FS
Namespace Advertisements
Todo
FIXME: all zeroses -> ?
An SBlock
with identifier all zeros is a signed advertisement for a
namespace. This special SBlock
contains metadata describing the
content of the namespace. Instead of the name of the identifier for a
potential update, it contains the identifier for the root of the
namespace. The URI should always be empty. The SBlock
is signed with
the content provider’s RSA private key (just like any other SBlock).
Peers can search for SBlock
s in order to find out more about a
namespace.
KSBlocks
GNUnet implements KSBlocks
which are KBlocks
that, instead of
encrypting a CHK and metadata, encrypt an SBlock
instead. In other
words, KSBlocks
enable GNUnet to find SBlocks
using the global
keyword search. Usually the encrypted SBlock
is a namespace
advertisement. The rationale behind KSBlock
s and SBlock
s is
to enable peers to discover namespaces via keyword searches, and, to
associate useful information with namespaces. When GNUnet finds
KSBlocks
during a normal keyword search, it adds the information to
an internal list of discovered namespaces. Users looking for interesting
namespaces can then inspect this list, reducing the need for out-of-band
discovery of namespaces. Naturally, namespaces (or more specifically,
namespace advertisements) can also be referenced from directories, but
KSBlock
s should make it easier to advertise namespaces for the
owner of the pseudonym since they eliminate the need to first create a
directory.
Collections are also advertised using KSBlock
s.
File-sharing persistence directory structure
This section documents how the file-sharing library implements
persistence of file-sharing operations and specifically the resulting
directory structure. This code is only active if the
GNUNET_FS_FLAGS_PERSISTENCE
flag was set when calling
GNUNET_FS_start
. In this case, the file-sharing library will try
hard to ensure that all major operations (searching, downloading,
publishing, unindexing) are persistent, that is, can live longer than
the process itself. More specifically, an operation is supposed to live
until it is explicitly stopped.
If GNUNET_FS_stop
is called before an operation has been stopped, a
SUSPEND
event is generated and then when the process calls
GNUNET_FS_start
next time, a RESUME
event is generated.
Additionally, even if an application crashes (segfault, SIGKILL, system
crash) and hence GNUNET_FS_stop
is never called and no SUSPEND
events are generated, operations are still resumed (with RESUME
events). This is implemented by constantly writing the current state of
the file-sharing operations to disk. Specifically, the current state is
always written to disk whenever anything significant changes (the
exception are block-wise progress in publishing and unindexing, since
those operations would be slowed down significantly and can be resumed
cheaply even without detailed accounting). Note that if the process
crashes (or is killed) during a serialization operation, FS does not
guarantee that this specific operation is recoverable (no strict
transactional semantics, again for performance reasons). However, all
other unrelated operations should resume nicely.
Since we need to serialize the state continuously and want to recover as
much as possible even after crashing during a serialization operation,
we do not use one large file for serialization. Instead, several
directories are used for the various operations. When
GNUNET_FS_start
executes, the master directories are scanned for
files describing operations to resume. Sometimes, these operations can
refer to related operations in child directories which may also be
resumed at this point. Note that corrupted files are cleaned up
automatically. However, dangling files in child directories (those that
are not referenced by files from the master directories) are not
automatically removed.
Persistence data is kept in a directory that begins with the
"STATE_DIR" prefix from the configuration file (by default,
"$SERVICEHOME/persistence/") followed by the name of the client as
given to GNUNET_FS_start
(for example, "gnunet-gtk") followed by
the actual name of the master or child directory.
The names for the master directories follow the names of the operations:
"search"
"download"
"publish"
"unindex"
Each of the master directories contains names (chosen at random) for each active top-level (master) operation. Note that a download that is associated with a search result is not a top-level operation.
In contrast to the master directories, the child directories are only consulted when another operation refers to them. For each search, a subdirectory (named after the master search synchronization file) contains the search results. Search results can have an associated download, which is then stored in the general "download-child" directory. Downloads can be recursive, in which case children are stored in subdirectories mirroring the structure of the recursive download (either starting in the master "download" directory or in the "download-child" directory depending on how the download was initiated). For publishing operations, the "publish-file" directory contains information about the individual files and directories that are part of the publication. However, this directory structure is flat and does not mirror the structure of the publishing operation. Note that unindex operations cannot have associated child operations.