Feature valkey radix tree

[yangbodong22011]

Abstract

This document proposes adding a general-purpose, binary-safe Radix Tree data type to Valkey. Each Valkey key corresponds to one Radix Tree, and each logical path in the tree maps to a field -> value map. The type provides exact lookup, longest-prefix matching, matching of all stored ancestor prefixes, atomic updates, and prefix traversal. It gains persistence and high availability through Valkey's existing RDB, AOF, primary-replica replication, and Cluster capabilities.

The first target use case is a KV Cache placement index for AI inference: Worker KV Cache events continuously update which Workers contain which cached prefixes. A Router queries all reusable prefixes along a request's cumulative block-hash path and then combines the results with real-time load to select a Worker. The type itself does not interpret tokens, models, Workers, or cache tiers. Applications encode those semantics into binary-safe paths, fields, and values, so the type can also serve hierarchical routing, longest-prefix matching, autocomplete indexes, and other prefix-oriented workloads.

1. Background and Inference Architecture

1.1 Problem Statement

Large-scale model inference services commonly reuse previously computed KV Cache through a prefix cache. When a request reaches a Router, the Router needs to know more than whether the prompt hits the cache. It also needs to know:

1. How deep the request's contiguous prefix matches on each Worker;
2. Whether the same prefix exists simultaneously on multiple Workers or cache tiers;
3. How placement changes continuously as blocks are created, migrated, evicted, and as Workers restart;
4. That multiple Routers must observe the same persistent and recoverable placement index;

A regular Hash can store block_hash -> workers, but it cannot express "find all stored ancestor prefixes of this query" in a single query. Storing every prefix as a separate top-level key also inflates the key count, makes atomic updates across keys difficult, and introduces many network round trips. The ordering of a Sorted Set is likewise not equivalent to a byte-prefix relationship.

A Radix Tree compresses paths by their shared byte prefixes, making it naturally suitable for longest-prefix matching and ancestor-prefix enumeration. Valkey already contains a mature internal rax implementation that supports path compression, binary-safe keys, values on internal nodes, insertion, deletion, and ordered iteration. Therefore, no new tree implementation dependency is required.

1.2 Goals

• Provide a general-purpose, binary-safe prefix-index data type.
• Query the longest prefix or all stored ancestor prefixes with one command.
• Allow multiple fields on the same path to be updated independently.
• Make writes idempotent and support atomic single-key batch application for event streams.
• Use existing Valkey persistence, replication, failover, and Cluster semantics.
• Reuse the path-compression and iteration capabilities of the current internal rax.
• Provide stable and explicit response structures for RESP2 and RESP3 clients.

1.3 Non-Goals

• The type does not provide vector similarity or semantic-similarity matching; all matching is exact byte-prefix matching.
• The type does not provide a global Radix Tree spanning multiple Valkey keys.
• The type does not replace the inference system's Worker snapshot, live-event, or event-reconciliation protocols.
• The type does not guarantee synchronous replication or zero RPO; data reliability still follows the selected Valkey deployment model.
• v1 does not provide a separate TTL for an individual Radix path; TTL still applies to the top-level Valkey key.

1.4 Inference Architecture

flowchart LR
    Client[Client] --> Router[Router]
    Router -->|prefix query| Valkey[Valkey Radix Tree]
    Router -->|inference request| Worker[Inference Worker]
    Worker -->|KV events| Bridge[KV Event Bridge]
    Bridge -->|atomic batch| Valkey
    Worker -->|load report| Router

Loading

The data plane has two paths:

• Update path: A Worker produces KV events such as block created, stored, and evicted. The Bridge converts each event into a complete Radix path and writes it idempotently to Valkey through RAXSET or batches updates to multiple paths through RAXMSET.
• Query path: The Router converts a prompt into a cumulative block-hash path, retrieves all matching prefixes and placements in one RAXPREFIXES call, and then combines them with Worker load for routing.

The Valkey Radix Tree is a placement index shared by multiple Routers. The Bridge is responsible for protocol adaptation, event ordering, batch coalescing, retries, generation management, and short-term in-memory buffering rather than serving queries.

1.5 Failure and Recovery Boundaries

Radix Tree data is persisted through RDB, AOF, and primary-replica replication. The Bridge uses bounded in-memory buffering and retries for unacknowledged events. If it detects a sequence gap or buffer overflow, it should reconcile against a Worker snapshot before continuing to consume live events.

A Radix placement index is generally rebuildable soft state. Losing an add usually produces a false miss and therefore reuses less cache. Losing a delete can produce a more dangerous stale positive. Applications should therefore include the Worker generation in the value and have the Router accept only the currently live generation. After a Worker restarts, an old generation will not participate in routing even if an old delete was lost; stale records can be reclaimed later by a background scan.

This strategy reduces application-level risk during a failure window but does not change the RPO of Valkey's asynchronous replication itself.

2. Data Model

2.1 Logical Structure

Each top-level Valkey key stores one tree:

Valkey key
└── Radix Tree
    ├── path P1
    │   ├── field F1 -> value V1
    │   └── field F2 -> value V2
    ├── path P1 || P2
    │   └── field F1 -> value V3
    └── path P1 || P3
        └── field F3 -> value V4

Formally:

Tree: BinaryString -> Map<BinaryString, BinaryString>

• Top-level key: A regular Valkey key responsible for the namespace, Cluster slot, and tree-wide TTL.
• path: A binary-safe byte string and the logical key in the Radix Tree.
• field: A binary-safe byte string identifying an owner or property that can be updated independently under the same path.
• value: A binary-safe opaque value encoded by the application.

An empty path is valid and represents the root payload. An internal navigation node is a "stored logical path" only when it has a non-empty field/value map. After its last field is deleted, the logical path disappears automatically, and rax can subsequently compress internal nodes that are no longer needed. After the final logical path is deleted, the top-level Valkey key should also disappear.

