When implementing an application with event sourcing, the persistence engine at work is an event store. That is, an append-only log of events, in past tense, in the order or occurrence. By simply replaying the events through the application, the state at any point in time can be reproduced.
My concern – doesn't this append-only event store inevitably lead to an append-only codebase? How can you maintain a codebase if removing, or even altering code, might leave the application unable to replay the sequence of events? Can the number of source lines of code ever decrease?
What if a business rule has to be modified, or perhaps worse, what if a nasty bug early in the early days of the application allowed it to enter into a forbidden state? Must the faulty code be kept alive indefinitely? Of course, a lot of these issues can – in theory – be dealt with using event versioning, event schemas, snapshot versioning etc. But hasn't event sourcing become a burden at that point?
Event sourcing is a fairly new technology, at least in production. I suspect that there are few applications that have been running on it for more than a couple of years. What will they look like in 10 years? That's not an unrealistic age for an enterprise application.
My concern – doesn't this append-only event store inevitably lead to an append-only codebase?
No, it implies an append-only schema, which is decoupled from your implementation.
What if a business rule has to be modified, or perhaps worse, what if a nasty bug early in the early days of the application allowed it to enter into a forbidden state? Must the faulty code be kept alive indefinitely?
Not really - the domain is decoupled from the durable representations.
Yes, there are some common scenarios that you need to incorporate into your design; like the idea that you may need to compensate for errors earlier in the event history.
It's not, fundamentally, different from what you would do if you were only storing current state. If you have a representation of an aggregate in your database that is in the wrong state, you just update it in place, right? by changing some of the fields to what they are supposed to be.
The idea is the same in event sourcing; you have an event stream that produces a state that you don't want to be in. You figure out what additional events are necessary to reach the state you should be in, and append them. Tada.
Of course, a lot of these issues can – in theory – be dealt with using event versioning, event schemas, snapshot versioning etc. But hasn't event sourcing become a burden at that point?
Not really? Yes, you need to design flexibility into your schema, so that you can evolve your model aggressively, but at it's core it's not different from storing current state - you can still migrate if you have to.
But you also have other levers to play with.
It does, perhaps, require more upfront design capital - you have to think about things like schema lifetimes, and the fact that your book of record accumulates data from multiple revisions of your model.
That doesn't mean it's a shoe for all feet. Designing good message schema is an investment. If the consumers of that schema (which in this case really means your model, and the subscribers) don't need to evolve independently, then maybe that investment doesn't make sense.
I have read a lot of postings and solutions here in stackoverflow.
I am very new to CAN networks and protocols and currently working on a project that entails communicating with the Vehicle engine Control Unit to cause the vehicle to decelerate to a preset speed.
Basically, I intend to establish a node in the CAN network where I can inject packets of data to the engine ECU to cause the car to decelerate to a predefined speed.
How do I translate signals received on the CAN bus, meant for the ECU, so as to make it possible for ECU to decode?
I plan to send two speed signals to the ECU.
speedSignal_1 = current vehicle speed
speedSignal_2 = target vehicle speed.
My intention is to make the ECU force the vehicle at current vehicle speed (speedSignal_1) to reduce to the target vehicle speed (speedSignal_2).
Can you advice me on how to proceed in achieving this?
Thanks
DISCLAIMER: Experimenting with the CAN bus on a moving vehicle is extremely dangerous. If you must audit an active vehicle, make sure that it is secured in a way in which it cannot escape and you cannot lose control (raised with wheels off the ground, on rollers, etc). Use the information in this post at your own risk. In no event shall I be liable for any direct, indirect, punitive, incidental, special consequential damages, to property or life, whatsoever arising out of or connected with the use or misuse of this information.
Your question assumes that the model of car that you're working on has this functionality built in. If your car has cruise control, you can most likely try and figure out what CAN messages are sent out when you activate the cruise control and modify the message to request the speed that you desire. The information regarding the exact message parameters is often proprietary to the manufacturer though, so you will most likely have to do some reverse engineering to figure out the correct messages by listening to the correct CAN bus and toggling the cruise control on/off.
