fastly/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.

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 |