2.2 Why Each Path Contains a Field/Value Map

If each path stored only one opaque value, updating placement for multiple Workers would require reading the entire value, modifying the list, and writing it back. Two Bridges or two event batches could then cause a lost update. A field/value map reduces the conflict granularity to an individual field:

path  = cumulative-hash-path
field = worker-a | hbm
value = generation 7 | component mask | metadata

worker-a and worker-b can independently execute RAXSET or RAXDEL operations, and the commands are naturally idempotent.

2.3 Definition of Prefix

Let p and q be byte strings. p is a prefix of q, written p ⪯ q, if and only if there is a byte string s such that q = p || s. Matching does not interpret UTF-8, integers, tokens, or segments.

For the set S of stored paths in the tree:

Exact(q)    = q, provided q belongs to S
Longest(q)  = arg max |p|, where p belongs to S and p ⪯ q
Prefixes(q) = all p in S satisfying p ⪯ q, ordered from shortest to longest

If an empty path is stored, it is the first match for every query. PREFIXES returns only ancestors of the query, not descendants for which the query is a prefix.

3. Inference Matching Flow and Example

3.1 From Token Blocks to a Radix Path

The application first divides the prompt into blocks using a fixed page size. Suppose there are three blocks, B1, B2, and B3, and a chained cumulative hash is used:

C1 = Hash(B1)
C2 = Hash(C1 || B2)
C3 = Hash(C2 || B3)

Each cumulative hash is encoded as a fixed-width big-endian byte string. For example, a 64-bit hash uses 8 bytes:

P1 = BE64(C1)
P2 = BE64(C1) || BE64(C2)
P3 = BE64(C1) || BE64(C2) || BE64(C3)

The request query path is:

Q = BE64(C1) || BE64(C2) || BE64(C3)

The stored representation is therefore a "cumulative hash sequence." If Hash(B1), Hash(B2), and Hash(B3) were treated as three unrelated keys, they would have no ancestor relationship at the byte level, and the Radix Tree could not derive the block order.

The server does not enforce an 8-byte segment size. Fixed-width encoding is simply how the application ensures that no logical path ends in the middle of a hash.

3.2 Complete Example

Suppose page_size = 4 and the request contains 12 tokens:

B1 = token 1..4
B2 = token 5..8
B3 = token 9..12

The placement tree has the following logical content:

P1
├── worker-a|hbm -> generation=7, components=1111
└── worker-b|hbm -> generation=3, components=1111

P2
├── worker-a|hbm -> generation=7, components=1111
└── worker-b|hbm -> generation=3, components=1111

P3
└── worker-b|hbm -> generation=3, components=1111

The shared-prefix relationship in the tree is:

flowchart TD
    Root[Root]
    P1[P1 cached by A and B]
    P2[P1 plus P2 cached by A and B]
    P3[P1 plus P2 plus P3 cached by B]
    Root --> P1
    P1 --> P2
    P2 --> P3

Loading

The Router executes the following operation for Q. LENGTHS avoids repeatedly returning the shared bytes of P1, P2, and P3; the Router can divide the matched length by 8 to obtain the page depth directly:

RAXPREFIXES kv:{model-id}:placement Q
    LENGTHS
    WITHVALUES
    COUNT 3
    MAXLEN 24

The response is conceptually equivalent to:

[
  [8,  [worker-a|hbm, value-a7, worker-b|hbm, value-b3]],
  [16, [worker-a|hbm, value-a7, worker-b|hbm, value-b3]],
  [24, [worker-b|hbm, value-b3]]
]

The Router aggregates by path length and obtains:

worker-a has a contiguous match of 2 pages = 8 tokens
worker-b has a contiguous match of 3 pages = 12 tokens

It then combines those results with signals such as load, queue length, and cross-machine communication cost for routing. The largest cache hit does not necessarily determine the final Worker. This is also why the Router needs all ancestor prefixes rather than only a global LONGEST result.

If the Router is considering only a set of healthy Workers, it can use FIELDS filtering to reduce the response size:

RAXPREFIXES kv:{model-id}:placement Q
    LENGTHS
    FIELDS 2 worker-a|hbm worker-b|hbm
    COUNT 3
    MAXLEN 24

3.3 How the Event Bridge Constructs a Path

The Radix Tree API accepts a complete path. It does not accept a parent_hash and then query the parent chain on the server. If a Worker event contains only the current block hash and parent hash, the Bridge has two options:

1. Maintain a short-lived hash -> full path cache and use it to construct the complete path;
2. Modify the event protocol so that the Worker carries either the cumulative hash-segment sequence or the fully encoded path directly.

The second approach makes a stateless Bridge easier to implement, while the first reduces event size. This trade-off belongs to the inference event protocol and is not part of the general-purpose Radix Tree data type.

4. API Design

4.1 Command Overview

In every command, key is the top-level Valkey key, while path, query, field, and value are binary-safe bulk strings. A command returns the standard WRONGTYPE error when an existing key has the wrong type.

Command	Syntax	Purpose	Time Complexity
RAXSET	RAXSET key path [FNX | FXX] FIELDS numfields field value [field value ...]	Atomically set one or more fields under one path	O(L + F·U)
RAXMSET	RAXMSET key path field value [path field value ...]	Atomically set field/value assignments across multiple paths	O(ΣL + M·U)
RAXGET	RAXGET key path field [field ...]	Read one or more fields by exact path	O(L + F·U)
RAXMGET	RAXMGET key path field [path field ...]	Read field values from multiple exact paths	O(ΣL + M·U)
RAXGETALL	RAXGETALL key path	Read all field/value pairs under a path	O(L + O)
RAXEXISTS	RAXEXISTS key path	Test whether an exact logical path exists	O(L)
RAXDEL	RAXDEL key path [field [field ...]]	Delete fields or an entire logical path	O(L + F·U)
RAXLONGEST	RAXLONGEST key query [LENGTH] [WITHVALUES | FIELDS numfields field [field ...]]	Return the longest stored prefix of a query	O(L + O)
RAXPREFIXES	RAXPREFIXES key query [LENGTHS] [WITHVALUES | FIELDS numfields field [field ...]] [COUNT count] [MAXLEN max-path-bytes]	Return all stored ancestor prefixes of a query	O(L + O)
RAXDELPREFIX	RAXDELPREFIX key prefix	Delete all logical paths under a prefix	O(L + N)
RAXSCAN	RAXSCAN key cursor [PREFIX prefix] [COUNT count] [WITHVALUES]	Incrementally traverse logical paths	O(L + C + O)
RAXCARD	RAXCARD key	Return the number of logical paths	O(1)

Where:

• L is the byte length of the input path or query;
• F is the number of requested fields;
• M is the number of (path, field) targets in a multi-path command; each RAXMSET target also carries a value;
• U is the lookup cost within a node payload: linear for a small listpack and amortized constant time for a dict;
• O is the number of bytes or fields actually returned;
• N is the total number of logical paths and payloads in the deleted subtree;
• C is the number of paths examined by the current scan.

