{"id":27511,"date":"2017-04-10T09:40:25","date_gmt":"2017-04-10T07:40:25","guid":{"rendered":"https:\/\/mamchenkov.net\/wordpress\/?p=27511"},"modified":"2017-04-10T09:40:25","modified_gmt":"2017-04-10T07:40:25","slug":"living-without-atomic-clocks","status":"publish","type":"post","link":"https:\/\/mamchenkov.net\/wordpress\/2017\/04\/10\/living-without-atomic-clocks\/","title":{"rendered":"Living Without Atomic Clocks"},"content":{"rendered":"\n

“Living Without Atomic Clocks<\/a>” is an interesting article that covers some design bits of distributed systems and CockroachDB<\/a> (what a name!), especially those related to time precision. \u00a0This part in particular is the one I’m sure I’ll came back to at some point:<\/p>\n

How does TrueTime provide linearizability?<\/b><\/p>\n

OK, back to Spanner and TrueTime. It\u2019s important to keep in mind that TrueTime does not guarantee perfectly synchronized clocks. Rather, TrueTime gives an upper bound for clock offsets between nodes in a cluster. Synchronization hardware helps minimize the upper bound. In Spanner\u2019s case, Google mentions an upper bound of 7ms<\/em>. That\u2019s pretty tight; by contrast, using NTP<\/a>\u00a0for clock synchronization is likely to give somewhere between 100ms<\/em> and 250ms<\/em>.<\/p>\n

So how does Spanner use TrueTime to provide linearizability given that there are still inaccuracies between clocks? It\u2019s actually surprisingly simple. It waits. Before a node is allowed to report that a transaction has committed, it must wait 7ms<\/em>. Because all clocks in the system are within 7ms<\/em> of each other, waiting 7ms<\/em> means that no subsequent transaction may commit at an earlier timestamp, even if the earlier transaction was committed on a node with a clock which was fast by the maximum 7ms<\/em>. Pretty clever.<\/p><\/blockquote>\n\n","protected":false},"excerpt":{"rendered":"\n

“Living Without Atomic Clocks” is an interesting article that covers some design bits of distributed systems and CockroachDB (what a name!), especially those related to time precision. \u00a0This part in particular is the one I’m sure I’ll came back to at some point: How does TrueTime provide linearizability? OK, back to Spanner and TrueTime. It\u2019s … Continue reading Living Without Atomic Clocks<\/span><\/a><\/p>\n\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"Living Without Atomic Clocks #databases #time #GPS #CockroachDB","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false,"_links_to":"","_links_to_target":""},"categories":[1,18,62],"tags":[3506,1559,1081,239],"keyring_services":[],"class_list":["post-27511","post","type-post","status-publish","format-standard","hentry","category-general","category-programming","category-technology","tag-cockroachdb","tag-databases","tag-gps","tag-time"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t