https://blog.modimeet.com/blog Engineering write-ups by Meet Modi: race conditions, idempotency, LLM pipelines, payments, and the bugs in between. en Mon, 13 Jul 2026 00:00:00 GMT https://blog.modimeet.com/blog/the-vector-database-was-innocent https://blog.modimeet.com/blog/the-vector-database-was-innocent Mon, 13 Jul 2026 00:00:00 GMT I built a RAG service over the OpenTelemetry docs, then pointed OpenTelemetry back at it to find out why answers took 16 seconds. It wasn't the LLM. It wasn't the vector search either. Observability SigNoz OpenTelemetry RAG https://blog.modimeet.com/blog/a-query-that-returned-nothing-nested-inside-it https://blog.modimeet.com/blog/a-query-that-returned-nothing-nested-inside-it Wed, 08 Jul 2026 00:00:00 GMT The exact same logical query returned full nested data through the REST layer and empty shells through the service layer underneath it. Same fields requested. Same backend. One shorthand parameter that only one of the two actually understood. CMS API Integration Backend https://blog.modimeet.com/blog/one-field-three-different-envelopes https://blog.modimeet.com/blog/one-field-three-different-envelopes Thu, 02 Jul 2026 00:00:00 GMT The same subscription-status field came back nested two levels deep, one level deep, or bare at the top, depending on account state. My fix didn't catch that. It added math on top of a check that was already wrong. API Design Data Modeling Debugging https://blog.modimeet.com/blog/the-redirect-that-forgot-where-you-were-going https://blog.modimeet.com/blog/the-redirect-that-forgot-where-you-were-going Fri, 26 Jun 2026 00:00:00 GMT A user clicked a link straight to a class recordings page. Their session had expired. They logged back in and landed on the dashboard home instead, three clicks away from where they started. Authentication Routing UX https://blog.modimeet.com/blog/three-ways-for-a-payment-to-finish https://blog.modimeet.com/blog/three-ways-for-a-payment-to-finish Fri, 19 Jun 2026 00:00:00 GMT A payment flow has three exits: the user finishes it, the user closes the window, or the component that started it is already gone. Mine only handled the first one. Payments React State Management https://blog.modimeet.com/blog/a-lock-made-of-css-and-a-mutation-observer https://blog.modimeet.com/blog/a-lock-made-of-css-and-a-mutation-observer Fri, 12 Jun 2026 00:00:00 GMT Say you're building a premium content gate. An overlay div is not a lock. It's a suggestion, and any browser's devtools can decline the suggestion in about four seconds. Frontend Security DOM UX Patterns https://blog.modimeet.com/blog/an-endpoint-built-to-be-called-twice https://blog.modimeet.com/blog/an-endpoint-built-to-be-called-twice Fri, 05 Jun 2026 00:00:00 GMT A reward endpoint has no way to tell a genuine retry from a genuinely repeated action. So we stopped pretending it could, and made it credit every call, on purpose. Distributed Systems API Design Backend https://blog.modimeet.com/blog/the-parameter-that-let-you-overwrite-someone-elses-quiz https://blog.modimeet.com/blog/the-parameter-that-let-you-overwrite-someone-elses-quiz Fri, 29 May 2026 00:00:00 GMT One internal caller needed to set a quiz's ID by hand. The public endpoint accepted the same parameter from anyone, and the storage layer used it as a primary key. Security API Design Backend https://blog.modimeet.com/blog/committing-the-balance-risking-the-ledger https://blog.modimeet.com/blog/committing-the-balance-risking-the-ledger Fri, 22 May 2026 00:00:00 GMT I wrapped a balance update and an audit log write in one transaction because that's what consistency is supposed to look like. Then a log insert failed and rolled back a reward the user should have already had. Transactions Database Backend https://blog.modimeet.com/blog/a-referral-credit-that-had-to-fire-exactly-once https://blog.modimeet.com/blog/a-referral-credit-that-had-to-fire-exactly-once Fri, 15 May 2026 00:00:00 GMT One endpoint, hit three or four times during onboarding, sometimes before the user record even finished writing. It still had to pay out a referral bonus exactly once. Idempotency Mobile Backend Backend