4.2 RAXSET

RAXSET key path [FNX | FXX]
    FIELDS numfields field value [field value ...]

Atomically set one or more field/value pairs in the field map for path. numfields must be greater than zero and must equal the number of field/value pairs that follow. Without a condition, a missing tree or path is created automatically and existing fields are overwritten.

• FNX: Apply the entire field/value group only if none of the specified fields exists under the path.
• FXX: Apply the entire field/value group only if every specified field exists under the path.
• FNX and FXX are mutually exclusive.
• Conditions are evaluated against all specified fields before any write is performed. If any field fails the condition, none of the supplied field/value pairs is written.
• If the key or path does not exist, FNX succeeds and creates it, while FXX fails without creating it.
• If the same field is specified more than once, assignments are applied from left to right and the last value wins.

Return: OK if the write succeeds; null if the condition is not satisfied.

For example, the following command writes both fields only if neither field-1 nor field-2 exists under path-a. The path may already exist with other fields. If either specified field exists, neither field is modified:

RAXSET tree path-a FNX FIELDS 2 field-1 value-1 field-2 value-2

4.3 RAXMSET

RAXMSET key path field value [path field value ...]

Atomically apply one or more (path, field, value) assignments within one Radix Tree. The arguments after key are parsed as fixed groups of three. Missing paths are created automatically and existing fields are overwritten.

RAXMSET does not support FNX or FXX; every assignment is unconditional. If the same (path, field) target appears more than once, assignments are applied from left to right and the last value wins. A syntax error rejects the entire command before any assignment is applied.

Return: OK after all assignments have been applied.

For example:

RAXMSET tree path-a field-1 value-1 path-b field-2 value-2

4.4 RAXGET

RAXGET key path field [field ...]

Perform one exact path lookup and read one or more fields from its payload.

• When one field is requested, return its value as a bulk string, or null if the key, path, or field does not exist.
• When multiple fields are requested, return an array of values in request order. A missing key or path produces null for every requested field, and each missing field produces null at its corresponding position.

4.5 RAXMGET

RAXMGET key path field [path field ...]

Read one or more (path, field) targets from one Radix Tree. The arguments after key are parsed as fixed groups of two. Each path lookup is exact; the command does not perform ancestor-prefix or descendant-prefix matching.

Return an array whose elements correspond to the requested (path, field) targets in request order. A missing key, path, or field produces null at the corresponding position. The command always returns an array, including when only one target is requested. Paths do not need to be distinct and may appear more than once.

For example, table-cache rows stored under a row field can be fetched by primary key in one command:

RAXMGET student:pk student-10001 row student-10002 row student-10003 row

4.6 RAXGETALL

RAXGETALL key path

Return all field/value pairs for the specified path as a flat array:

[field-1, value-1, field-2, value-2, ...]

Field order is undefined. Return an empty array if the path or key does not exist.

4.7 RAXEXISTS

RAXEXISTS key path

Perform an exact path lookup and return 1 if path is a stored logical path, or 0 if the key or path does not exist. Prefix ancestors and descendants do not count as an exact match.

4.8 RAXDEL

RAXDEL key path [field [field ...]]

• When fields are provided, delete only those fields and return the number of fields actually deleted.
• When no field is provided, delete the entire payload for the path and return 1 or 0.
• Deleting a path payload does not delete descendant paths.
• After the last field is deleted, the path is automatically removed from the logical tree.

The command is an idempotent no-op for a target that does not exist.

4.9 RAXLONGEST

RAXLONGEST key query [LENGTH] [WITHVALUES | FIELDS numfields field [field ...]]

Find the longest stored path that is a byte prefix of query.

• By default, return the matching path; return null if there is no match.
• LENGTH: Replace the matching path with its byte length, for cases where the caller already holds the query.
• WITHVALUES: Return [path, [field, value, ...]].
• FIELDS: Read only the specified fields. The inner value array preserves request order and places null for a missing field. Return [path, [value-or-null, ...]].
• WITHVALUES and FIELDS are mutually exclusive.

An empty path can be returned as a match if it is stored.

4.10 RAXPREFIXES

RAXPREFIXES key query
    [LENGTHS]
    [WITHVALUES | FIELDS numfields field [field ...]]
    [COUNT count]
    [MAXLEN max-path-bytes]

Return all stored paths satisfying path ⪯ query, ordered from shortest to longest.

• By default, return [path-1, path-2, ...].
• LENGTHS: Replace each matching path with its byte length to avoid repeating the shared bytes of a long query in the response.
• WITHVALUES: Return [[path-1, [field, value, ...]], ...].
• FIELDS: Return [[path-1, [value-or-null, ...]], ...], with each value array aligned to the requested field order.
• COUNT: Return at most count matches. This is a hard limit, not a hint. If more matches exist, retain the deepest count prefixes; return the selected results in ascending path-length order.
• MAXLEN: Match only paths whose byte length does not exceed max-path-bytes. Apply this limit before COUNT.
• Return an empty array when there is no match.

The command visits only ancestor nodes along the query path and does not scan the entire tree.

