Elvis Sautet, engineering writingNotes from building and operating production systems. Engine design, interface craft, and what broke on the way.https://elvissautet.com/enWhat I write down, and whyhttps://elvissautet.com/writing/what-i-write-down-and-why/https://elvissautet.com/writing/what-i-write-down-and-why/Two documents, doing different jobs. A plan that tracks reality while work is in flight, and a handoff that records what surprised me once it ships.Sun, 26 Jul 2026 00:00:00 GMTThree correct fixes that all failedhttps://elvissautet.com/writing/three-correct-fixes-that-all-failed/https://elvissautet.com/writing/three-correct-fixes-that-all-failed/A site ranked as one page. Three commits added the right tag and changed nothing, because everybody read the source and the truth was in the output.Fri, 24 Jul 2026 00:00:00 GMTHow to make a system explain itself when you are the only one askinghttps://elvissautet.com/solutions/observability-when-you-are-the-only-engineer/https://elvissautet.com/solutions/observability-when-you-are-the-only-engineer/Not dashboards. Four questions the system must answer without you reading logs, and the cheapest way to make each one answerable.Wed, 22 Jul 2026 00:00:00 GMTAstro against Next for content sites, dogfooded on this onehttps://elvissautet.com/decisions/astro-vs-nextjs-for-content-sites/https://elvissautet.com/decisions/astro-vs-nextjs-for-content-sites/This site is the argument. Zero JavaScript on every content page, a canonical that cannot be got wrong, and a build that fails on SEO mistakes.Mon, 20 Jul 2026 00:00:00 GMTHow to move off a legacy system without a big banghttps://elvissautet.com/solutions/legacy-system-migration/https://elvissautet.com/solutions/legacy-system-migration/Run both, compare in production, and move traffic one surface at a time. The comparison log is the most valuable artefact in the whole project.Sat, 18 Jul 2026 00:00:00 GMTHow to ship alone without the wheels coming offhttps://elvissautet.com/solutions/shipping-alone-without-a-team/https://elvissautet.com/solutions/shipping-alone-without-a-team/With no reviewer, correctness has to be structural rather than checked. Six practices that replace the things a team does for you.Thu, 16 Jul 2026 00:00:00 GMTHow to make a site that ranks, verified against the outputhttps://elvissautet.com/solutions/ssr-seo-and-core-web-vitals/https://elvissautet.com/solutions/ssr-seo-and-core-web-vitals/Read the served HTML rather than the source, make the invariants structural, and fail the build on the mistakes that quietly cost you every ranking.Tue, 14 Jul 2026 00:00:00 GMTHow to build for connections that drop, on devices that are oldhttps://elvissautet.com/solutions/building-for-low-bandwidth-markets/https://elvissautet.com/solutions/building-for-low-bandwidth-markets/The primary answer must survive the worst connection your users have. Everything visual is an enhancement on top of it, and the ordering is not negotiable.Sun, 12 Jul 2026 00:00:00 GMTRead the output, not the sourcehttps://elvissautet.com/writing/reading-the-output-not-the-source/https://elvissautet.com/writing/reading-the-output-not-the-source/The source is what somebody intended. The output is what is true. Stuck debugging is usually time spent in the first when the answer is in the second.Fri, 10 Jul 2026 00:00:00 GMTLoading, empty and error, the three states every demo skipshttps://elvissautet.com/interface/loading-empty-and-error-states/https://elvissautet.com/interface/loading-empty-and-error-states/A demo shows the filled state. Real users meet the other three on day one, and those are the ones that decide whether a product feels solid or broken.Wed, 08 Jul 2026 00:00:00 GMTWhen one number has four ownershttps://elvissautet.com/writing/when-one-number-has-four-owners/https://elvissautet.com/writing/when-one-number-has-four-owners/Four subsystems each computed the same balance and three disagreed. I spent two weeks making them agree, which held for a month.Tue, 30 Jun 2026 00:00:00 GMTForms that do not lose work, and errors people can act onhttps://elvissautet.com/interface/forms-that-do-not-lose-work/https://elvissautet.com/interface/forms-that-do-not-lose-work/Surface every