## Why “just use Redis” is no longer a safe answer A few years ago, cache-layer meetings lasted five minutes. One line — “let’s put Redis in front of it” — closed the discussion. That changed in March 2024, when Redis moved from BSD 3-Clause to a RSALv2/SSPLv1 dual license. The fallout did not stay inside the source tree. - Major Linux distros started pulling Redis out of their default package repos and replacing it with a fork (e.g. Fedora, openSUSE, and Debian-family packaging-policy discussions). - **Valkey**, the community fork backed by AWS, Google Cloud, Oracle, and others, moved under the Linux Foundation and showed up as a separate engine option in cloud managed services. - Cloud vendors are pushing the fork at **a lower unit price**. License is now both a legal issue and a **billing issue**. - In 2025 Redis added AGPLv3 as a license option, but that is not a rollback — it is an extra choice. Depending on the organization, AGPL itself can be more burdensome than RSAL. This is not a concepts primer. It is a **decision document that scores Redis / Valkey / Memcached on six axes and names a single winner per scenario**. If you need a similar judgment, read [Terraform vs OpenTofu in Practice: What to Use After BSL and When to Move](/blog/terraform-vs-opentofu-실무-선택-bsl-이후-뭘-쓰고-언제-옮길까) with the same frame. --- ## 30-second scorecard: use this when… Grab the conclusion without scrolling. | # | Our situation | Verdict | One-line rationale | |---|---|---|---| | ① | Simple string/byte cache only, horizontal scale first | **Memcached** | Slab allocation is memory-efficient + multithreading is native; smallest ops surface | | ② | Depends on Sorted Set, Stream, HyperLogLog, Bitmap, etc. | **Valkey** (keeping Redis is also viable) | Memcached has no data structures. Valkey is Redis 7.2 API compatible | | ③ | Used as Pub/Sub or a lightweight queue stand-in | **Valkey**, but a real queue needs a real broker | Streams can hold, but if you need reprocessing and a DLQ, Kafka/SQS/RabbitMQ is the answer | | ④ | Per-core throughput is the bottleneck; need vertical scale on a single node | **Valkey** or **Memcached** | Valkey 8 multithreaded I/O, Memcached native multithreading — both use the cores | | ⑤ | Avoiding license risk is contractually mandatory | **Valkey** | Stays BSD 3-Clause + Linux Foundation governance | | ⑥ | No one to run a cluster | **Managed service + single shard (1 replica)** | Who operates it is a bigger risk variable than which engine you pick | ### The three variables that actually flip the verdict The axes that reverse the table are really just three. 1. **Value-size distribution** — large values saturate NIC bandwidth first and wash out engine differences. 2. **Data-structure dependence** — if you only use `SETEX`/`GET`, Memcached is a serious candidate. A single `ZADD` knocks it out. 3. **Whether you resell** — offering cache as a service to customers is the only knockout punch on the license axis. The sections below validate each of those three. --- ## License judgment: what is actually a problem for *your* company ### Timeline | When | Event | What gets constrained | |---|---|---| | ~2024.02 | Redis: BSD 3-Clause | Effectively none. Resell, modify, bundle freely | | 2024.03 | Redis 7.4~: **RSALv2 / SSPLv1 dual** | Restricts offering Redis “as a managed service to third parties.” Choosing SSPL raises the whole-stack source-disclosure debate | | 2024.03~ | **Valkey fork**, Linux Foundation transfer | Based on Redis 7.2.4-era code, stays BSD 3-Clause | | 2024~2025 | Distro package swaps, cloud engine split | `apt install redis` may no longer mean latest Redis | | 2025 | Redis 8: **AGPLv3 option added** | Relicensing is eased, but AGPL’s network-distribution clause is still a burden for some orgs | ### Risk by usage pattern | Usage