I wanted to use drools for one of our projects - Online Buying and Selling.
Events are like Buying a book, Buying a pen, Buying a kindle.
These events are stored in a database.
Now, based on the events happened before, I want to decide the consequence.
Like say if a person had the following sequence,
1. Buy a book at a price.
2. Sell the same book at a higher price.
Then
Do something based on that.
If someone has done this,
1. Buy a kindle.
2. Purchase a book in Science Section of books.
Then
show him the relevant content in the UI.
I have all the listed things as Events in the database.
Now I have written an interface for the Actions to be done and I have also done the interface to pass a Customer when an event happens.
Now what would give me the best performance to process the events and make a decision based on the sequence of events. I cannot store all the events in memory for sure as I have a whole lot of those.
There are different aspects to consider:
For recommending additional items to customers, there are Recommendation Engines. You may want to use one of those, if most/all of Your use-cases are recommendations.
Storing "all the events in memory" is not neccessarily required. In fact, Drools removes events that are no longer relevant to the rule base. The documentation says
"Events may be automatically expired after some time in the working memory. Typically this happens when, based on the existing rules in the knowledge base, the event can no longer match and activate any rules. Although, it is possible to explicitly define when an event should expire."
To allow early removal of events, I would use Drools to generate aggregated data like "likes science topics", "owns a e-book reader", etc. Those can be inferred from the events but consume less memory.
Closed. This question is off-topic. It is not currently accepting answers.
Want to improve this question? Update the question so it's on-topic for Stack Overflow.
Closed 10 years ago.
Improve this question
The question is quite generic. What are the points that should be kept in mind to identify a valid use case? How to approach a use case?
A use case identifies, with specificity, a task or goal that users will be able to accomplish using a program. It should be written in terms that users can understand.
Wikipedia's description is overly formal. I'll dig through my other texts shortly.
In contrast, the original wiki's article is much more accessible.
An early article by Alastair Cockburn, cited positively by The Pragmatic Programmer, contains a good template.
This question, from just a few days ago, is very closely related, but slightly more specific.
The definition of use case is simple:
An actor's interactions with a system to create something of business value.
More formally:
a sequence of transactions performed
by a system that yield a measurable
set of values for a particular actor.
They're intended to be very simple: Actor, Interaction, Value. You can add some details, but not too many.
Using use cases is easy. Read this: http://www.gatherspace.com/static/use_case_example.html
The biggest mistake is overlooking the interaction between actor and system. A use case is not a place to write down long, detailed, technical algorithm designs. A use case is where an actor does something.
People interact with systems so they can take actions (place orders, approve billing, reject an insurance claim, etc.) To take an action, they first make a decision. To make a decision, they need information
Information
Decision
Action
These are the ingredients in the "Interaction" portion of a use case.
A valid use case could describe:
intended audience / user
pre-requisites (ie must have logged in, etc)
expected outcome(s)
possible points of failure
workflow of user
From Guideline: Identify and Outline Actors and Use Cases by the Eclipse people:
Identifying actors
Find the external entities with which
the system under development must
interact. Candidates include groups of
users who will require help from the
system to perform their tasks and run
the system's primary or secondary
functions, as well as external
hardware, software, and other systems.
Define each candidate actor by naming
it and writing a brief description.
Includes the actor's area of
responsibility and the goals that the
actor will attempt to accomplish when
using the system. Eliminate actor
candidates who do not have any goals.
These questions are useful in
identifying actors:
Who will supply, use, or remove
information from the system?
Who will
use the system?
Who is interested in a
certain feature or service provided by
the system?
Who will support and
maintain the system?
What are the
system's external resources?
What
other systems will need to interact
with the system under development?
Review the list of stakeholders that
you captured in the Vision statement.
Not all stakeholders will be actors
(meaning, they will not all interact
directly with the system under
development), but this list of
stakeholders is useful for identifying
candidates for actors.
Identifying use cases
The best way to find use cases is to
consider what each actor requires of
the system. For each actor, human or
not, ask:
What are the goals that the actor will
attempt to accomplish with the system?