error in three places, validate when it helps rather than when it is easy, and never punish somebody for a slow network they did not choose.Sun, 28 Jun 2026 00:00:00 GMTWhen to build an API and when to build the web apphttps://elvissautet.com/solutions/when-to-build-an-api-vs-a-web-app/https://elvissautet.com/solutions/when-to-build-an-api-vs-a-web-app/Building an API first for a single web client is a cost with no buyer. Here is the test for whether a second consumer is real or imagined.Wed, 24 Jun 2026 00:00:00 GMTHow to add realtime without four sockets per browser tabhttps://elvissautet.com/solutions/realtime-without-websocket-sprawl/https://elvissautet.com/solutions/realtime-without-websocket-sprawl/One connection per application, events multiplexed by type, and a resume token so a reconnect does not lose the events that happened while you were away.Sat, 20 Jun 2026 00:00:00 GMTTables people actually use, and the four rules behind themhttps://elvissautet.com/interface/tables-people-actually-use/https://elvissautet.com/interface/tables-people-actually-use/One search per table, search where the data lives, keep the previous page while the next loads, and let the table own its empty state. Four rules.Tue, 16 Jun 2026 00:00:00 GMTWhy adding more workers made it slowerhttps://elvissautet.com/writing/why-more-workers-made-it-slower/https://elvissautet.com/writing/why-more-workers-made-it-slower/I doubled the workers and throughput fell. The work was contention bound, so every additional worker was buying lock contention instead of capacity.Sun, 14 Jun 2026 00:00:00 GMTHow to change a database schema while people are using ithttps://elvissautet.com/solutions/zero-downtime-migrations/https://elvissautet.com/solutions/zero-downtime-migrations/The expand, migrate, contract sequence, what each phase must never do, and why the dangerous migration is the one that looks trivial.Wed, 10 Jun 2026 00:00:00 GMTHow to build an approval workflow an auditor cannot breakhttps://elvissautet.com/solutions/approval-workflows-that-survive-audit/https://elvissautet.com/solutions/approval-workflows-that-survive-audit/Snapshot the policy at filing, make the decision log the truth, fingerprint the intent, and never let a timeout count as agreement.Sat, 06 Jun 2026 00:00:00 GMTDesigning dense screens for people who use them all dayhttps://elvissautet.com/interface/designing-dense-operational-screens/https://elvissautet.com/interface/designing-dense-operational-screens/Software used all day by somebody with a queue in front of them obeys different rules from software used once a week. Whitespace is not always kindness.Thu, 04 Jun 2026 00:00:00 GMTEarly work, kept on the site deliberatelyhttps://elvissautet.com/work/early-work/https://elvissautet.com/work/early-work/Small business sites from the start of my career. They are here on purpose, because the trajectory is more informative than any single project.Tue, 02 Jun 2026 00:00:00 GMTHow to make a slow Postgres query fast, in orderhttps://elvissautet.com/solutions/postgres-query-and-index-tuning/https://elvissautet.com/solutions/postgres-query-and-index-tuning/Read the plan before changing anything. Then fix the access path, then the join, then the index, and measure the tail rather than the average.Sat, 30 May 2026 00:00:00 GMTAt least once is the only honest guaranteehttps://elvissautet.com/writing/at-least-once-is-the-only-honest-guarantee/https://elvissautet.com/writing/at-least-once-is-the-only-honest-guarantee/Nobody can sell you exactly once across a network. Stop trying to buy it and make running twice harmless, which is a problem you can actually solve.Tue, 26 May 2026 00:00:00 GMTHow to design a rules engine, and when not to build onehttps://elvissautet.com/engines/designing-a-rules-engine/https://elvissautet.com/engines/designing-a-rules-engine/A rules engine is how you stop shipping a release for every policy change. It is also how you build a programming language by accident. Here is the line.Tue, 26 May 2026 00:00:00 GMTAn internet provider site where the first question is coveragehttps://elvissautet.com/work/tabasamu-fibre/https://elvissautet.com/work/tabasamu-fibre/Every visitor arrives with one question, can