pattern | Risk | Verdict and what to check | |---|---|---| | (a) Internal infra, cache only | Low | Fine in practice. Still refresh the license entry in your software BOM | | (b) **Internal** component of your own SaaS | Medium | The fight is “are we providing Redis itself, or our service that happens to use Redis?” The clause is ambiguous — **legal review required** | | (c) Reselling cache itself as a managed service | High | Exactly the shape RSALv2 targets. Commercial license or move to Valkey | | (d) Bundled in an appliance / on-prem package | High | Distribution happens, so source-disclosure / notice obligations must be reviewed | ### Internal-docs checklist to pull before the meeting - [ ] **Open-source clause** in customer contracts — any “no GPL-family or similar copyleft licenses” language? - [ ] **Software BOM (SBOM)** — is the Redis version pre-relicense (7.2 or earlier) or after? - [ ] Redis binaries baked into container image bases (including sidecars and bundled images) - [ ] License-table consistency in the **open-source notice docs** you filed for public-sector / financial deliveries > ⚠️ This section is not legal advice. Final interpretation and application of license terms must go through in-house counsel or outside counsel. What you get here is a list of **what to ask**. **Remember one principle.** Do not migrate because “licenses are scary.” An (a)-type org that rushes a migration and pages itself loses far more. Conversely, if you are (c) or (d), you need a date on the calendar now. --- ## Features, performance, and cost — three-way comparison ### Feature comparison | Item | Redis 8 | Valkey 8 | Memcached 1.6 | |---|---|---|---| | String / Hash / List / Set | ✅ | ✅ | String (bytes) only | | Sorted Set / Stream / Bitmap / HyperLogLog | ✅ | ✅ | ❌ | | Multithreaded I/O | ✅ (io-threads) | ✅ (improved async I/O) | ✅ **multithreaded by default** | | Cluster mode | ✅ | ✅ | ❌ (client-side sharding) | | Replication | ✅ | ✅ | ❌ | | Persistence (RDB/AOF) | ✅ | ✅ | ❌ (full loss on restart) | | TLS / ACL | ✅ | ✅ | TLS yes; ACL is SASL-level | | Modules (Search/JSON/TimeSeries) | ✅ official bundle | Separate modules (e.g. valkey-search); ecosystem still early | ❌ | | Memory efficiency | jemalloc, object overhead exists | Same + key-offload improvements | **Slab allocation; lowest overhead for plain KV** | | License | RSALv2/SSPL or AGPLv3 | **BSD 3-Clause** | BSD 3-Clause | | Governance | Redis Ltd. | Linux Foundation | Community | **Where Memcached clearly wins**: operational simplicity (far fewer knobs), memory efficiency on pure KV, multithreading by default, and a stateless design where “on failure, just drop it” is correct. If this is a **DB query-result cache** rather than a session store, Memcached is still the right answer. **Where Memcached loses**: no persistence, no data structures, no replication, no compound ops beyond atomic counters. ### Benchmarks: use our values, not someone else’s numbers Vendor benches are mostly measured under **small values + high pipelining**. Change the conditions and the conclusion changes. Reproduce it yourself with `memtier_benchmark`. ```bash # 1) 기준선: 파이프라이닝 없음, 작은 값(32B) — 지연 시간 중심 memtier_benchmark -s 10.0.1.10 -p 6379 \ --protocol=redis --clients=50 --threads=4 \ --data-size=32 --ratio=1:9 --pipeline=1 \ --test-time=60 --hide-histogram ``` ```bash # 2) 처리량 상한: 파이프라이닝 16 — 엔진의 I/O 스레드 효과가 드러나는 구간 memtier_benchmark -s 10.0.1.10 -p 6379 \ --clients=50 --threads=8 \ --data-size=32 --ratio=1:9 --pipeline=16 \ --test-time=60 ``` ```bash # 3) 큰 값 구간: 100KB — 여기서부터는 NIC 대역폭이 먼저 포화된다 memtier_benchmark -s 10.0.1.10 -p 6379 \ --clients=20 --threads=4 \ --data-size=102400 --ratio=1:4 --pipeline=1 \ --test-time=60 ``` **How to read the results** | Condition | Dominating variable | Practical implication | |---|---|---| | value ≤ 1KB + pipeline ≥ 8 | I/O thread count, CPU cores | The band where Valkey multithreaded I/O gains show up most | | value ≤ 1KB + pipeline = 1 | RTT (network round trip) | AZ placement and connection pools matter more than the engine | | value ≥ 100KB | NIC bandwidth | Engine gap washes out. Compression / value splitting help more | | mostly GET/SET | Memory efficiency | Memcached holds more keys in the same RAM | **How to extract our value-size distribution** (on a production node, always run this on a replica or a snapshot-restored instance. `--bigkeys` / `MEMORY USAGE` add load): ```bash # 큰 키 상위 목록 (샘플링, 부하 낮음) redis-cli -h --bigkeys # 랜덤 샘플 200개의 실제 메모리 사용량 히스토그램 for i in $(seq 1 200); do k=$(redis-cli -h RANDOMKEY) redis-cli -h MEMORY USAGE "$k" done | sort -n | awk '{a[NR]=$1} END { print "p50:", a[int(NR*0.5)]; print "p90:", a[int(NR*0.9)]; print "p99:", a[int(NR*0.99)]; }' ``` If p90 is over 10KB, “multithreaded benchmark numbers” are irrelevant to our workload. ### Real cost math: the formula, in the open > 💰 **The unit prices below are example figures as of an August 2026 lookup and change constantly by region, commitment, and engine. Re-check the latest rates on the official AWS ElastiCache pricing page, then plug them into the formula.** The formula is simple. ``` 월 노드 비용 = 시간당 온디맨드 단가(USD) × 노드 수 × 730시간 ``` Seoul region (ap-northeast-2), `cache.r7g.large` × 2 nodes (1 primary + 1 replica) template: | Engine | Hourly rate (USD, fill in lookup) | Formula | Monthly total (USD) | |---|---|---|---| | Redis OSS | `$A` | `$A × 2 × 730` | `= $A × 1,460` | | **Valkey** | `$B` (typically priced below A) | `$B × 2 × 730` | `= $B × 1,460` | | Memcached | `$C` (no replication → re-count nodes) | `$C × N × 730` | `= $C × N × 730` | Put the savings rate in the meeting deck as-is. ``` 절감률(%) = (A - B) / A × 100 ``` If Valkey is priced 20% below Redis OSS, monthly savings on 2 nodes is `A × 1,460 × 0.20`. It scales linearly with node count, so **if you already run 10+ shards, cost alone is enough reason to evaluate a move — license aside.** **Rough TCO: ElastiCache vs self-managed on EC2** (same spec, 2 nodes) | Cost item | ElastiCache | Self-managed on EC2 | |---|---|---| | Compute | node rate × 730h | instance rate × 730h (cheaper with RI/SP) | | Storage | included | EBS gp3 volume extra | | Backup | snapshot charges (after free allowance) | S3 storage + script upkeep | | Failover | automatic (Multi-AZ) | Sentinel/Cluster you build yourself | | **Ops hours** | ≈ 2–4h/month | **≈ 8–16h/month** (patches, monitoring, failover drills) | | Hours converted | hourly loaded cost × hours above | same math | Multiply those hours by your team’s loaded hourly rate and the instance-price gap usually flips. If you run failover yourself, the procedure in [Redis Sentinel — High Availability and Automatic Failover](/engineer/redis-sentinel-high-availability) applies to Valkey as-is (config syntax is compatible). --- ## Migration runbook: Redis 7.2 → Valkey 8 with zero downtime Valkey is based on Redis 7.2.4 code, so protocol and command compatibility is very high. “Very high” is not “complete.” ### Stage 0: pre-inventory (can we even cut over?) ```bash # 서버 정보 및 모듈 목록 redis-cli -h INFO server redis-cli -h MODULE LIST # 지원 명령어 수 (전후 비교용) redis-cli -h COMMAND COUNT # 실제로 어떤 명령을 쓰고 있는지 (짧게, 부하 주의) redis-cli -h --stat timeout 30 redis-cli -h MONITOR | awk '{print $4}' | sort | uniq -c | sort -rn | head -30 ``` **Expected healthy result**: `MODULE LIST` is an empty array (`(empty array)`) → proceed. **Branch**: if Redis Stack modules such as `search`, `json`, `timeseries`, `bloom` show up, **this is not a simple cutover**. Your options are ① keep Redis (re-read the license terms) ② split that capability onto OpenSearch/PostgreSQL/etc. ③ validate