1wrap_log_reader(3) Erlang Module Definition wrap_log_reader(3)
2
6 wrap_log_reader - A service to read internally formatted wrap disk
7 logs.
8
11 This module makes it possible to read internally formatted wrap disk
12 logs, see disk_log(3). wrap_log_reader does not interfere with disk_log
13 activities; there is however a bug in this version of the
14 wrap_log_reader, see section Known Limitations.
15 A wrap disk log file consists of many files, called index files. A log
17 file can be opened and closed. Also, a single index file can be opened
18 separately. If a non-existent or non-internally formatted file is
19 opened, an error message is returned. If the file is corrupt, no at‐
20 tempt is made to repair it, but an error message is returned.
21 If a log is configured to be distributed, it is possible that all items
23 are not logged on all nodes. wrap_log_reader only reads the log on the
24 called node; it is up to the user to be sure that all items are read.
25
27 continuation()
28 Continuation returned by open/1,2 or chunk/1,2.
30
32 chunk(Continuation) -> chunk_ret()
33 chunk(Continuation, N) -> chunk_ret()
35 Types:
37 Continuation = continuation()
39 N = infinity | integer() >= 1
40 chunk_ret() =
41 {Continuation2 :: term(), Terms :: [term()]} |
42 {Continuation2 :: term(),
43 Terms :: [term()],
44 Badbytes :: integer() >= 0} |
45 {Continuation2 :: term(), eof} |
46 {error, Reason :: term()}
47 Enables to efficiently read the terms that are appended to a
49 log. Minimises disk I/O by reading 64 kilobyte chunks from the
50 file.
51 The first time chunk() is called, an initial continuation re‐
53 turned from open/1 or open/2 must be provided.
54 When chunk/3 is called, N controls the maximum number of terms
56 that are read from the log in each chunk. Defaults to infinity,
57 which means that all the terms contained in the 8K chunk are
58 read. If less than N terms are returned, this does not necessar‐
59 ily mean that end of file is reached.
60 Returns a tuple {Continuation2, Terms}, where Terms is a list of
62 terms found in the log. Continuation2 is yet another continua‐
63 tion that must be passed on to any subsequent calls to chunk().
64 With a series of calls to chunk(), it is then possible to ex‐
65 tract all terms from a log.
66 Returns a tuple {Continuation2, Terms, Badbytes} if the log is
68 opened in read only mode and the read chunk is corrupt. Badbytes
69 indicates the number of non-Erlang terms found in the chunk. No‐
70 tice that the log is not repaired.
71 Returns {Continuation2, eof} when the end of the log is reached,
73 and {error, Reason} if an error occurs.
74 The returned continuation either is or is not valid in the next
76 call to this function. This is because the log can wrap and
77 delete the file into which the continuation points. To ensure
78 this does not occur, the log can be blocked during the search.
79 close(Continuation) -> ok | {error, Reason}
81 Types:
83 Continuation = continuation()
85 Reason = file:posix()
86 Closes a log file properly.
88 open(Filename) -> open_ret()
90 open(Filename, N) -> open_ret()
92 Types:
94 Filename = string() | atom()
96 N = integer()
97 open_ret() =
98 {ok, Continuation :: continuation()} |
99 {error, Reason :: tuple()}
100 Filename specifies the name of the file to be read.
102 N specifies the index of the file to be read. If N is omitted,
104 the whole wrap log file is read; if it is specified, only the
105 specified index file is read.
106 Returns {ok, Continuation} if the log/index file is opened suc‐
108 cessfully. Continuation is to be used when chunking or closing
109 the file.
110 Returns {error, Reason} for all errors.
112
114 This version of wrap_log_reader does not detect if disk_log wraps to a
115 new index file between a call to wrap_log_reader:open() and the first
116 call to wrap_log_reader:chunk(). If this occurs, the call to chunk()
117 reads the last logged items in the log file, as the opened index file
118 was truncated by disk_log.
119
121 disk_log(3)
122Ericsson AB kernel 9.1 wrap_log_reader(3)