mirror of
https://github.com/klzgrad/naiveproxy.git
synced 2024-11-24 22:36:09 +03:00
68 lines
3.3 KiB
Markdown
68 lines
3.3 KiB
Markdown
|
# disk_cache API
|
||
|
|
|
||
|
|
The disk_cache API provides for caches that can store multiple random-access
|
||
|
|
byte streams of data associated with a key on disk (or in memory).
|
||
|
|
|
||
|
|
There are two kinds of entries that can be stored: regular and sparse.
|
||
|
|
|
||
|
|
Regular entries contain up to 3 separate data streams. Usually stream 0
|
||
|
|
would be used for some kind of primary small metadata (e.g. HTTP headers);
|
||
|
|
stream 1 would contain the main payload (e.g. HTTP body); and stream 2 would
|
||
|
|
optionally contain some auxiliary metadata that's needed only some of the time
|
||
|
|
(e.g. V8 compilation cache). There is no requirement that these streams be used
|
||
|
|
in this way, but implementations may expect similar size and usage
|
||
|
|
characteristics.
|
||
|
|
|
||
|
|
Sparse entries have a stream 0 and a separate sparse stream that's accessed with
|
||
|
|
special methods that have `Sparse` in their names. It's an API misuse to try to
|
||
|
|
access streams 1 or 2 of sparse entries or to call `WriteSparseData` on entries
|
||
|
|
that have contents in those streams. Calling `SparseReadData` or
|
||
|
|
`GetAvailableRange` to check whether entries are sparse is, however, permitted.
|
||
|
|
An added entry becomes a regular entry once index 1 or 2 is written to, or it
|
||
|
|
becomes a sparse entry once the sparse stream is written to. Once that is done,
|
||
|
|
it cannot change type and the access/modification restrictions relevant to the
|
||
|
|
type apply. Type of an entry can always be determined using `SparseReadData` or
|
||
|
|
`GetAvailableRange`.
|
||
|
|
|
||
|
|
The sparse streams are named as such because they are permitted to have holes in
|
||
|
|
the byte ranges of contents they represent (and implementations may also drop
|
||
|
|
some pieces independently). For example, in the case of a regular entry,
|
||
|
|
starting with an empty entry, and performing `WriteData` on some stream at
|
||
|
|
offset = 1024, length = 1024, then another `WriteData` at offset = 3072,
|
||
|
|
length = 1024, results in the stream having length = 4096, and the areas not
|
||
|
|
written to filled in with zeroes.
|
||
|
|
|
||
|
|
In contrast, after the same sequence of `WriteSparseData` operations, the entry
|
||
|
|
will actually keep track that [1024, 2048) and [3072, 4096) are valid, and will
|
||
|
|
permit queries with `GetAvailableRange`, and only allow reads of the defined
|
||
|
|
ranges.
|
||
|
|
|
||
|
|
[`net/disk_cache/disk_cache.h`](/net/disk_cache/disk_cache.h) is the only
|
||
|
|
include you need if you just want to use this API.
|
||
|
|
`disk_cache::CreateCacheBackend()` is the first method you'll need to call.
|
||
|
|
|
||
|
|
[TOC]
|
||
|
|
|
||
|
|
## The implementations
|
||
|
|
|
||
|
|
### [disk_cache/blockfile directory](/net/disk_cache/blockfile/)
|
||
|
|
|
||
|
|
This implementation backs the HTTP cache on Windows and OS X. It tries to pack
|
||
|
|
many small entries caches typically have into "block" files, which can help
|
||
|
|
performance but introduces a lot of complexity and makes recovery from
|
||
|
|
corruption very tricky.
|
||
|
|
|
||
|
|
### [disk_cache/memory directory](/net/disk_cache/memory/)
|
||
|
|
|
||
|
|
This contains the in-memory-only implementation. It's used for incognito
|
||
|
|
mode.
|
||
|
|
|
||
|
|
### [disk_cache/simple directory](/net/disk_cache/simple/)
|
||
|
|
|
||
|
|
This implementation backs the HTTP cache on Android, ChromeOS, and Linux, and
|
||
|
|
is used to implement some features like CacheStorage on all platforms. The
|
||
|
|
design is centered around roughly having a single file per cache entry (more
|
||
|
|
precisely for streams 0 and 1), with a compact and simple in-memory index for
|
||
|
|
membership tests, which makes it very robust against failures, but also highly
|
||
|
|
sensitive to OS file system performance.
|