See [code changes](https://github.com/etcd-io/etcd/compare/v3.4.0...v3.5.0) and [v3.5 upgrade guide](https://github.com/etcd-io/etcd/blob/master/Documentation/upgrades/upgrade_3_5.md) for any breaking changes.
**Again, before running upgrades from any previous release, please make sure to read change logs below and [v3.5 upgrade guide](https://github.com/etcd-io/etcd/blob/master/Documentation/upgrades/upgrade_3_5.md).**
-`curl -L http://localhost:2379/v3beta/kv/put -X POST -d '{"key": "Zm9v", "value": "YmFy"}'` does work in v3.5. Use `curl -L http://localhost:2379/v3/kv/put -X POST -d '{"key": "Zm9v", "value": "YmFy"}'` instead.
- **`etcd --experimental-backend-bbolt-freelist-type` flag has been deprecated.** Use **`etcd --backend-bbolt-freelist-type`** instead. The default type is hashmap and it is stable now.
- Previously, if etcd parent process ID (PPID) is 1 (e.g. run with systemd), `etcd --logger=capnslog --log-outputs=default` redirects server logs to local systemd journal. And if write to journald fails, it writes to `os.Stderr` as a fallback.
- However, even with PPID 1, it can fail to dial systemd journal (e.g. run embedded etcd with Docker container). Then, [every single log write will fail](https://github.com/etcd-io/etcd/pull/9729) and fall back to `os.Stderr`, which is inefficient.
-`etcd --enable-v2=true --enable-v2v3=''` by default, to enable v2 API server that is backed by **v2 store**.
-`etcd --enable-v2=true --enable-v2v3=/aaa` to enable v2 API server that is backed by **v3 storage**.
-`etcd --enable-v2=false --enable-v2v3=''` to disable v2 API server.
-`etcd --enable-v2=false --enable-v2v3=/aaa` to disable v2 API server. TODO: error?
- Automatically [create parent directory if it does not exist](https://github.com/etcd-io/etcd/pull/9626) (fix [issue#9609](https://github.com/etcd-io/etcd/issues/9609)).
- v4.0 will configure `etcd --enable-v2=true --enable-v2v3=/aaa` to enable v2 API server that is backed by **v3 storage**.
- Deprecate v2 apply on cluster version. [Use v3 request to set cluster version and recover cluster version from v3 backend](https://github.com/etcd-io/etcd/pull/11427).
- Add [TryLock](https://github.com/etcd-io/etcd/pull/11104) method to `clientv3/concurrency/Mutex`. A non-blocking method on `Mutex` which does not wait to get lock on the Mutex, returns immediately if Mutex is locked by another session.
- Fix [client balancer failover against multiple endpoints](https://github.com/etcd-io/etcd/pull/11184).
- Fix ["kube-apiserver: failover on multi-member etcd cluster fails certificate check on DNS mismatch" (kubernetes#83028)](https://github.com/kubernetes/kubernetes/issues/83028).
- Fix [IPv6 endpoint parsing in client](https://github.com/etcd-io/etcd/pull/11211).
- Fix ["1.16: etcd client does not parse IPv6 addresses correctly when members are joining" (kubernetes#83550)](https://github.com/kubernetes/kubernetes/issues/83550).
- Use [ServerName as the authority](https://github.com/etcd-io/etcd/pull/11574) after bumping to grpc v1.26.0. Remove workaround in [#11184](https://github.com/etcd-io/etcd/pull/11184).
-`curl -L http://localhost:2379/v3beta/kv/put -X POST -d '{"key": "Zm9v", "value": "YmFy"}'` does work in v3.5. Use `curl -L http://localhost:2379/v3/kv/put -X POST -d '{"key": "Zm9v", "value": "YmFy"}'` instead.
- Fix [NoPassword check when adding user through GRPC gateway](https://github.com/etcd-io/etcd/pull/11418) ([issue#11414](https://github.com/etcd-io/etcd/issues/11414))
- Upgrade [`google.golang.org/grpc`](https://github.com/grpc/grpc-go/releases) from [**`v1.23.0`**](https://github.com/grpc/grpc-go/releases/tag/v1.23.0) to [**`v1.26.0`**](https://github.com/grpc/grpc-go/releases/tag/v1.26.0).