fastlike

Trigger and scope

Trigger on: Fastly Compute, Compute@Edge, WASM on Fastly, fastlike, XQD ABI, Compute request lifecycle, 508 loop detection, backend subrequests, body streaming, profiling a local Compute app, embedding a Fastly Compute runtime in Go code, or any question about how Compute platform primitives work internally (caching, KV stores, edge rate limiting, ACLs, geolocation, secret stores, config stores, dictionaries, logging, dynamic backends, request collapsing, async I/O).

Do NOT use for: Fastly VCL (use falco), Fastly CLI/API (use fastly-cli or fastly), Viceroy, CDN comparison, WAF, Terraform, cache purging via API, or Fastly logging/stats configuration.

Fastlike — Local Compute Runtime & Reference

Fastlike is a Go implementation of the Fastly Compute ABI. It runs compiled WebAssembly programs locally, implementing the same 249+ host functions that Fastly's production Compute platform provides: backends, dictionaries, KV stores, caching, geolocation, rate limiting, ACLs, secret stores, and more.

Equally important, the fastlike source code is the most complete programmatic specification of how Fastly Compute works — its ABI implementations document every platform primitive, request lifecycle detail, and data structure as executable code.

Fastlike documentation: https://github.com/avidal/fastlike

Source Code as Compute Reference

When you have access to the fastlike source code locally (default: ~/src/fastlike), use these paths to answer specific Compute questions:

Question	Read This File	Why
"How does the request lifecycle work?"	instance.go, xqd_http_downstream.go	Per-request setup, execution, downstream handling
"What ABI functions exist for X?"	xqd_*.go files	Each file implements a group of related ABI functions
"How do backend subrequests work?"	xqd_backend.go, backend.go	Request sending, dynamic backends, timeouts
"How does caching work?"	xqd_cache.go, xqd_http_cache.go, cache.go	Cache operations, Vary, surrogate keys, request collapsing
"How does KV store work?"	xqd_kv_store.go, kv_store.go	CRUD operations, pagination, generation-based concurrency
"How does rate limiting work?"	xqd_erl.go, erl.go	Rate counters, penalty boxes, threshold checks
"How do ACLs work?"	xqd_acl.go, acl.go	CIDR-based IP filtering, most-specific match
"What configuration options exist?"	options.go	Every With* functional option for the runtime
"What error codes can operations return?"	constants.go	All XQD status codes and error types
"How does the profiler work / what does a trace look like?"	profile.go, profile_json.go, docs/profiling.md	Trace data model, JSON wire format, deep-mode metrics, encoders

For a comprehensive guide, see understanding-compute-from-source.md.

Install from Source

Requires Go 1.24+.

# Clone and build
git clone https://github.com/avidal/fastlike.git ~/src/fastlike
cd ~/src/fastlike
make build        # Creates bin/fastlike

# Or install to GOPATH/bin
make install

# Or install directly
go install fastlike.dev/cmd/fastlike@latest

Quick Start

# Minimal: WASM + single backend. The wasm path is positional.
bin/fastlike -backend localhost:8000 app.wasm

Flags can appear on either side of the wasm path.

On macOS, avoid binding to port 5000 — the AirTunes/AirPlay Receiver listens there by default and will steal the connection. Pick another port (e.g. -bind localhost:8000) or disable the AirPlay Receiver in System Settings.

Fastlike vs Viceroy

Feature	Fastlike	Viceroy
Language	Go	Rust
Geolocation	Custom JSON file (-geo)	Built-in defaults
Hot reload	SIGHUP (-reload)	Restart required
Install	go install or make build	cargo install or fastly compute serve
Local backends	-backend name=host:port	[local_server.backends] in fastly.toml

When to use Fastlike: Non-Rust Compute apps, want custom geo data, need hot reload, debugging. When to use Viceroy: Rust Compute apps with cargo-nextest, Component Model projects, using fastly compute serve.

Common Configurations

With named backends:

bin/fastlike \
  -backend api=api.example.com:8080 \
  -backend cache=redis:6379 \
  -backend localhost:8000 \
  app.wasm

Development mode with hot-reload:

bin/fastlike -backend localhost:8000 -reload -v 2 app.wasm

Send SIGHUP to reload the WASM without restarting.

With the built-in profiler:

bin/fastlike -backend localhost:8000 -profile-ui localhost:6060 app.wasm

Open http://localhost:6060/ for the trace index. Each request lands as /r/{id} (HTML) or /r/{id}.json (canonical native JSON; also .chrome.json, .firefox.json, .pprof). Add -profile deep for body byte / cache outcome / header / wasm heap metrics. Non-loopback -profile-ui requires -profile-auth TOKEN (or explicit -profile-insecure-ui). See profiling.md.

Full configuration:

bin/fastlike \
  -bind 0.0.0.0:5000 \
  -backend localhost:8000 \
  -dictionary config=./config.json \
  -kv store=./data.json \
  -config-store settings=./settings.json \
  -secret-store secrets=./secrets.json \
  -acl blocklist=./acl.json \
  -logger output=./logs.txt \
  -geo ./geodata.json \
  -compliance-region us-eu \
  -v 2 \
  -reload \
  app.wasm

Required Arguments

Argument	Description
<wasm-file>	Positional path to the WebAssembly program (required)
-backend VALUE or -b VALUE	Backend server (required, repeatable)

Optional Flags

Flag	Default	Description
-bind ADDR	localhost:8000	Server bind address
-reload	false	Enable SIGHUP hot-reload
-v INT	0	Verbosity (0-2)
-dictionary NAME=FILE or -d	-	Load dictionary from JSON
-kv NAME[=FILE]	-	KV store (empty or from JSON)
-config-store NAME=FILE	-	Config store from JSON
-secret-store NAME=FILE	-	Secret store from JSON
-acl NAME=FILE	-	ACL from JSON
-logger NAME[=FILE]	-	Log endpoint (file or stdout)
-geo FILE	-	Geolocation JSON file
-compliance-region REGION	-	Compliance region (none, us-eu, us)
-profile MODE	trace	off, trace, native, combined, deep. See profiling.md.
-profile-ui ADDR	-	Bind the profile UI listener on ADDR (separate socket from -bind).
-profile-auth TOKEN	-	Bearer token required on UI requests. Mandatory for non-loopback -profile-ui unless -profile-insecure-ui is set.
-profile-insecure-ui	false	Permit a non-loopback -profile-ui without -profile-auth (use only behind external auth).
-profile-retain N	256	LRU size for completed traces.
-profile-backend-cap N	512	Per-request cap on recorded backend calls.
-profile-async-grace DUR	100ms	How long finalize waits for in-flight async backends. Pass 0 to disable.
-profile-dir PATH	cwd	Directory for wasm-symbols-{pid}.json and per-process profile artifacts.

References

Topic	File	Use when...
Compute from Source	understanding-compute-from-source.md	Understanding Compute internals by reading fastlike's implementation
Profiling	profiling.md	Per-request hostcall + backend traces, deep metrics, native JSON schema, pprof export
Backends	backends.md	Setting up named backends, catch-all backends, microservices routing
Config	config.md	Creating JSON config files for dictionaries, KV stores, secrets, ACLs, geolocation
Build	build.md	Building Fastlike from source, running linters, make targets
Test	test.md	Running Go tests, Fastly Compute ABI spec tests
ABI	abi.md	Fastly Compute ABI internals, implementing new ABI functions, handle system