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34 lines
1.7 KiB
Go
34 lines
1.7 KiB
Go
// Package restorer contains code to restore data from a repository.
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//
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// The Restorer tries to keep the number of backend requests minimal. It does
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// this by downloading all required blobs of a pack file with a single backend
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// request and avoiding repeated downloads of the same pack. In addition,
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// several pack files are fetched concurrently.
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//
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// Here is high-level pseudo-code of the how the Restorer attempts to achieve
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// these goals:
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//
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// while there are packs to process
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// choose a pack to process [1]
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// get the pack from the backend or cache [2]
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// write pack blobs to the files that need them [3]
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// if not all pack blobs were used
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// cache the pack for future use [4]
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//
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// Pack download and processing (steps [2] - [4]) runs on multiple concurrent
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// Goroutines. The Restorer runs all steps [2]-[4] sequentially on the same
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// Goroutine.
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//
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// Before a pack is downloaded (step [2]), the required space is "reserved" in
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// the pack cache. Actual download uses single backend request to get all
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// required pack blobs. This may download blobs that are not needed, but we
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// assume it'll still be faster than getting individual blobs.
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//
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// Target files are written (step [3]) in the "right" order, first file blob
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// first, then second, then third and so on. Blob write order implies that some
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// pack blobs may not be immediately used, i.e. they are "out of order" for
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// their respective target files. Packs with unused blobs are cached (step
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// [4]). The cache has capacity limit and may purge packs before they are fully
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// used, in which case the purged packs will need to be re-downloaded.
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package restorer
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