On October 19 2025 AWS experienced an outage that lasted over a day, 10 days later we finally got the root cause analysis and we know exactly what caused the DNS to fail0:00 Summary 5:30 How did Dynamo lost its DNS?13:41 EC2 Errors 16:16 Network Load Balancer ErrorsRCA here https://aws.amazon.com/message/101925/
There are cases where the backend may need to close the connection to prevent unexpected situations, prevent bad actors or simply just free up resources. Closing a connection gracefully allows clients and backends to clean up and finish any pending requests.
In this episode of the backend engineering show I discuss graceful connections in both HTTP/1.1 via the connection header and HTTP/2 via the GOAWAY frame.
Links
https://www.youtube.com/watch?v=fVKPrDrEwTI&t=1s
https://issues.chromium.org/issues/40555364
https://issues.chromium.org/issues/40501721
Postgres 18 has been released with many exciting features such as UUIDv7, Over explain module, composite index skip scans, and the most anticipated asynchronous IO with worker and io_uring mode which I uncover in this show. Hope you enjoy it
0:00 Intro
1:30 Synchronous vs Asynchronous calls
3:00 Synchronous IO
6:30 Asynchronous IO
10:00 Postgres 17 synchronous io
17:20 The challenge of Async IO in Postgres 18
20:00 io_method worker23:00
io_method io_uring
29:30 io_method sync
31:08 Async IO isn’t done! 3
1:30 Support for backend writers
32:36 Improve worker io_method
33:00 direct io support
37:00 Summary
Fundamentals of Operating Systems Course https://oscourse.winktls is brilliant.TLS encryption/decryption often happens in userland. While TCP lives in the kernel. With ktls, userland can hand the keys to the kernel and the kernel does crypto. When calling write, the kernel encrypts the packet and send it to the NIC.When calling read, the kernel decrypts the packet and handed it to the userspace. This mode still taxes the host’s CPU of course, so there is another mode where the kernel offloads the crypto to the NIC device! Host CPU becomes free. Incoming packets to the NIC are decrypted in device before they are DMAed to the kernel. outgoing packets are encrypted before they leave the NIC to the network.ktls still need handshake to happen in userspace. There is also enabling zerocopy in some cases (now that kernel has context) Deserves a video. So much good stuff.0:00 Intro2:00 Userspace SSL Libraries 3:00 ktls 6:00 Kernel Encrypts/Decrypts (TLS_SW)8:20 NIC offload mode (TLS_HW)10:15 NIC does it all (TLS_HW_RECORD)12:00 Write TX Example13:50 Read RX Example17:00 Zero copy (sendfile)https://docs.kernel.org/networking/tls-offload.html
If you are bored of contemporary topics of AI and need a breather, I invite you to join me to explore a mundane, fundamental and earthy topic.
The CPU.
A reading of my substack article https://hnasr.substack.com/p/the-beauty-of-the-cpu
This new PostgreSQL 17 feature is game changer. They know can combine IOs when performing sequential scan.
Grab my database course
https://courses.husseinnasser.com
No technical video today, just talking about the idea of discipline and consistency.
Fundamentals of Operating Systems Course
This video is an overview of how the operating system kernel does socket management and the different data structures it utilizes to achieve that.
timestamps
0:00 Intro
1:38 Socket vs Connections
7:50 SYN and Accept Queue
18:56 Socket Sharding
23:14 Receive and Send buffers
27:00 Summary
Polling is the ability to interrogate a backend to see if a piece of information is ready. It can introduce a chatty system and as a result long polling was born. In this video I explain the beauty of this design pattern and how we can push it to its limit.
0:00 Intro
0:45 Polling
2:30 Problem with Polling
3:50 Long Polling
8:18 Timeouts
10:00 Long Polling Benefits
12:00 Make requests into Long Polling
17:36 Request Resumption
21:40 Summary
You get better as a software engineer when you go through these stages.
0:00 Intro
1:15 Understand a technology
7:07 Articulate how it works
15:30 Understand its’ limitations
19:48 Try to build something better
27:45 Realize what you built also has limitations
32:48 Appreciate the original tech as is
We use technologies all the time without knowing how it works. And it is ok not knowing how things work if interests isn’t there. But when there is interest to understand how something works, pursue it. It feels good when you understand how something works because you work better with it, you swim with the tide instead of against it.
When I learned how TCP/IP work.. you would appreciate every connection request, how you read requests. You will ask questions,
what is my code doing here?
When exactly I’m creating connections?
When am I reading from the connection?
Is it safe to share connections?
This one is not easy, you might think you understand something until you try to explain how it works. If you find yourself using jargon you probably don’t understand and you just try to impress others. Have you seen people who want to talk about something to show they understand it? It’s the opposite. Try to truly articlate how it works, you will really understand it , back to 1.
I thought I understand how backend reads requests until I tried to speak to it.