For example, suppose the matching path lengths are [8, 16, 24, 32]:

RAXPREFIXES key query LENGTHS COUNT 2 MAXLEN 24

MAXLEN 24 first produces [8, 16, 24]; COUNT 2 then retains the two deepest matches, so the final result is [16, 24].

4.11 RAXDELPREFIX

RAXDELPREFIX key prefix

Delete every logical path satisfying prefix ⪯ path and return the number of paths deleted. An empty prefix clears the entire tree.

This is an @slow command whose execution cost is proportional to the size of the subtree.

4.12 RAXSCAN

RAXSCAN key cursor [PREFIX prefix] [COUNT count] [WITHVALUES]

Incrementally traverse paths in lexicographic order. The cursor is an opaque bulk string: pass 0 on the first call; the server returns [next-cursor, entries], where next-cursor = 0 means the traversal is complete. A nonzero cursor encodes the previous position and must not be interpreted by the client.

• PREFIX restricts traversal to the specified subtree.
• COUNT is a hint for the number of paths to examine in each call and does not guarantee an exact result count.
• WITHVALUES returns each entry as [path, [field, value, ...]].
• As with Valkey SCAN, concurrent modifications can cause duplicates or omissions, and clients must process results idempotently.

The implementation can encode a version number and the last returned path in the cursor, then use raxSeek to resume at the lexicographic position without retaining an iterator session on the server.

4.13 RAXCARD

RAXCARD key

Return the number of logical paths in the tree that carry a non-empty payload. Return 0 if the key does not exist.

5. Possible Implementation Using the Current Internal rax

5.1 Overall Memory Layout

typedef struct RadixPayload {
    uint8_t encoding;          /* listpack or dict */
    uint32_t num_fields;
    void *data;
} RadixPayload;

typedef struct RadixObject {
    rax *index;                /* path -> RadixPayload* */
    uint64_t num_paths;
    uint64_t num_fields;
} RadixObject;

Valkey key
└── RadixObject
    ├── rax index
    │   └── data pointer ─────────┐
    └── counters                  │
                                 ▼
                         RadixPayload
                         └── listpack or dict
                             └── field -> value

The rax data pointer points to a RadixPayload. rax already allows a key to terminate at an internal node that still has descendants, so P1 and P1 || P2 can both carry payloads. A small payload uses a listpack to reduce memory overhead when each block belongs to only a few Workers. After the entry or value threshold is exceeded, the payload is promoted one-way to a dict to provide near-O(1) field updates. It is not automatically demoted after deletion, avoiding encoding oscillation.

5.2 rax Capabilities That Can Be Reused Directly

• raxNew creates an empty tree;
• raxInsert and raxTryInsert insert a path and payload pointer;
• raxFind supports exact-path operations such as RAXGET and RAXEXISTS;
• raxRemove removes a logical path after its payload becomes empty and recompresses compressible nodes;
• raxStart, raxSeek, raxNext, and raxPrev support lexicographic scan, RDB save, and AOF rewrite;
• raxSize provides the number of logical keys;
• raxAllocSize can be included in MEMORY USAGE;
• Path compression stores the shared bytes of many cumulative hash prefixes only once.

5.3 Prefix-Walk Capability to Add

Currently, raxFind returns data only when the query exactly matches a stored key. raxSeek provides lexicographic positioning but does not efficiently enumerate ancestors of the query. If a client calls raxFind separately for every byte prefix of the query, the worst-case complexity degrades from O(L) to O(L²).

Two helpers should be added at the internal rax layer, or a callback-based walk should be added and shared by both:

void *raxFindLongestPrefix(
    rax *rt,
    const unsigned char *query,
    size_t len,
    size_t *matched_len);

int raxForEachPrefix(
    rax *rt,
    const unsigned char *query,
    size_t len,
    raxPrefixCallback callback,
    void *context);

The algorithm walks the query only once:

1. Start at the head; if the root is a logical key, record length 0.
2. At a regular node, select the child for the next query byte.
3. At a compressed node, compare the entire compressed edge at once; stop immediately if any byte differs.
4. Whenever an iskey node is reached, record the number of bytes consumed and the data pointer.
5. LONGEST retains only the final record; PREFIXES invokes the callback for every record in order.
6. Stop when the query is exhausted or the required branch does not exist.

The time complexity is O(L + K), where K is the number of logical paths actually matched; siblings and descendants are not scanned. The implementation must correctly handle a root key, a mismatch in the middle of a compressed edge, a key that is also an ancestor of another key, an empty query, and binary zero.

The current raxLowWalk already contains most of the path-descent logic, but it is static inline and returns only the stopping node and split position. The implementation can extract a shared internal walker without changing the existing insertion semantics, or add a separate read-only prefix walker. The latter has a smaller change surface and is easier to validate with differential tests.

5.4 RAXDELPREFIX and RAXSCAN

RAXSCAN PREFIX p can use raxSeek(">=", p) to locate the first candidate, then call raxNext until a key no longer starts with p.

RAXDELPREFIX cannot iterate and delete without accounting for iterator invalidation. Possible implementations include using a safe iterator or collecting paths in chunks before calling raxRemove. Before applying any deletion, the command must confirm that the work does not exceed the configured limit, so it cannot fail after deleting only half of a subtree.

If truly nonblocking deletion of a large subtree is needed in the future, rax can gain a subtree-detach capability and hand detached nodes to the lazy-free thread. This is not required for v1.

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• src/commands/rgetall.json
• src/commands/rlongest.json
• src/commands/rmget.json
• src/commands/rprefixes.json
• src/commands/rscan.json
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---

📝 Walkthrough

Walkthrough

Adds a radix-tree data type backed by rax, ten radix commands, prefix operations, RDB/AOF persistence, runtime integration, notifications, module support, and comprehensive unit and integration tests.

