Universal fallback:// URI plugin for @suaveplan/storage-uri. Ships ordered multi-source resolution with cycle detection, depth cap, mirror writes, list union, AbortSignal propagation, and OTel + EventBusLike observability.

Documentation tree

classify — The classify module hosts the pure error-classification function that governs fall-through eligibility for every walk…
- classify — The classify module hosts the pure error-classification function that governs fall-through eligibility for every walk…
conformance — The fallback-conformance module exports runFallbackConformance, a black-box harness consumers call from their own i…
- fallback-conformance — The fallback-conformance module exports runFallbackConformance, a black-box harness consumers call from their own i…
error-codes — The error-codes module hosts the structured-error registry for the @suaveplan/storage-uri-plugin-fallback package.…
- error-codes — The error-codes module hosts the structured-error registry for the @suaveplan/storage-uri-plugin-fallback package.…
events — The observability/events module exposes thin emit helpers that forward the package's five canonical event topics to a…
otel — The observability/otel module owns the OpenTelemetry instrument bundle for the package: the per-candidate `storage_ur…
delete — The ops/delete module implements fallbackDelete, the mirror-aware removal operation. It dispatches remove to ever…
list — The ops/list module implements fallbackList, the enumeration operation for fallback:// URIs. It invokes list on…
metadata — The ops/metadata module hosts fallbackExists and fallbackStat, the metadata-side adapter operations. Both walk th…
read — The ops/read module implements the read-side of the fallback adapter surface. It wires the fallback:// URI parser,…
write — The ops/write module implements the write-side semantics of the fallback adapter: fallbackSet for byte writes and `…
abort-any — The abort-any module provides a universal AbortSignal composition helper. It prefers the native `AbortSignal.a…
parse-fallback-uri — The parse-fallback-uri module owns the canonical grammar for the fallback:// URI scheme. It parses input strings in…
plugin — The plugin module hosts the fallbackUriPlugin factory — the public entry point of the package. The factory validate…
- plugin — The plugin module hosts the fallbackUriPlugin factory — the public entry point of the package. The factory validate…
recursion — The depth-context module tracks the recursion state threaded through nested fallback:// dispatches: the per-call de…
- depth-context — The depth-context module tracks the recursion state threaded through nested fallback:// dispatches: the per-call de…
parallel — The walk/parallel module implements the Promise.any-style race strategy activated when policy.maxParallel > 1. It…
sequential — The walk/sequential module implements the default ordered fall-through walk strategy. It iterates the candidate list…

Install

bun add @suaveplan/storage-uri-plugin-fallback

URI grammar

fallback://?u=<pct-encoded-uri>&u=<pct-encoded-uri>[&u=...]

Always empty authority and empty path.
One or more ordered u= parameters.
Encoding via URLSearchParams (RFC 3986-compliant).
Default maxCandidates is 16; hard ceiling 64.
Each candidate may itself be a fallback:// URI (nested up to policy.maxDepth, default 4 / hard ceiling 8).

Use the bundled buildFallbackUri helper rather than handcrafting:

import { buildFallbackUri } from "@suaveplan/storage-uri-plugin-fallback";

const uri = buildFallbackUri([
    "cdn://zone-eu/avatars/alice.jpg",
    "drive://primary/avatars/alice.jpg",
]);

Simple example

import { createStorageUriResolver } from "@suaveplan/storage-uri";
import {
    buildFallbackUri,
    fallbackUriPlugin,
} from "@suaveplan/storage-uri-plugin-fallback";

// Late-bind the resolver so the fallback plugin can recurse through it.
let resolver: ReturnType<typeof createStorageUriResolver> | undefined;
const lazy = new Proxy({}, {
    get(_t, prop) {
        const v = (resolver as unknown as Record<string, unknown>)[prop as string];
        return typeof v === "function" ? v.bind(resolver) : v;
    },
}) as ReturnType<typeof createStorageUriResolver>;

resolver = createStorageUriResolver([
    cdnPlugin,
    drivePlugin,
    fallbackUriPlugin({ resolver: lazy }),
]);

const uri = buildFallbackUri([
    "cdn://zone-eu/avatars/alice.jpg",
    "drive://primary/avatars/alice.jpg",
]);
const bytes = await resolver.read(uri);

Advanced example — mirror writes + dedup list

import { createEventBus } from "@suaveplan/events";
import {
    buildFallbackUri,
    fallbackUriPlugin,
    type FallbackEventMap,
} from "@suaveplan/storage-uri-plugin-fallback";

const bus = createEventBus<FallbackEventMap>();
bus.on("storage-uri.fallback.write-mirror-partial", (p) => {
    console.warn("mirror drift:", p);
});

const plugin = fallbackUriPlugin({
    resolver,
    policy: {
        writeMode: "all",
        listFailureMode: "partial",
        maxParallel: 2,
    },
    events: bus,
});

const uri = buildFallbackUri([
    "cdn://zone-eu/photos/2026/",
    "drive://primary/photos/2026/",
]);
await resolver.write(uri, photoBytes);   // mirrors to both
const { uris } = await resolver.list(uri); // union, deduped

Policy reference

Knob	Default	Notes
`errorClass`	`"transient"`	`"any"` re-classifies semantic errors as fall-through
`maxParallel`	`1`	`>1` enables `Promise.any`-style race
`stopOnAbort`	`true`	`false` lets the current candidate finish before throwing `ABORTED`
`writeMode`	`"all"`	`"first-success"` returns on first ok + background remainder
`deleteMode`	`"any-success"`	`"all"` raises `WRITE_MIRROR_FAILED` on any reject
`listMode`	`"union"`	`"concat"` returns raw concatenation
`listFailureMode`	`"throw"`	`"partial"` skips failed legs
`maxDepth`	`4`	Hard ceiling 8
`maxCandidates`	`16`	Hard ceiling 64

Error codes

FALLBACK_URI.NO_CANDIDATES, INVALID_URI, ALL_CANDIDATES_FAILED, WRITE_MIRROR_FAILED, LIST_UNION_FAILED, ABORTED, MAX_DEPTH_EXCEEDED, CIRCULAR_REFERENCE — every code is reachable from a real call site exercised by the test suite.

Complementarity with `@suaveplan/storage-mux`

storage-mux composes pre-constructed StorageLike backends at the adapter layer; storage-uri-plugin-fallback composes URI strings at the scheme layer. They share a classification table so transient/semantic/ abort semantics line up across the stack. Use storage-mux when you own the backend instances; use fallback:// when callsites receive URIs from manifests, config, or the wire.

Modules

error-codes — FALLBACK_URI.* registry + FallbackUriError.
types/policy.types — FallbackPolicy, FallbackPluginOptions, event payloads.
types/candidate.types — CandidateError, CandidateAttempt, MirrorOutcome.
parse-uri/parse-fallback-uri — grammar, parser, builder.
parse-uri/abort-any — AbortSignal.any shim.
classify/classify — error class table.
recursion/depth-context — depth + visited set + registry.
walk/sequential — ordered fall-through.
walk/parallel — race + abort.
ops/read, write, metadata, delete, list — per-operation surfaces.
observability/events — EventBusLike emit helpers.
observability/otel — OTel instruments.
plugin/plugin — fallbackUriPlugin factory.
conformance/fallback-conformance — runFallbackConformance consumer harness.

README.md

@suaveplan/storage-uri-plugin-fallback - Universal fallback:// URI plugin for @suaveplan/storage-uri: ordered multi-source resolution with cycle detection, depth cap, mirror writes, list union, AbortSignal propagation, and OTel + EventBus observability.