mirror of
https://github.com/tstack/lnav
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123 lines
5.3 KiB
Markdown
123 lines
5.3 KiB
Markdown
# Architecture
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This document covers the internal architecture of the Logfile Navigator (lnav),
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a terminal-based tool for viewing and analyzing log files.
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## Goals
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The following goals drive the design and implementation of lnav:
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- Don't make the user do something that can be done automatically.
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Example: Automatically detect log formats for files instead of making them
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specify the format for each file.
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- Be performant on low-spec hardware.
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Example: Prefer single-threaded optimizations over trying to parallelize
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- Operations should be "live" and not block the user from continuing to work.
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Example: Searches are run in the background.
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- Provide context-sensitive help.
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Example: When the cursor is over a SQL keyword/function, the help text for
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that is shown above.
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- Show a preview of operations so the user knows what is going to happen.
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Example: When entering a `:filter-out` command, the matched parts of the
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lines are highlighted in red.
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## Overview
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The whole of lnav consists of a
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[log file parser](https://lnav.readthedocs.io/en/latest/formats.html),
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[text UI](https://lnav.readthedocs.io/en/latest/ui.html),
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[integrations with SQLite](https://lnav.readthedocs.io/en/latest/sqlext.html),
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[command-line interface](https://lnav.readthedocs.io/en/latest/cli.html),
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and
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[commands for operating on logs](https://lnav.readthedocs.io/en/latest/commands.html).
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Since the majority of lnav's operations center around logs, the core
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data-structure is the combined log message index. The message index
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is populated when new messages are read from log files. The text UI
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displays a subset of messages from the index. The SQLite virtual-tables
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allow for programmatic access to the messages and lnav's internal state.
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[](https://whimsical.com/lnav-architecture-UM594Qo4G3nt2XWaSZA1mh)
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## File Monitoring
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Each file being monitored by lnav has an associated [`logfile`](src/logfile.hh)
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object, be they plaintext files or files with a recognized format. These
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objects are periodically polled by the main event loop to check if the file
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was deleted, truncated, or new lines added. While reading new lines, if no
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log format has matched yet, each line will be passed through the log format
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regular expressions to try and find a match. Each line that is read is added
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to an index
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#### Why is `mmap()` not used?
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Note that file contents are consumed using `pread(2)`/`read(2)` and not
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`mmap(2)` since `mmap(2)` does not react well to files changing out from
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underneath it. For example, a truncated file would likely result in a
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`SIGBUS`.
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## Log Messages
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As files are being indexed, if a matching format is found, the file is
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"promoted" from a plaintext file to a log file. When the file is promoted,
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it is added to the [logfile_sub_source](src/logfile_sub_source.hh), which
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collates all log messages together into a single index.
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### Timestamp Parsing
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Since all log messages need to have a timestamp, timestamp parsing needs to be
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very efficient. The standard `strptime()` function is quite expensive, so lnav
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includes an optimized custom parser and code-generator in the
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[ptimec](src/ptimec.hh) component. The code-generator is used at compile-time
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to generate parsers for several [common formats](src/time_formats.am).
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## Log Formats
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[log_format](src/log_format.hh) instances are used to parse lines from files
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into `logline` objects. The majority of log formats are
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[external_log_format](src/log_format_ext.hh) objects that are create from
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[JSON format definitions](https://lnav.readthedocs.io/en/latest/formats.html).
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The built-in definitions are located in the [formats](src/formats) directory.
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Log formats that cannot be handled through a simple regular expression are
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implemented in the [log_format_impls.cc](src/log_format_impls.cc) file.
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## User Interface
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The lnav text-user-interface is built on top of
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[ncurses](https://invisible-island.net/ncurses/announce.html).
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However, the higher-level functionality of panels, widgets, and such is not
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used. Instead, the following custom components are built on top of the ncurses
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primitives:
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- [view_curses](src/view_curses.hh) - Provides the basics for text roles, which
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allows for themes to color and style text. The `mvwattrline()` function does
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all the heavy lifting of drawing ["attributed" lines](src/attr_line.hh),
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which are strings that have attributes associated with a given range of
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characters.
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- [listview_curses](src/listview_curses.hh) - Displays a list of items that are
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provided by a source.
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- [textview_curses](src/textview_curses.hh) - Builds on the list view by adding
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support for searching, filtering, bookmarks, etc... The main panel that
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displays the logs/plaintext/help is a textview.
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- [statusview_curses](src/state-extension-functions.cc) - Draws the status bars
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at the top and bottom of the TUI.
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- [vt52_curses](src/vt52_curses.hh) - Adapts vt52 escape codes to the ncurses
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API.
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- [readline_curses](src/readline_curses.hh) - Provides access to the readline
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library. The readline code is executed in a child process since readline
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does not get along with ncurses. The child process and readline is set to
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use a vt52 terminal and the vt52_curses view is uses to translate those
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escape codes to ncurses.
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The following diagram shows the underlying components that make up the TUI:
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[](https://whimsical.com/lnav-tui-MQjXc7Vx23BxQTHrnuNp5F)
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