Changes

Radix tree data type

Layer / File(s)	Summary
Type contracts and command wiring src/server.h, src/commands/*, src/commands.def, src/acl.c, src/notify.c, src/valkeymodule.h, utils/*, src/Makefile, cmake/*, valkey.conf	Defines radix object, ACL, notification, module, command metadata, generator, configuration, and build integration contracts.
Radix operations and prefix traversal src/t_radix.c, src/rax.c, src/rax.h	Implements radix lifecycle, field operations, prefix matching, scanning, deletion, cardinality, and AOF rewriting.
Persistence and runtime integration src/rdb.c, src/rdb.h, src/aof.c, src/object.c, src/db.c, src/debug.c, src/defrag.c, src/lazyfree.c, src/module.c, src/valkey-check-rdb.c	Adds RDB version 81 support and integrates radix objects with copying, memory accounting, diagnostics, freeing, modules, and RDB inspection.
Unit and integration validation src/unit/test_rax.cpp, tests/unit/type/radix.tcl, tests/unit/cluster/radix.tcl, tests/integration/cross-version-replication.tcl, .github/workflows/external.yml	Tests prefix traversal, command behavior, persistence, malformed data, notifications, replication, cluster operation, and external test startup timing.

Estimated code review effort: 5 (Critical) | ~90+ minutes

Merge Risk: 🟠 High · up to 9a668

The PR adds a persisted Radix data type, but the current implementation may serialize it in a format that older consumers cannot parse, creating compatibility and recovery risk. Digest ambiguity could also conceal replica divergence, so merge should wait for these issues to be fixed or explicitly accepted.

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant serverCommandTable
  participant rsetCommand
  participant radixObject
  participant rax
  Client->>serverCommandTable: Dispatch RSET
  serverCommandTable->>rsetCommand: Invoke handler
  rsetCommand->>radixObject: Update path field
  radixObject->>rax: Insert or locate path
  rax-->>rsetCommand: Return mutation result
  rsetCommand-->>Client: Send response and notification

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Check name	Status	Explanation
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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: adding a Valkey radix tree feature. It is concise and related to the changeset.
Description check	✅ Passed	The description directly explains the proposed radix tree data type, its commands, use case, data model, persistence, replication, and implementation objectives.

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[valkey-review-bot]

The two commits in this PR are missing Signed-off-by: trailers, so the DCO check will fail. Please sign off both commits.

[valkey-review-bot]

Found several compatibility and API-shape issues in the new native type. The server build succeeds; the unit-test target could not run in this workspace because googletest is unavailable.

[coderabbitai]

Actionable comments posted: 2

🧹 Nitpick comments (4)

src/defrag.c (1)

712-713: 🚀 Performance & Scalability | 🔵 Trivial | ⚡ Quick win

Consider defragmenting the radix container and rax nodes now.

The empty branch prevents the panic, which is correct. It also means active defrag never relocates any radix allocation, so fragmentation in radix keys is never reclaimed.

The container struct and the rax nodes can be relocated safely without touching payloads. Pass defrag_data = 0 to defragRadixTree so the payload robj pointers stay in place, and keep the payload hashes deferred.

♻️ Proposed minimal defrag support

     } else if (ob->type == OBJ_RADIX) {
-        /* Radix payload defragmentation is intentionally deferred. */
+        /* Defrag the container and the rax nodes. Payload (hash) defragmentation
+         * is intentionally deferred, so pass defrag_data = 0 to keep the payload
+         * object pointers stable. */
+        radixObject *radix = objectGetVal(ob), *newradix;
+        if ((newradix = activeDefragAlloc(radix))) {
+            objectSetVal(ob, newradix);
+            radix = newradix;
+        }
+        defragRadixTree(&radix->index, 0, NULL, NULL);
     } else if (ob->type == OBJ_MODULE) {

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In `@src/defrag.c` around lines 712 - 713, Implement the OBJ_RADIX branch in the
defragmentation logic by calling defragRadixTree with defrag_data set to 0,
allowing the radix container and rax nodes to relocate while leaving payload
robj pointers and payload hashes untouched.

tests/unit/type/radix.tcl (2)

217-233: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Compare digests instead of only asserting a non-empty digest.

Line 232 asserts $digest_before ne "", which passes for any dataset. The test therefore does not verify that COPY and RESTORE reproduce the radix value.

Use DEBUG DIGEST-VALUE to compare the source key against the copy and the restored key. This turns radixTypeDigest into an oracle for both paths, including the root payload and the multi-field path.

💚 Proposed stronger assertions

-        set digest_before [r debug digest]
+        set digest_before [r debug digest-value tree]
         assert {[r memory usage tree] > 0}
         assert_equal 1 [r copy tree tree-copy]
         assert {[r pttl tree-copy] > 0}
         assert_equal {root value} [r rgetall tree-copy {}]
         assert_equal 2 [r rcard tree-copy]
+        assert_equal $digest_before [r debug digest-value tree-copy]
         set dumped [r dump tree]
         assert_equal OK [r restore tree-restored 0 $dumped]
         assert_equal [r rprefixes tree abc withvalues] [r rprefixes tree-restored abc withvalues]
-        assert {$digest_before ne ""}
+        assert_equal $digest_before [r debug digest-value tree-restored]

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In `@tests/unit/type/radix.tcl` around lines 217 - 233, Update the test around the
existing digest_before assertion to use DEBUG DIGEST-VALUE comparisons for tree
versus tree-copy and tree-restored, verifying both copied and restored radix
values while preserving the existing root and multi-field data setup.

---

250-277: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Make the corruption offsets self-checking and reset the debug flag reliably.

Two points in this test.

1. Lines 255, 263, and 271 patch the DUMP payload at fixed character offsets. Those offsets depend on the exact byte layout that rdbSaveRawString produces for these specific short strings. If the layout changes, the patched byte lands elsewhere. The command still fails with "Bad data format", so the test keeps passing while no longer covering the intended load-time checks (empty payload, duplicate path, duplicate field). Add a comment that documents the expected byte layout for each offset, or assert the pre-patch byte value before replacing it.

2. Line 251 enables debug set-skip-checksum-validation and line 276 disables it. If an assertion between them fails, the flag stays enabled for the remaining tests on the same server. Reset it in a way that also runs on failure.

