ncro

Nix Cache Route Optimizer

Synopsis
Quick Start | configuration
Contributing

Synopsis

ncro (pronounced Necro) is a lightweight HTTP proxy, inspired by Squid and several other projects in the same domain, optimized for Nix binary cache routing. It routes narinfo requests to the fastest available upstream using EMA latency tracking, persists routing decisions in SQLite and optionally gossips routes to peer nodes over a mesh network. How cool is that!

Unlike ncps, ncro does not store NARs on disk. It streams NAR data directly from upstreams with zero local storage. The tradeoff is simple: repeated downloads of the same NAR always hit an upstream, but routing decisions (which upstream to use) are cached and reused. Though, this is desirable for what ncro aims to be. The optimization goal is extremely domain-specific.

Motivation

During a Nix build, binaries are downloaded from configured substituters, also known as binary caches. When multiple caches serve the same paths or you have multiple caches configured in your Nix setup, there is additional wait time and overhead to every build. ncro solves this by acting as an intelligent local proxy that measures upstream latency in real time and routes each request to the fastest responder. To keep ncro small and lightweight, routing metadata is persisted on disk; NAR content is streamed through with zero local storage. This keeps the proxy stateless on the data path and eliminates cache-invalidation complexity.

For a deeper look at the system design, see the architechture document.

How It Works

flowchart TD
    A[Nix client] --> B[ncro proxy :8080]

    B --> C[/hash.narinfo request/]
    B --> D[/nar/*.nar request/]

    C --> E[Parallel HEAD race]
    E --> F[Fastest upstream wins]
    F --> G[Result cached in SQLite TTL]

    D --> H[Try upstreams in latency order]
    H --> I{404?}
    I -- yes --> J[Fallback to next upstream]
    I -- no --> K[Zero copy stream to client]

    J --> H
    K --> A

Loading

The request flow follows two distinct paths depending on the request type:

Narinfo Lookups

1. Nix requests /<hash>.narinfo
2. ncro checks the SQLite route cache; on a hit, it re-fetches from the cached upstream without probing others
3. On a miss, it races HEAD requests to all configured upstreams in parallel
4. The fastest upstream wins; the full narinfo body is fetched from that upstream and returned to the client
5. The winning route is persisted with a configurable TTL; subsequent requests for the same hash use the cached route directly

NAR Streaming

1. Nix requests /nar/<hash>.nar
2. ncro looks up the route for the corresponding narinfo hash; if no route is found (e.g. the narinfo was requested directly from an upstream), it tries upstreams in latency order
3. The NAR body is streamed chunk-by-chunk from the selected upstream to the client with zero buffering on disk
4. If the upstream returns 404, ncro falls through to the next upstream in latency order
5. After all upstreams are exhausted with no success, a 404 is returned

Background probes (HEAD /nix-cache-info) run every 30 seconds to keep latency measurements current and detect unhealthy upstreams. System design is covered further in the architechture document.

Runtime Endpoints

• GET /nix-cache-info: proxy capability advertisement used by Nix
• GET /<hash>.narinfo: route lookup and upstream selection
• GET /nar/<path>.nar: streamed NAR content from the chosen upstream
• GET /metrics: Prometheus metrics
• GET /health: JSON health summary of configured upstreams

Routing Notes

• Route cache decisions are stored in SQLite and reused until their TTL expires (or they are evicted by the LRU policy when max_entries is reached).
• Latency is tracked using an Exponentially Weighted Moving Average (EMA) with a configurable smoothing factor (cache.latency_alpha, default 0.3). Higher alpha values react faster to changes; lower values filter out measurement noise.
• Lower latency wins the race. When two upstreams are within 10% of each other, the lower priority value acts as a tiebreaker.
• Background probes (HEAD /nix-cache-info) update latency estimates every 30 seconds even when no client traffic is flowing, ensuring warm routing data.
• On a cache miss, ncro races all configured upstreams in parallel and returns the first successful response. Unhealthy upstreams (detected by consecutive probe failures) are excluded from the race until they recover.

Quick Start

# Run with defaults (upstreams: cache.nixos.org, listen: :8080)
$ ncro

# Point at a config file
$ ncro --config /etc/ncro/config.toml

# Tell Nix to use it. The trusted key must match the upstream narinfo signer.
$ nix-shell -p hello \
    --substituters http://localhost:8080 \
    --extra-trusted-public-keys cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=

Deployment instructions are in installation document.

Tip

If you are testing locally, point only a single Nix client at ncro first. That makes it easier to see cache behavior and upstream selection in logs.