What are the primary tasks that the
actor wants the system to perform?
Will the actor create, store, change,
remove, or read data in the system?
Will the actor need to inform the
system about sudden external changes?
Does the actor need to be informed
about certain occurrences, such as
unavailability of a network resource,
in the system?
Will the actor perform
a system startup or shutdown?
Understanding how the target
organization works and how this
information system might be
incorporated into existing operations
gives an idea of system's
surroundings. That information can
reveal other use case candidates.
Give a unique name and brief
description that clearly describes the
goals for each use case. If the
candidate use case does not have
goals, ask yourself why it exists, and
then either identify a goal or
eliminate the use case.
Outlining Use Cases
Without going into details, write a
first draft of the flow of events of
the use cases identified as being of
high priority. Initially, write a
simple step-by-step description of the
basic flow of the use case. The
step-by-step description is a simple
ordered list of interactions between
the actor and the system. For example,
the description of the basic flow of
the Withdraw Cash use case of an
automated teller machine (ATM) would
be something like this:
The customer inserts a bank card.
The system validates the card and prompts
the person to enter a personal
identification number (PIN).
The customer enters a PIN.
The system
validates the PIN and prompts the
customer to select an action.
The customer selects Withdraw Cash.
The system prompts the customer to choose
which account.
The customer selects
the checking account.
The system
prompts for an amount.
The customer
enters the amount to withdraw.
The
system validates the amount (assuming
sufficient funds), and then issues
cash and receipt.
The customer takes the cash and receipt, and then
retrieves the bank card.
The use case ends.
As you create this step-by-step
description of the basic flow of
events, you can discover alternative
and exceptional flows. For example,
what happens if the customer enters an
invalid PIN? Record the name and a
brief description of each alternate
flow that you identified.
Representing relationships between
actors and use cases
The relationship between actors and
use cases can be captured, or
documented. There are several ways to
do this. If you are using a use-case
model on the project, you can create
use-case diagrams to show how actors
and use cases relate to each other.
Refer to Guideline: Use-Case
Model for more information.
If you are not using a use-case model
for the project, make sure that each
use case identifies the associated
primary and secondary actors.
Closed. This question is opinion-based. It is not currently accepting answers.
Want to improve this question? Update the question so it can be answered with facts and citations by editing this post.
Closed 4 years ago.
Improve this question
This is a perennial question for me that I've never really resolved so I'd like your input. If I have actions that I know a user will not be able to perform due to insufficient privileges or object state, should the UI elements for those actions be hidden from the user, visible but disabled, or visible and result in an error if attempted? What would be the rationale for your answer? If disabled, would you communicate the reason why and, if so, how?
This is a web interface so I already know that I need to check the incoming post/get for permissions and handle errors there anyway. I'm primarily talking about how to handle the UI.
This is similar to Rules about disabling or hiding menu items, though I am interested in all types of UI elements not just menus.
Examples:
I have a New page that allows a user to create a new Event. Events can be master events or subevents. Creating a master event requires "EditMasterEvent" privilege, while creating a subevent requires only "EditEvent" privilege. I have a drop down that allows one to choose an existing event as the parent (master event) or no parent (this is a master event). Should the "Create Master Event" choice be shown on the dropdown or omitted if the user only has "EditEvent" privileges.
Deleting events requires that you be an application administrator or have the appropriate edit permission for the event type. In the latter case, the event must also be more than 5 years old. Deleting an event causes major cascading deletes of related data in the system and for legal reasons this data must be kept for at least 5 years after the event. Since this operation is rare for the normal user, the typical case is that the action is not available. Should it be shown always or only when actually possible?
Hidden - This is the best approach for actions that are never available to the current user. There is no point in having the user waste mental effort figuring out why something is disabled if there is no action they can take to change this.
Disabled - This is the best approach for actions that are sometimes available, but not at the moment or in the current context. A disabled option should convey two things: first, the action is not available right now, and second, there is something the user could do to make the action available (change some setting or permission, select an item, enter prerequisite data, etc.). If you can indicate what needs to be done to enable the action in a tooltip - all the better. Enabling/disabling actions as the user enters data or changes context provides excellent feedback about what the program requires.
