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a few edits to architecture.txt and related docs
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@ -9,9 +9,10 @@ virtual drive, and the application that sits on top.
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The lowest layer is the "grid", basically a DHT (Distributed Hash Table)
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The lowest layer is the "grid", basically a DHT (Distributed Hash Table)
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which maps URIs to data. The URIs are relatively short ascii strings
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which maps URIs to data. The URIs are relatively short ascii strings
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(currently about 140 bytes), and each is used as a reference to an immutable
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(currently about 140 bytes), and each is used as a reference to an immutable
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arbitrary-length sequence of data bytes. This data is distributed around the
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arbitrary-length sequence of data bytes. This data is encrypted and
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grid across a large number of nodes, such that a statistically unlikely number
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distributed around the grid across a large number of nodes, such that a
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of nodes would have to be unavailable for the data to become unavailable.
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statistically unlikely number of nodes would have to be unavailable for the
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data to become unavailable.
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The middle layer is the virtual drive: a tree-shaped data structure in which
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The middle layer is the virtual drive: a tree-shaped data structure in which
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the intermediate nodes are directories and the leaf nodes are files. Each
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the intermediate nodes are directories and the leaf nodes are files. Each
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@ -27,9 +28,9 @@ later, a user can recover older versions of their files. Other sorts of
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applications can run on top of the virtual drive, of course -- anything that
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applications can run on top of the virtual drive, of course -- anything that
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has a use for a secure, robust, distributed filestore.
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has a use for a secure, robust, distributed filestore.
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Note: some of the description below indicates design targets rather than
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Note: some of the text below describes design targets rather than actual code
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actual code present in the current release. Please take a look at roadmap.txt
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present in the current release. Please take a look at roadmap.txt to get an
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to get an idea of how much of this has been implemented so far.
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idea of how much of this has been implemented so far.
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THE BIG GRID OF PEERS
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THE BIG GRID OF PEERS
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@ -46,11 +47,11 @@ StorageServer, which offers to hold data for a limited period of time (a
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that would cause it to consume more space than it wants to provide. When a
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that would cause it to consume more space than it wants to provide. When a
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lease expires, the data is deleted. Peers might renew their leases.
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lease expires, the data is deleted. Peers might renew their leases.
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This storage is used to hold "shares", which are themselves used to store
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This storage is used to hold "shares", which are encoded pieces of files in
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files in the grid. There are many shares for each file, typically between 10
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the grid. There are many shares for each file, typically between 10 and 100
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and 100 (the exact number depends upon the tradeoffs made between
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(the exact number depends upon the tradeoffs made between reliability,
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reliability, overhead, and storage space consumed). The files are indexed by
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overhead, and storage space consumed). The files are indexed by a
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a "StorageIndex", which is derived from the encryption key, which may be
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"StorageIndex", which is derived from the encryption key, which may be
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randomly generated or it may be derived from the contents of the file. Leases
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randomly generated or it may be derived from the contents of the file. Leases
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are indexed by StorageIndex, and a single StorageServer may hold multiple
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are indexed by StorageIndex, and a single StorageServer may hold multiple
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shares for the corresponding file. Multiple peers can hold leases on the same
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shares for the corresponding file. Multiple peers can hold leases on the same
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@ -90,7 +91,7 @@ be used to reconstruct the whole file. The shares are then deposited in
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StorageServers in other peers.
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StorageServers in other peers.
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A tagged hash of the encryption key is used to form the "storage index",
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A tagged hash of the encryption key is used to form the "storage index",
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which is used for both peer selection (described below) and to index shares
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which is used for both server selection (described below) and to index shares
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within the StorageServers on the selected peers.
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within the StorageServers on the selected peers.
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A variety of hashes are computed while the shares are being produced, to
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A variety of hashes are computed while the shares are being produced, to
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@ -173,10 +174,10 @@ accurate. The plan is to store this capability next to the URI in the virtual
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drive structure.
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drive structure.
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PEER SELECTION
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SERVER SELECTION
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When a file is uploaded, the encoded shares are sent to other peers. But to
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When a file is uploaded, the encoded shares are sent to other peers. But to
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which ones? The "peer selection" algorithm is used to make this choice.
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which ones? The "server selection" algorithm is used to make this choice.
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In the current version, the verifierid is used to consistently-permute the
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In the current version, the verifierid is used to consistently-permute the
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set of all peers (by sorting the peers by HASH(verifierid+peerid)). Each file
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set of all peers (by sorting the peers by HASH(verifierid+peerid)). Each file
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@ -3,12 +3,9 @@ CODE OVERVIEW
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A brief map to where the code lives in this distribution:
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A brief map to where the code lives in this distribution:
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src/zfec: the erasure-coding library, turns data into shares and back again.
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src/allmydata: the code for this project. When installed, this provides the
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When installed, this provides the 'zfec' package.
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'allmydata' package. This includes a few pieces copied from
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the PyCrypto package, in allmydata/Crypto/* .
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src/allmydata: the bulk of the code for this project. When installed, this
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provides the 'allmydata' package. This includes a few pieces
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copied from the PyCrypto package, in allmydata/Crypto/* .
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Within src/allmydata/ :
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Within src/allmydata/ :
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@ -29,12 +26,13 @@ Within src/allmydata/ :
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storageserver.py: provides storage services to other nodes
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storageserver.py: provides storage services to other nodes
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codec.py: low-level erasure coding, wraps zfec
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codec.py: low-level erasure coding, wraps the zfec library
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encode.py: handles turning data into shares and blocks, computes hash trees
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encode.py: handles turning data into shares and blocks, computes hash trees
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upload.py: upload-side peer selection, reading data from upload sources
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download.py: download-side peer selection, share retrieval, decoding
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upload.py: upload server selection, reading data from upload sources
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download.py: download server selection, share retrieval, decoding
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dirnode.py: implements the directory nodes. One part runs on the
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dirnode.py: implements the directory nodes. One part runs on the
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global vdrive server, the other runs inside a client
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global vdrive server, the other runs inside a client
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