Chris Sereno builds at the intersection of AI, cloud, entrepreneurship and faith. Co-founder and CTO of Cowboy Christian, a health cost sharing platform; founder of Sereno Tech; formerly the lead AWS Solutions Architect on McDonald’s. He writes Actual State, four moves for leading systems nobody can see.https://chrissereno.com/en-ushttps://chrissereno.com/writing/there-is-always-a-middle-account/https://chrissereno.com/writing/there-is-always-a-middle-account/I spent months designing a system around "person to person" before I drew how a dollar actually moves. Every transfer is two instructions with an account in between.Tue, 25 Aug 2026 00:00:00 GMThealthcareactual statepaymentssystemshttps://chrissereno.com/writing/imprints/https://chrissereno.com/writing/imprints/Two worn spots on a pillow, pressed in by the same two small heads over months. We leave imprints on people whether or not we intend to.Thu, 09 May 2019 00:00:00 GMTleadershipinfluencehttps://chrissereno.com/writing/clear-cloudflare-cache-from-lambda/https://chrissereno.com/writing/clear-cloudflare-cache-from-lambda/Purging a CDN cache on every deploy, with the API key in Parameter Store rather than in the function, because that one key opened most of the account.Fri, 19 Apr 2019 00:00:00 GMTawslambdaautomationhttps://chrissereno.com/writing/case-study-smtp-limits/https://chrissereno.com/writing/case-study-smtp-limits/Mail stopped after exactly three messages, repeatably. The firewall never saw the SYN. Every component the customer managed was clean, which left one nobody owned.Fri, 15 Mar 2019 00:00:00 GMTawssmtplimitsdiagnosishttps://chrissereno.com/writing/relentless/https://chrissereno.com/writing/relentless/Amazon was almost called Relentless: showing no abatement of severity, intensity, strength or pace. A better description of a working rhythm than of a personality.Tue, 01 Jan 2019 00:00:00 GMTleadershipdisciplinehttps://chrissereno.com/writing/cloudshark-halloween-challenge/https://chrissereno.com/writing/cloudshark-halloween-challenge/CloudShark hid a puzzle inside a packet capture. Carving the objects back out of it and following the conversation until the answer fell out.Fri, 02 Nov 2018 00:00:00 GMTpacket captureforensicsdiagnosishttps://chrissereno.com/writing/case-study-out-of-memory/https://chrissereno.com/writing/case-study-out-of-memory/A website went down weekly and a reboot always fixed it. I am a network person, so I looked at the network. It was a 1 GB instance with no swap file.Sat, 25 Aug 2018 00:00:00 GMTawsec2diagnosishttps://chrissereno.com/writing/diagram-your-service/https://chrissereno.com/writing/diagram-your-service/You cannot choose a capture point until you understand the service, and the fastest way to understand one is to draw it. The drawing is the diagnosis, not documentation.Sat, 07 Jul 2018 00:00:00 GMTarchitecturedocumentationmethodhttps://chrissereno.com/writing/command-for-dozing/https://chrissereno.com/writing/command-for-dozing/A Cat D11T bulldozer in Arizona, driven live from a trade-show floor in Málaga. Five thousand miles between an operator's hands and several hundred tonnes of moving steel.Tue, 03 Jul 2018 00:00:00 GMTcaterpillarnetworkinglatencyhttps://chrissereno.com/writing/case-of-an-fqdn-issue/https://chrissereno.com/writing/case-of-an-fqdn-issue/"I can't access my shared drive." The finger pointed at the network, as it does. An unfiltered capture caught two packets that were not part of the conversation.Sat, 02 Jun 2018 00:00:00 GMTdnsdiagnosispacket capturehttps://chrissereno.com/writing/thoughts-from-behind-a-mower/https://chrissereno.com/writing/thoughts-from-behind-a-mower/My dad taught me that straight rows come from watching the end of the row, not