Once 1,2 are done you will truly understand the tech, now you are confidant, you are excited about the tech and you will truly see when you can use the tech to its full potential and also know the weak points of the tech where it breaks, this happens a lot with TCP/IP. We know tcps limitations.
This one is optional and can be skipped, but attempting to design or building something better then the tech because you know the limitations will truly reveal how you became better. But the challenge here is the ego, you might understand the limitations but you problem is thinking that what you will build is flawless. This step must be proceed with caution.
Dust settles.. this step hurts, and you may take a while to realize it, but whatever you build will have flaws… and when you realize this it is when you get better as an engineer.
This is when you are back full circle you are back to the first stage, look at the technology and understand it but don’t judge it.. just know the limitations and its strength and flow with it. Stop fighting and instead build around a tech, does that mean you shouldn’t build anything new, of course not. Go build, but don’t stress around making something better to defeat existing tech. But actually build it for building it.
Fundamentals of Operating Systems Course
https://oscourse.win
Very clever! We often call read/rcv system call to read requests from a connection, this copies data from kernel receive buffer to user space which has a cost.
This new patch changes this to allow zero copy with notification.
“Reading' data out of a socket instead becomes a “notification” mechanism, where the kernel tells userspace where the data is.”
This kernel patch enables zero copy from the receive queue.
https://lore.kernel.org/io-uring/ZwW7_cRr_UpbEC-X@LQ3V64L9R2/T/
0:00 Intro
1:30 patch summary
7:00 Normal Connection Read (Kernel Copy)
12:40 Zero copy Read
15:30 Performance
Cloudflare built a global cache purge system that runs under 150 ms.
This is how they did it.
Using RockDB to maintain local CDN cache, and a peer-to-peer data center distributed system and clever engineering, they went from 1.5 second purge, down to 150 ms.
However, this isn’t full picture, because that 150 ms is just actually the P50. In this video I explore Clouldflare CDN work, how the old core-based centralized quicksilver, lazy purge work compared to the new coreless, decentralized active purge. In it I explore the pros and cons of both systems and give you my thoughts of this system.
0:00 Intro
4:25 From Core Base Lazy Purge to Coreless Active
12:50 CDN Basics
16:00 TTL Freshness
17:50 Purge
20:00 Core-Based Purge
24:00 Flexible Purges
26:36 Lazy Purge
30:00 Old Purge System Limitations
36:00 Coreless / Active Purge
39:00 LSM vs BTree
45:30 LSM Performance issues
48:00 How Active Purge Works
50:30 My thoughts about the new system
58:30 Summary
Cloudflare blog
https://blog.cloudflare.com/instant-purge/
Mentioned Videos
Cloudflare blog
https://blog.cloudflare.com/instant-purge/
Percentile Tail Latency Explained (95%, 99%) Monitor Backend performance with this metric
https://www.youtube.com/watch?v=3JdQOExKtUY
How Discord Stores Trillions of Messages | Deep Dive
https://www.youtube.com/watch?v=xynXjChKkJc
Fundamentals of Operating Systems Course
https://os.husseinnasser.com
Backend Troubleshooting Course
https://performance.husseinnasser.com
Fundamentals of Database Engineering udemy course https://databases.win
MySQL has been having bumpy journey since 2018 with the release of the version 8.0. Critical crashes that made to the final product, significant performance regressions, and tons of stability and bugs issues. In this video I explore what happened to MySql, are these issues getting fixed? And what is the current state of MySQL at the end of 2024.
0:00 Intro
2:00 MySQL 8.0 vs 5.7 Performance
11:00 Critical Crash in 8.0.38, 8.4.1 and 9.0.0
15:40 Is 8.4 better than 8.0.36?
16:30 More Features = More Bugs
22:30 Summary and my thoughts
resources
https://x.com/MarkCallaghanDB/status/1786428909376164263
https://www.percona.com/blog/do-not-upgrade-to-any-version-of-mysql-after-8-0-37/
http://smalldatum.blogspot.com/2024/09/mysql-innodb-vs-sysbench-on-large-server.html
https://www.percona.com/blog/mysql-8-0-vs-5-7-are-the-newer-versions-more-problematic/
Fundamentals of Operating Systems Course
https://oscourse.win
In this video I use strace a performance tool that measures how many system calls does a process makes. We compare a simple task of reading from a file, and we run the program in different runtimes, namely nodejs, buns , python and native C.
We discuss the cost of kernel mode switches, system calls and pe
0:00 Intro
5:00 Code Explanation
6:30 Python
9:30 NodeJS
12:30 BunJS
13:12 C
16:00 Summary
Fundamentals of Operating Systems Course
https://os.husseinnasser.com
When do you use threads?
I would say in scenarios where the task is either
1) IO blocking task
2) CPU heavy
3) Large volume of small tasks
In any of the cases above, it is favorable to offload the task to a thread.
1) IO blocking task
When you read from or write to disk, depending on how you do it and the kernel interface you used, the write might be blocking. This means the process that executes the IO will not be allowed to execute any more code until the write/read completes.
