Tag Archives: servers

Enterprise Overhaul: Resolving DNS

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Everyone assumes all software engineers are great with numbers. If only they knew the truth. How many people’s phone numbers can you recite? No peeking and emergency numbers don’t count! Don’t worry if you couldn’t name that many. Here’s the real embarrassing test of the day: How many sites’ IP addresses can you name? No pinging and local subnets don’t count!

telephone
Most telephones still looked like this when DNS was invented. Not pictured: the phonebook.

Back in the mid-1980s, the first Domain Name System (DNS) implementations started putting our IP addresses into server-based contact lists and the Internet has never looked the same since. These days, we may associate DNS with large-scale networks, but it’s important to remember that DNS really came from a very human distaste for numbers. Thirty years later, we engineers use it so much in normal Internet usage that it’s easy to take for granted.

DNS may be a mature, but the fact of networks is that it always takes at least two to tango. As new technologies and deployments emerge, the implications of integrating with DNS must still be revisited. Your datacenter is not the Internet, even if it’s in the cloud. Continuing the enterprise themes of our previous posts, this post looks at how to resolve a few of the DNS pitfalls preying on precious reliability and performance.

A protocol precaution

The client side of DNS, resolution, is virtually all UDP. This is interesting because UDP is designed as a lightweight, unreliable transport. However, in many of the most common use cases, DNS calls precede TCP-backed HTTP and other protocols based on reliable transports. This fundamental difference changes many things. Looking upstream, UDP does not load-balance like TCP. Because UDP is not connection-oriented or congestion-controlled, DNS traffic will act very differently at scale.

So our first lesson is to stay true to the stateless nature of UDP and avoid putting stateful load balancers in front of DNS infrastructure. Instead, configure clients and servers to conform to the built-in load-handling architecture of DNS. The Internet’s DNS “deployment” is load balanced via its inherent hierarchy and IP Anycast.

Client integration

Back on the client side, you can do a lot to optimize and robustify your application’s DNS integration. The first step is to take a hard look at your stack. Whether you’re running Python, Java, JavaScript, or C++, the defaults may not be for you, especially when working with traffic within the datacenter.

For example, while not supported here at PayPal, it’s safe to say Tornado is a popular Python web framework, with many asynchronous networking features. But, silently and subtly, DNS is not one of them. Tornado’s default DNS resolution behavior will block the entire IO event loop, leading to big issues at scale.

And that’s just one example of library DNS defaults jeopardizing application reliability. Third-party packages and sometimes even builtins in Java, Node.js, Python, and other stacks are full of hidden DNS faux pas.

For instance, the average off-the-shelf HTTP client seems like a neutral-enough component. Where would we be without reliable standbys like wget? And that is how the trouble starts. The DNS defaults in most tools are designed to make for good Internet citizens, not reliable and performant enterprise foundations.

DNS_in_the_real_world.svg
The hops Internet-connected applications make for you. It’s no wonder the default timeout is 5000 milliseconds.

The first difference is name resolution timeouts. By default, resolve.conf, netty, and c-ares (gevent, node.js, curl) are all configured to a whopping 5 seconds. But this is your enterprise, your service, and your datacenter. Look at the SLA of your service and the reliability of your DNS. If your service can’t take an extra 500 milliseconds some percentage of the time, then you should lower that timeout. I’ve usually recommended 200 milliseconds or less. If your infrastructure can’t resolve DNS faster than that, do one or more of the following:

  1. Put the authoritative DNS servers topologically closer.
  2. Add caching DNS servers, maybe even on the same machine.
  3. Build application-level DNS caching.

Option #1 is purely a network issue, and a matter for network operations to discuss. For brevity’s sake, option #2 is outside the scope of this article. But option #3 is the one we recommend most, because it is bureaucracy-free and relatively easy to implement, even with enterprise considerations.

Application-level DNS caching

When designing an enterprise application-level DNS cache, we must recognize that we are not discussing standard-issue web components like scrapers and browsers. Most enterprise services talk to a fixed set of relatively few machines. Even the most powerful and complex production PayPal services communicate with fewer than 200 addresses, partly due to the prevalence of load balancing LTMs in our architecture.

For our gevent-based Python stack, we use an asynchronous DNS cache that refreshes those addresses every five minutes. Plus, the stack warms up our application’s DNS cache by kicking off preresolution of many known DNS-addressed hosts at startup, ensuring that the first requests are as fast as later ones.

