diff --git a/src/mocks/TimelineMock.ts b/src/mocks/TimelineMock.ts
new file mode 100644
index 00000000..4932e61e
--- /dev/null
+++ b/src/mocks/TimelineMock.ts
@@ -0,0 +1,255 @@
+import {createEvent, withTextBody, withSender} from "./event.js";
+import {TimelineEvent} from "../matrix/storage/types";
+
+export const TIMELINE_START_TOKEN = "timeline_start";
+
+export class TimelineMock {
+    private _counter: number;
+    private _dagOrder: TimelineEvent[];
+    private _syncOrder: TimelineEvent[];
+    private _defaultSender: string;
+
+    constructor(defaultSender: string) {
+        this._counter = 0;
+        this._dagOrder = [];
+        this._syncOrder = [];
+        this._defaultSender = defaultSender;
+    }
+
+    _defaultEvent(id: string): TimelineEvent {
+        return withTextBody(`This is event ${id}`, withSender(this._defaultSender, createEvent("m.room.message", id)));
+    }
+
+    _createEvent(func?: (eventId: string) => TimelineEvent): TimelineEvent {
+        const id = `$event${this._counter++}`;
+        return func ? func(id) : this._defaultEvent(id);
+    }
+
+    _createEvents(n: number, func?: (eventId: string) => TimelineEvent) {
+        const events: TimelineEvent[] = [];
+        for (let i = 0; i < n; i++) {
+            events.push(this._createEvent(func));
+        }
+        return events;
+    }
+
+    insertAfter(token: string, n: number, func?: (eventId: string) => TimelineEvent) {
+        const events = this._createEvents(n, func);
+        const index = this._findIndex(token, "f", this._dagOrder);
+        this._dagOrder.splice(index, 0, ...events);
+        this._syncOrder.push(...events);
+        return events[events.length - 1]?.event_id;
+    }
+
+    append(n: number, func?: (eventId: string) => TimelineEvent) {
+        const events = this._createEvents(n, func);
+        this._dagOrder.push(...events);
+        this._syncOrder.push(...events);
+        return events[events.length - 1]?.event_id;
+    }
+
+    _getStep(direction: "f" | "b") : 1 | -1 { 
+        return direction === "f" ? 1 : -1;
+    }
+
+    _findIndex(token: string, direction: "f" | "b", eventOrdering: TimelineEvent[]): number {
+        const step = this._getStep(direction);
+        if (token === TIMELINE_START_TOKEN) {
+            const firstSyncEvent = this._syncOrder[0];
+            if (!firstSyncEvent) {
+                // We have no events at all. Wherever you start looking,
+                // you'll stop looking right away. Zero works as well as anything else.
+                return 0;
+            }
+            const orderIndex = eventOrdering.findIndex(e => e.event_id === firstSyncEvent.event_id);
+            return orderIndex;
+        } 
+        // All other tokens are (non-inclusive) event indices
+        const index = eventOrdering.findIndex(e => e.event_id === token);
+        if (index === -1) {
+            // We didn't find this event token at all. What are we
+            // even looking at?
+            throw new Error("Invalid token passed to TimelineMock");
+        }
+        return index + step;
+    }
+
+    messages(begin: string, end: string | undefined, direction: "f" | "b", limit: number = 10) {
+        const step = this._getStep(direction);
+        let index = this._findIndex(begin, direction, this._dagOrder);
+        const chunk: TimelineEvent[] = [];
+        for (; limit > 0 && index >= 0 && index < this._dagOrder.length; index += step, limit--) {
+            if (this._dagOrder[index].event_id === end) {
+                break;
+            }
+            chunk.push(this._dagOrder[index]);
+        }
+        return {
+            start: begin,
+            end: chunk[chunk.length - 1]?.event_id || begin,
+            chunk,
+            state: []
+        };
+    }
+
+    context(eventId: string, limit: number = 10) {
+        if (limit <= 0) {
+            throw new Error("Cannot fetch zero or less events!");
+        }
+        let eventIndex = this._dagOrder.findIndex(e => e.event_id === eventId);
+        if (eventIndex === -1) {
+            throw new Error("Fetching context for unknown event");
+        }
+        const event = this._dagOrder[eventIndex];
+        limit -= 1;
+        let offset = 1;
+        const eventsBefore: TimelineEvent[] = [];
+        const eventsAfter: TimelineEvent[] = [];
+        while (limit !== 0 && (eventIndex - offset >= 0 || eventIndex + offset < this._dagOrder.length)) {
+            if (eventIndex - offset >= 0) {
+                eventsBefore.push(this._dagOrder[eventIndex - offset]);
+                limit--;
+            }
+            if (limit !== 0 && eventIndex + offset < this._dagOrder.length) {
+                eventsAfter.push(this._dagOrder[eventIndex + offset]);
+                limit--;
+            }
+            offset++;
+        }
+        return {
+            start: eventsBefore[eventsBefore.length - 1]?.event_id || eventId,
+            end: eventsAfter[eventsAfter.length - 1]?.event_id || eventId,
+            event,
+            events_before: eventsBefore,
+            events_after: eventsAfter,
+            state: []
+        };
+    }
+
+    sync(since?: string, limit: number = 10) {
+        const startAt = since ? this._findIndex(since, "f", this._syncOrder) : 0;
+        const index = Math.max(this._syncOrder.length - limit, startAt);
+        const limited = this._syncOrder.length - startAt > limit;
+        const events: TimelineEvent[] = [];
+        for(let i = index; i < this._syncOrder.length; i++) {
+            events.push(this._syncOrder[i]);
+        }
+        return {
