---

title: Extension Points

weight: 40

Extension Points

libfossil exposes three surfaces for consumers that want to instrument, extend, or replace pieces of the stack:

1. Observer interfaces for sync and checkout lifecycle events (SyncObserver, CheckoutObserver).
2. SQLite driver registration (db.Register) — pick the SQLite backend at link time.
3. The db.DB abstraction — an opaque wrapper around database/sql with Fossil-specific helpers. Treat it as internal; it is less stable than the observer and driver surfaces.

Observer interfaces

Observers are the supported way to attach telemetry, logging, or metrics to a Repo without forking or wrapping the public API. Both interfaces are defined in the root github.com/danmestas/libfossil package and are context-free by design so that they can be reused across synchronous and asynchronous callers.

SyncObserver

Source: observer.go.

type SyncObserver interface {
    Started(info SessionStart)
    RoundStarted(round int)
    RoundCompleted(round int, stats RoundStats)
    Completed(info SessionEnd)
    Error(err error)
    HandleStarted(info HandleStart)
    HandleCompleted(info HandleEnd)
    TableSyncStarted(info TableSyncStart)
    TableSyncCompleted(info TableSyncEnd)
}

Method	Fires when
Started	A client-side sync or clone session begins, before any rounds run.
RoundStarted	A new sync round starts (there are typically multiple rounds per session).
RoundCompleted	A sync round finishes — RoundStats carries per-round totals.
Completed	The sync/clone session ends successfully.
Error	The sync machinery reports a per-error event (protocol or transport).
HandleStarted	Server-side: an inbound sync/clone HTTP request is accepted.
HandleCompleted	Server-side: an inbound sync/clone request finishes.
TableSyncStarted	An extension table sync (for example peer_registry) is about to run.
TableSyncCompleted	An extension table sync finishes.

Libfossil ships NopSyncObserver() and StdoutSyncObserver() as drop-in implementations.

CheckoutObserver

Source: observer.go.

type CheckoutObserver interface {
    ExtractStarted(info ExtractStart)
    ExtractFileCompleted(name string, change UpdateChange)
    ExtractCompleted(info ExtractEnd)
    ScanStarted(dir string)
    ScanCompleted(info ScanEnd)
    CommitStarted(info CommitStart)
    CommitCompleted(info CommitEnd)
    Error(err error)
}

Method	Fires when
ExtractStarted	An extract or update begins writing files from a checkin into the working tree.
ExtractFileCompleted	One file has been written — change is ChangeAdded, ChangeModified, or ChangeDeleted.
ExtractCompleted	The extract/update finishes.
ScanStarted	A working-tree scan (status/change detection) begins for dir.
ScanCompleted	The working-tree scan finishes.
CommitStarted	A checkin commit begins after staging.
CommitCompleted	The commit lands — info.UUID and info.RID identify the new checkin.
Error	A per-error event fires from the checkout pipeline.

Minimal logging observer

package main

import (
    "log"

    libfossil "github.com/danmestas/libfossil"
)

type logSync struct{ libfossil.SyncObserver } // embed nop for defaults

func (logSync) Started(info libfossil.SessionStart) {
    log.Printf("sync start project=%s push=%v pull=%v", info.ProjectCode, info.Push, info.Pull)
}

func (logSync) RoundCompleted(round int, s libfossil.RoundStats) {
    log.Printf("round %d sent=%d recv=%d", round, s.FilesSent, s.FilesRecvd)
}

func newLogSyncObserver() libfossil.SyncObserver {
    return logSync{SyncObserver: libfossil.NopSyncObserver()}
}

The embedded NopSyncObserver() provides no-op implementations of the methods you do not override, so you only need to define the events you care about. The same pattern applies to CheckoutObserver via NopCheckoutObserver().

For a full real-world observer that emits spans and counters, see observer/otel/otel.go. It implements both interfaces and is the reference you should copy from when wiring up your own telemetry backend.