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In `@tests/unit/type/radix.tcl` around lines 250 - 277, Make the corruption
mutations in test RESTORE rejects malformed radix payloads self-checking by
asserting each target byte has the expected pre-patch value, or documenting the
exact payload layout that makes offsets 4 and 10 target the intended fields.
Ensure debug set-skip-checksum-validation is reset even when an assertion or
restore check fails by wrapping the test body in the Tcl test framework’s
failure-safe cleanup mechanism.

src/rdb.c (1)

1190-1213: 🚀 Performance & Scalability | 🔵 Trivial | ⚡ Quick win

Use allocation-free hash iterator accessors for radix payloads.

Replace hashTypeCurrentObjectNewSds with hashTypeCurrentFromListpack and hashTypeCurrentFromHashTable. Use rdbSaveLongLongAsStringObject for listpack integers and rdbSaveRawString for strings. This removes two temporary allocations per hash entry from BGSAVE and rdbSavedObjectLen.

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In `@src/rdb.c` around lines 1190 - 1213, Update the hash iteration block using
hashTypeCurrentFromListpack and hashTypeCurrentFromHashTable instead of
hashTypeCurrentObjectNewSds, avoiding temporary SDS allocations. Serialize
listpack integer values with rdbSaveLongLongAsStringObject and string values
with rdbSaveRawString, while preserving the existing error cleanup and
byte-counting behavior.

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Inline comments:
In `@src/rdb.c`:
- Line 779: Update RDB_VERSION and RDB_VERSION_MAP to define version 81, then
modify rdbGetObjectType so OBJ_RADIX returns -1 when rdbver is below 81 and
RDB_TYPE_RADIX otherwise, following the existing RDB_TYPE_HASH_2 version-gating
pattern.

In `@src/t_radix.c`:
- Around line 93-116: Update radixTypeDigest to mix each path, field, and value
length into the per-entry digest alongside its bytes, ensuring concatenation
boundaries are unambiguous while preserving the existing XOR aggregation and
iterator behavior.

---

Nitpick comments:
In `@src/defrag.c`:
- Around line 712-713: Implement the OBJ_RADIX branch in the defragmentation
logic by calling defragRadixTree with defrag_data set to 0, allowing the radix
container and rax nodes to relocate while leaving payload robj pointers and
payload hashes untouched.

In `@src/rdb.c`:
- Around line 1190-1213: Update the hash iteration block using
hashTypeCurrentFromListpack and hashTypeCurrentFromHashTable instead of
hashTypeCurrentObjectNewSds, avoiding temporary SDS allocations. Serialize
listpack integer values with rdbSaveLongLongAsStringObject and string values
with rdbSaveRawString, while preserving the existing error cleanup and
byte-counting behavior.

In `@tests/unit/type/radix.tcl`:
- Around line 217-233: Update the test around the existing digest_before
assertion to use DEBUG DIGEST-VALUE comparisons for tree versus tree-copy and
tree-restored, verifying both copied and restored radix values while preserving
the existing root and multi-field data setup.
- Around line 250-277: Make the corruption mutations in test RESTORE rejects
malformed radix payloads self-checking by asserting each target byte has the
expected pre-patch value, or documenting the exact payload layout that makes
offsets 4 and 10 target the intended fields. Ensure debug
set-skip-checksum-validation is reset even when an assertion or restore check
fails by wrapping the test body in the Tcl test framework’s failure-safe cleanup
mechanism.

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📒 Files selected for processing (34)

• src/Makefile
• src/acl.c
• src/aof.c
• src/commands.def
• src/commands/rcard.json
• src/commands/rdel.json
• src/commands/rdelprefix.json
• src/commands/rget.json
• src/commands/rgetall.json
• src/commands/rlongest.json
• src/commands/rmget.json
• src/commands/rprefixes.json
• src/commands/rscan.json
• src/commands/rset.json
• src/db.c
• src/debug.c
• src/defrag.c
• src/fuzzer_command_generator.c
• src/lazyfree.c
• src/module.c
• src/object.c
• src/rax.c
• src/rax.h
• src/rdb.c
• src/rdb.h
• src/redismodule.h
• src/server.h
• src/t_radix.c
• src/unit/test_rax.cpp
• src/valkey-check-rdb.c
• src/valkeymodule.h
• tests/unit/cluster/radix.tcl
• tests/unit/type/radix.tcl
• utils/generate-command-code.py

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[soloestoy]

As I mentioned in the original proposal discussion — KV cache × Valkey with our internal radix tree is a natural fit, and this is exactly the kind of evolution caching needs in the AI era.

Some additional context on why we're driving this: our team (Alibaba Tair) has been actively collaborating with the SGLang community on the co-design of the KV cache placement index. The Radix Tree data type isn't just a standalone feature — its command semantics, prefix-walk behavior, and persistence model are all being shaped by real production requirements from SGLang's disaggregated prefill/decode architecture. We've also accumulated significant experience in the KV cache space, which informs decisions like the field/value map per path (for multi-Worker idempotent writes), the RPREFIXES "deepest-N" semantics, and the generation-based stale-record strategy.

Going forward, @yangbodong22011 will be dedicating major effort to both the radix tree implementation and ongoing communication with the SGLang community to ensure the design evolves in sync with upstream inference architecture changes. Our target is to land this in Valkey 9.2 and present it at LF Open Source Summit Europe (Oct) — positioning it as another major capability for Valkey in the AI infrastructure domain, following the momentum of vector search and beyond.

Since introducing a new data type is a major decision, cc @valkey-io/core-team — please take a look, vote, and review. Would love to get alignment early so we can move fast on the implementation.

Excited to see this come together. Let's make Valkey a first-class building block for AI inference. 🚀

[coderabbitai]

Actionable comments posted: 2

Caution

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⚠️ Outside diff range comments (1)

valkey.conf (1)

1227-1228: 📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Document the new radix ACL category.

This PR adds ACL_CATEGORY_RADIX and the @radix category. The command-category list in this file ends at stream and does not mention radix. Add an entry so the config documentation matches the ACL categories the server exposes.