Configuration

Default config is embedded; create a TOML file to override any field.

[server]
listen = ":8080"
read_timeout = "30s"
write_timeout = "30s"

[[upstreams]]
url = "https://cache.nixos.org"
priority = 10 # lower = preferred on latency ties (within 10%)
public_key = "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="

[[upstreams]]
url = "https://nix-community.cachix.org"
priority = 20
public_key = "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="

# S3-compatible cache (Garage, MinIO etc.)
[[upstreams]]
url      = "s3://my-bucket?endpoint=minio.example.com&scheme=https"
priority = 15

# Private HTTP cache requiring Basic Auth
[[upstreams]]
url      = "https://cache.internal.example.com"
priority = 5
username = "ncro"
password = "hunter2" # it says ******* on my screen it's secure!

[cache]
db_path = "/var/lib/ncro/routes.db"
max_entries = 100000 # LRU eviction above this
ttl = "1h"           # how long a routing decision is trusted
negative_ttl = "10m" # cache misses for a short window
latency_alpha = 0.3  # EMA smoothing factor (0 < alpha < 1)

[cache.mass_query]
max_concurrent_races = 64     # total concurrent narinfo races
per_upstream_max_inflight = 8 # per-upstream narinfo head concurrency
in_memory_negative_ttl = "5s" # short-lived miss suppression
upstream_cooldown = "15s"     # cooldown on transient upstream network errors

[logging]
level = "info" # debug | info | warn | error
format = "json" # json | text

[discovery]
enabled = false
service_name = "_nix-serve._tcp" # mDNS service type to browse
domain = "local"                 # mDNS domain
discovery_time = "5s"            # how long to listen per discovery cycle
priority = 20                    # priority assigned to discovered upstreams
address_family = "any"           # "any" | "ipv4" | "ipv6"

[mesh]
enabled = false
bind_addr = "0.0.0.0:7946"
peers = []       # list of {addr, public_key} peer entries
private_key = "" # path to ed25519 key file; empty = ephemeral
gossip_interval = "30s"

Environment Overrides

Variable	Config field
NCRO_LISTEN	server.listen
NCRO_DB_PATH	cache.db_path
NCRO_LOG_LEVEL	logging.level

Environment overrides are useful for containerized or Systemd deployments where you want a fixed config file but still need to tweak one or two settings.

S3 Upstreams

ncro accepts Nix-style s3:// URLs in the url field and fetches narinfo/NAR objects through the native AWS S3 SDK. Credentials are loaded through the standard AWS provider chain: environment variables, shared config/credentials files, profile=, or instance/task identity where available.

Supported query parameters:

Parameter	Description
endpoint	Custom S3-compatible host (MinIO, Garage, Backblaze, ...).
scheme	http or https. Only meaningful with endpoint. Default: https.
region	AWS region. Default: us-east-1.
profile	AWS credential profile name for the standard AWS config/credentials files.
addressing-style	auto, path, or virtual. Default: auto; custom endpoints and dotted bucket names use path-style in auto.

# S3-compatible store with a custom endpoint
[[upstreams]]
url      = "s3://my-bucket?endpoint=minio.example.com&scheme=https"
priority = 15

# AWS S3 bucket with explicit region and credential profile
[[upstreams]]
url      = "s3://my-nix-cache?region=eu-west-1&profile=cache-readonly"
priority = 20

Note

username/password are for HTTP Basic Auth upstreams only. S3 upstreams use AWS credentials, for example AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY, or a named profile=.

Upstream Authentication

Any upstream can carry username and password fields. ncro itself sends HTTP Basic Auth on every request to that upstream: health probes, narinfo races, and NAR streaming.

[[upstreams]]
url      = "https://cache.internal.example.com"
priority = 5
username = "ncro"
password = "hunter2"

password is optional. Omit it for token-only schemes where the token goes in the username field.

NixOS Integration

{lib, ...}: {
  services.ncro = {
    enable = true;
    settings = {
      upstreams = [
        {
          url = "https://cache.nixos.org";
          priority = 10;
          public_key = "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=";
        }

        {
          url = "https://nix-community.cachix.org";
          priority = 20;
          public_key = "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=";
        }
      ];
    };
  };

  # Point Nix at the proxy. By default the module appends every configured
  # upstream public_key to nix.settings.trusted-public-keys; set
  # services.ncro.addUpstreamPublicKeys = false to manage those keys yourself.
  # NOTE: ncro needs to be the *only* substituter if you wish to benefit
  # from its capabilities fully. If there are other substituters in your
  # list, or if you don't mkForce this option, ncro will perform less
  # efficiently.
  nix.settings.substituters = lib.mkForce [ "http://localhost:8080" ];
}

