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Why did consistency matter when building a large payments system? Data in the system needed to be consistent. But just how consistent? For some parts of the system, only strongly consistent data would do. For example, knowing if a payment has been initiated is something that needed to be stored in a strongly consistent way. For other parts, that were less mission-critical, eventual consistency is something that was considered as a reasonable tradeoff. A good example is listing recent transactions: these could be implemented with eventual consistency (meaning, the latest transaction might only show up in parts of the system after a while - in return, the operation will be return with lower latency or be less resource intensive).

Why did consistency matter when building a large payments system?

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means that once data is successfully added to a data store, it will be available going forward. This will be the case even if nodes in the system go offline, crash or have their data corrupted.

Different distributed databases have different levels of durability. Some support machine/node level durability, some do this at cluster level and some just don't provide it out of the box. Some form of replication is usually used to increase durability - if the data is stored on multiple nodes, if one or more nodes go down, the data will still be available. Here is a good article on why achieving durability in distributed systems can be challenging.

Why did data durability matter when building a payments system? For many parts of the system, no data could be lost, given this being something critical, like payments. The distributed data stores we built on needed to support cluster level data durability - so even if instances would crash, completed transactions would persist. These days, most distributed data storage services, like Cassandra, MongoDB, HDFS or Dynamodb all support durability at various levels and can be all configured to provide cluster level durability.

Why did data durability matter when building a payments system?

Nodes in distributed systems perform computations, store data and send messages to each other. A key characteristic of sending messages is how reliably these messages arrive. For mission-critical systems, there is often a need for having zero messages being lost.

For distributed systems, messaging is usually done by some distributed messaging service, such as RabbitMQ, Kafka or others. These messaging services can support (or be configured to support) different levels of reliability in delivering messages.

Message persistence means that when some failure happens on a node that is processing the message, the message will still be there to process after the failure is resolved. Message durability is usually used at message queue level. With a durable message queue, if the queue (or node) goes offline when the message is sent, it will still get the message when it comes back online. An good article to read more on this topic is this one .

Ceylon also forces us to declare that an attribute or method refines (overrides) an attribute or method of a superclass by annotating it actual ( Heidi Klum Intimates Woman Sabine Lowrise Scalloped Lace Briefs Royal Blue Size M Heidi Klum Intimates Sale Cost 1M0F3Fx
). All this annotating stuff costs a few extra keystrokes, but it helps the compiler detect errors. We can't inadvertently refine a member of the superclass, or inadvertently fail to refine it.

Notice that Ceylon goes out of its way to repudiate the idea of "duck" typing or structural typing. If it walks() like a Duck , then it should be a subtype of Duck and must explicitly refine the definition of walk() in Duck . We don't believe that the name of a method or attribute alone is sufficient to identify its semantics. And, more importantly, Mimi Holliday By Damaris Woman Bowdetailed Lace And Satin Underwired Bra Black Size 34 D Mimi Holliday Big Sale Sale Online luiyU
.

There's a more compact way to refine a default member of a superclass: simply specify its refined implementation using => , like this:

Or assign a value to it using = , like this:

You can refine any method or non- variable attribute using this streamlined syntax.

Note that this shortcut syntax does not allow annotations. If you need to add documentation or other annotations to the refining member, you must use the more verbose syntax.

Our Polar class is an implicit subtype of the class Essential Top Petals by VIDA VIDA Pick A Best 2018 Newest For Sale New Arrival Cheap Price Classic For Sale Fake Online BRvBw
in the package ceylon.language . If you take a look at this class, you'll see that it has a default attribute named Free Shipping Latest Collections Clearance Latest Cupro Skirt Dash by VIDA VIDA Outlet Store Locations Big Discount Cheap Price Sale 5fyQRcdFje
. It's common to refine this attribute to provide a developer-friendly representation of the object.

Polar is also a subtype of the interface Heidi Klum Intimates Woman Zoe Lace Underwired Balconette Bra Light Blue Size 38 C Heidi Klum Intimates Wholesale Price For Sale waHPj
which defines default implementations of Sleeveless Top Native by VIDA VIDA Outlet Newest khlSuB
and . We should definitely refine those:

It's the first time we've seen this syntax:

As you've probably guessed, if (is ... ) works just like if (exists ... ) , testing and narrowing the type of a value. In this case it tests the type of that and narrows to Polar if that is indeed an instance of Polar . We'll come back to this construct Derek Lam 10 Crosby Woman Paneled Striped Jersey And Cottonpoplin Wrap Top Multicolor Size 6 Derek Lam Quality Free Shipping For Sale Fw30nsBe
.

Using the shortcut syntax for refinement that all over logo boxer Philipp Plein Wholesale Price Online Cheap Online Outlet Online Shop d8FZNAoL
, we could abbreviate the above code like this:

(But in this case, the shortcut syntax is perhaps not an improvement.)

Now let's consider a much more interesting problem: abstracting over the polar and cartesian coordinate systems. Since a cartesian coordinate isn't just a special kind of polar coordinate, this is a case for introduction of an abstract superclass:

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