the Valkey-side counterpart module, then migrate. For ③, budget validation effort as its own line item. ### Procedure 1. **Bring up a new Valkey 8 node** (same subnet as existing Redis, same spec or better) ```bash valkey-server /etc/valkey/valkey.conf --port 6379 --daemonize yes ``` 2. **Point it at existing Redis as master** ```bash valkey-cli -h REPLICAOF 6379 # 인증이 있다면 valkey-cli -h CONFIG SET masterauth "" ``` 3. **Confirm replication** — these values must be healthy before you continue. ```bash valkey-cli -h INFO replication ``` **Expected healthy result**: `master_link_status:up`, `master_sync_in_progress:0`, and `lag:0` on the master’s `INFO replication`. **Branch**: if `master_link_status:down` persists, check security groups / `bind` / `protected-mode` / `requirepass` first. If `master_sync_in_progress:1` hangs around, an RDB transfer is in flight — wait proportional to dataset size. 4. **Optionally shift a slice of read traffic to Valkey** — canary 5–10%, watch latency and error rate. 5. **Flip the client endpoint** — DNS CNAME or a config switch. Drop TTL to 30 seconds or less ahead of time. 6. **Promote** ```bash valkey-cli -h REPLICAOF NO ONE ``` 7. **Observation window (minimum 24 hours)** — hang the rollback thresholds below on the dashboard. 8. **Retire old Redis** — keep it stopped, with snapshots, for at least 7 days (stay on 3-2-1 backup). ### Client-library checkpoints | Library | What to check | Common failure point | |---|---|---| | **redis-py** (Python) | `HELLO 3` RESP3 handshake, `decode_responses` behavior, `CLUSTER SLOTS` parsing if you use Cluster | Custom code that regex-parses the server version string | | **Lettuce / Spring Data Redis** (Java) | Command-support logic keyed off server version, Sentinel topology refresh interval, `ClientResources` reconnect policy | Assuming the engine-name string in Sentinel `INFO` | | **go-redis** (Go) | `Options.Protocol` (RESP2/3), cluster topology refresh, `ReadTimeout` tuning | MOVED retry handling on pipeline + cluster | Common check command: ```bash valkey-cli -h INFO server | grep -E "redis_version|valkey_version|server_name" ``` Some clients assume a `redis_version` field exists. Valkey keeps that field for compatibility, but **grep your homegrown health-check and monitoring scripts for hardcoded strings**. ### Rollback conditions (quantitative thresholds) If **any one** of these holds for 5+ minutes, immediately revert the step-5 endpoint. | Metric | Threshold | |---|---| | Cache-read p99 latency | **>+30%** vs pre-cutover baseline | | Client error rate | **>+0.1 pp** vs baseline | | Replication lag (`lag`) | **>10 seconds** and not recovering | | Cache hit ratio | **>−5 pp** vs baseline | | Node CPU | **sustained 80%** | Rollback procedure: point DNS back at old Redis → put Valkey back as a replica with `REPLICAOF` → if you had already promoted, **check whether both sides took writes first** (if dual-write happened, invalidate and reload is the safe path). ### Constraints when you are on ElastiCache ElastiCache sometimes **cannot in-place change a cluster’s engine** (whether the console offers an upgrade path depends on the engine/version combo — confirm in the console and official docs). If there is no path, the workaround is: 1. Take a manual snapshot of the existing cluster 2. Create a **new Valkey-engine cluster from that snapshot** (seed RDB restore) 3. Catch up the post-snapshot delta via application-level warming, or via `REPLICAOF` against the source (through a self-managed node) 4. Flip the endpoint → observe → delete the old cluster If this is a pure cache and delta catch-up is hard, **natural warming via TTL expiry** is simpler. Right after cutover the origin DB will get slammed, so put **cache-stampede protection (request coalescing or jittered TTLs)** in place first. --- ## Conclusion: final recommendation by three scenarios ### ① Startup, single instance (low traffic, 1–2 infra people) **Verdict: Valkey (managed, single shard + 1 replica)** Simple: feature-parity with Redis, cheaper managed rates, and you will not think about the license again. No need to rush if you already run Redis, but **when you create a new cluster, create it as Valkey.