https://blog.modimeet.com/blog/the-reward-that-fired-twice https://blog.modimeet.com/blog/the-reward-that-fired-twice Fri, 08 May 2026 00:00:00 GMT A challenge creator got a bonus for winning. They also got it for winning again, on the same challenge, because a retry doesn't know it's a retry. Idempotency Backend Concurrency https://blog.modimeet.com/blog/building-the-quiz-before-anyone-presses-play https://blog.modimeet.com/blog/building-the-quiz-before-anyone-presses-play Fri, 01 May 2026 00:00:00 GMT A student is enrolled in six classes. On a given day they open one, maybe two. We were generating all six anyway, and paying for five nobody would ever see. System Design LLM Cost Backend https://blog.modimeet.com/blog/ten-were-requested-the-model-decided-otherwise https://blog.modimeet.com/blog/ten-were-requested-the-model-decided-otherwise Fri, 24 Apr 2026 00:00:00 GMT A ten-question quiz request sometimes came back with eight questions, sometimes twelve, sometimes ten where two didn't have a correct answer among the options. LLM Output Validation Data Integrity Backend https://blog.modimeet.com/blog/a-dollar-sign-with-two-jobs https://blog.modimeet.com/blog/a-dollar-sign-with-two-jobs Fri, 17 Apr 2026 00:00:00 GMT "Solve $3x + 7 = 22$" and "a shirt costs $20" use the same character to mean two different things. The renderer couldn't tell them apart, and neither could our regex on the first try. Text Sanitization LLM Output Rendering https://blog.modimeet.com/blog/the-question-the-model-kept-repeating https://blog.modimeet.com/blog/the-question-the-model-kept-repeating Fri, 10 Apr 2026 00:00:00 GMT A student asked for more practice questions on the same topic four times in a row. The third batch had a question nearly identical to one from the first. LLM Prompting Quiz Generation Backend https://blog.modimeet.com/blog/the-quiz-where-the-answer-was-always-c https://blog.modimeet.com/blog/the-quiz-where-the-answer-was-always-c Thu, 02 Apr 2026 00:00:00 GMT Across a batch of auto-generated quizzes, the correct answer kept landing in the same slot. Prompting the model to vary it helped. It didn't fix it. LLM Prompting Testing Backend https://blog.modimeet.com/blog/state-that-existed-only-sometimes https://blog.modimeet.com/blog/state-that-existed-only-sometimes Fri, 27 Mar 2026 00:00:00 GMT A page crashed reading state that should have existed. The bug: a Redux slice got registered in exactly one bootstrap path, and a second, equally valid path to the same page never went through it. Micro-Frontends Redux Frontend Architecture https://blog.modimeet.com/blog/success-on-retry-meant-silence https://blog.modimeet.com/blog/success-on-retry-meant-silence Wed, 18 Mar 2026 00:00:00 GMT A user tapped accept twice and landed in a quiz flow that expected a fresh acceptance. The bug: calling accept a second time returned success silently, and success was the UI's cue to barge ahead. Idempotency Error Handling Backend https://blog.modimeet.com/blog/the-error-that-blamed-the-network https://blog.modimeet.com/blog/the-error-that-blamed-the-network Wed, 11 Mar 2026 00:00:00 GMT Users kept being told to check their internet connection. Their internet connection was fine. The bug: a switch statement built for 3 error codes was still running the show after the backend's real list grew to 6. Error Handling API Contracts Frontend https://blog.modimeet.com/blog/two-names-for-one-field https://blog.modimeet.com/blog/two-names-for-one-field Tue, 03 Mar 2026 00:00:00 GMT A quiz report showed every answer as wrong. A carousel on the same page threw and took the whole surface down with it. Same root cause: a call site that skipped a step every other call site took for granted. API Contracts Defensive Coding Frontend https://blog.modimeet.com/blog/off-by-one-in-the-fallback-branch https://blog.modimeet.com/blog/off-by-one-in-the-fallback-branch Tue, 24 Feb 2026 00:00:00 GMT A results report marked three question types as wrong, every time, no exceptions. The bug: one branch subtracted 1 from an index. The fallback copied that subtraction without asking why it was there. Data Mapping Testing Frontend