📝 Proposed documentation addition

 # * stream - Data type: streams related.
+# * radix - Data type: radix trees related.

As per coding guidelines: "If behavior or commands change, check whether related documentation also needs updating."

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In `@valkey.conf` around lines 1227 - 1228, Update the ACL command-category
documentation near the existing stream entry to add the new radix category,
using the same formatting and description style as the surrounding entries.

Source: Coding guidelines

🧹 Nitpick comments (1)

src/notify.c (1)

113-113: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Run clang-format-18 on this line.

The added NOTIFY_RADIX term makes this condition line about 150 columns wide. The repository formatting limit wraps such lines. Format the file before finalizing.

As per coding guidelines: "Follow repository formatting conventions using clang-format-18; format modified C/C++ sources and headers before finalizing when available."

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In `@src/notify.c` at line 113, Run clang-format-18 on the modified condition in
notify.c so the long notification-type expression, including NOTIFY_RADIX, is
wrapped according to repository formatting conventions.

Source: Coding guidelines

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@src/t_radix.c`:
- Around line 299-305: Update src/t_radix.c lines 299-305 in rdelCommand to emit
the radix deletion event followed by a generic "del" keyspace event after
dbDelete. Apply the same generic "del" notification ordering in the empty-prefix
branch at src/t_radix.c lines 491-499 and after the empty-tree dbDelete at lines
524-529 within rdelprefixCommand.

In `@tests/unit/type/radix.tcl`:
- Around line 319-329: Add the {external:skip} test-level tag to the test named
“valkey-check-rdb validates and reports the native radix type” so it is excluded
when running against an external server.

---

Outside diff comments:
In `@valkey.conf`:
- Around line 1227-1228: Update the ACL command-category documentation near the
existing stream entry to add the new radix category, using the same formatting
and description style as the surrounding entries.

---

Nitpick comments:
In `@src/notify.c`:
- Line 113: Run clang-format-18 on the modified condition in notify.c so the
long notification-type expression, including NOTIFY_RADIX, is wrapped according
to repository formatting conventions.

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• src/rdb.c
• src/rdb.h
• src/server.h
• src/t_radix.c
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[codecov]

Codecov Report

❌ Patch coverage is 94.24084% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.93%. Comparing base (6bb9651) to head (c1bfb9b).
⚠️ Report is 4 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/t_radix.c	95.43%	24 Missing ⚠️
src/rdb.c	77.64%	19 Missing ⚠️
src/module.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #4506      +/-   ##
============================================
+ Coverage     78.77%   78.93%   +0.15%     
============================================
  Files           170      172       +2     
  Lines         89805    90615     +810     
============================================
+ Hits          70746    71527     +781     
- Misses        19059    19088      +29     

Files with missing lines	Coverage Δ
src/acl.c	92.66% <ø> (ø)
src/aof.c	80.43% <100.00%> (+0.09%)	⬆️
src/commands.def	100.00% <ø> (ø)
src/db.c	95.23% <100.00%> (+<0.01%)	⬆️
src/debug.c	56.03% <100.00%> (+0.07%)	⬆️
src/defrag.c	80.13% <ø> (ø)
src/fuzzer_command_generator.c	77.12% <100.00%> (+0.06%)	⬆️
src/lazyfree.c	88.60% <100.00%> (+0.22%)	⬆️
src/notify.c	97.46% <100.00%> (+0.06%)	⬆️
src/object.c	91.88% <100.00%> (-0.56%)	⬇️
... and 8 more

... and 20 files with indirect coverage changes

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Actionable comments posted: 1

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Inline comments:
In @.github/workflows/external.yml:
- Line 45: Replace the fixed sleep in all three Valkey workflow jobs with finite
retry loops that run ./src/valkey-cli ... -t 1 ping before the standalone and
nodebug runtest commands and the cluster addslots command. Proceed only after
readiness succeeds; when retries are exhausted, print external-server.log and
fail the job.

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[stockholmux]

Overall, I like this. The use case (table result caching) I have in mind for this is quite different than what you're aiming for @yangbodong22011 .

1. For it to be useful for my use case, I'd like the field de-duplication optimization that is present in streams.
2. I'd like to handle null/nil values somehow.

[yangbodong22011]

@stockholmux Thanks for sharing your ideas. table result caching is a very interesting use case. I’d first like to confirm that Streams do not fully meet your requirements because:

1. Stream IDs must increase monotonically and entries are append-oriented, while table rows usually need arbitrary primary keys and in-place updates or deletions.

2. Streams are indexed by Stream ID/time and do not support exact or prefix lookup using arbitrary row keys.

Is my understanding correct?

In the current KV Cache design, fields represent a sparse and frequently changing set of Workers. Different paths often have different Worker sets, so Stream-style SAMEFIELDS compression does not fit the current payload model directly.

NULL is also more complicated because a missing field already returns null, while stored values are currently binary strings. We would need to define how to represent a stored NULL, how it affects NX/XX, and how clients distinguish it from a missing field.

Both ideas are valuable, but they likely require a separate payload encoding and some additional API design.

[murphyjacob4]

SAMEFIELDS compression does not fit the current payload model directly

I think @stockholmux is considering how this data structure might be used in other use cases. I do agree that field de-duplication would be a neat feature. You can imagine various use cases might reuse the same set of fields over each entry. But I also think it isn't necessary for v1.

The deduplication for streams and radix trees should probably look different. The SAMEFIELDS style used in Streams is contingent on the "master entry" concept of the nodes, whereas this design uses a single node per path. So the "SAMEFIELDS" management would have to travel through the rax spine which is a bit awkward (e.g. you inherit the fields from the parent node).

Probably, if we want to do it in v2, we could add a rax-level field interning map, where fields are mapped to ints. But again - it seems incremental to me.

I'd like to handle null/nil values somehow.

I would say we should probably match Hashes and Streams, where NULL is not supported. But you can just use an empty string value, which is pretty cheap memory-wise since it is encoded as a listpack.

