diff --git a/sync/dataset/dataset.go b/sync/dataset/dataset.go
new file mode 100644
index 0000000..4d18d90
--- /dev/null
+++ b/sync/dataset/dataset.go
@@ -0,0 +1,254 @@
+// Package dataset manages values that are periodically re-fetched from an
+// upstream source and hot-swapped behind atomic pointers. Consumers read via
+// View.Value (lock-free); a single Load drives any number of views off a
+// shared set of Fetchers, so upstreams (one git pull, one tar.gz download)
+// don't get re-fetched per view.
+//
+// Typical lifecycle:
+//
+//	s := dataset.NewSet(repo) // *gitshallow.Repo satisfies Fetcher
+//	inbound  := dataset.Add(s, func(ctx context.Context) (*ipcohort.Cohort, error) { ... })
+//	outbound := dataset.Add(s, func(ctx context.Context) (*ipcohort.Cohort, error) { ... })
+//	if err := s.Load(ctx); err != nil { ... }       // initial populate
+//	go s.Tick(ctx, 47*time.Minute, onError)         // background refresh
+//	current := inbound.Value()                      // lock-free read
+package dataset
+
+import (
+	"context"
+	"errors"
+	"io"
+	"os"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// Fetcher reports whether an upstream source has changed since the last call.
+// Implementations should dedup rapid-fire calls internally (e.g. gitshallow
+// skips redundant pulls within a short window; httpcache uses ETag).
+type Fetcher interface {
+	Fetch() (updated bool, err error)
+}
+
+// FetcherFunc adapts a plain function to Fetcher.
+type FetcherFunc func() (bool, error)
+
+func (f FetcherFunc) Fetch() (bool, error) { return f() }
+
+// NopFetcher always reports no update. Use for sets whose source never
+// changes (test fixtures, embedded data).
+type NopFetcher struct{}
+
+func (NopFetcher) Fetch() (bool, error) { return false, nil }
+
+// PollFiles returns a Fetcher that stat's the given paths and reports
+// "updated" whenever any file's size or modtime has changed since the last
+// call. The first call always reports updated=true.
+//
+// Use for Sets whose source is local files that may be edited out of band
+// (e.g. a user-provided --inbound list) — pair with Set.Tick to pick up
+// changes automatically.
+func PollFiles(paths ...string) Fetcher {
+	return &filePoller{paths: paths, stats: make(map[string]fileStat, len(paths))}
+}
+
+type fileStat struct {
+	size    int64
+	modTime time.Time
+}
+
+type filePoller struct {
+	mu    sync.Mutex
+	paths []string
+	stats map[string]fileStat
+}
+
+func (p *filePoller) Fetch() (bool, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	changed := false
+	for _, path := range p.paths {
+		info, err := os.Stat(path)
+		if err != nil {
+			return false, err
+		}
+		cur := fileStat{size: info.Size(), modTime: info.ModTime()}
+		if prev, ok := p.stats[path]; !ok || prev != cur {
+			changed = true
+			p.stats[path] = cur
+		}
+	}
+	return changed, nil
+}
+
+// Set ties one or more Fetchers to one or more views. A Load call fetches
+// each source and, on the first call or when any source reports a change,
+// reloads every view and atomically swaps its current value. Use multiple
+// fetchers when a single logical dataset is spread across several archives
+// (e.g. GeoLite2 City + ASN); a single fetcher is the common case (one git
+// repo, one tar.gz).
+type Set struct {
+	fetchers []Fetcher
+	views    []reloader
+	loaded   atomic.Bool
+}
+
+// reloader is a type-erased handle to a View's reload function.
+type reloader interface {
+	reload(ctx context.Context) error
+}
+
+// NewSet creates a Set backed by fetchers. All fetchers are called on every
+// Load; the set reloads its views whenever any one of them reports a change.
+func NewSet(fetchers ...Fetcher) *Set {
+	return &Set{fetchers: fetchers}
+}
+
+// Loaded reports whether Load has completed successfully at least once.
+func (s *Set) Loaded() bool {
+	return s.loaded.Load()
+}
+
+// Load fetches upstream and, on the first call or whenever any fetcher
+// reports a change, reloads every view and atomically installs the new values.
+func (s *Set) Load(ctx context.Context) error {
+	updated := false
+	for _, f := range s.fetchers {
+		u, err := f.Fetch()
+		if err != nil {
+			return err
+		}
+		if u {
+			updated = true
+		}
+	}
+	if s.loaded.Load() && !updated {
+		return nil
+	}
+	for _, v := range s.views {
+		if err := ctx.Err(); err != nil {
+			return err
+		}
+		if err := v.reload(ctx); err != nil {
+			return err
+		}
+	}
+	s.loaded.Store(true)
+	return nil
+}
+
+// Close closes every view's currently-held value. Call on shutdown to
+// release any OS resources held by the final snapshots (file handles,
+// network connections). Safe to call on a Set that hasn't been loaded
+// or whose views hold pure in-memory values — non-Closer values are
+// skipped. Returns joined errors from any Close failures.
