Pratique du Shadowing: Everything You Need to Know About DNS: Crash Course System Design #4 - Apprendre l'anglais à l'oral avec la vidéo
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DNS or domain name system is the backbone of the internet,
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but few know exactly how it works.
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In this video, we'll learn all about the system design of DNS.
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Let's dive right in.
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DNS is the internet's directory.
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It translates human readable domain names such as google.com to machine readable IP addresses.
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DNS is a little confusing because there are different types of DNS server in the DNS hierarchy,
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each serving a different purpose.
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When a browser makes a DNS query,
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it's asking a DNS resolver.
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Now this DNS resolver could be from our ISP or from popular DNS providers like Cloudflare's 1.1.1.1 or Google's 8.8.8.8.
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If the DNS resolver does not have the answer in its cache,
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it finds the right authoritative nameserver and asks it.
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The authoritative nameserver is the one that holds the answer.
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When we update a domain's DNS records as a site owner,
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we are updating its authoritative nameserver.
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How does the DNS resolver find the authoritative nameserver?
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This is where the system of DNS gets interesting.
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There are three main levels of authoritative DNS servers.
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There are the root name servers,
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the top level domain or TLD name servers,
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and the authoritative name servers for the domains.
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The root name servers store the IP addresses of the TLD name servers.
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There are 13 logical root name servers.
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Each root name server has a single IP address assigned to it.
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There are actually many physical servers behind each IP address.
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Through the magic of Anycast,
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we get routed to the one closest to us.
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The TLD name servers store the IP addresses of the authoritative name servers for all the domains under them.
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There are many types of TLD names.
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We are all familiar with .com, .org, and .edu.
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There are also country code TLDs like .de and .uk.
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There are many others.
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The authoritative nameservers for a domain provide well,
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authoritative answers to DNS queries.
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When we register a domain,
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the registrar hosts the authoritative nameservers by default,
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but we can change them to others.
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Cloud providers like AWS and Cloudflare run robust authoritative nameservers.
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This hierarchical design makes DNS highly decentralized and robust.
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Now let's walk through the life of a typical DNS query.
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The user types google.com into the browser.
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The browser first checks its cache.
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If it has no answer,
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it makes an operating system call to try to get the answer.
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The operating system call would most likely have its own cache too.
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If the answer isn't there,
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it reaches out to the DNS resolver.
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The DNS resolver first checks its cache.
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If it is not there or if the answer has expired,
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it asks the root name server.
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The root name server responds with the list of the .com TLD name servers.
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Note that since .com is such a common TLD,
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the resolver most likely already caches the IP addresses for those .com TLD name servers.
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The DNS resolver then reaches out to the .com TLD name server and the .com TLD name server
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returns the authoritative name servers for google.com.
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And finally, the DNS resolver reaches out to google.com's authoritative name server,
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and it returns the IP addresses of google.com.
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Now the DNS resolver then returns the IP addresses to the operating system,
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and the operating system returns them to the browser.
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Finally, let's go over some gotchas when updating DNS records for a live high traffic production system.
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DNS propagation is slow because there is a TTL on each DNS record.
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And some of the default TTLs are pretty long.
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Also, not every DNS resolver is a good citizen.
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There are some buggy ones out there that don't honor the TTL.
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To mitigate the risk, there are two practical steps to take.
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First, reduce the TTL for the record that we want to change to something very short,
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say 60 seconds, well in advance before the update actually happens.
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This gives ample time for all the DNS servers to receive the shortened TTL,
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which would allow the actual record update to take effect based on the new shortened TTL.
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Second, leave the server running on the old IP addresses for a while.
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Only decommission the server when traffic dies down to an acceptable level.
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Because some DNS resolvers don't honor the TTL,
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this could take a bit of time and patience.
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This concludes our video on DNS.
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We hope you have a better understanding of how its hierarchical design makes it decentralized and robust.
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Remember, DNS is what translates domain names to IP addresses,
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making it a critical component of the Internet backbone.
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You like our videos, you may like our system design newsletter as well.
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It covers topics and trends in large-scale system design.
