For another question, I'm running into a misconception that seems to arise here at SO occasionally. Some questioners seem to think that Triggers are to Databases as Events are to OOP.
Does anyone have a good analogy to explain why this is a flawed comparison, and the consequences of misapplying it?
EDIT:
Bill K. has hit it correctly, but maybe doesn't see the importance of the critical differeence between the event and the callback function that strikes me, anyway. Triggers actually cause code to execute every time the event occurs; callbacks only occur whenever one has been registered for an event (which is not true for the vast majority of events); and even then, in most cases the callback's first action is to deregister itself (or at least the callback contains a qualifcation exit so it only executes once.)
If you write a trigger, it will unfailingly execute every time the event occurs, because there's no way to register or deregister to code segment.
Triggers are a way to interpose repeating logic synchronously into the thread of execution (i.e. synchronicity). Events are a means to defer logic until later (i.e. implement asynchronicity).
There are exceptions and mitigations in both cases, but the basic patterns of triggers and callbacks are mostly opposite in intention and implementation. Often the distinction doesn't seem to have fully sunk in. (IMHO, YMMV). :D
They're not the same thing, but they're not unrelated.
In both cases, the mechanism can be described approximately as follows:
Some block of code declares "interest" for changes in state.
Your application affects some change.
The system runs the block of code in response to the change.
Perhaps a database trigger is more like a callback function that has registered interest in a specific event.
Here's an analogy: the event is a rubber ball that you throw. The trigger is a dog that chases after a thrown ball.
If there's some other difference that you have in mind that makes it "dangerous" (note: OP has edited this choice of word out of the question) to compare triggers and events, you can describe what you mean.
Triggers are a way to interpose
repeating logic synchronously into the
thread of execution (i.e.
synchronicity). Events are a means to
defer logic until later (i.e.
implement asynchronicity).
Okay, I see what you mean more clearly. But I think it's in some ways subject to the implementation. I wouldn't assume an event handler has to deregister itself; it depends on the system you're using. A UNIX signal handler, for example, has to prevent itself from catching a new signal while it's already handling one. But a Java servlet inside a Tomcat container should be thread-safe because it may be called concurrently by multiple threads. They're both event handlers, of different kinds.
Event handlers may be synchronous or asynchronous. Can a handler in a publish/subscribe system read messages that were posted recently, but prior to the handler registering its interest? Or only messages posted concurrently?
There's another important reason to treat triggers as different from event handlers: I frequently recommend against doing anything in a trigger that affects state outside the database.
For example, sending an email, writing to a file, posting to a web service, or forking a process is inappropriate inside a trigger. If for no other reason than the transaction that spawned the trigger may be rolled back, but you can't roll back those external effects. You may not even be using explicit transactions, but say you send an email in a BEFORE trigger, but the operation fails because of a NOT NULL constraint or something.
Instead, all such work should be done by code in one's application, after one has confirmed that the SQL operation was successful and the transaction committed.
It's too bad that people keep trying to do inappropriate work inside a trigger. There are senior developers at MySQL who promote UDFs to read and write data in memcached. Wow -- I just noticed these have made it into the MySQL 6.0 product!! Shocking!
So here's another attempt at an analogy, comparing triggers and events to the process of a criminal trial:
A BEFORE trigger is an allegation.
An AFTER trigger is an indictment.
COMMIT is a conviction after a guilty verdict.
ROLLBACK is an acquittal after an innocent verdict.
You only want to put the perpetrator in prison after they are convicted.
Whereas an EVENT is the crime itself.
Related
I'm currently looking at implementing CQRS driven by events (not yet event sourcing) in for a service at work; the reasoning being:
I need aggregate data to support a RestAPI coming out of this service (which will be used to populate views)- however the aggregated data will not be used by the application logic/processing (ie the data originating outside this service, the bits that of the aggregate originating within it will be used)
I need to stream events to other systems so that they can react to the data (will produce to a Kafka topic, so the 'read'/'projection' side of this system will consume the same events as the external systems, from these Kafka topics
I will be consuming events from internal systems to help populate the aggregate for the views in first point (ie it's data from this service and other's)
The reason for not going event sourced currently is that a) we're in a bit of a time crunch, and b) due to still learning about it. Having said which, it is something that we are looking to do in the future- though currently, we have a static DB in the 'Command' side of the system, which will just store current state
I'm pretty confident with the concept of using the aggregate data to provide the Rest API; however my confusion is coming from when I want to change a resource from within the system (for example via a cron job triggered 5 times a day) Example:
If I have resource of class x, which (given some data), wants a piece of state changing
I need to select instances of the class x which meet the requirements (from one of the DB's). Think select * from {class x} where last_changed_ date > 5 days ago;
Then create a command to change the state of these instances of x (in my case, the static command DB would be updated, as well as an event made to update the read DB)
The middle bullet point is what is confusing me. If I pull the data out of the Read DB, and check some information on it, then decide to change a property; I then have to convert the object from the 'Read Object' to the 'Command Object', so that I can then persist it and create an event? With my current architecture- I could query the command DB no problem, to find all the instances of {class x} that match the criteria, however I don't know if a) this is the right thing to do, and b) how this would work if I was using an event store as a DB? I'd have to query a table with millions of rows to find the most recent bit of state about the objects, to then see if they match?