I get this at my address. Answering it in the first screen changed what the whole site was for.Fri, 22 May 2026 00:00:00 GMTPostgres against Mongo for multi-tenant systemshttps://elvissautet.com/decisions/postgres-vs-mongodb-for-multi-tenant/https://elvissautet.com/decisions/postgres-vs-mongodb-for-multi-tenant/The question is rarely the data model. It is whether the system has money in it, because money means constraints, and constraints are what Postgres is for.Mon, 18 May 2026 00:00:00 GMTThe second dealership, and what I reused rather than rebuilthttps://elvissautet.com/work/lamad-motors/https://elvissautet.com/work/lamad-motors/The second build of a similar thing is where you find out which decisions were principles and which were accidents that happened to work once.Thu, 14 May 2026 00:00:00 GMTHow to build role based access that survives five yearshttps://elvissautet.com/solutions/role-based-access-that-scales/https://elvissautet.com/solutions/role-based-access-that-scales/Permissions on actions rather than roles, checked in one place, with the interface derived from the same source. The four rules that stop it rotting.Tue, 12 May 2026 00:00:00 GMTSnapshot the approver, not the rolehttps://elvissautet.com/writing/snapshot-the-approver/https://elvissautet.com/writing/snapshot-the-approver/An auditor asked who was allowed to approve a payment in March. The system answered with who is allowed today, which is a different question entirely.Sun, 10 May 2026 00:00:00 GMTHow to design a notification engine with one ledger and real laneshttps://elvissautet.com/engines/designing-a-notification-engine/https://elvissautet.com/engines/designing-a-notification-engine/Many channels, one message ledger, and priority lanes so a one time password never waits behind a bulk send of forty thousand messages.Fri, 08 May 2026 00:00:00 GMTHow to fix a queue that is slower after you added workershttps://elvissautet.com/solutions/background-jobs-and-queue-throughput/https://elvissautet.com/solutions/background-jobs-and-queue-throughput/Concurrency only helps when the workers are waiting. When they are competing, more workers buy contention. Here is how to tell which one you have.Mon, 04 May 2026 00:00:00 GMTA dealership platform the owner could run without mehttps://elvissautet.com/work/charlies-motors/https://elvissautet.com/work/charlies-motors/A public inventory site with real search and real leads, built so the owner could run it alone. Handover is a design requirement, not a document.Sat, 02 May 2026 00:00:00 GMTMeasure the tail, not the averagehttps://elvissautet.com/writing/measuring-the-tail-not-the-average/https://elvissautet.com/writing/measuring-the-tail-not-the-average/The average said the page was fast. One request in twenty took two seconds, every day, and nobody reported it because complaining requires a comparison.Thu, 30 Apr 2026 00:00:00 GMTHow to make a payment webhook safe to receive twicehttps://elvissautet.com/solutions/idempotent-payment-webhooks/https://elvissautet.com/solutions/idempotent-payment-webhooks/A step by step build of a webhook that survives retries, crashes and duplicates, with the failure each step exists to prevent and how to verify it.Fri, 24 Apr 2026 00:00:00 GMTHow to design a reconciliation engine that assumes the worker diedhttps://elvissautet.com/engines/designing-a-reconciliation-engine/https://elvissautet.com/engines/designing-a-reconciliation-engine/Money arrives over a network you do not control, twice, out of order, or not at all. A reconciler is the component that assumes all three happened at once.Mon, 20 Apr 2026 00:00:00 GMTDatabase per tenant against a shared schema, decided properlyhttps://elvissautet.com/solutions/database-per-tenant-vs-shared-schema/https://elvissautet.com/solutions/database-per-tenant-vs-shared-schema/The choice is not about isolation, both can be safe. It is about how many tenants you will have and what a migration across all of them costs you.Thu, 16 Apr 2026 00:00:00 GMTMigrations while people are using ithttps://elvissautet.com/writing/migrations-while-people-are-using-it/https://elvissautet.com/writing/migrations-while-people-are-using-it/The migration that took the system down was one