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70 lines
3 KiB
Markdown
70 lines
3 KiB
Markdown
# SeaweedMQ Message Queue on SeaweedFS (WIP, not ready)
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## What are the use cases it is designed for?
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Message queues are like water pipes. Messages flow in the pipes to their destinations.
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However, what if a flood comes? Of course, you can increase the number of partitions, add more brokers, restart,
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and watch the traffic level closely.
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Sometimes the flood is expected. For example, backfill some old data in batch, and switch to online messages.
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You may want to ensure enough brokers to handle the data and reduce them later to cut cost.
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SeaweedMQ is designed for use cases that need to:
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* Receive and save large number of messages.
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* Handle spike traffic automatically.
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## What is special about SeaweedMQ?
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* Separate computation and storage nodes to scale independently.
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* Unlimited storage space by adding volume servers.
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* Unlimited message brokers to handle incoming messages.
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* Offline messages can be operated as normal files.
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* Scale up and down with auto split and merge message topics.
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* Topics can automatically split into segments when traffic increases, and vice verse.
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* Pass messages by reference instead of copying.
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* Clients can optionally upload the messages first and just submit the references.
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* Drastically reduce the broker load.
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# Design
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# How it works?
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Brokers are just computation nodes without storage. When a broker starts, it reports itself to masters.
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Among all the brokers, one of them will be selected as the leader by the masters.
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A topic needs to define its partition key on its messages.
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Messages for a topic are divided into segments. One segment can cover a range of partitions. A segment can
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be split into 2 segments, or 2 neighboring segments can be merged back to one segment.
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During write time, the client will ask the broker leader for a few brokers to process the segment.
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The broker leader will check whether the segment already has assigned the brokers. If not, select a few brokers based
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on their loads, save the selection into filer, and tell the client.
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The client will write the messages for this segment to the selected brokers.
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## Failover
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The broker leader does not contain any state. If it fails, the masters will select a different broker.
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For a segment, if any one of the selected brokers is down, the remaining brokers should try to write received messages
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to the filer, and close the segment to the clients.
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Then the clients should start a new segment. The masters should assign other healthy brokers to handle the new segment.
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So any brokers can go down without losing data.
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## Auto Split or Merge
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(The idea is learned from Pravega.)
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The brokers should report its traffic load to the broker leader periodically.
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If any segment has too much load, the broker leader will ask the brokers to tell the client to
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close current one and create two new segments.
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If 2 neighboring segments have the combined load below average load per segment, the broker leader will ask
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the brokers to tell the client to close this 2 segments and create a new segment.
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