the ground in front of the blade. It works for architecture and careers too.Fri, 25 May 2018 00:00:00 GMTleadershipfocushttps://chrissereno.com/writing/case-of-the-large-header/https://chrissereno.com/writing/case-of-the-large-header/The HTTP spec sets no header limit. Every server, proxy and load balancer in the path does, and none of them logged it.Wed, 21 Mar 2018 00:00:00 GMThttpprotocolsdiagnosishttps://chrissereno.com/writing/case-of-the-named-pipes/https://chrissereno.com/writing/case-of-the-named-pipes/"The network path was not found." Vague errors blame the network. This one blamed the network and meant something else entirely.Thu, 01 Feb 2018 00:00:00 GMTsqlprotocolsdiagnosishttps://chrissereno.com/writing/case-of-the-tired-firewall/https://chrissereno.com/writing/case-of-the-tired-firewall/Unreproducible symptoms, no root cause. The device in the path was not failing; it was tired, and its own health checks were never going to say so.Mon, 01 Jan 2018 00:00:00 GMTfirewalllatencydiagnosishttps://chrissereno.com/writing/case-of-the-rogue-server/https://chrissereno.com/writing/case-of-the-rogue-server/Several hundred users lost connectivity one by one while every device reported healthy. Clearing the ARP table helped for a moment, the only real clue in the incident.Fri, 10 Nov 2017 00:00:00 GMTdhcparpdiagnosishttps://chrissereno.com/writing/captures-at-both-ends/https://chrissereno.com/writing/captures-at-both-ends/A client sent a TLS 1.2 Client Hello. The server received TLS 1.0. Neither capture alone could have told you that.Sat, 23 Sep 2017 00:00:00 GMTtlspacket capturediagnosishttps://chrissereno.com/writing/tcp-performance-options/https://chrissereno.com/writing/tcp-performance-options/Window scaling, MSS, Nagle, congestion control. The options every application performance question eventually bottoms out in, and what each one costs you.Fri, 25 Aug 2017 00:00:00 GMTtcpperformancemethodhttps://chrissereno.com/writing/performing-a-capture/https://chrissereno.com/writing/performing-a-capture/You picked the point and set the filters. Now start it, generate the traffic that reproduces the problem, and stop before the evidence buries itself.Fri, 30 Jun 2017 00:00:00 GMTpacket capturewiresharkplaybookhttps://chrissereno.com/writing/preparing-for-the-capture/https://chrissereno.com/writing/preparing-for-the-capture/Where to capture, how, and what to filter: the prep work that decides whether an analysis is possible at all.Sun, 28 May 2017 00:00:00 GMTpacket captureplaybookwiresharkhttps://chrissereno.com/writing/capturing-light/https://chrissereno.com/writing/capturing-light/Electricity, light and radio become ones and zeros become packets. How you catch the fastest thing there is, at an exact point, repeatably.Sat, 08 Apr 2017 00:00:00 GMTpacket capturefundamentalsmethodhttps://chrissereno.com/writing/classic-performance-example/https://chrissereno.com/writing/classic-performance-example/A transfer running at 80 Mbps on a 10 Gbps link, weeks instead of days. Thirty zero window segments in a few seconds said the problem was not the network.Fri, 10 Mar 2017 00:00:00 GMTtcpperformancediagnosishttps://chrissereno.com/writing/ipv6/https://chrissereno.com/writing/ipv6/My ISP had no native IPv6, so I tunnelled it and then captured the traffic to see whether the performance benefits everyone claims for it actually showed up.Wed, 22 Feb 2017 00:00:00 GMTipv6packet captureperformancehttps://chrissereno.com/writing/online-privacy/https://chrissereno.com/writing/online-privacy/A former FBI investigator spoofed the guest wifi and harvested phones in a room full of IT professionals. What I changed about my own data afterwards.Tue, 07 Feb 2017 00:00:00 GMTprivacysecurityfamily