Fail with an Error - This is the worst choice. You should only resort to an error report for operations that might work: you can't tell that it will fail except by trying.
As with nearly all UI questions, the answer is "it depends".
You need to weigh discoverability with user satisfaction, among other things. For example, allowing an invalid action gives you an opportunity to explain why something is invalid. This is particularly useful if the answer to "why is this disabled" isn't obvious. For an application where most users are beginners, that's important.
On the other hand, it can be mightily frustrating to see a control, click on it, only to be rewarded with a "sorry, you can't do that now" message. An app I inherited a couple years back was rife with that sort of stuff and it made using the UI an exercise in frustration.
Completely hiding functionality is probably rarely a good idea. Imagine knowing some feature "was there a minute ago" but now it's gone. Whether it's a menu item or a toolbar button or something else entirely, making it hidden can be an exercise in frustration for the end user.
Try doing a little usability testing, if only by asking the next person you see "hey, does it make sense to disable this or show you an informative dialog". Just one other opinion is often enough to get you to look at the problem from another direction.
Bottom line: do what best serves the user. All the scenarios you mention are valid under certain circumstances. As with all UI questions, ask yourself (or better, your users) what best serves their needs.
I disable the elements instead of hiding them. That way the user knows the option would normally be available, and I provide a tooltip to explain why the element isn't currently available.
It depends. Do you want the user to be aware that the action is possible, just not for them? In that case, show them the button, but disable it. An example might be if a user doesn't have delete authority, but other users do, they should know that entries CAN be deleted, so they can ask someone to do it for them if they need the action.
On the other hand, if the user is not supposed to even know about the action (for example, a user who does not have read access to audit logs probably shouldn't know that these logs exist) should not be able to see the button, so hide it completely.
Great question!
A couple of considerations:
If you place the elements on the page but disable them, there's still a remote chance that the user could doctor the system and enable them using a javascriptlet.
If you do not show them at all, the overall functionality may be a bit confusing to the general user. "Shouldn't there be an edit button here?"
If you're going to either display and disable or display and verify the elements, I would definitely do server-side validation. Don't leave the validation in the hands of JavaScript; I think the reasons for this are obvious.
I tend to handle the two different types of situations differently. Is this an action that is governed by privilege and by state of the object.
If the person does not have enough privileges to do an action, I hide the option, they do not know they can perform the action.
If the option is not available because the object is not in a state that can use that option, I disable it, allowing the option to be visible to the user, but no action can be done.
From your examples:
I would not have "Create Master Event" as an option. The user has insufficient privileges to view it.
I would have the Delete button visible to the administrators. Then depending on how you do the rest of the site (a lot of visible text, tooltips, help icon, etc) I would follow that convention about informing the user why the button is not usable at this time. And possibly putting a timer on, above, near the button with either how old the post is or how long until it can be deleted.
Depending on the item, we will either hide them or disable them. If the user has access to a large feature, but not to a smaller piece inside it, then we will hide the smaller piece. However, if the user has access to several large features, but not to others, we'll leave them visible but disabled as a marketing ploy to remind them that the features are available for purchase if they should decide they want them.
I've also seen some programs that disable the menu item and change the text of it to "Log in to do blah..."
I like this because it doesn't leave me with the "why isn't this working?" feeling and tells me immediately what to do to get it working. Not applicable in every case, but this is a nice approach if you can implement it.
The general rule is use disabling if the user can do something in the UI to get the privilege. Disabled means “you can do this command, but just not right now the way things are.” The “way things are” includes the current selection, so use enabling/disabling if the user has the EditEvent privilege for old objects but not for new objects. There should be a clear indication which objects are delete-able so users understand why the associated commands are disabled for some objects (e.g., if users generally know that records must be kept for 5 years, a simple Age field maybe be sufficient, perhaps reinforced with a graphic difference for records over 5 years old).
