# Design

1. Flatten binary key-value space
    
2. Keep the event history until compaction
    - access to old version of keys
    - user controlled history compaction
    
3. Support range query
    - Pagination support with limit argument
    - Support consistency guarantee across multiple range queries
    
4. Replace TTL key with Lease
    - more efficient/ low cost keep alive
    - a logical group of TTL keys
    
5. Replace CAS/CAD with multi-object Txn
    - MUCH MORE powerful and flexible
    
6. Support efficient watching with multiple ranges

7. RPC API supports the completed set of APIs. 
    - more efficient than JSON/HTTP
    - additional txn/lease support

8. HTTP API supports a subset of APIs.
    - easy for people to try out etcd
    - easy for people to write simple etcd application


## Protobuf Defined API

[api protobuf](../../etcdserver/etcdserverpb/rpc.proto)

[kv protobuf](../../storage/storagepb/kv.proto)

## Examples

### Put a key (foo=bar)
```
// A put is always successful
Put( PutRequest { key = foo, value = bar } )

PutResponse { 
    cluster_id = 0x1000,
    member_id = 0x1,
    revision = 1,
    raft_term = 0x1,
}
```

### Get a key (assume we have foo=bar)
```
Get ( RangeRequest { key = foo } )

RangeResponse {
    cluster_id = 0x1000,
    member_id = 0x1,
    revision = 1,
    raft_term = 0x1,
    kvs = {
      {
          key = foo,
          value = bar,
          create_revision = 1,
          mod_revision = 1,
          version = 1;
      },
    },
}
```

### Range over a key space (assume we have foo0=bar0… foo100=bar100)
```
Range ( RangeRequest { key = foo, end_key = foo80, limit = 30  } )

RangeResponse {
    cluster_id = 0x1000,
    member_id = 0x1,
    revision = 100,
    raft_term = 0x1,
    kvs = {
      {
          key = foo0,
          value = bar0,
          create_revision = 1,
          mod_revision = 1,
          version = 1;
      },
         ...,
      {
          key = foo30,
          value = bar30,
          create_revision = 30,
          mod_revision = 30,
          version = 1;
      },
    },
}
```

### Finish a txn (assume we have foo0=bar0, foo1=bar1)
```
Txn(TxnRequest {
    // mod_revision of foo0 is equal to 1, mod_revision of foo1 is greater than 1
    compare = {
        {compareType = equal, key = foo0, mod_revision = 1}, 
        {compareType = greater, key = foo1, mod_revision = 1}}
    },
    // if the comparison succeeds, put foo2 = bar2
    success = {PutRequest { key = foo2, value = success }},
    // if the comparison fails, put foo2=fail
    failure = {PutRequest { key = foo2, value = failure }},
)

TxnResponse {
    cluster_id = 0x1000,
    member_id = 0x1,
    revision = 3,
    raft_term = 0x1,
    succeeded = true,
    responses = {
      // response of PUT foo2=success
      {
            cluster_id = 0x1000,
            member_id = 0x1,
            revision = 3,
            raft_term = 0x1,
        }
    }
}
```

### Watch on a key/range

```
Watch( WatchRequest{
           key = foo,
           end_key = fop, // prefix foo
           start_revision = 20,
           end_revision = 10000,
           // server decided notification frequency
           progress_notification = true,
       } 
       … // this can be a watch request stream
      )

// put (foo0=bar0) event at 3
WatchResponse {
    cluster_id = 0x1000,
    member_id = 0x1,
    revision = 3,
    raft_term = 0x1,
    event_type = put,
    kv = {
              key = foo0,
              value = bar0,
              create_revision = 1,
              mod_revision = 1,
              version = 1;
          },
    }
    …
    
    // a notification at 2000
    WatchResponse {
        cluster_id = 0x1000,
        member_id = 0x1,
        revision = 2000,
        raft_term = 0x1,
        // nil event as notification
    }
    
    … 
    
    // put (foo0=bar3000) event at 3000
    WatchResponse {
        cluster_id = 0x1000,
        member_id = 0x1,
        revision = 3000,
        raft_term = 0x1,
        event_type = put,
        kv = {
                key = foo0,
                value = bar3000,
                create_revision = 1,
                mod_revision = 3000,
                version = 2;
          },
    }
    …
    
```