[murphyjacob4]

In Valkey's command grammar, R is already strongly associated with "Right" (for Lists):

• RPUSH / LPUSH
• RPOP / LPOP
• RPUSHX
• RPOPLPUSH

Further, Valkey has LSET (which sets a list element at an index). People would naturally assume RSET means "set a list element indexed from the right", rather than "Radix Set". People might also mistake RDEL for a list-trimming or right-side list deletion helper (similar to LREM or RPOP).

Some alternatives:

1. RAX* (RAXSET, RAXGET, RAXPREFIXES): Mirrors the internal engine name rax. The three letter prefix is used in commands like BIT* and GEO* so there is precedent.
2. TRIE* (TRIESET, TRIEGET, TRIEPREFIXES): If we want a real word, not an abbreviation.
3. RADIX* (RADIXSET, RADIXGET, RADIXPREFIXES): Seems a bit verbose to me? But it is an option
4. Namespaced (RADIX.SET, RADIX.GET, RADIX.PREFIXES): Follows the module style (JSON., FT.). We haven't taken anything like this into the core yet, but if we plan on promoting something like JSON in the future, this would make sense.

My vote is on the first one (RAX*) but curious what others think.

[zuiderkwast]

RAX* sounds good to me.

Another idea: Use P for Prefix tree. PSET, PGET, PPREFIXES, etc.

[stockholmux]

In the past, I've worked on a prototype for a stripped down radix tree data type so I think this has a lot promise. Ultimately, I'd prefer a single new data type that could be used for multiple use-cases (yours, table result caching, others).

@yangbodong22011 Streams aren't a great fit. The time-based IDs, APIs, and consumer group additions make it just enough off from being cleanly usable. Append only is fine since I think updates to tables results would be extremely rare. Searching between bounds would all really needed, so a radix tree would work, I think.

@murphyjacob4 I don't think an empty string works semantically. In a relational database, data could be NULL, a 0-length string, or non-existent. None of these are equivalent.

[murphyjacob4]

Supporting only a single field and value seems like it would be constraining in the future. We can choose to do just one field/value for v1, but we should design the syntax to support variadic in the future.

On that note, NX and XX is ambiguous to me. Is this key level, path level, or field level? I could imagine any of those might be useful. For HSETEX, we support key-level and field-level: https://valkey.io/commands/hsetex/. Would it make sense to support it like:

RSET key path [NX | XX] [PNX | PXX] [FNX | FXX] FIELDS numfields field value [field value ...]

@ranshid maybe you can chime in. We don't need to ship it all right now, but just trying to plan ahead.

[yangbodong22011]

I changed NX/XX to FNX/FXX. The actual NX/XX and PNX/PXX will be determined based on future needs.

[murphyjacob4]

@murphyjacob4 I don't think an empty string works semantically. In a relational database, data could be NULL, a 0-length string, or non-existent. None of these are equivalent.

Yeah, I am getting flashbacks to #68. I don't know if we ever really landed on a consensus solution there.

There are really three distinct flavors of "empty" to consider for Radix:

1. Empty Radix (Key-level): Similar to an empty set. If the Radix tree is a cache of an upstream query, an empty key confirms the query was successfully executed and cached with 0 results (avoiding cache penetration).
2. Empty Path (Node-level): Allows using Radix as a pure "Prefix Set" for fast existence/LPM checks. E.g., for an IP blocklist, storing 1,000,000 IP prefixes with dummy field payloads wastes ~50-100MB of robj and listpack allocations, whereas a pure path trie in rax would only consume ~15MB.
3. Empty Value (Field-level): The use case to differentiate "", non-existent, and existent-but-empty values.

Of those three, the second seems like the most compelling practical feature. But because it is primarily a memory optimization, we don't need to block v1 on it, we could optimize it down the line internally (e.g., using a shared singleton object or isnull = 1 when a path only holds a placeholder payload like {"": ""}).

The first one is the closest to our previous discussion, and I think we can punt solving it here to if/when we solve it at the Valkey-level (e.g. a config to preserve empty data structures).

The third one (field-level NULL) seems like the easier one to workaround by using some kind of application-configured sentinel (whether empty string, or a special value). Given that none of the other data types support NULL values, solving it just for Radix feels a bit incongruent.

Otherwise, how do you propose we would handle NULL fields?

[yangbodong22011]

@murphyjacob4 Thank you. My vote is also for the RAX* command prefix, and I strongly agree that this data structure should support more use cases.

1. Field deduplication is important, but it does not need to be completed in v1.
2. Your description of the different empty states is very clear. I think the second case—an empty path—has the most practical value.
3. I will revisit the code changes you suggested, reorganize the implementation, and temporarily rename the commands with the RAX* prefix so that more people can review the API.

For v1, we can just do the "absolutely correct" thing(Part of API with full consideration for future expansion), and then move on.

[stockholmux]

@murphyjacob4:

There are really three distinct flavors of "empty" to consider for Radix:

1. Empty Radix (Key-level): Similar to an empty set. If the Radix tree is a cache of an upstream query, an empty key confirms the query was successfully executed and cached with 0 results (avoiding cache penetration).
2. Empty Path (Node-level): Allows using Radix as a pure "Prefix Set" for fast existence/LPM checks. E.g., for an IP blocklist, storing 1,000,000 IP prefixes with dummy field payloads wastes ~50-100MB of robj and listpack allocations, whereas a pure path trie in rax would only consume ~15MB.
3. Empty Value (Field-level): The use case to differentiate "", non-existent, and existent-but-empty values.

My gut says that 3 is most important and hardest to solve later elegantly.

The third one (field-level NULL) seems like the easier one to workaround by using some kind of application-configured sentinel (whether empty string, or a special value).

This is the inelegant part. ;)

Given that none of the other data types support NULL values, solving it just for Radix feels a bit incongruent.

Repeating mistakes is an anti pattern. But seriously, it's ironic that, for a database that prides itself on efficiency, we struggle with nothing-ness.

I need to play around with the implementation more - I think there might relatively simple samefields/null solution now that I think more deeply about it.