line and looked completely safe. Here is what I actually check now, before running anything.Tue, 14 Apr 2026 00:00:00 GMTScreens for people with a queue standing in front of themhttps://elvissautet.com/work/operations-interface/https://elvissautet.com/work/operations-interface/Dense operational software used all day by staff under pressure. Where clarity beats beauty, where it does not, and how the two get measured.Sun, 12 Apr 2026 00:00:00 GMTServer sent events against WebSockets, and why I usually pick SSEhttps://elvissautet.com/decisions/sse-vs-websockets/https://elvissautet.com/decisions/sse-vs-websockets/Most realtime features only need the server to talk to the browser. That is one direction, and SSE does it over plain HTTP with no new infrastructure.Wed, 08 Apr 2026 00:00:00 GMTDense screens are a different crafthttps://elvissautet.com/writing/dense-screens-are-a-different-craft/https://elvissautet.com/writing/dense-screens-are-a-different-craft/I made an operational screen prettier and it tested worse. Whitespace pushed content below the fold and added a scroll to a constant task.Mon, 06 Apr 2026 00:00:00 GMTHow to design an approval engine that survives an audithttps://elvissautet.com/engines/designing-an-approval-engine/https://elvissautet.com/engines/designing-an-approval-engine/Permissions decide who gets the screen. They must never decide who has authority. Here is the separation, and the guarantees an engine owes you.Thu, 02 Apr 2026 00:00:00 GMTHow to build multi-tenant SaaS where isolation cannot be forgottenhttps://elvissautet.com/solutions/multi-tenant-saas-architecture/https://elvissautet.com/solutions/multi-tenant-saas-architecture/Isolation as a structural property rather than a filter somebody remembers. Six layers, in the order to build them, with the test that proves each one.Sat, 28 Mar 2026 00:00:00 GMTQueues against cron, and when cron is still the right answerhttps://elvissautet.com/decisions/queues-vs-cron/https://elvissautet.com/decisions/queues-vs-cron/Cron is not obsolete and a queue is not always an upgrade. The line is whether you need to know what happened, and whether a failure can be retried.Fri, 20 Mar 2026 00:00:00 GMTThe alias you forgothttps://elvissautet.com/writing/the-alias-you-forgot/https://elvissautet.com/writing/the-alias-you-forgot/Soft deletes are convenient until you write raw SQL. One join without the predicate and deleted records reappear in a report somebody will sign.Wed, 18 Mar 2026 00:00:00 GMTHow to design a scheduler engine that holds ten million future jobshttps://elvissautet.com/engines/designing-a-scheduler-engine/https://elvissautet.com/engines/designing-a-scheduler-engine/Most schedulers put every pending job in Redis and fall over. Three tier time routing keeps memory flat while the table grows into the millions.Sat, 14 Mar 2026 00:00:00 GMTA scheduler holding ten million future jobs on flat memoryhttps://elvissautet.com/work/scheduling-and-automation/https://elvissautet.com/work/scheduling-and-automation/Every pending job in Redis is the default design and it does not survive scale. Three tier time routing cut memory by about ninety percent.Fri, 06 Mar 2026 00:00:00 GMTWhen one number has four owners, three of them are wronghttps://elvissautet.com/work/financial-correctness/https://elvissautet.com/work/financial-correctness/Four subsystems each calculated the same financial figure and three disagreed. Making them agree failed. Removing their right to an opinion worked.Tue, 24 Feb 2026 00:00:00 GMTThe two minute rule that decides whether a site surviveshttps://elvissautet.com/writing/the-two-minute-rule-for-client-work/https://elvissautet.com/writing/the-two-minute-rule-for-client-work/Most small business sites die the same way. If a routine update cannot be done on a phone in two minutes, the content goes stale and starts doing harm.Wed, 18 Feb 2026 00:00:00 GMTSole engineer on a school platform used by 112 schoolshttps://elvissautet.com/work/enaton-school-platform/https://elvissautet.com/work/enaton-school-platform/Zero customers to 112 schools, one engineer. What being the only person who can fix it forces you to design differently, and what it costs.Tue, 10 Feb 2026 00:00:00 GMT