Some may be asking, why use a custom, application-level DNS cache when virtually every operating system caches DNS automatically? In short, when the OS cache expires, the next DNS resolution will block, causing stacks without this asynchronous DNS cache to block on the next resolution. Our DNS cache allows us to use mildly stale addresses while the cache is refreshing, making us robust to many DNS issues. For our use cases both the chances and consequences of connecting to the wrong server are so minute that it’s not worth inflating outlier response times by inlining DNS. This arrangement also makes services much more robust to network glitches and DNS outages, as well as allowing for more logging and instrumentation around the explicit DNS resolution so you can see when DNS is performing badly.

Denecessitizing DNS?

The overhaul wouldn’t be complete without exploring one final scenario. What’s it like to not use DNS at all? It may sound odd, given the number of technologies built on DNS in the last 30 years. But even today, PayPal production services still communicate to each other using a statically generated IP-address-based system, like a souped-up hosts file. This design decision long predates my tenure here, and for a long time I considered it technical debt. But after collaborating with architects here and at other enterprise datacenters, I’ve come to appreciate the advantages of skipping DNS. DNS was designed for multi-authority, federated, eventually-consistent networks, like the Internet. Even the biggest datacenters are not the Internet. A datacenter is topologically smaller, has only one operational authority, and must meet much tighter reliability requirements.

pp_midtier
A little peek at PayPal’s midtier-to-midtier traffic. Each shrunken line of text is a service endpoint. It looks like a lot, but each endpoint only talks to a few others.

Whether or not your system uses DNS, when you own the entire network it’s still best practice to maintain a central, version-controlled, “single source of truth” repository for networking configurations. After all, even DNS server configurations have to come from somewhere. If it were possible to efficiently and reliably push that same information to every client, would you?

Explicit preresolution of all service names reduces the window of inconsistency while saving the datacenter billions of network requests. If you already have a scalable deployment system, could it also fill the network topology gap, saving you the trouble of overhauling, scaling, and maintaining an Internet system for enterprise use? There’s a lot packed in a question like that, but it’s something to consider when designing your service ecosystem.

In short

So, to sum it all up, here are the key takeaways:

  • Beware the pitfalls of stateful load-balancing for DNS and UDP.
  • Tighten up your timeouts according to your SLAs.
  • Consider an in-application DNS cache with explicit resolution.
  • The fastest and most reliable request is the request you don’t have to make.
  • A datacenter is not the Internet.

It may be obvious now, but it bears repeating. If you’re not careful, out-of-box solutions will fill your inbox with avoidable problems. Quality enterprise engineering means taking a microscope to libraries, with deliberate overhauling for your organization’s needs.

If you found this interesting, have some experience, and know a thing or two about security, have we got the job for you! See this description and contact mahmoud@paypal.com and kurose@paypal.com with your résumé/portfolio.

 

Introducing SuPPort

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In our last post, Ten Myths of Enterprise Python, we promised a deeper dive into how our Python Infrastructure works here at PayPal and eBay. There is a problem, though. There are only so many details we can cover, and at the end of the day, it’s just so much better to show than to tell.

support_logoSo without further ado, we’re pleased to introduce SuPPort, an in-development distillation of our PayPal Python Infrastructure.

Started in 2010, Python Infrastructure initially powered PayPal’s internal price-setting interfaces, then grew to support payment orchestration interfaces, and now in 2015 it supports dozens of projects at PayPal and eBay, having handled billions of production-critical requests for a wide range of teams and tiers. So what does it mean to distill this functionality into SuPPort?

SuPPort is an event-driven server framework designed for building scalable and maintainable services and clients. It’s built on top of several open-source technologies, so before we dig into the workings of SuPPort, we ought to showcase its foundations:

Some or all of these may be new to many developers, but all-in-all they comprise a powerful set of functionality. With power comes complexity, and while Python as a language strives for technical convergence, there are many ways to approach the problem of developing, scaling, and maintaining components. SuPPort is one way gevent and the libraries above have been used to build functional services and products with anywhere from 100 requests per day to 100 requests per second and beyond.

Enterprise Ideals, Flexible Features

Many motivations have gone into building up a Python stack at PayPal, but as in any enterprise environment, we continuously aim to achieve the following:

Of course organizations of all sizes want these features as well, but the key difference is that large organizations like PayPal usually end up building more. All while demanding a higher degree of redundancy and risk mitigation from their processes. This often results in great cost in terms of both hardware and developer productivity. Fortunately for us, Python can be very efficient in both respects.

So, let’s take a stroll through a selection of SuPPort’s feature set in the context of these criteria! Note that if you’re unfamiliar with evented programming, nonblocking sockets, and gevent in particular, some of this may seem quite foreign. The gevent tutorial is a good entry point for the intermediate Python programmer, which can be supplemented with this well-illustrated introduction to server architectures.

