45 lines
2.0 KiB
Markdown
45 lines
2.0 KiB
Markdown
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title = "Request routing logic"
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weight = 10
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Data retrieval requests to Garage endpoints (S3 API and websites) are resolved
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to an individual object in a bucket. Since objects are replicated to multiple nodes
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Garage must ensure consistency before answering the request.
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## Using quorum to ensure consistency
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Garage ensures consistency by attempting to establish a quorum with the
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data nodes responsible for the object. When a majority of the data nodes
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have provided metadata on a object Garage can then answer the request.
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When a request arrives Garage will, assuming the recommended 3 replicas, perform the following actions:
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- Make a request to the two preferred nodes for object metadata
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- Try the third node if one of the two initial requests fail
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- Check that the metadata from at least 2 nodes match
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- Check that the object hasn't been marked deleted
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- Answer the request with inline data from metadata if object is small enough
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- Or get data blocks from the preferred nodes and answer using the assembled object
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Garage dynamically determines which nodes to query based on health, preference, and
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which nodes actually host a given data. Garage has no concept of "primary" so any
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healthy node with the data can be used as long as a quorum is reached for the metadata.
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## Node health
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Garage keeps a TCP session open to each node in the cluster and periodically pings them. If a connection
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cannot be established, or a node fails to answer a number of pings, the target node is marked as failed.
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Failed nodes are not used for quorum or other internal requests.
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## Node preference
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Garage prioritizes which nodes to query according to a few criteria:
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- A node always prefers itself if it can answer the request
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- Then the node prioritizes nodes in the same zone
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- Finally the nodes with the lowest latency are prioritized
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For further reading on the cluster structure look at the [gateway](@/documentation/cookbook/gateways.md)
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and [cluster layout management](@/documentation/reference-manual/layout.md) pages.
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