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syncthing/relnotes/v2.0.md
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Jakob BorgandRoss Smith II 836045ee87 feat: switch logging framework (#10220)
This updates our logging framework from legacy freetext strings using
the `log` package to structured log entries using `log/slog`. I have
updated all INFO or higher level entries, but not yet DEBUG (😓)... So,
at a high level:

There is a slight change in log levels, effectively adding a new warning
level:

- DEBUG is still debug (ideally not for users but developers, though
this is something we need to work on)
- INFO is still info, though I've added more data here, effectively
making Syncthing more verbose by default (more on this below)
- WARNING is a new log level that is different from the _old_ WARNING
(more below)
- ERROR is what was WARNING before -- problems that must be dealt with,
and also bubbled as a popup in the GUI.

A new feature is that the logging level can be set per package to
something other than just debug or info, and hence I feel that we can
add a bit more things into INFO while moving some (in fact, most)
current INFO level warnings into WARNING. For example, I think it's
justified to get a log of synced files in INFO and sync failures in
WARNING. These are things that have historically been tricky to debug
properly, and having more information by default will be useful to many,
while still making it possible get close to told level of inscrutability
by setting the log level to WARNING. I'd like to get to a stage where
DEBUG is never necessary to just figure out what's going on, as opposed
to trying to narrow down a likely bug.

Code wise:

- Our logging object, generally known as `l` in each package, is now a
new adapter object that provides the old API on top of the newer one.
(This should go away once all old log entries are migrated.) This is
only for `l.Debugln` and `l.Debugf`.
- There is a new level tracker that keeps the log level for each
package.
- There is a nested setup of handlers, since the structure mandated by
`log/slog` is slightly convoluted (imho). We do this because we need to
do formatting at a "medium" level internally so we can buffer log lines
in text format but with separate timestamp and log level for the API/GUI
to consume.
- The `debug` API call becomes a `loglevels` API call, which can set the
log level to `DEBUG`, `INFO`, `WARNING` or `ERROR` per package. The GUI
is updated to handle this.
- Our custom `sync` package provided some debugging of mutexes quite
strongly integrated into the old logging framework, only turned on when
`STTRACE` was set to certain values at startup, etc. It's been a long
time since this has been useful; I removed it.
- The `STTRACE` env var remains and can be used the same way as before,
while additionally permitting specific log levels to be specified,
`STTRACE=model:WARN,scanner:DEBUG`.
- There is a new command line option `--log-level=INFO` to set the
default log level.
- The command line options `--log-flags` and `--verbose` go away, but
are currently retained as hidden & ignored options since we set them by
default in some of our startup examples and Syncthing would otherwise
fail to start.

Sample format messages:

```
2009-02-13 23:31:30 INF A basic info line (attr1="val with spaces" attr2=2 attr3="val\"quote" a=a log.pkg=slogutil)
2009-02-13 23:31:30 INF An info line with grouped values (attr1=val1 foo.attr2=2 foo.bar.attr3=3 a=a log.pkg=slogutil)
2009-02-13 23:31:30 INF An info line with grouped values via logger (foo.attr1=val1 foo.attr2=2 a=a log.pkg=slogutil)
2009-02-13 23:31:30 INF An info line with nested grouped values via logger (bar.foo.attr1=val1 bar.foo.attr2=2 a=a log.pkg=slogutil)
2009-02-13 23:31:30 WRN A warning entry (a=a log.pkg=slogutil)
2009-02-13 23:31:30 ERR An error (a=a log.pkg=slogutil)
```

---------

Co-authored-by: Ross Smith II <ross@smithii.com>
2025-08-07 11:19:36 +02:00

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Markdown

## Major changes in 2.0
- Database backend switched from LevelDB to SQLite. There is a migration on
first launch which can be lengthy for larger setups. The new database is
easier to understand and maintain and, hopefully, less buggy.
- The logging format has changed to use structured log entries (a message
plus several key-value pairs). Additionally, we can now control the log
level per package, and a new log level WARNING has been inserted between
INFO and ERROR (which was previously known as WARNING...). The INFO level
has become more verbose, indicating the sync actions taken by Syncthing. A
new command line flag `--log-level` sets the default log level for all
packages, and the `STTRACE` environment variable and GUI has been updated
to set log levels per package. The `--verbose` and `--logflags` command
line options have been removed and will be ignored if given.
- Deleted items are no longer kept forever in the database, instead they are
forgotten after six months. If your use case require deletes to take
effect after more than a six month delay, set the
`--db-delete-retention-interval` command line option or corresponding
environment variable to zero, or a longer time interval of your choosing.
- Modernised command line options parsing. Old single-dash long options are
no longer supported, e.g. `-home` must be given as `--home`. Some options
have been renamed, others have become subcommands. All serve options are
now also accepted as environment variables. See `syncthing --help` and
`syncthing serve --help` for details.
- Rolling hash detection of shifted data is no longer supported as this
effectively never helped. Instead, scanning and syncing is faster and more
efficient without it.
- A "default folder" is no longer created on first startup.
- Multiple connections are now used by default between v2 devices. The new
default value is to use three connections: one for index metadata and two
for data exchange.
- The following platforms unfortunately no longer get prebuilt binaries for
download at syncthing.net and on GitHub, due to complexities related to
cross compilation with SQLite:
- dragonfly/amd64
- illumos/amd64 and solaris/amd64
- linux/ppc64
- netbsd/*
- openbsd/386 and openbsd/arm
- windows/arm
- The handling of conflict resolution involving deleted files has changed. A
delete can now be the winning outcome of conflict resolution, resulting in
the deleted file being moved to a conflict copy.