Adding an OpenTelemetry (or other) observer

observer/otel lives in its own Go module (separate go.mod) so that the OpenTelemetry SDK dependency does not leak into consumers that do not want it. To use it, import the subpackage explicitly and hand the observer to each Repo operation that accepts one (SyncOpts.Observer, CloneOpts.Observer, HandleOpts.Observer, CheckoutCreateOpts.Observer, CheckoutOpenOpts.Observer).

import (
    libfossil "github.com/danmestas/libfossil"
    "github.com/danmestas/libfossil/observer/otel"
)

obs := otel.NewSyncObserver() // implements libfossil.SyncObserver
_, err := repo.Sync(ctx, transport, libfossil.SyncOpts{
    Pull:     true,
    Observer: obs,
})

otel.NewCheckoutObserver() returns the matching CheckoutObserver. Any third-party telemetry library (Prometheus, Datadog, zap, etc.) should be vendored the same way: a sibling module that imports libfossil and implements the observer interfaces.

SQLite driver interface

Source: db/register.go, db/db.go, db/doc.go.

libfossil never imports a SQLite driver directly. Instead, the db package exposes a single registration hook that a driver package calls from its init():

// db/register.go
type DriverConfig struct {
    Name     string
    BuildDSN func(path string, pragmas map[string]string) string
}

func Register(cfg DriverConfig)

Each db.Open(path) / db.OpenWith(path, cfg) call routes through the currently registered driver. Exactly one driver may be registered per process — Register panics if called twice, if Name is empty, or if BuildDSN is nil. Likewise, calling Open with no driver registered panics with a message pointing at the driver import paths.

The driver contract is intentionally small:

• Name: the name passed to sql.Open. The driver package is responsible for registering this name with database/sql (typically via a blank import of the underlying SQLite library).
• BuildDSN(path, pragmas): return a DSN string that opens path and applies the given pragmas. libfossil calls db.DefaultPragmas() and merges any caller overrides before handing them to BuildDSN.

Built-in drivers

Driver	Import	When to use
modernc	_ "github.com/danmestas/libfossil/db/driver/modernc"	Default. Pure Go, no cgo, works on every GOOS/GOARCH libfossil supports.
ncruces	_ "github.com/danmestas/libfossil/db/driver/ncruces"	Uses github.com/ncruces/go-sqlite3 (WASM-based SQLite). Pick this for WASM builds or environments where you need the ncruces feature set.

Registering a custom driver

Use this pattern when you want to wrap an instrumented SQLite build, redirect to a network-attached SQLite, or inject additional pragmas. Model the package after db/driver/modernc/modernc.go:

package mydriver

import (
    "fmt"
    "strings"

    "github.com/danmestas/libfossil/db"
    _ "example.com/my/sqlite" // registers "mysqlite" with database/sql
)

func init() {
    db.Register(db.DriverConfig{
        Name:     "mysqlite",
        BuildDSN: buildDSN,
    })
}

func buildDSN(path string, pragmas map[string]string) string {
    var parts []string
    for k, v := range pragmas {
        parts = append(parts, fmt.Sprintf("_pragma=%s(%s)", k, v))
    }
    if len(parts) == 0 {
        return path
    }
    return fmt.Sprintf("file:%s?%s", path, strings.Join(parts, "&"))
}

Consumers then blank-import your package (_ "example.com/mydriver") exactly like they would for modernc or ncruces.

The DB abstraction

github.com/danmestas/libfossil/db wraps *sql.DB with Fossil-aware helpers: DSN construction, default-pragma setup, WAL/nolock handling for WASM, transaction scoping via DB.WithTx, and the Querier interface (satisfied by both DB and Tx) so repository code can be written transaction-agnostic. The exported surface is stable for reading — SqlDB(), Path(), Driver(), Exec/Query* — but the struct itself is treated as internal: the public Repo API is the intended extension point. Prefer writing a custom driver (above) over implementing DB yourself. See db/db.go, db/config.go, and db/scan.go for the source of truth.