milvus/docs/design_docs/20211215-milvus_timesync.md

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# Timesync -- All The things you should know
`Time Synchronization` is the kernel part of Milvus 2.0; it affects all components of the system. This document describes the detailed design of `Time Synchronization`.
There are 2 kinds of events in Milvus 2.0:
- DDL events
- create collection
- drop collection
- create partition
- drop partition
- DML events
- insert
- search
- etc
All events have a `Timestamp` to indicate when this event occurs.
Suppose there are two users, `u1` and `u2`. They connect to Milvus and do the following operations at the respective timestamps.
| ts | u1 | u2 |
| --- | -------------------- | ------------ |
| t0 | create Collection C0 | - |
| t2 | - | search on C0 |
| t5 | insert A1 into C0 | - |
| t7 | - | search on C0 |
| t10 | insert A2 | - |
| t12 | - | search on C0 |
| t15 | delete A1 from C0 | - |
| t17 | - | search on C0 |
Ideally, `u2` expects `C0` to be empty at `t2`, and could only see `A1` at `t7`; while `u2` could see both `A1` and `A2` at `t12`, but only see `A2` at `t17`.
It's easy to achieve this in a `single-node` database. But for a `Distributed System`, such as `Milvus`, it's a little difficult; the following problems need to be solved:
1. If `u1` and `u2` are on different nodes, and their time clock is not synchronized. To give an extreme example, suppose that the time of `u2` is 24 hours later than `u1`, then all the operations of `u1` can't be seen by `u2` until the next day.
2. Network latency. If `u2` starts the `Search on C0` at `t17`, then how can it be guaranteed that all the `events` before `t17` have been processed? If the events of `delete A1 from C0` have been delayed due to the network latency, then it would lead to an incorrect state: `u2` would see both `A1` and `A2` at `t17`.
`Time synchronization system` is used to solve the above problems.
## Timestamp Oracle(TSO)
Like [TiKV](https://github.com/tikv/tikv), Milvus 2.0 provides `TSO` service. All the events must alloc timestamp from `TSO`, not from the local clock, so the first problem can be solved.
`TSO` is provided by the `RootCoord` component. Clients could alloc one or more timestamp in a single request; the `proto` is defined as follows.
```proto
service RootCoord {
...
rpc AllocTimestamp(AllocTimestampRequest) returns (AllocTimestampResponse) {}
...
}
message AllocTimestampRequest {
common.MsgBase base = 1;
uint32 count = 3;
}
message AllocTimestampResponse {
common.Status status = 1;
uint64 timestamp = 2;
uint32 count = 3;
}
```
`Timestamp` is of type `uint64`, and contains physical and logical parts.
This is the format of `Timestamp`
![Timestamp struct](./graphs/time_stamp_struct.jpg)
In an `AllocTimestamp` request, if `AllocTimestampRequest.count` is greater than `1`, `AllocTimestampResponse.timestamp` indicates the first available timestamp in the response.
## Time Synchronization
To better understand `Time Synchronization`, let's introduce the data operation of Milvus 2.0 briefly.
Take `Insert Operation` as an example.
- User can configure lots of `Proxy` to achieve load balancing, in `Milvus 2.0`
- User can use `SDK` to connect to any `Proxy`
- When `Proxy` receives `Insert` Request from `SDK`, it splits `InsertMsg` into different `MsgStream` according to the hash value of `Primary Key`
- Each `InsertMsg` would be assigned with a `Timestamp` before sending to the `MsgStream`
>*Note: `MsgStream` is the wrapper of message queue, the default message queue in `Milvus 2.0` is `pulsar`*
![proxy insert](./graphs/timesync_proxy_insert_msg.png)
Based on the above information, we know that the `MsgStream` has the following characteristics:
- In `MsgStream`, `InsertMsg` from the same `Proxy` must be incremented in timestamp
- In `MsgStream`, `InsertMsg` from different `Proxy` have no relationship in timestamp
The following figure shows an example of `InsertMsg` in `MsgStream`. The snippet contains 5 `InsertMsg`, 3 of them from `Proxy1` and others from `Proxy2`.
The 3 `InsertMsg` from `Proxy1` are incremented in timestamp, and the 2 `InsertMsg` from `Proxy2` are also incremented in timestamps, but there is no relationship between `Proxy1` and `Proxy2`.
![msgstream](./graphs/timesync_msgstream.png)
So the second problem has turned into this: After reading a message from `MsgStream`, how to make sure that all the messages with smaller timestamp have been consumed?
For example, when reading a message with timestamp `110` produced by `Proxy2`, but the message with timestamp `80` produced by `Proxy1`, is still in the `MsgStream`. How can this situation be handled?
The following graph shows the core logic of `Time Synchronization System` in `Milvus 2.0`; it should solve the second problem.
- Each `Proxy` will periodically report its latest timestamp of every `MsgStream` to `RootCoord`; the default interval is `200ms`
- For each `Msgstream`, `Rootcoord` finds the minimum timestamp of all `Proxy` on this `Msgstream`, and inserts this minimum timestamp into the `Msgstream`
- When the consumer reads the timestamp inserted by the `RootCoord` on the `MsgStream`, it indicates that all messages with smaller timestamp have been consumed, so all actions that depend on this timestamp can be executed safely
- The message inserted by `RootCoord` into `MsgStream` is of type `TimeTick`
![upload time tick](./graphs/timesync_proxy_upload_time_tick.png)
This is the `Proto` that is used by `Proxy` to report timestamp to `RootCoord`:
```proto
service RootCoord {
...
rpc UpdateChannelTimeTick(internal.ChannelTimeTickMsg) returns (common.Status) {}
...
}
message ChannelTimeTickMsg {
common.MsgBase base = 1;
repeated string channelNames = 2;
repeated uint64 timestamps = 3;
uint64 default_timestamp = 4;
}
```
After inserting `Timetick`, the `Msgstream` should look like this:
![msgstream time tick](./graphs/timesync_msgtream_timetick.png)
`MsgStream` will process the messages in batches according to `TimeTick`, and ensure that the output messages meet the requirements of timestamp. For more details, please refer to the `MsgStream` design details.