That is why you see most logging operations are done on a secondary thread (like libuv that Node uses) this way the thread is blocked but the main process/thread can resume its work.
If you can do file reads/writes asynchronously with say io_uring then you technically don't need threading.
Now notice how I said file IO because it is different than socket IO which is always done asynchronously with epoll/select etc.
2) CPU heavy
The second use case is when the task requires lots of CPU time, which then starves/blocks the rest of the process from doing its normal job. So offloading that task to a thread so that it runs on a different core can allow the main process to continue running on its the original core.
3) Large volume of small tasks
The third use case is when you have large amount of small tasks and single process can't deliver as much throughput. An example would be accepting connections, a single process can only accept connections so fast, to increase the throughput in case where you have massive amount of clients connecting, you would spin multiple threads to accept those connections and of course read and process requests. Perhaps you would also enable port reuse so that you avoid accept mutex locking.
Keep in mind threads come with challenges and problems so when it is not required.
0:00 Intro
1:40 What are threads?
7:10 IO blocking Tasks
17:30 CPU Intensive Tasks
22:00 Large volume of small tasks
I am fascinated by how timeouts affect backend and frontend programming.
When a party is waiting on something you can place a timeout to break the wait. This is useful for freeing resources to more critical processes, detecting slow operations and even avoiding DOS attacks.
Contrary to common beliefs, timeouts are not exclusive to request processing, they can be applied to other parts of the frontend-backend communications. Let us explore this briefly.
0:00 Intro
2:30 Connection Timeout
5:00 Request Read timeout
10:00 Wait Timeout
12:00 Usage Timeout
14:00 Response Timeout
16:00 Canceling a request
19:50 Proxies and timeouts
Learn more about database and OS internals, check out my courses
Fundamentals of database engineering https://databases.win
Fundamentals of operating systems https://oscourse.win
This new PostgreSQL 17 feature is game changer.
You see, postgres like most databases work with fixed size pages. Pretty much everything is in this format, indexes, table data, etc. Those pages are 8K in size, each page will have the rows, or index tuples and a fixed header. The pages are just bytes in files and they are read and cached in the buffer pool.
To read page 0, for example, you would call read on offset 0 for 8192 bytes, To read page 1 that is another read system call from offset 8193 for 8192, page 7 is offset 57,345 for 8192 and so on.
If table is 100 pages stored a file, to do a full table scan, we would be making 100 system calls, each system call had an overhead (I talk about all of that in my OS course).
The enhancement in Postgres 17 is to combine I/Os you can specify how much IO to combine, so technically while possible you can scan that entire table in one system call doesn’t mean its always a good idea of course and Ill talk about that.
This also seems to included a vectorized I/O, with preadv system call which takes an array of offsets and lengths for random reads.
The challenge will become how to not read too much, say I’m doing a seq scan to find something, I read page 0 and found it and quit I don’t need to read any more pages. With this feature I might read 10 pages in one I/O and pull all its content, put in shared buffers only to find my result in the first page (essentially wasting disk bandwidth, memory etc)
It is going to be interesting to balance this out.
Fundamentals of Operating Systems Course
https://os.husseinnasser.com
Why Windows Kernel connects slower than Linux
I explore the behavior of TCP/IP stack in Windows kernel when it receives a RST from the backend server especially when the host is available but the port we are trying to connect to is not. This behavior is exacerbated by having both IPv6 and IPv4 and if the happy eye ball protocol is in place where IPv6 is favorable.
0:00 Intro
0:30 Fundamentals TCP/IP
3:00 Unreachable Port Behavior
6:00 Client Kernel Behavior (Linux vs Windows)
11:40 Slow TCP Connect on Windows
15:00 localhost, IPv6 and IPv4
20:00 Happy Eyeballs
28:00 Registry keys to change the behavior
31:00 Port Unreachable vs Host Unreachable
https://daniel.haxx.se/blog/2024/08/14/slow-tcp-connect-on-windows/
In this episode of the backend engineering show I describe an interesting bug I ran into where the web server ran out of ephemeral ports causing the system to halt.
0:00 Intro
0:30 System architecture
2:20 The behavior of the bug
4:00 Backend Troubleshooting
7:00 The cause
15:30 Ephemeral ports on loopback
Fundamentals of Operating Systems Course
https://os.husseinnasser.com
Linux I/O expert and subsystem maintainer Jens Axboe has submitted all of the IO_uring feature updates ahead of the imminent Linux 6.10 merge window.
In this video I explore this with a focus on what zerocopy.
0:00 Intro
0:30 IO_uring gets faster
2:00 What is io_uring
7:00 How Normal Copying Work
12:00 How Zero Copy Works
13:50 ZeroCopy and TLS
https://www.phoronix.com/news/Linux-6.10-IO_uring
https://lore.kernel.org/io-uring/fef75ea0-11b4-4815-8c66-7b19555b279d@kernel.dk/?s=09
