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