Interoperability

Python usage here at PayPal has spread to virtually every imaginable use case: administrative interfaces, midtier services, operations automation, developer tools, batch jobs; you name it, Python has filled a gap in that area. This legacy has resulted in a few rather interesting abstractions exposed in SuPPort.

BufferedSocket

PayPal has hundreds of services across several tiers. Interoperating between these means having to implement over half a dozen network protocols. The BufferedSocket type eliminated our inevitable code duplication, handling a lot of the nitty-gritty of making a socket into a parser-friendly data source, while retaining timeouts for keeping communications responsive. A must-have primitive for any gevent protocol implementer.

ConnectionManager

Errors happen in live environments. DNS requests fail. Packets are lost. Latency spikes. TCP handshakes are slow. SSL handshakes are slower. Clients rarely handle these problems gracefully. This is why SuPPort includes the ConnectionManager, which provides robust error handling code for all of these cases with consistent logging and monitoring. It also provides a central point of configuration for timeouts and host fallbacks.

Introspectability

As part of a large organization, we can afford to add more machines, and are even required to keep a certain level of redundancy and idle hardware. And while DevOps is catching on in many larger-scale environments, there are many cases in enterprise environments where developers are not allowed to attend to their production code.

SuPPort currently comes with all the same general-purpose introspection capabilities that PayPal Python developers enjoy, meaning that we get you as much structured information about your application as possible without actually requiring login privileges. Of course almost every aspect of this is configurable, to suit a wide variety of environments from development to production.

Context management

Python famously has no global scope: all values are namespaced in module scope. But there are still plenty of aspects of the runtime that are global. Some are out of our control, like the OS-assigned process ID, or the VM-managed garbage collection counters. Others aspects are in our control, and best practice in concurrent programming is to keep these as well-managed as possible.

SuPPort uses a system of Contexts to explicitly manage nonlocal state, eliminating difficult-to-track implicit global state for many core functions. This has the added benefit of creating opportunities to centrally manage and monitor debugging data and statistics (some charts of which are shown below), all made available through the MetaApplication, detailed further down.

Accept SSL Charts

Charting quantiles and recent timings for incoming SSL connections from a remote service.

SuPPort Stats table

Values are also available in table-based and JSON formats, for easy human and machine readability.

MetaApplication

While not exclusively a web server framework, SuPPort leverages its strong roots in the web to provide both a web-based user interface and API full of useful runtime information.

As you can see below, there is a lot of information exposed through this default interface. This is partly because of restricted environments not allowing local login on machines, and another part is the relative convenience of a browser for most developers. Not pictured is the feature that the same information is available in JSON format for easy programmatic consumption. Because this application is such a rich source of information, we recommend using SuPPort to run it on a separate port which can be firewalled accordingly, as seen in this example.

MetaApplication Screenshot #1

A screenshot of the MetaApplication, showing load averages and other basic information, as well as subroutes to further info.

MetaApplication Screenshot 2

Another shot of the MetaApplication, showing process and runtime info

 

Infallibility

At the end of the day, reliability over long periods of time is what earns a stack approval and adoption. At this point, the SuPPort architecture has a billion production requests under its belt here at PayPal, but on the way we put it through the proverbial paces. At various points, we have tested and confirmed several edge behaviors. Here are just a few key characteristics of a well-behaved application:

  • Gracefully sheds traffic under load (no unbounded queues here)
  • Can and has run at 90%+ CPU load for days at a time
  • Is free from framework memory leaks
  • Is robust to memory leakage in user code

To illustrate, a live service handling millions of requests per day had a version of OpenSSL installed which was leaking memory on every handshake. Thanks to preemptive worker cycling on excessive process memory usage, no intervention was required and no customers were impacted. The worker cycling was noted in the logs, the leak was traced to OpenSSL, and operations was notified. The problem was fixed with the next regularly scheduled release rather than being handled as a crisis.

No monkeypatching

One of the first and sometimes only ways that people experience gevent is through monkeypatching. At the top of your main module you issue a call to gevent that automatically swaps out virtually all system libraries with their cooperatively concurrent ones. This sort of magic is relatively rare in Python programming, and rightfully so. Implicit activities like this can have unexpected consequences. SuPPort is a no-monkeypatching approach to gevent. If you want to implement your own network-level code, it is best to use gevent.socket directly. If you want gevent-incompatible libraries to work with gevent, best to use SuPPort’s gevent-based threading capabilities.

