August 2016
Intermediate to advanced
384 pages
9h 20m
Japanese
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11 章 ネットワーキングとサービスディスカバリ
11.2.3
Consul
Consul
（
https://www.consul.io
）は、サービスディスカバリの問題に対する
Hashicorp
の回
答です。高可用性を持つ分散キーバリューストアであることに加えて、デフォルトでヘルスチェッ
ク機能と
DNS
サーバーを持つことを誇ります。
キーバリューストアとサービスディスカバリを見ていくと、すぐに CAP 定理（https://ja.wikipe
dia.org/wiki/CAP 定理）にぶつかることになります。おおまかに言って、CAP 定理が述べている
のは、分散システムは、ということで
す
†
。
AP システムは、一貫性よりも可用性を優先させるので、読み書きはほぼ常にできるはずです（そ
して通常は高速です）が、情報が常に最新であるとは限りません（古いデータが返される場合があ
ります）。CP システムは一貫性を優先するので、書き込みに失敗することがありますが、返された
データは、必ず正しくて最新です。
実際には、特に Consul に関しては、もう少し話はややこしくなります。etcd と Consul はどち
らも CP のソリューションを提供する Raft アルゴリズムを基盤としています。しかし、Consul
には複数の動作モード（"default"、"consistent"、"stale"）があり、さまざまなレベルでの一貫
性と可用性のトレードオフを提供できます。 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.