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new 261234f feat: add docs for compaction offload mechanism (#141)
261234f is described below
commit 261234fc603675f0c3a3f208946133f393e2eb60
Author: Leslie Su <[email protected]>
AuthorDate: Mon Oct 28 10:12:35 2024 +0800
feat: add docs for compaction offload mechanism (#141)
Co-authored-by: Jiacai Liu <[email protected]>
---
content/en/docs/design/compaction_offload.md | 92 ++++++++++++++++++++++++++++
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+---
+title: "Compaction Offload"
+---
+
+**Note: This feature is still in development.**
+
+This chapter discusses compaction offload, which is designed to separate the
compaction workload from the local horaedb nodes and delegate it to external
compaction nodes.
+
+## Overview
+
+```plaintext
+┌─────────────────────────────────────────────────────────────────────────┐
+│ │
+│ HoraeMeta Cluster │
+│ │
+└─────────────────────────────────────────────────────────────────────────┘
+ ▲ ▲ | |
+ │ │1.Fetch compaction │(Monitor compaction│
+ │ │ node info │node) │
+ | │ ▼ ▼
+┌────────────┐ ┌────────────┐ ┌────────────┐ ┌────────────┐
+│ │ │ │2.Offload Task│ │ │ │
+│ HoraeDB │ │ HoraeDB │ ─────────▶ │ Compaction │ │ Compaction │
+│ │ │ │ ◀───────── │ Node │ │ Node │
+└────────────┘ └────────────┘ 4.Ret Result └────────────┘ └────────────┘
+ | | | |
+ │ 5.Update the │ │ 3.Compact │
+ │ SSTable │ │ │
+ ▼ ▼ ▼ ▼
+┌─────────────────────────────────────────────────────────────────────────┐
+│ ┌─────────────────────┐ │
+│ Object Storage │ Temporary Workspace │ │
+│ └─────────────────────┘ │
+└─────────────────────────────────────────────────────────────────────────┘
+```
+
+The diagram above describes the architecture of cluster for compaction
offload, where some key concepts need to be explained:
+
+- `Compaction Node`: Takes responsibility to handle offloaded compaction
tasks. The compaction node receives the compaction task and performs the actual
merging of SSTables, then sends back the task result to HoraeDB.
+- `HoraeMeta Cluster`: HoraeMeta acts as a compaction nodes manager in the
compaction offload scenario. It monitors the compaction nodes cluster and
schedule the compaction nodes.
+
+The procedure of remote compaction based above architecture diagram is:
+
+1. HoraeDB servers fetch the information of suitable compaction nodes from the
HoraeMeta.
+2. HoraeDB submit the compaction task to the remote compaction node, according
to the information fetched from HoraeMeta.
+3. Compaction node executes the task and write results to the temporary
workspace.
+4. Compaction node sends compaction results back to HoraeDB.
+5. HoraeDB receives the result, installs the data in temporary workspace and
purges compaction input files.
+
+The architecture above makes it easy to implement some wonderful features like
load balancing and high availability. Let's dive into the key components in the
architecture and talking about how these features are implemented.
+
+### Compaction Node
+
+`Compaction Node` runs the main logic of compaction. It is implemented based
on HoraeDB and distinguished by:
+
+- `NodeType`: A config parameter used to distinguished the `HoraeDB` and
`CompactionNode`. This info would be sent to HoraeMeta through heartbeat.
+
+The compaction service is implemented as grpc service.
+
+### HoraeMeta
+
+`HoraeMeta` manages the compaction nodes cluster with `CompactionNodeManager`,
which takes responsibilities for compaction nodes metadata management and
scheduling.
+
+The compaction nodes metadata includes:
+
+- Compaction node information, such as node name, node state;
+- A compaction node name list, used as the key to access compaction node info,
for better scheduling with round-robin strategy;
+- ...
+
+As for the compaction nodes scheduling work, it mainly includes:
+
+- Receiving the heartbeats from the compaction node and determining the online
status of these registered nodes.
+- Performing load balancing according to the compaction nodes cluster info.
+- Providing the info of suitable compaction node for HoraeDB when remote
compaction execution is needed.
+
+## Load Balancing
+
+Load Balancing is critical for compaction nodes cluster to make their overall
processing more efficient. The effect of load balancing mainly based on the
schedule algorithm for compaction nodes impl in `CompactionNodeManager`.
+
+_(ps: The current implementation of schedule algorithm is round-robin strategy
for easiness.)_
+
+The main process for the schedule algorithm based on real load is:
+
+- HoraeMeta collects the compaction nodes load information through the
heartbeats to create a load overview of the compaction nodes cluster.
+- Pick a compaction node with low load according to the load overview.
+
+## High Availability
+
+The fault tolerance of above architecture can be achieved by such a procedure:
+
+- When detecting that the heartbeat is broken, `HoraeMeta` determines that the
compaction node is offline.
+- When `HoraeMeta` can not provide suitable compaction node for HoraeDB or
compaction node doesn't return the task result successfully, HoraeDB would
switches to run compaction task locally.
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