Serverless Docker Beta

The focus of our ZEIT Day Keynote this year was on the new capabilities of the Now cloud platform. In particular, we emphasized our focus on Serverless Docker Deployments.

Today, we are announcing their availability as a public beta, which features:

  • A 10x-20x improvement in cold boot performance, based on data from 1.5M+ deployments

    • This translates into a sub-second cold boot (full round trip) for most workloads
  • A new slot configuration property which defines the resource allocation in terms of CPU and Memory, defaulting to c.125-m512 (.125 of a vCPU and 512MB of memory)

    • This enables fitting your application into the most appropriate set of constraints, paving the road to special CPU features, GPU cores, etc.
  • Strictly specified tunable limits

    • Maximum execution time (defaulting to 5 minutes, with a maximum of 30 minutes)
    • A shutdown timeout after the last request (defaulting to 1 minute, max 5)
    • Maximum request concurrency before automatically scaling (defaulting to 10)
  • Support for HTTP/2.0 and WebSocket connections to the deployments

  • Automatic port discovery. We no longer rely on the EXPOSE instruction. We automatically forward traffic to the port of the process started by CMD
Read on to learn how it works or head directly to our examples.


Serverless Node.js Functions

Let's deploy a simple function exposed as an HTTP service using micro:

A simple function accessible via, built with npm ci

  • A deployment is created, diffing our local file system with the cloud
  • A simple Dockerfile is used to hold the instructions to build the project
  • We build with our own preference of Node.js version (10) and package manager (npm ci)
  • The index.js file contains our main function
  • The serverless container is limited to just .125 CPUs and 512MB of memory
  • DNS lookup + TLS handshake + Cold Boot + Full roundtrip happens in 600ms~
  • Once the deployment instance is warm, subsequent requests take 100ms~

The only requirement to make this work is for your now.json to activate the beta via a feature flag:

{ "type": "docker", "features": { "cloud": "v2" }

Serverless Go + Websockets

Let's go a bit deeper into the power of this technology. The next example will take an image right from the Docker registry, of a program written in Go.

A serverless shell, powered by HTTP/2.0 and WebSockets, available at

  • The usage of an unmodified Dockerfile from the public Docker registry
  • A different programming language and runtime: Go
  • Transient statefulness, as evidenced by our ability to inspect the filesystem - After 5 minutes (the default duration), the state will recycle
  • Sub-500ms cold roundtrip. Go exhibits better startup performance, even though this is a larger application (usually 400ms-500ms for this example)
  • The service responds to HTTP requests to serve the initial HTML and then a WebSocket connection to exchange the PTY data

Serverless Rust (Global)

This infrastructure works remarkably well in combination with Global Now. In other words, it takes one flag to deploy serverlessly to all our global locations.
Here is an example of deploying Rust + Hyper:

A Rust microservice instantly available in every region at

This is equivalent to the rest of the examples, but we scaled right from the get-go to all regions by running now --regions all.

It's also possible to scale after you have already deployed, by running:

// scale to sfo
now scale sfo // scale to all regions
now scale all // disable everywhere
now scale 0

Automatic Horizontal Scalability

To underline the ability of this system to automatically scale with the parameters you define, within the boundaries that you define, here is an example that stress tests with wrk, a load-testing tool:

Instant and predictable horizontal scalability

This is, in our opinion, the most important defining characteristic of Serverless Deployments. However, it's not the only one, as we will see next.

Principles of Serverless Computing

We selected these demos in particular to underline a very important point. We think Serverless can be a very general computing model. One that does not require new protocols, new APIs and can support every programming language and framework without large rewrites.

Here are three of the underlying ideas behind this new architecture.

1. Focus on Code, Not Infrastructure

Serverless enables engineers to focus on code rather than managing servers, VMs, registries, clusters, load balancers, availability zones, and so on.

This, in turn, allows you to define your engineering workflow solely around source control and its associated tools (like pull requests). Our recent GitHub integration, therefore, makes it possible to deploy a Docker container in the cloud solely by creating a Dockerfile.

It is not sufficient to ignore that the infrastructure is there, or forget about it. The execution model must make it so that manual intervention, inspection, replication, and monitoring or alert-based uptime assurance is completely unnecessary, which takes us to our next two points.

What Are the Benefits?

When we deployed the examples above, we didn't have to deal with:

  • Clusters or federations of clusters
  • Build nodes or build farms
  • Container registries and authentication
  • Container image storage, garbage collection and distributed caching

2. Deployments Should Self-Heal

A very common category of failure of software applications is associated with failures that occur after programs get into states that the developers didn't anticipate, usually arising after many cycles.

In other words, programs can fail unexpectedly from accumulating state over a long lifespan of operation. Perhaps the most common example of this is a memory leak: the unanticipated growth of irreclaimable memory that ultimately concludes in a faulty application.

Serverless means never having to "try turning it off and back on again"

Serverless models completely remove this category of issues, ensuring that no request goes unserviced during the recycling, upgrading or scaling of an application, even when it encounters runtime errors.

How Does Now Ensure This?

Your deployment instances are constantly recycling and rotating. Because of the request-driven nature of scheduling execution, combined with limits such as maximum execution length, you avoid many common operational errors completely.

3. Deployments Should Automatically and Predictably Scale

Perhaps the most important or appealing aspect of the serverless paradigm is the promise of automatic scalability.

In its most basic form, a function automatically scales with a 1:1 mapping of requests to resource allocations. A request comes in, a new function is provisioned or an existing one is re-used.

We have taken this a step further, by allowing you to customize the concurrency your process can handle.

Technical Details

This new infrastructure is already available to Docker deployments made in the free tier, or for paying subscriptions that opt-into the feature via now.json:

{ "type": "docker", "features": { "cloud": "v2" }


Hard Limits

These limits are fixed. They are subject to change once the feature goes into General Availability.

  • A maximum of 3 concurrent deployment instances for OSS
  • A maximum of 10 concurrent deployment instances per subscription
  • A maximum of 500 concurrent requests/connections across deployments per subscription
Soft Limits

These limits are configurable in now.json as part of a limits object.

  • maxConcurrentReqs max concurrency of each process (min 1, max 256, default 10)
  • duration max amount of time in ms your process can run (min/default 5 minutes, max 30 minutes)
  • timeout how long in ms to wait after the last request to downscale (min/default 1 minute, max 30 minutes)


While in beta, we require a paid subscription to be able to go over the maximum of 3 concurrent deployment instances. Current rates apply and are subject to change.

Next up: Faster, More Capable, Smaller

Faster Cold Start

Despite having so dramatically sped up instantiation times, we still have very significant room for improvement.

We are excited about unveiling some of these over the coming weeks before the new infrastructure goes into General Availability.


We will introduce new slot identifiers so that you can fit your applications into other CPU/memory combinations.

This is important for resource-intensive applications.

Smaller Images

When your code is built, we post-process the resulting snapshot and let you know what the total size is.

We are confident that in its present form, our system can fit the vast majority of our customers' workloads without any issues.

However, we are currently developing improvements to optimize this dimension further, without you having to make any changes.


This beta contains the lessons and the experiences of a massively distributed and diverse user base, that has completed millions of deployments, over the past two years.

To get started, we suggest you take a look at the comprehensive list of examples we put together for this release.

Over the coming weeks, we will share more in-depth articles and documentation about our new offering.

Your feedback is crucial during this period. Please let us know how well it works for you.