Quick recap before jumping into the problem: Microservices produce artifacts; artifacts are either send downstream through some message broker to other microservices or kept in the same microservice for future requests; microservices can listen to more than one data input to build their artifact.
Previously I mentioned an example of a score microservice that produces an artifact with the current game score for each team and the names of the players that scored. This microservice could listen to:
- The teams queue: this may be needed so we can show the team name or its abbreviation in the score request; once a team appears in the championship, the microservice adds it to its state for future reference1.
- The players queue: same as above, so the microservice can return the player name, nickname, shirt number or something player related if the player scores; again, it keeps listening to the player queue and stores the players in its state.
- The match queue: if a match will happen, it has to have a score, probably starting at 0 with no players in the goal list; this is just to avoid any issues with the services requesting scores of matches that didn't start or haven't had any goals yet; in any case, the artifact will be ready to be retrieved.
- The narration queue: by listening to the narration queue, the score microservice will detect any goals, update its state and generate the new artifact.
The keyword to take from the above list is "could": Depending on the way the microservice and the messages are built, it may not be necessary to have all this.
Using full-blown messages
Let's start with the easiest way to avoid listening to all those queues: Full-blown messages.
In a full-blown message, all the related data is sent along with the main information. Using the previous example, the service could listen to just the match and narration queue, but expect that the "NewMatch" message will contain the names of the teams, their abbreviation, logo, probably some id and so on; the same for the "NewNarration" message: it will contain the player name, nickname, shirt name, player id and so on.
The problem with full-blown messages is that they tend to become bigger and bigger: As more microservices are plugged in the system, more fields may be required -- and dropped by services that don't need those fields.
The pro side of full-blown messages is that a microservice will always have all the information necessary, while keep the number of listening queues low. This would also help if you just add a new service in the pool: if it starts with a blank state, it will be able to build all the state from scratch, 'cause all the information is already there.
Listen to base queues, request the rest
Almost like the solution before, the service would listen to the narrations and matches, but once it detects a missing information (for example, the narration event says player with ID, but this ID doesn't exist in its state), the service would request the more "stale" information (players, teams and products are not added all the time, for example) for some other service and fill the lacking information in its state.
This means that this microservice now, instead of knowing only about queues, now has to have information about other services (the ones that process and store the "stale" data) and their interfaces -- and, in general, it would also require some service discovery in the system. Those microservices would be the "two faced" type of microservice, which receives information, store in the state, build the artifact but also has an interface for it to be retrieved instead of simply receiving, processing and passing it along. Caching would also be advised here, so one service can't flood the other requesting the same data over and over -- and updates from time to time would make sense in some situations.
The messages are shorter now ('cause you receive only the team/player ID instead of everything) and retrieval of information happen when necessary, but where you reduce the number of listeners, you increase the number of requests. As will full-blown messages, a new service can easily build its own state from scratch without any issues -- it will do a lot of requests, but it will, eventually, have all the necessary information.
Listen to all
This is exactly same solution as presented in the example above: the microservice keeps listening to the queues of all related events and build the state with them.
One problem with this solution: since the queues are asynchronous, there could be a problem with the ordering of the data, with goals coming before players (for different reasons). In this case... what would the service do? Reject the goal in the hopes the player will appear, to avoid any inconsistencies, and that the message broker requeue the event?
One solution would have services that, along with this one, listen to one specific data: the score microservice listens to all four, but one microservice listens only to the player queue. This service would process things way faster than the score, and serve as sort of "fallback" in case some data is missing, kinda like the solution above. This will reduce the traffic in the network, but it'd create have duplicate data in different services -- although that last point shouldn't be a problem in the first place.
New services will find it problematic, 'cause although they are receiving the main data, they are were not alive when the more "stale" data was processed; they will need to either communicate with other services to get this information, or someone will have to manually duplicate the sources.
I just describe solutions in which every data has its own queue, but what if we put all the events in the same queue? This way, order is assured (players will be queue before the goals, and the services will process players before they even see there is a goal).
This reduces the number of listeners, but it requires some good message design, specially in statically typed languages, which usually require a well-defined structure for serialization and deseralization.
But it solves almost everything else: there is no issue with the processing order, the number of listeners is low and the messages are small. But it will also make new services suffer from the lack of stale data, forcing them to communicate with other services or to have the data manually copied when they are brought up.
The best one?
Honestly, I have no idea. I'm siding with "Full-blown messages" simply 'cause it simplifies the structure of the services, even knowing that network is not free; if I used some non-statically typed language, I'd probably side with the single queue one. But, again, I don't think there is any "one size fits all".
Probably there are more architectural options for this, and those are the ones I can remember discussing with my coworkers.
It's worth noting that the microservice may simply drop some of the information. For example, if the artifact produced only requires the abbreviated name, it full name may be completely dropped from the state.