Use message boxes instead of disabling if there is no way to make the reason for the disabling clear to the user assuming they have average knowledge of the domain. Tooltips for disabled controls, BTW, are a great idea, but may not be sufficient by themselves.
Use hiding if the user never has the privilege no matter what they do in the UI given their current position in the organization (e.g., they are not an Application Administrator). It is cluttering and frustrating to use disabling or message boxes for this case. As far as the users are concerned, actions they don’t have the privilege for are not their job (otherwise they’d have the privilege), and so the associated controls should simply not exist in their UI. Documentation or organization procedure manuals may tell users how such actions are accomplished (e.g., “Your supervisor creates new events for you.”).
I’ve more details at http://www.zuschlogin.com/?p=40.
I would say disable with a hover containing the reason.
It prevents the user from wondering what the hell is going on while at the same time letting them know certain actions are possible under the right conditions.
I have a particular hatred of applications that disable buttons. If you're an end user - you want to know why you can't use that button. Having it greyed out doesn't tell you anything. How do you get to the state to enable it? Tooltips are one solution, but they aren't the best, a lot of users will struggle with tooltips (unless you're working with experienced users).
My personal feeling is that the elements should always be present. If the user doesn't have enough permissions to do them, they should generate an error when clicked upon.
I know that translators don't really enjoy creating a zillion different "permission denied" error messages, so this is often not done in localised applications, which tend to hide the elements instead.
In practice a lot of people tend to hide the options instead even in non-localised apps.
Other people have provided good answers with valid suggestion to avoid hiding elements and instead disable them and provide some hints for the reasons.
So, I would like to look at it from different perspective - but how to hide some UI elements in cases when user does not need to see them, no matter if he has or has no permissions for particular actions related to the elements?
For example, let's say, users of some role are given access to sellers records in the system.
But then business analyst says: "Look, there is a dropdown with sellers list in this form and we should not allow some specific roles to see it".
Developer asks: "So, we just remove the "Read sellers" permission from this role, right?" But the analyst replies: "No! This role should still be able to view the sellers on the Sellers page. It's just this single form where we should hide the list for some roles and show it to some other roles."
So, the developer adds permission called "Show sellers dropdown on the form X".
Ooops, now we have a problem. Access to the same data is being controlled by two separate permissions. Now we have to figure out how to combine both of them. And what if there are more than one form where seller's list should be hidden for some roles? How do we combine it with "Read seller's list"? For us, developers, it is somewhat clear that "Read" permission should have higher priority above "View", so even if a user can "View" a list, he still should not see it (or see empty or disabled with a helpful hint) if he does not have "Read" permission. We, developers and analysts of the system know it. But how should the system administrator know it? Should we teach him this? How can we guarantee that the admin won't confuse all those "View" and "Read" for the single data list?
As you see, it all gets messy for one reason - we are mixing data processing permissions with UI conveniences in the list of role permissions.
I have seen many projects where it gets messy because permissions on the server side get coupled too much to UI, which asks for troubles and possible security holes (because you have multiple items in your role permission editor for the same actions on the same data).
Permissions are about access and operations on some specific data. UI can only react to permissions in consistent way throughout entire system (disabling with hints, hiding etc.). We should never invent new permission entries just for UI purposes.
Now the question remains - but how do we actually hide UI elements for some system users to avoid overwhelming them with huge amount of always disabled items? One solution might be role workspaces. If we clearly know that users of some role will never ever need access to some specific data, we create a set of UI control entries, similar to permissions, but this time we don't call them permissions. And we can get really fancy here, even allowing users themselves to freely customize their workspace and choose what they can or cannot see. Of course, permissions will always take the highest priority, but it will only affect the data and state of UI elements and not visibility.
That's my two cents. Unfortunately, I myself haven't worked on such a system where permissions and UI workspace options are neatly separated because I always somehow come too late to a project, when the "damage has been done". But I hope some day I'll have a chance. I just hope to find a good example how to do this right, but somehow internet searches do not give me anything useful. Does it really mean that nobody else has came to the same conclusions as me? I don't believe it, somebody in the enterprise design pattern world should have noticed this UI<->permission impedance mismatch long ago.