Alternatively, if you're not using NixOS, create a Systemd service similar to this. You'll also want to harden this, but for the sake of brevity I will not cover that here. Make sure you have ncro in your PATH, and then write the Systemd service:

[Unit]
Description=Nix Cache Route Optimizer

[Service]
ExecStart=ncro --config /etc/ncro/config.toml
DynamicUser=true
StateDirectory=ncro
Restart=on-failure

[Install]
WantedBy=multi-user.target

Place it in /etc/systemd/system/ and enable the service with systemctl enable. In the case you want to test out first, run the binary with a sample configuration instead.

Discovery Mode

When discovery.enabled = true, ncro browses the local network for mDNS services matching service_name (default _nix-serve._tcp) and registers each discovered instance as a dynamic upstream with priority.

Every routable address advertised by a discovered service is registered separately. When address_family = "any" (default), both IPv4 and IPv6 addresses are added so the router's race engine can try them in parallel. Set address_family = "ipv4" or address_family = "ipv6" to restrict to one family. This is generally useful when your binary cache server only listens on one stack (e.g. nix-serve binds 0.0.0.0 by default and does not accept IPv6 connections.)

Discovered upstreams are removed when they have not been seen for three discovery_time intervals.

[discovery]
enabled = true
service_name = "_nix-serve._tcp"
domain = "local"
discovery_time = "5s"
priority = 20
address_family = "ipv4" # restrict to IPv4-only caches

Mesh Mode

When mesh.enabled = true, ncro creates an ed25519 identity, binds a UDP socket on bind_addr, and gossips recent route decisions to configured peers on gossip_interval. Messages are signed with the node's ed25519 private key and serialized with msgpack. Received routes are merged into an in-memory store using a lower-latency-wins / newer-timestamp-on-tie conflict resolution policy.

Each peer entry takes an address and an optional ed25519 public key. When a public key is provided, incoming gossip packets are verified against it; packets from unlisted senders or with invalid signatures are silently dropped.

If mesh.private_key is left empty, ncro generates an ephemeral identity on startup. That is fine for testing, but persistent gossip requires a stable key so peers can recognize the node across restarts.

[mesh]
enabled = true
private_key = "/var/lib/ncro/node.key"

[[mesh.peers]]
addr = "100.64.1.2:7946"
public_key = "a1b2c3..." # hex-encoded ed25519 public key (32 bytes)

[[mesh.peers]]
addr = "100.64.1.3:7946"
public_key = "d4e5f6..."

The node logs its public key on startup (mesh node identity log line). You can share it with peers so they can add it to their config.

Tip

Keep mesh traffic on a private network. The gossip protocol is signed, but it is still meant for trusted peers. ncro's mesh network feature was designed with Tailscale in mind.

Metrics

Prometheus metrics are available at /metrics.

Metric	Type	Description
ncro_narinfo_cache_hits_total	counter	Narinfo requests served from route cache
ncro_narinfo_cache_misses_total	counter	Narinfo requests requiring upstream race
ncro_narinfo_requests_total{status}	counter	Narinfo requests by status (200/error)
ncro_nar_requests_total	counter	NAR streaming requests
ncro_upstream_race_wins_total{upstream}	counter	Race wins per upstream
ncro_upstream_latency_seconds{upstream}	histogram	Race latency per upstream
ncro_route_entries	gauge	Current route entries in SQLite

Tip

If you are tuning upstreams, watch ncro_upstream_latency_seconds and ncro_upstream_race_wins_total together. The first shows raw response timing; the second shows which cache host is actually being chosen.

Operational Tips

• Use priority to break ties between similarly fast caches, not to override a clearly slower upstream.
• Put db_path on persistent storage if you want routing decisions to survive restarts.
• Use a small ttl while testing and a larger one in production to reduce upstream probing.
• Keep cache.nix.org and any private caches in the upstream list, with the most trusted cache first.
• If you run behind a firewall or container network, make sure the listen port is reachable from your Nix clients.

Hacking

This project is built with NixOS in mind and naturally the primary means of working on this project is using Nix for a reproducible developer environment. Use nix develop to enter a development shell, or direnv allow to use the provided .envrc if you use Direnv.

Building

# With Nix (recommended)
$ nix build

# With Cargo directly
$ cargo build --release

# Development shell
$ nix develop
$ cargo test

License

This project is made available under European Union Public Licence (EUPL) version 1.2. See LICENSE for more details on the exact conditions. An online copy is provided here.

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