T4 is an open-source key-value database built on object storage.

It is designed for durable infrastructure state: config, coordination, watches, leases, transactions, recovery, and branching. T4 stores data locally for fast access, persists WAL segments and checkpoints to S3-compatible object storage, and can speak the etcd v3 API for existing infrastructure clients.

Infrastructure systems often need a small, reliable place to keep control-plane state: configuration, service metadata, locks, leases, and change notifications.

etcd is the standard answer, but operating a consensus cluster is not always the shape you want. For small platforms, edge deployments, development environments, ephemeral nodes, and systems that already trust object storage, a full Raft membership model can be more machinery than the state store deserves.

T4 explores a different tradeoff: keep the familiar key-value and watch primitives, store hot data locally, and use object storage as the durable recovery layer. Nodes can disappear, lose their disks, or be replaced; the database can rebuild from WAL segments and checkpoints in S3-compatible storage.

• Object-storage durable — WAL segments and periodic checkpoints are uploaded to S3-compatible object storage. A node that loses its disk recovers automatically.
• Infrastructure primitives — Revisions, watches, leases, transactions, and prefix scans are built in.
• Standalone or embedded — Run t4 as a server, or call t4.Open(cfg) inside a Go process. No sidecar required.
• Multi-node — Leader elected via an S3 lock. Followers stream the WAL in real time and forward writes transparently.
• etcd v3 compatible — The standalone binary speaks the etcd v3 gRPC protocol, including multi-key transactions.
• Twelve-factor config — CLI flags can be supplied through T4_* environment variables.
• Branches — Fork a database at any checkpoint with zero S3 copies. Each branch writes to its own prefix; shared SST files are deduplicated automatically.

import "github.com/t4db/t4"

node, err := t4.Open(t4.Config{
    DataDir: "/var/lib/myapp/t4",
})
defer node.Close()

rev, err := node.Put(ctx, "/config/timeout", []byte("30s"), 0)

kv, err := node.Get("/config/timeout")
fmt.Println(string(kv.Value)) // 30s

events, _ := node.Watch(ctx, "/config/", 0)
for e := range events {
    fmt.Printf("%s %s=%s\n", e.Type, e.KV.Key, e.KV.Value)
}

import (
    "github.com/t4db/t4"
    "github.com/t4db/t4/pkg/object"
)

store, err := object.NewS3StoreFromConfig(ctx, object.S3Config{
    Bucket: "my-bucket",
    Prefix: "t4/",
    Region: "us-east-1",
    // Endpoint: "http://localhost:9000", // MinIO or another S3-compatible store
})
if err != nil {
    return err
}

node, err := t4.Open(t4.Config{
    DataDir:     "/var/lib/myapp/t4",
    ObjectStore: store,
})

The t4 binary exposes the etcd v3 gRPC protocol. Use etcdctl, the official Go client, or any other etcd v3 compatible tool.

go install github.com/t4db/t4/cmd/t4@latest

# Single node, local only
t4 run --data-dir /var/lib/t4 --listen 0.0.0.0:3379

# Single node with S3
t4 run --data-dir /var/lib/t4 --listen 0.0.0.0:3379 \
           --s3-bucket my-bucket --s3-prefix t4/

# The same configuration can come from environment variables.
T4_DATA_DIR=/var/lib/t4 \
T4_LISTEN=0.0.0.0:3379 \
T4_S3_BUCKET=my-bucket \
T4_S3_PREFIX=t4/ \
T4_S3_REGION=us-east-1 \
t4 run

# Verify
etcdctl --endpoints=localhost:3379 put /hello world
etcdctl --endpoints=localhost:3379 get /hello

For offline inspection of a local data directory, use t4 inspect:

# Show local metadata without starting a server.
t4 inspect meta --data-dir /var/lib/t4

# Explore current keys.
t4 inspect list --data-dir /var/lib/t4 --prefix /config/
t4 inspect get --data-dir /var/lib/t4 /config/timeout

# Explore revision history and changes over time.
t4 inspect history --data-dir /var/lib/t4 /config/timeout
t4 inspect diff --data-dir /var/lib/t4 --from-rev 100 --to-rev 120 --prefix /config/

Multi-node and production setup: see Operations.

Branches fork a database from an existing S3 checkpoint without copying shared SST files.

# Register the branch against the source prefix.
checkpoint_key=$(t4 branch fork \
  --s3-bucket my-bucket \
  --s3-prefix t4/ \
  --branch-id experiment)

# Start the branch in its own prefix, using the source prefix as its ancestor.
t4 run \
  --data-dir /var/lib/t4-experiment \
  --listen 0.0.0.0:3379 \
  --s3-bucket my-bucket \
  --s3-prefix t4-experiment/ \
  --branch-prefix t4/ \
  --branch-checkpoint "$checkpoint_key"

When the branch is retired, remove its registry entry so future GC can reclaim unneeded source objects:

t4 branch unfork --s3-bucket my-bucket --s3-prefix t4/ --branch-id experiment

Full documentation is available at t4db.github.io/t4.