+func (s *Set) Close() error {
+	var errs []error
+	for _, v := range s.views {
+		if c, ok := v.(io.Closer); ok {
+			if err := c.Close(); err != nil {
+				errs = append(errs, err)
+			}
+		}
+	}
+	return errors.Join(errs...)
+}
+
+// Tick calls Load every interval until ctx is done. Load errors are passed to
+// onError (if non-nil) and do not stop the loop; callers choose whether to log,
+// count, page, or ignore. Run in a goroutine: `go s.Tick(ctx, d, onError)`.
+func (s *Set) Tick(ctx context.Context, interval time.Duration, onError func(error)) {
+	t := time.NewTicker(interval)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			if err := s.Load(ctx); err != nil && onError != nil {
+				onError(err)
+			}
+		}
+	}
+}
+
+// View is a read-only handle to one dataset inside a Set.
+type View[T any] struct {
+	loader   func(ctx context.Context) (*T, error)
+	ptr      atomic.Pointer[T]
+	loadedAt atomic.Pointer[time.Time] // nil until first successful reload
+}
+
+// Value returns the current snapshot. Nil before the Set is first loaded
+// unless the view was registered via AddInitial.
+func (v *View[T]) Value() *T {
+	return v.ptr.Load()
+}
+
+// LoadedAt returns the time of the most recent successful reload, or the
+// zero time if the view has never loaded.
+func (v *View[T]) LoadedAt() time.Time {
+	if t := v.loadedAt.Load(); t != nil {
+		return *t
+	}
+	return time.Time{}
+}
+
+// Close clears the view and closes the currently-held value if it
+// implements io.Closer. Idempotent — subsequent calls are no-ops.
+func (v *View[T]) Close() error {
+	prev := v.ptr.Swap(nil)
+	if prev == nil {
+		return nil
+	}
+	if closer, ok := any(prev).(io.Closer); ok {
+		return closer.Close()
+	}
+	return nil
+}
+
+func (v *View[T]) reload(ctx context.Context) error {
+	t, err := v.loader(ctx)
+	if err != nil {
+		return err
+	}
+	prev := v.ptr.Swap(t)
+	// Close the replaced value if it holds OS resources (open file handles,
+	// network connections). Geoip readers and similar wrappers implement
+	// io.Closer; cohort and other pure-in-memory values don't — the type
+	// assertion filters to only the ones that need it. The `prev != nil`
+	// guard is required because any(typedNilPtr) is a non-nil interface
+	// wrapping a nil pointer — the type assertion succeeds and we'd
+	// call Close on a nil receiver.
+	if prev != nil {
+		if closer, ok := any(prev).(io.Closer); ok {
+			_ = closer.Close()
+		}
+	}
+	now := time.Now()
+	v.loadedAt.Store(&now)
+	return nil
+}
+
+// Add registers a new view in s and returns it. Call after NewSet and before
+// the first Load. View.Value() returns nil until Set.Load succeeds.
+// The loader receives the ctx passed to Set.Load, so long-running parses
+// should honor ctx.Err() to support graceful shutdown.
+func Add[T any](s *Set, loader func(ctx context.Context) (*T, error)) *View[T] {
+	v := &View[T]{loader: loader}
+	s.views = append(s.views, v)
+	return v
+}
+
+// AddInitial is like Add but pre-populates the view with initial, so
+// View.Value() returns a usable (possibly empty) value before the first
+// Load completes. Use when the initial state is benign (e.g. an empty
+// cohort matches nothing) and you want to start serving before the
+// first load finishes.
