Caching Techniques - Web Development Udacity English (auto-generated) Japanese

Caching Techniques - Web DevelopmentYouTube↗ IA↗(long shot)
Education 34.4K views 216 likes 7 dislikes 23 Feb 2015

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okay I'd like to go over each of the

次にお話しするのは

approaches we've talked about and kind of the different properties they have so

これまで見てきた手法とそれぞれの特徴です

the first approach was no caching and and this meant on every page view we

まずまったくキャッシュをしないという手法です

were doing a DB read and when we

データベースの読み込みは閲覧の度に行い

submitted a new piece of art there was

作品の投稿時には行いません

no DB reads then we did this kind of

実際に採用したのはナイーブなキャッシングです

what I would describe as the naive caching approach which was basically have the basic cache if the cache is

キャッシュが空の時にはデータベースを読み込み

empty through the DB read and if it's not

キャッシングして空でない場合は結果を返す手法です

return the result and what we'll add a

表にもう1列加えて"バグ"の欄を作ります

third column on the edge here called bugs so the naive caching only does a DB

ナイーブなキャッシングではデータベースの読み込みは

read on a cache miss and doesn't do any

キャッシュミスの時だけで投稿時は行いません

reading on a submit and it's full of bugs or at least has one bug where the

実はナイーブなキャッシングにはバグがよく発生してそうなると

the front page would become out of out

トップページは最新でなくなります

of date that the front page becomes stale so then we started clearing the

キャッシュのクリアという手法については

cache this has the same property of doing a DB read on a page view no DB

データベース読み込みはページビューごとで

read on a submit and no bugs then we

投稿時には行わずバグはありません

improve to the kind of the refreshing of

キャッシュの復元については

the cache and so this means we're no

データベースの読み込みは

longer doing any DB reads on page views or very rarely basically only the first

非常にまれに行います

time our app turns on and the cache is

初めてサイトに訪問したために

empty and that first page you every

キャッシュが空という時だけで

other page view after that is cached

その他はキャッシュされます

which is a really nice property to have and we're doing one DB read per

データベースの読み込みは投稿する度に行います

submission and it works now the difference between between these two and

データベースの読み込みを"キャッシュミス"の時

this one is the notion that a page view

あるいは"まれに"しか行わないということは

doesn't hit the database hardly ever and

閲覧時にはデータベースにほぼアクセスしないということです

that's a really nice property to have

これはすばらしい特性です

you should always be striving to have

サイトの運営者にとって望ましいのは

the situation where a normal unlogged in

正常でログインされていないシンプルな状態です

basic you know viewer of your website

つまり閲覧者がデータベースに

doesn't touch the database and I'm going

アクセスしていないという状況です

to kind of condense that down to the

その状態を保つためには

notion that simple users shouldn't touch

閲覧者をデータベースに近づけないことが大切です

the database basically you know they're

彼らは投稿せず

just lurkers they're just reading so

ただ閲覧するだけなので

they're not changing the site so they shouldn't be touching the database

データベースに近づかせてはダメです

everything should be cached and ready to go for them that makes the user experience better because the requests