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Vocabulaire et conseils d’expression pour cette leçon
Cette leçon d’expression orale de niveau C1 s’appuie sur la vidéo « Everything You Need to Know About DNS: Crash Course System Design ». Les mots qui reviennent le plus souvent : Dns, server, authoritative, resolver, Tld. Cette vidéo contient 81 phrases et 794 mots à répéter en shadowing. La partie parlée dure 5:29. Le locuteur parle à un rythme régulier d’environ 145 mots par minute, confortable pour le shadowing. Seuls 69 % des mots font partie des 3 000 mots les plus courants en anglais, le vocabulaire est donc exigeant.
Vocabulaire clé de cette vidéo
Les 15 mots les plus avancés de la vidéo, avec leur prononciation et leur sens :
| Mot | Prononciation | Sens |
|---|---|---|
| authoritative adjectif | /əˈθɔɹɪˌteɪtɪv/ | officiel, autoritaire |
| cache nom | /kæʃ/ | cache, mémoire cache |
| query nom | /ˈkwɪɹ.i/ | question |
| robust adjectif | /ɹəʊˈbʌst/ | robuste, costaud |
| decentralize verbe | /diːˈsɛntɹəlaɪz/ | décentraliser |
| hierarchical adjectif | /ˌhaɪəˈɹɑːkɪkəl/ | hiérarchique |
| shorten verbe | /ˈʃɔɹtn̩/ | raccourcir, écourter |
| backbone nom | /ˈbækˌboʊn/ | colonne vertébrale, rachis |
| translate verbe | /tɹænzˈleɪt/ | traduire |
| decommission verbe | /diːkəˈmɪʃən/ | déclasser |
| gotcha | /ˈɡɑ.tʃə/ | j’t’ai eu |
| registrar nom | /ˈɹɛd͡ʒ.ɪsˌtɹɑɹ/ | registraire |
| mitigate verbe | /ˈmɪt.ɪ.ɡeɪt/ | réduire, atténuer |
| expire verbe | /ɪkˈspaɪə(ɹ)/ | expirer, périmer |
| assign verbe | /əˈsaɪn/ | désigner |
Prononciation à surveiller
Le locuteur utilise 4 contractions et formes réduites, comme don't, isn't, we'll. Prononcez-les sous leur forme courte, telles que vous les entendez.
- Les sons « sh » et « zh »: cache /kæʃ/, shorten /ˈʃɔɹtn̩/, decommission /diːkəˈmɪʃən/, propagation /pɹɑpəˈɡeɪʃən/
- Mots longs — placez bien l’accent: authoritative /əˈθɔɹɪˌteɪtɪv/, decentralize /diːˈsɛntɹəlaɪz/, hierarchical /ˌhaɪəˈɹɑːkɪkəl/, decommission /diːkəˈmɪʃən/, propagation /pɹɑpəˈɡeɪʃən/
Les sons difficiles pour les francophones :
- /h/ — il se souffle, il n’est pas muet: hierarchical /ˌhaɪəˈɹɑːkɪkəl/, hierarchy /ˈhaɪ.ə.ɹɑː(ɹ).ki/
- /tʃ/ et /dʒ/ — à ne pas adoucir en « ch » et « j »: gotcha /ˈɡɑ.tʃə/, registrar /ˈɹɛd͡ʒ.ɪsˌtɹɑɹ/
- /r/ anglais — langue recourbée, sans frotter la gorge: authoritative /əˈθɔɹɪˌteɪtɪv/, browser /ˈbɹaʊ.zəɹ/, query /ˈkwɪɹ.i/, robust /ɹəʊˈbʌst/, decentralize /diːˈsɛntɹəlaɪz/
Comment s’entraîner avec cette vidéo
- Écoutez la vidéo en entier une fois sans parler et notez les mots que vous ne connaissez pas.
- Répétez phrase par phrase à vitesse normale, en reprenant chacune jusqu’à ce que votre rythme corresponde à celui du locuteur.
- Enregistrez-vous et comparez avec l’original, en faisant attention à des mots comme authoritative, cache, query.
Qu'est-ce que la technique du Shadowing ?
Le Shadowing est une technique d'apprentissage des langues fondée sur la science, développée à l'origine pour la formation des interprètes professionnels. Le principe est simple mais puissant : vous écoutez de l'anglais natif et le répétez immédiatement à voix haute — comme une ombre suivant le locuteur avec un décalage de 1 à 2 secondes. Les recherches montrent une amélioration significative de la précision de la prononciation, de l'intonation, du rythme, des liaisons, de la compréhension orale et de la fluidité.