+            next_batch: events[events.length - 1]?.event_id || since || TIMELINE_START_TOKEN,
+            prev_batch: events[0]?.event_id || since || TIMELINE_START_TOKEN,
+            events,
+            limited
+        }
+    }
+}
+
+export function tests() {
+    const SENDER = "@alice:hs.tdl";
+
+    function eventId(i: number): string {
+        return `$event${i}`;
+    }
+
+    function eventIds(from: number, to: number): string[] {
+        return [...Array(to-from).keys()].map(i => eventId(i + from));
+    }
+
+    return {
+        "Append events are returned via sync": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(10);
+            const syncResponse = timeline.sync();
+            const events = syncResponse.events.map(e => e.event_id);
+            assert.deepEqual(events, eventIds(0, 10));
+            assert.equal(syncResponse.limited, false);
+        },
+        "Limiting a sync properly limits the returned events": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(20);
+            const syncResponse = timeline.sync(undefined, 10);
+            const events = syncResponse.events.map(e => e.event_id);
+            assert.deepEqual(events, eventIds(10, 20));
+            assert.equal(syncResponse.limited, true);
+        },
+        "The context endpoint returns messages in DAG order around an event": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const context = timeline.context(eventId(15));
+            assert.equal(context.event.event_id, eventId(15));
+            assert.deepEqual(context.events_before.map(e => e.event_id).reverse(), eventIds(10, 15));
+            assert.deepEqual(context.events_after.map(e => e.event_id), eventIds(16, 20));
+        },
+        "The context endpoint returns the proper number of messages": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            for (const i of new Array(29).keys()) {
+                const middleFetch = timeline.context(eventId(15), i + 1);
+                assert.equal(middleFetch.events_before.length + middleFetch.events_after.length + 1, i + 1);
+                const startFetch = timeline.context(eventId(1), i + 1);
+                assert.equal(startFetch.events_before.length + startFetch.events_after.length + 1, i + 1);
+                const endFetch = timeline.context(eventId(28), i + 1);
+                assert.equal(endFetch.events_before.length + endFetch.events_after.length + 1, i + 1);
+            }
+        },
+        "The previous batch from a sync returns the previous events": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(20);
+            const sync = timeline.sync(undefined, 10);
+            const messages = timeline.messages(sync.prev_batch, undefined, "b");
+            const events = messages.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events, eventIds(0, 10));
+        },
+        "Two consecutive message fetches are continuous if no new events are inserted": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const sync = timeline.sync(undefined, 10);
+            const messages1 = timeline.messages(sync.prev_batch, undefined, "b");
+            const events1 = messages1.chunk.map(e => e.event_id).reverse();
+            const messages2 = timeline.messages(messages1.end, undefined, "b");
+            const events2 = messages2.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events1, eventIds(10, 20));
+            assert.deepEqual(events2, eventIds(0, 10));
+        },
+        "Two consecutive message fetches detect newly inserted event": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const messages1 = timeline.messages(eventId(20), undefined, "b", 10);
+            const events1 = messages1.chunk.map(e => e.event_id).reverse();
+            timeline.insertAfter(eventId(9), 1);
+            const messages2 = timeline.messages(eventId(10), undefined, "b", 10);
+            const events2 = messages2.chunk.map(e => e.event_id).reverse();
+            assert.deepEqual(events1, eventIds(10, 20));
+            const expectedEvents2 = eventIds(1, 10);
+            expectedEvents2.push(eventId(30));
+            assert.deepEqual(events2, expectedEvents2);
+        },
+        "A sync that receives no events has the same next batch as it started with": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(10);
+            const sync1 = timeline.sync();
+            const sync2 = timeline.sync(sync1.next_batch);
+            assert.equal(sync1.next_batch, sync2.next_batch);
+        }, 
+        "An event inserted in the midle still shows up in a sync": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const sync1 = timeline.sync(undefined, 10);
+            const sync2 = timeline.sync(sync1.next_batch, 10)
+            assert.deepEqual(sync2.events, []);
+            assert.equal(sync2.limited, false);
+            timeline.insertAfter(TIMELINE_START_TOKEN, 1);
+            const sync3 = timeline.sync(sync2.next_batch, 10)
+            const events = sync3.events.map(e => e.event_id);
+            assert.deepEqual(events, eventIds(30, 31));
+        },
+        "An event inserted in the midle does not show up in a message fetch": assert => {
+            const timeline = new TimelineMock(SENDER);
+            timeline.append(30);
+            const sync1 = timeline.sync(undefined, 10);
+            const messages1 = timeline.messages(sync1.prev_batch, undefined, "f", 10);
+            timeline.insertAfter(TIMELINE_START_TOKEN, 1);
+            const messages2 = timeline.messages(sync1.prev_batch, undefined, "f", 10);
+            assert.deepEqual(messages1.chunk, messages2.chunk);
+        },
+    }
+}