Lots of what I read online has been very conceptual- so I think when it comes to implementations it maybe seems more difficult than it is? Anyhow, if anyone has any advice it would be hugely appreciated!
TIA :)
CQRS can be interpreted in a "permissive" way: rather than saying "thou shalt not query the command/write side", it says "it's OK to have a query/read side that's separate from the command/write side". Because you have this permission to do such separation, it follows that one can optimize the command/write side for a more write-heavy workload (in practice, there are always some reads in the command/write side: since command validation is typically done against some state, that requires some means of getting the state!). From this, it's extremely likely that there will be some queries which can be performed efficiently against the command/write side and some that can't be (without deoptimizing the command/write side). From this perspective, it's OK to perform the first kind of query against the command/write side: you can get the benefit of strong consistency by doing that, though be sure to make sure that you're not affecting the command/write side's primary raison d'etre of taking writes.
Event sourcing is in many ways the maximally optimized persistence model for a command/write side, especially if you have some means of keeping the absolute latest state cached and ensuring concurrency control. This is because you can then have many times more writes than reads. The tradeoff in event sourcing is that nearly all reads become rather more expensive than in an update-in-place model: it's thus generally the case that CQRS doesn't force event sourcing but event sourcing tends to force CQRS (and in turn, event sourcing can simplify ensuring that a CQRS system is eventually consistent, which can be difficult to ensure with update-in-place).
In an event-sourced system, you would tend to have a read-side which subscribes to the event stream and tracks the mapping of X ID to last updated and which periodically queries and issues commands. Alternatively, you can have a scheduler service that lets you say "issue this command at this time, unless canceled or rescheduled before then" and a read-side which subscribes to updates and schedules a command for the given ID 5 days from now after canceling the command from the previous update.
In concurrent systems, domain events are typically handled asynchronously. In Go, a simple approach for asynchronous event handling can be implemented via channels, but the issue is that if something bad happens for handling an event, or worst, for the whole program, the event will be lost.
How asynchronous domain events can be handled properly in a Go program, i.e.:
When an event handler fails, the event should not be purged from the event queue, in order to be handled properly in a later time.
If the whole program goes down, the events have to be recovered and processed accordingly.
The first is relatively easy; you can have an error handler within the worker that re-queues the work in the event of an error.
The second is much harder; your options are a) roll your own bulletproof mechanism for writing events to disk and purging them when they're completed in a thread-safe way or b) use one of the many, many popular systems available that's already proven reliable, e.g. RabbitMQ or Kafka, with the appropriate replication and redundancy to ensure the level of reliability you require. I would strongly recommend the latter.
I have a class that implements a file-monitoring service to detect when a file I am interested in has been changed by something other than my application. I use the standard technique of opening the file (with the O_EVTONLY flag) and binding the file descriptor to a Grand Central Dispatch source of type DISPATCH_SOURCE_TYPE_VNODE. When I get an event, I notify my main thread with NSNotificationCenter's postNotificationName:object:userInfo: which calls an observer in my app delegate. So far so good. It works great. But, in general, if the triggering event is an attributes change (i.e. the DISPATCH_VNODE_ATTRIB flag is set on return from dispatch_source_get_data()) then I usually get two closely-spaced events. The behaviour is easily exhibited if I touch(1) the object I am monitoring. I hypothesise this is due to the file's mtime and atime being set non-atomically although I can't verify this. This can lead to spurious notifications being sent to my observer and this raises the possibility of race conditions etc.
What is the best way of dealing with this? I thought of storing a timestamp for the last event received and only sending a notification if the current event is later than this timestamp by some amount (a few tens of milliseconds?) Does this sound like a reasonable solution?
You can't ever escape the "race condition" in this situation, because the notification of your GCD event source in your process is not synchronous with the other process's modification of the underlying file. So, no matter what, you must always be tolerant of the possibility that the change you're being notified for could already be "gone."
