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I'm reading a blog post regarding encapsulation, after reading it I think I get what encapsulation is and why we need to use it, but at the end, the author talks about what are the advantages of encapsulation and he lists "Decoupling Implementation Details" as one of the benefits of using encapsulation. I'm quoting it here:

Since Encapsulation enables operating on data using a public interface, it is easier to update the implementation because the tight coupling is eliminated. This is an excellent way to always code against an interface. This also means that with Encapsulation, the object is scalable to accommodate future changes without breaking compatibility. The Coupling means the extent to which two modules are dependent on each other. Decoupling refers to eliminating such dependencies.

I'm trying to work this out in my head, and thought that maybe a SO member could help me understand what he is talking about. Can someone explain this to me?

Thanks in advance!

randle
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2 Answers2

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Encapsulation is a conscious decision to give your code - your implementation of some functionality - a particular interface, such that the consumer of your implementation only assume what they have to.

I'll start with a non-JS example, but will show one at the end.

Hostnames are an interface. IP addresses are implementation details.

When you go to Stack Overflow, you go to stackoverflow.com, you don't go to 151.101.193.69. And if you did go to 151.101.193.69, you'd notice that it's a Fastly CDN address, not a Stack Overflow address.

It's likely that when Stack Overflow just started, it implemented its web access using its own server, on a different IP address, say, for example, 198.51.100.253.

If everyone bookmarked 198.51.100.253, then when Stack Overflow started using Fastly CDN, suddenly everyone who bookmarked it - millions of people - would have to adjust.
That is a case of broken compatibility, because those millions of people would have been coupled to the IP address 198.51.100.253.

By encapsulating the IP address 198.51.100.253 - the actual detail of the implementation of web access - behind only the thing that users need to know - the name of the website - stackoverflow.com - the public interface, Stack Overflow was able to migrate to Fastly CDN, and all those millions of users were none the wiser.
This was possible because all these users were not coupled to the IP address 198.51.100.253, so when it changed to 151.101.193.69, nobody was affected.

This principle applies in many areas. Here are some examples:

  • Energy: You pay for electricity. The supplier can provide it using coal, gas, diesel, nuclear energy, hydro, they can change it from one to the other, and you're none the wiser, you're not coupled to hydro, because your interface is the electric socket, not a generator.
  • Business: When an office building gets cleaning company to keep the building clean, they only have a contract with the company; They cleaners get hired and fired, their salary changes, but that's all encapsulated by the cleaning company and does not affect the building.
  • Money: You don't need money, you need food and shelter and clothes. But those are implementation details. The interface you export to your employer is money, so they don't have to pay you in food, and if you change your diet or style, they don't have to adjust what food or clothes they buy you.
  • Engineering: When an office building gets HVAC, and it breaks, the owner just calls the HVAC company, they don't try to fix it themselves. If they did, they void the warranty, because the HVAC company can't guarantee good product if someone else touches the HVAC. Their public interface is the maintenance contract and the HVAC user-facing controls - you're not allowed to access the implementation directly.

And of course, software: Let's say you have a distributed key-value store which has the following client API:

client = kv.connect("endpoint.my.db");
bucket = crc(myKey)
nodeId = bucket % client.nodeCount();
myValue = client.get(nodeId, bucket, myKey);

This interface:

  • allows the caller to directly and easily find the node which will store the key.
  • allows the caller to cache bucket information to further save calls.
  • allows the caller to avoid extra calls to map a key to a bucket.

However, it leaks a ton of implementation details into the interface:

  • the existence of buckets
  • the usage of CRC to map keys to buckets
  • the bucket distribution and rebalancing strategy - the usage of bucket % nodeCount as the logic to map buckets to nodes
  • the fact that buckets are owned by individual nodes

And now the caller is coupled with all these implementation details. If the maintainer of the DB wants to make certain changes, they will break all existing users. Examples:

  • Use CRC32 instead of CRC, presumably because it's faster. This would cause existing code to use the wrong bucket and/or node, failing the queries.
  • Instead of round-robin buckets, allocate buckets based on storage nodes' free space, free CPU, free memory, etc. - that breaks bucket % client.nodeCount() - likewise leads to wrong bucket/node and fails queries.
  • Allow multiple nodes to own a bucket - requests will still go to a single node.
  • Change the rebalancing strategy - if a node goes down, then nodeCount goes from e.g. 3 to 2, so all the buckes have to be rebalanced such that bucket % client.nodeCount() finds the right node for that bucket.
  • Allow reading from any node instead of the bucket owner - requests will still go to a single node.

To decouple the caller from the implementation, you don't allow them to cache anything, save calls, or assume anything:

client = kv.connect("endpoint.my.db");
myValue = client.get(myKey);

The caller doesn't know about CRC, buckets, or even nodes.

Here, the client has to do extra work to figure out which node to send the request to. Perhaps with Zookeeper or using a gossip protocol.

With this interface:

  • Hashing logic e.g. CRC isn't hard-coded in the caller, it's on the server side and changing it won't break the caller.
  • Any bucket distribution strategy is likewise only on the server side.
  • Any rebalancing logic is likewise not in the client.

Even other changes are possible by just upgrading the client, but not changing any code in the caller:

  • Allow multiple nodes to own a bucket
  • Read from any node (e.g. choosing the one with the lowest latency).
  • Switching from a Zookeeper-based node finding infrastructure to a gossip-based one.
root
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Let's take the Map class as an example. it has an API for adding key/value entries to the map, getting the value for a key, etc. You don't know the details of how the Map stores its key/value pairs, you just know that if you use set with a key, then later do get with the same key, you'll get back the value. The detail of the implementation are hidden from you, and largely don't matter to your code.¹ That's an example of how encapsulation decouples the API from the implementation details.

¹ You do know a little bit about the implementation in this specific example: The specification say "Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection." So you know that a Map isn't implemented as (say) an array of key/value pairs, because that implementation wouldn't offer sublinear access times on the number of elements. But you don't know if it's a hash table or B-tree or what. Those are implementation details.

T.J. Crowder
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