Using threads with gevent

“Threads? In my gevent? I thought the whole point of greenlets and gevent was to eliminate evil, evil threads!” –Countless strawmen

Originating in Stackless and ported over in 2004 by Armin Rigo (of PyPy fame), greenlets are mature and powerful concurrency primitives. We wanted to add that power to the process- and thread-based world of POSIX. There’s no point running from standard OS capabilities; threads have their place. Many architectures adopt a thread-per-request or process-per-request model, but the last thing we want is the number of threads going up as load increases. Threads are expensive; each thread adds a bit of contention to the mix, and in many environments the memory overhead alone, typically 4-8MB per thread, presents a problem. At just a few kilobytes apiece, greenlet’s microthreads are three orders of magnitude less costly.

Furthermore, thread usage in our architecture is hardly about parallelism; we use worker processes for that. In the SuPPort world, threads are about preemption. Cooperative greenlets are much more efficient overall, but sometimes you really do need guarantees about responsiveness.

One excellent example of how threads provide this responsiveness is the ThreadQueueServer detailed below. But first, there are two built-in Threadpools with decorators worth highlighting, io_bound and cpu_bound:

io_bound

This decorator is primarily used to wrap opaque clients built without affordances for cooperative concurrent IO. We use this to wrap cx_Oracle and other C-based clients that are built for thread-based parallelization. Other major use cases for io_bound is when getting input from standard input (stdin) and files.

A rough sketch of what threads inside a worker look like. The outer box is a process, inner boxes are threads/threadpools, and each text label refers to a coroutine/greenlet.

A rough sketch of what threads inside a worker look like. The outer box is a process, inner boxes are threads/threadpools, and each text label refers to a coroutine/greenlet.

cpu_bound

The cpu_bound decorator is used to wrap expensive operations that would halt the event loop for too long. We use it to wrap long-running cryptography and serialization tasks, such as decrypting private SSL certificates or loading huge blobs of XML and JSON. Because the majority of use cases’ implementations do not release the Global Interpreter Lock, the cpu_bound ThreadPool is actually just a pool of one thread, to minimize CPU contention from multiple unparallelizable CPU-intensive tasks.

It’s worth noting that some deserialization tasks are not worth the overhead of dispatching to a separate thread. If the data to be deserialized is very short or a result is already cached. For these cases, we have the cpu_bound_if decorator, which conditionally dispatches to the thread, yielding slightly higher responsiveness for low-complexity requests.

Also note that both of these decorators are reentrant, making dispatch idempotent. If you decorate a function that itself eventually calls a decorated function, performance won’t pay the thread dispatch tax twice.

ThreadQueueServer

The ThreadQueueServer exists as an enhanced approach to pulling new connections off of a server’s listening socket. It’s SuPPort’s way of incorporating an industry-standard practice, commonly associated with nginx and Apache, into the gevent WSGI server world.

If you’ve read this far into the post, you’re probably familiar with the standard multi-worker preforking server architecture; a parent process opens a listening socket, forks one or more children that inherit the socket, and the kernel manages which worker gets which incoming client connection.

Preforking architecture

Basic preforking architecture. The OS balances traffic between workers, monitored by an arbiter.

The problem with this approach is that it generally results in inefficient distribution of connections, and can lead to some workers being overloaded while others have cycles to spare. Plus, all worker processes are woken up by the kernel in a race to accept a single inbound connection, in what’s commonly referred to as the thundering herd.

The solution implemented here uses a thread that sleeps on accept, removing connections from the kernel’s listen queue as soon as possible, then explicitly pushing accepted connections to the main event loop. The ability to inspect this user-space connection queue enables not only even distribution but also intelligent behavior under high load, such as closing incoming connections when the backlog gets too long. This fail-fast approach prevents the kernel from holding open fully-established connections that cannot be reached in a reasonable amount of time. This backpressure takes the wait out of client failure scenarios leading to a more responsive extrinsic system, as well.

What’s next for SuPPort

The sections above highlight just a small selection of the features already in SuPPort, and there are many more to cover in future posts. In addition to those, we will also be distilling more code from our internal codebase out into the open. Among these we are particularly excited about:

  • Enhanced human-readable structured logging
  • Advanced network security functionality based on OpenSSL
  • Distributed online statistics collection
  • Additional generalizations for TCP client infrastructure

And of course, more tests! As soon as we get a couple more features distilled out, we’ll start porting out more than the skeleton tests we have now. Suffice to say, we’re really looking forward to expanding our set of codified concurrent software learnings, and incorporating as much community feedback as possible, so don’t forget to subscribe to the blog and the SuPPort repo on GitHub.

Mahmoud Hashemi, Kurt Rose, Mark Williams, and Chris Lane