View Source wrap_log_reader (kernel v10.1.2)
A service to read internally formatted wrap disk logs.
This module makes it possible to read internally formatted wrap disk logs, see
disk_log
. wrap_log_reader
does not interfere with disk_log
activities;
there is however a bug in this version of the wrap_log_reader
, see section
Known Limitations.
A wrap disk log file consists of many files, called index files. A log file can be opened and closed. Also, a single index file can be opened separately. If a non-existent or non-internally formatted file is opened, an error message is returned. If the file is corrupt, no attempt is made to repair it, but an error message is returned.
If a log is configured to be distributed, it is possible that all items are not
logged on all nodes. wrap_log_reader
only reads the log on the called node; it
is up to the user to be sure that all items are read.
Known Limitations
This version of wrap_log_reader
does not detect if disk_log
wraps to a new
index file between a call to wrap_log_reader:open/1
and the first call to
wrap_log_reader:chunk/1
. If this occurs, the call to chunk/1
reads the last
logged items in the log file, as the opened index file was truncated by
disk_log
.
Summary
Functions
Equivalent to chunk(Continuation, infinity)
.
Enables to efficiently read the terms that are appended to a log. Minimises disk I/O by reading 64 kilobyte chunks from the file.
Closes a log file properly.
Equivalent to open(Filename, ...)
except that the whole
wrap log file is read.
Filename
specifies the name of the file to be read.
Types
-opaque continuation()
Continuation returned by open/1,2
or chunk/1,2
.
-type open_ret() :: {ok, Continuation :: continuation()} | {error, Reason :: tuple()}.
Functions
-spec chunk(Continuation) -> chunk_ret() when Continuation :: continuation().
Equivalent to chunk(Continuation, infinity)
.
-spec chunk(Continuation, N) -> chunk_ret() when Continuation :: continuation(), N :: infinity | pos_integer().
Enables to efficiently read the terms that are appended to a log. Minimises disk I/O by reading 64 kilobyte chunks from the file.
The first time chunk/2
is called, an initial continuation returned from
open/1
or open/2
must be provided.
When chunk/2
is called, N
controls the maximum number of terms that are read
from the log in each chunk. infinity
means that all the
terms contained in the 8K chunk are read. If less than N
terms are returned,
this does not necessarily mean that end of file is reached.
Returns a tuple {Continuation2, Terms}
, where Terms
is a list of terms found
in the log. Continuation2
is yet another continuation that must be passed on
to any subsequent calls to chunk()
. With a series of calls to chunk()
, it is
then possible to extract all terms from a log.
Returns a tuple {Continuation2, Terms, Badbytes}
if the log is opened in read
only mode and the read chunk is corrupt. Badbytes
indicates the number of
non-Erlang terms found in the chunk. Notice that the log is not repaired.
Returns {Continuation2, eof}
when the end of the log is reached, and
{error, Reason}
if an error occurs.
The returned continuation either is or is not valid in the next call to this function. This is because the log can wrap and delete the file into which the continuation points. To ensure this does not occur, the log can be blocked during the search.
-spec close(Continuation) -> ok | {error, Reason} when Continuation :: continuation(), Reason :: file:posix().
Closes a log file properly.
Equivalent to open(Filename, ...)
except that the whole
wrap log file is read.
Filename
specifies the name of the file to be read.
N
specifies the index of the file to be read. Use open/1
to read the entire
wrap log.
Returns {ok, Continuation}
if the log/index file is opened successfully.
Continuation
is to be used when chunking or closing the file.
Returns {error, Reason}
for all errors.