As for coalescing, do whatever makes sense for your app. There are two obvious strategies. You can act immediately on a received event, and then drop subsequent events received in some time window on the floor, or you can delay every event for some time period during which you will drop other events for the same file on the floor. It really just depends on what's more important, acting quickly, or having a higher likelihood of a quiescent state (knowing that you can never be sure things are quiescent.)
The only thing I would add is to suggest that you do all your coalescence before dispatching anything to the main thread. The main thread has things like tracking loops, etc that will make it harder to get time-based coalescing right in certain cases.
When we call fsm.process_event('eventname');
is there a way to return true if the transition occured and false if "no_transition" was called or an exception occurred?
Thanks
Seeing as no one has answered so far I'll post my quite humble suggestion. You could try calling the current_state() method before and after calling fsm.process_event() and compare the results. This however would not cover the case of self transitions or internal transitions and is not something I would use if there are other alternatives (its a hack at best).
If you are trying to catch the case of an event not being handled by any state and just propagating through you could add one more bottom layer superstate which reports events that reach it (i.e. are ignored by all states they propagated through).
I have had situations where I needed to know if some event actually did something and when it did it (maybe it was deferred first and then executed). In that case I made my MSM post "ACK" messages to an outside queue, I'm not sure if this applies to your problem.
In my humble knowledge interrupts and state machines don't mix very well, I usually either simply swallow them or try and turn them into some event depending on the context. You should never allow you sates (the underlying function objects) to throw.
At work, we have a huge framework and use events to send data from one part of it to another. I recently started a personal project and I often think to use events to control the interactions of my objects.
For example, I have a Mixer class that play sound effects and I initially thought I should receive events to play a sound effect. Then I decided to only make my class static and call
Mixer.playSfx(SoundEffect)
in my classes. I have a ton of examples like this one where I initially think of an implementation with events and then change my mind, saying to myself it is too complex for nothing.
So when should I use events in a project? In which occasions events have a serious advantage over others techniques?
You generally use events to notify subscribers about some action or state change that occurred on the object. By using an event, you let different subscribers react differently, and by decoupling the subscriber (and its logic) from the event generator, the object becomes reusable.
In your Mixer example, I'd have events signal the start and end of playing of the sound effect. If I were to use this in a desktop application, I could use those events to enable/disable controls in the UI.
The difference between Calling a subroutine and raising events has to do with: Specification, Election, Cardinality and ultimately, which side, the initiator or the receiver has Control.
With Calls, the initiator elects to call the receiving routine, and the initiator specifies the receiver. And this leads to many-to-one cardinality, as many callers may elect to call the same subroutine.
With Events on the other hand, the initiator raises an event that will be received by those routines that have elected to receive that event. The receiver specifies what events it will receive from what initiators. This then leads to one-to-many cardinality as one event source can have many receivers.
So the decision as to Calls or Events, mostly has to do with whether the initiator determines the receiver is or the receiver determines the initiator.
Its a tradeoff between simplicity and re-usability. Lets take an metaphor of "Sending the email" process:
If you know the recipients and they are finite in number that you can always determine, its as simple as putting them in "To" list and hitting the send button. Its simple as thats what we use most of the time. This is calling the function directly.
However, in case of mailing list, you don't know in advance that how many users are going to subscribe to your email. In that case, you create a mailing list program where the users can subscribe to and the email goes automatically to all the subscribed users. This is event modeling.
Now, even though, in both above option, emails are sent to users, you are a better judge of when to send email directly and when to use the mailing list program. Apply the same judgement, hope that you would get your answer :)
Cheers,
Ajit.
I have been working with a huge code base at my previous work place and have seen, that using events can increase the complexity quite a lot and often unnecessarily.
I had often to reverse engineer existing code in order to fix it or to extend it.
In both cases, it is a lot easier to understand what is going on, when you can simply read a list of function calls instead of just seeing the raise of an event.
The event forces you to look for usages in order to fully understand what is happening. Not a problem with modern IDEs, but if you then encounter many functions, which also raise events, it quickly becomes complex. I had encountered cases, where it mattered in what order functions did subscribe to an event, even though most languages don't even gurantee a calling order...
There are cases when it is a really good idea to use events. But before you start eventing, consider the alternative. It is probably easier to read and mantain.
A Classic example for the use of events is a UI framework, which provides elements like buttons etc.
You want the function "ButtonPressed()" of the framework to call some of your functions, so that you can react to the user action.
The alternative to an event that you can subscribe to, would for example be a public bool "buttonPressed", which the UI framework exposes
and which you can regurlary check for beeing true or false. This is of course very ineffecient, when there are hundreds of UI elements.