** - **One thing to do today**: change the default engine on the next cache cluster you create to Valkey, and update the `engine` value in the IaC template (Terraform module, etc.) at the same time. ### ② High-traffic SaaS (multi-tenant, some resale element) **Verdict: Valkey (full cutover)** Multi-tenant SaaS sits on the (b)–(c) boundary of the license table. Shard count is high enough that savings are material. Nodes keep growing while legal interpretation is still pending. - **Cutover priority**: ① no modules + pure cache clusters (safest) → ② session store → ③ rate limiter / distributed lock → ④ module-dependent clusters (split out as a separate project) - Anything tied to Redis Stack modules comes off the cutover list and gets **redefined as “move the capability to another store.”** ### ③ Regulated-industry on-prem (air-gapped, vendor-audit response) **Verdict: Valkey (or Memcached if it is pure KV)** This is where bundled distribution and open-source notice obligations bite, so the simplicity of BSD 3-Clause is audit-cost reduction. - **License evidence pack**: ① copy of the LICENSE file for the version in use ② refreshed SBOM (CycloneDX/SPDX) ③ open-source notice (NOTICE) ④ package provenance (official release URL / checksum) on record --- ## Our team’s decision checklist (copy this) ``` [ ] 1. 우리는 캐시를 고객에게 "서비스로" 제공하거나 제품에 번들 배포하는가? (Yes → Valkey 필수) [ ] 2. Sorted Set / Stream / Bitmap 중 하나라도 쓰는가? (No → Memcached 후보 진입) [ ] 3. Redis Stack 모듈(Search/JSON/TimeSeries)에 의존하는가? (Yes → 단순 전환 불가, 별도 과제) [ ] 4. 값 크기 p90이 10KB를 넘는가? (Yes → 멀티스레드 성능 마케팅은 무시) [ ] 5. 캐시 노드가 10개를 넘는가? (Yes → 라이선스와 무관하게 비용만으로 전환 검토) ``` ### Three data points you must have before the decision meeting 1. **Value-size distribution** — p50/p90/p99 from the `MEMORY USAGE` sampling script above 2. **Commands actually in use** — top 30 from a 30-second `MONITOR` sample + `MODULE LIST` 3. **Current monthly cache bill** — last 3 months actuals from Cost Explorer, tagged to ElastiCache Without those three, the meeting ends at “but Redis is more stable, right?” With numbers, you close in 30 minutes. --- ## FAQ **Q. Is Valkey 100% compatible with Redis?** A. Valkey forked from Redis 7.2.4, so compatibility is very high at the command, protocol, config-file, and replication-protocol level. Separately verify ① Redis Stack modules (Search/JSON/TimeSeries, etc.) ② features added after Redis 8 ③ homegrown scripts that hardcode the server version string. The two projects are evolving independently, so the gap will widen over time. **Q. We only use Redis as an internal cache. Do we need to move to Valkey urgently?** A. In most cases, no. Internal-infra-only use is pattern (a) on the license table and is fine in practice. The rational order is: create new clusters as Valkey, then evaluate a cutover when the managed-price gap becomes material (node count grows). Make the final call after legal reviews the open-source clause in customer contracts with you. **Q. Can we use Memcached as a session store?** A. Not recommended. Memcached has no persistence and no replication, so a node restart wipes everything and every user is logged out. Data whose loss immediately breaks UX — sessions — belongs on Valkey/Redis, which have replication and persistence. Memcached is optimal for “we can recompute it” DB query-result caches.

#캐시 마이그레이션

Redis vs Valkey vs Memcached: A Selection Guide and Migration Runbook