers - because of unmatching selectors.
This is correct behavior.
However the problem occurs when the unmatched messages reach a certain quantity threshold, because at that point no other messages are being delivered to the consumers
activemq tries to dispatch those old unmatchable messages, but since there is no consumer for them everything is stuck
can anybody help with this?
My setup is ActiveMq 5.5
is there some configuration option, or is it just a flawed design?
I'd say this is a flawed design given there are better alternatives and perhaps a bug in ActiveMQ.
First question: is your producer publishing to this topic setting the JMSExpiration header on those messages?
If yes, the first thing I'd do is create a Jira issue detailing the scenario you described above, because it does seem incorrect that ActiveMQ will continue hold on to and continue send messages for which no selectors apply.
As for flawed design, the minute you hear yourself saying "I need durable subscribers" and you are using ActiveMQ, you should immediately turn to using Virtual Destinations instead. Virtual Destinations have the advantages of topics in that a producer can send a message to a destination and have that message propagated to N number of other destinations for consumption, but doesn't have the disadvantages that come with having durable subscribers on a topic. Read more about Virtual Destinations here.
This is related to the way that ActiveMQ handles sparse selectors. The current implementation doesn't page into the store to look for message matching sparse selectors so you need to make some configuration changes to try and work around this. You can set the maxBrowsePageSize in the configured destination policy, the default is 400. See this page.
Virtual destinations in ActiveMQ is probably the better choice, it almost always is when thinking about using durable subscribers. You could however add some message expiration to you messages and configure the policy to expire messages on inactive durable subscribers if you use a SNAPSHOT version of ActiveMQ 5.6.
It does seem like a bug, (or dashedly inconvenient at the least) but there is a work around using Virtual Destinations.
Quote:
Note that making a topic virtual does add a small CPU overhead when
sending messages to the topic but it is fairly small. From version
5.4, dispatch from virtual topics to subscription queues can be
selectorAware such that only messages that match one of the existing
subscribers are actually dispatched. Using this option prevents the
build up of unmatched messages when selectors are used by exclusive
consumers.
I am looking for a way to send requests and receive call backs from another party.
The only gotcha is that we do not now how it will be designed/deployed on the receiver side.
We do have the text/JSON based messages defined and agreed upon.
Looked at RabbitMQ and others, but each requires a server that would need to be maintained.
Thanks,
RabbitMQ is pretty easy to maintain. You would use two queues, one for requests and the other for replies. Use the AMQP correlation_id header to tag requests and replies so that when a reply message is received it can be matched with the orginal request.
However, if a broker is not for you, then use ZeroMQ. It is a client library available for a dozen or more languages and it enforces messaging patterns over top of sockets. This means that your app does not have to do all the low level socket management. Instead you declare the socket as REQ/REP and ZeroMQ handles all the rest. You just send messages in any format you desire, and you get messages back.
I've used ZeroMQ to implement a memcache style application in Python using REQ/REP.
#user821692: You have to agree not only message format but also destination/transport protocol. For e.g. if both communicating parties has access to same queue physically located anywhere, then they can communicate pre-defined messages. You may also look of sending messages over HTTP..