+func AddInitial[T any](s *Set, initial *T, loader func(ctx context.Context) (*T, error)) *View[T] {
+	v := &View[T]{loader: loader}
+	v.ptr.Store(initial)
+	s.views = append(s.views, v)
+	return v
+}
diff --git a/sync/dataset/dataset_test.go b/sync/dataset/dataset_test.go
new file mode 100644
index 0000000..44ba257
--- /dev/null
+++ b/sync/dataset/dataset_test.go
@@ -0,0 +1,189 @@
+package dataset_test
+
+import (
+	"context"
+	"errors"
+	"os"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/therootcompany/golib/sync/dataset"
+)
+
+type countFetcher struct {
+	calls   atomic.Int32
+	updated bool
+	err     error
+}
+
+func (f *countFetcher) Fetch() (bool, error) {
+	f.calls.Add(1)
+	return f.updated, f.err
+}
+
+func TestSet_LoadPopulatesAllViews(t *testing.T) {
+	f := &countFetcher{}
+	g := dataset.NewSet(f)
+
+	var aCalls, bCalls int
+	a := dataset.Add(g, func(_ context.Context) (*string, error) {
+		aCalls++
+		v := "a"
+		return &v, nil
+	})
+	b := dataset.Add(g, func(_ context.Context) (*int, error) {
+		bCalls++
+		v := 42
+		return &v, nil
+	})
+
+	if err := g.Load(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	if f.calls.Load() != 1 {
+		t.Errorf("Fetch called %d times, want 1", f.calls.Load())
+	}
+	if aCalls != 1 || bCalls != 1 {
+		t.Errorf("loaders called (%d,%d), want (1,1)", aCalls, bCalls)
+	}
+	if got := a.Value(); got == nil || *got != "a" {
+		t.Errorf("a.Value() = %v", got)
+	}
+	if got := b.Value(); got == nil || *got != 42 {
+		t.Errorf("b.Value() = %v", got)
+	}
+}
+
+func TestSet_SecondLoadSkipsUnchanged(t *testing.T) {
+	f := &countFetcher{updated: false}
+	g := dataset.NewSet(f)
+	calls := 0
+	dataset.Add(g, func(_ context.Context) (*string, error) {
+		calls++
+		v := "x"
+		return &v, nil
+	})
+	if err := g.Load(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	if calls != 1 {
+		t.Fatalf("initial load ran loader %d times, want 1", calls)
+	}
+	if err := g.Load(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	if calls != 1 {
+		t.Errorf("second load ran loader %d times, want 1 (no upstream change)", calls)
+	}
+}
+
+func TestSet_LoadOnUpdateSwaps(t *testing.T) {
+	f := &countFetcher{updated: true}
+	g := dataset.NewSet(f)
+	n := 0
+	v := dataset.Add(g, func(_ context.Context) (*int, error) {
+		n++
+		return &n, nil
+	})
+	if err := g.Load(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	if err := g.Load(t.Context()); err != nil {
+		t.Fatal(err)
+	}
+	if got := v.Value(); got == nil || *got != 2 {
+		t.Errorf("v.Value() = %v, want 2", got)
+	}
+}
+
+func TestSet_ValueBeforeLoad(t *testing.T) {
+	g := dataset.NewSet(dataset.NopFetcher{})
+	v := dataset.Add(g, func(_ context.Context) (*string, error) {
+		s := "x"
+		return &s, nil
+	})
+	if v.Value() != nil {
+		t.Error("Value() before Load should be nil")
+	}
+}
+
+func TestSet_FetchError(t *testing.T) {
+	f := &countFetcher{err: errors.New("offline")}
+	g := dataset.NewSet(f)
+	dataset.Add(g, func(_ context.Context) (*string, error) {
+		s := "x"
+		return &s, nil
+	})
+	if err := g.Load(t.Context()); err == nil {
+		t.Error("expected fetch error")
+	}
+}
+
+func TestSet_LoaderError(t *testing.T) {
+	g := dataset.NewSet(dataset.NopFetcher{})
+	dataset.Add(g, func(_ context.Context) (*string, error) {
+		return nil, errors.New("parse fail")
+	})
+	if err := g.Load(t.Context()); err == nil {
+		t.Error("expected loader error")
+	}
+}
+
+func TestPollFiles(t *testing.T) {
+	dir := t.TempDir()
+	a := dir + "/a.txt"
+	b := dir + "/b.txt"
+	if err := os.WriteFile(a, []byte("1"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(b, []byte("2"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	p := dataset.PollFiles(a, b)
+
+	if u, err := p.Fetch(); err != nil || !u {
+		t.Fatalf("first Fetch: updated=%v err=%v, want true/nil", u, err)
+	}
+	if u, err := p.Fetch(); err != nil || u {
+		t.Fatalf("unchanged Fetch: updated=%v err=%v, want false/nil", u, err)
+	}
+
+	// Bump mtime + change contents on b.
+	future := time.Now().Add(2 * time.Second)
+	if err := os.WriteFile(b, []byte("22"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chtimes(b, future, future); err != nil {
+		t.Fatal(err)
+	}
+	if u, err := p.Fetch(); err != nil || !u {
+		t.Errorf("after change: updated=%v err=%v, want true/nil", u, err)
+	}
+	if u, err := p.Fetch(); err != nil || u {
+		t.Errorf("steady Fetch: updated=%v err=%v, want false/nil", u, err)
+	}
+}
+
+func TestPollFiles_MissingFile(t *testing.T) {
+	p := dataset.PollFiles(t.TempDir() + "/nope.txt")
+	if _, err := p.Fetch(); err == nil {
+		t.Error("expected error for missing file")
+	}
+}
+
+func TestFetcherFunc(t *testing.T) {
+	var called bool
+	f := dataset.FetcherFunc(func() (bool, error) {
+		called = true
+		return true, nil
+	})
+	updated, err := f.Fetch()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !called || !updated {
+		t.Errorf("FetcherFunc: called=%v updated=%v", called, updated)
+	}
+}
diff --git a/sync/dataset/go.mod b/sync/dataset/go.mod
new file mode 100644
index 0000000..b22cfc7
--- /dev/null
+++ b/sync/dataset/go.mod
@@ -0,0 +1,3 @@
+module github.com/therootcompany/golib/sync/dataset
+
+go 1.26.0