すべてキャッシュしておくことで

will be faster and it makes keeps your

リクエストに応える速度もユーザ体験も向上し

load down because you can add many many of those users and because they're just

負荷を少なく保ってユーザを増やせます

bouncing off the cache you don't have to

彼らはキャッシュを取り出せばいいからです

do very much work to serve them you don't have to actually hit the database

では4番目の手法を紹介します

now there's a fourth approach that we didn't implement yet which is the most

これはまだ実装していませんが

aggressive of all of these and I'm going to kind of refer to this as distinct

これまでの手法の中で最も積極的なものです

キャッシュの復元と区別して"更新"とします

I'm going to call it updating the cache and I'll and I'll talk about this

具体的な内容はこのあと説明しますが

approach in just a sec and we can get to the state we're on a page view on a

閲覧の度にデータベースを読み込むことは

simple page view we do 0 DB reads ever and this is slightly better than rarely

まったくありません "まれに"読み込むより効率がいいですね

and we don't do any database reads on submission either and it works this is a

また投稿時も読み込みはまったく行われません理想的な手法ですね　しかもデータベースは

really nice property I have now of course we still do our database right

最適化をしていませんが

you notice we haven't been optimizing rights at all because you've got to

正常な状態に保たれています

store this you got to store the submission at some point but you can cut

ある時点で投稿データを格納します

down on the database reads lower them to both 0 by keeping your cache completely

そしてキャッシュを最新にして読み込み回数をゼロにします

up to date and I'll show you how we

どうしてこのようにできるのか説明しましょう

might do that right now ok we're going to look at this picture one more time

ではおなじみの図を使って説明しましょう

here we know all the pieces now the user ASCII Chan the database in our cache

ご存じのとおりユーザ、ASCII Chan、データベース、キャッシュです

we're going to talk about a new

状況を説明しましょう

situation here so let's like let's pretend our cache is already warm you

キャッシュはすでに保存済みとします

know it's got some pictures in there and

新しく投稿されたトップページを読み込むと

reading the front page which hits our cache which you know returns the result

キャッシュにアクセスして結果を得ます

that we sent back to the user nothing too complex there we're not hitting the

そしてその結果をユーザに返します

database because we're only doing reads now what happens is when we do a

閲覧だけなのでデータベースにはアクセスしません

database right we're going to send that

一方データベースに書き込みをすると

right to the database we're simultaneously going to send that

同時にキャッシュにもそのデータが送られます

right to our cache as well and so this

ここで少し複雑になりますが

this gets a little bit more complex we're going to send the right to the

データベースに書き込みをすると

database and instead of immediately rereading from the database to update

即座にキャッシュを更新かクリアするために

our cache or clearing our cache we're

データベースが再読み出しをする代わりに

just going to update the cache we're going to say ok this affects the front

直接キャッシュの更新だけをするのです

page so let's find that front page cache

ASCII Chanを使って考えます

insert our new piece of you know ASCII art into the cache and then from then on

トップページに投稿されると作品をキャッシュします

that follow-up request that redirect

トップページがリダイレクトされ再びリロードされると

that follow-up requests to slash is basically going to bounce right off the

キャッシュから書き込みデータが引き出されます

cache again and so we never did we never

つまりこの一連のプロセスで実行されたのは

did a database read during this whole process we're just writing only the only

データベースの書き込みだけで読み出しは一切ありません

time we would do a database read is when

例外は初めてサイトへアクセスして

we that you know start off the app for the first time and do that first request

最初のリクエストを処理した時です

or maybe we have a program that that

しかしそれはプログラムに処理させることも可能です

does that for us so no user ever does a database read and this is actually how

つまりデータベースの読み出しは一切行わないのです

we do things on reddit now every listing

実はこの手法はまさにRedditが採用しているものです

you can look at is stored in its own cache and when you submit a link or do a

すべてのリスティングは各キャッシュに保存され

vote we update all of the appropriate

リンクが張られたり投票されたりすると

caches all all the different cache keys that may be affected so so we've kind of

適切なキャッシュが更新されるのです

introduced the trade-off here of you know complex inserts versus database

つまり複雑な挿入とデータベース読み出しの

reads unread we actually do this we have you know

トレードオフと言えるでしょう

different cash key for every every

実際にRedditでは

listing you might look at you know for every sorting for every subreddit all

各リスティングに対して様々なキャッシュキーを使用します

big cetera et cetera and when you submit a link or you vote we have to update all

そしてリンクや投票結果が投稿されると

of all the possible listings that could

影響を受けそうなリスティングをすべて更新します

be affected by that action on the flip side you know users are pummeling the

ユーザはサイトにアクセスしますが

site all the time and they never read

データベースの読み出しはしません

from the database and you know so we have complex inserts plus speed which is

サイトが複雑な挿入と速度に対応しているのは好ましいことですが

nice but you know complexity is

しかし複雑さに対応するのは難しいことです

complexity on ASCII Chan we probably don't need to do this right now our site

ASCII Chanはサイトの規模から言って

just isn't at that scale that doesn't

その技術は必要ないでしょう

you know a cache Stampede isn't a

ユーザ数も少ないのでキャッシュ･スタンピードは

realistic threat because we don't have that many users but if we did this is

現実的な脅威ではないでしょう

the kind of approach we'd want to take and and so this is kind of the name of

しかしこの問題と解決法はとても重要です

the game when caching you know if you want to keep this cache totally accurate

データベースへの書き込みなしに

without doing database writes we're

完璧なキャッシュをするなら

going to have to do you know complex

複雑なコードが必要です

code so one thing to keep in mind is the more accurate the cache the more complex

キャッシュの正確さはコードの複雑さに比例します

the code and these are the decisions

サイトの構築時には両者のトレードオフが重要です

you'll make as you build your website and as you're scaling you know this is

スケーリングに際して

probably the ultimate solution you want

データベースのキャッシュを

to look to when you're kind of caching a

検討する時キャッシュの更新は

database you know if the solution works for you

非常に有効でしょう

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