diff --git a/client/lib/gumnut/data.ts b/client/lib/gumnut/data.ts
new file mode 100644
index 0000000..766394c
--- /dev/null
+++ b/client/lib/gumnut/data.ts
@@ -0,0 +1,174 @@
+import { SimpleCache, todoSignal } from 'thorish';
+import type { FloorReadApi, GumnutFloor } from './floor.ts';
+import { emptySymbol, type GumnutData, InoPosRef } from './types.ts';
+
+const leadingRef = '^';
+
+export type DataTypeInterface = DataMap | DataArray | DataString;
+
+export type DataType =
+  | DataTypeInterface
+  // native types
+  | null
+  | undefined
+  | number
+  | boolean
+  | string
+  | bigint
+  // not-regular-JS types
+  | InoPosRef
+  | typeof emptySymbol;
+
+export function buildDataApi(gf: GumnutFloor): DataApi {
+  return new DataApiImpl(gf);
+}
+
+export interface DataApi {
+  root(): DataMap;
+}
+
+class DataApiImpl implements DataApi {
+  public readonly byId: (id: string) => DataTypeInterface;
+
+  constructor(public readonly gf: GumnutFloor) {
+    this.byId = this.cache.get.bind(this.cache);
+  }
+
+  private readonly cache = new SimpleCache<string, DataTypeInterface>((name) => {
+    if (name.endsWith(':m')) {
+      return new DataMapImpl(this, name);
+    }
+    throw new Error(`TODO: unhandled ${name}`);
+  });
+
+  root(): DataMap {
+    return this.cache.get('root:m') as DataMap;
+  }
+
+  /**
+   * Exists to steal/ref to other "...data" types.
+   */
+  convertToDataType(data: GumnutData): DataType {
+    if (typeof data === 'string') {
+      if (data[0] === leadingRef) {
+        if (data[1] !== leadingRef) {
+          return this.byId(data.slice(1));
+        }
+        data = data.slice(1); // unescape leading ref
+      }
+      return data; // intern string
+    }
+
+    return data;
+  }
+
+  fromDataType(data: DataType): GumnutData {
+    if (typeof data === 'string' && data[0] === leadingRef) {
+      return leadingRef + data; // escape leading ref
+    }
+
+    if (data instanceof DataMap) {
+      return leadingRef + (data as DataMapImpl).name;
+    } else if (data instanceof DataArray) {
+      throw 'TODO DataArray';
+    } else if (data instanceof DataString) {
+      throw 'TODO DataString';
+    }
+
+    return data;
+  }
+}
+
+export abstract class DataMap {
+  abstract length(): number;
+  abstract get(key: DataType): DataType;
+  abstract set(key: DataType, value: DataType): void;
+}
+
+class DataMapImpl extends DataMap {
+  private readonly ino: FloorReadApi;
+  private readonly cache = new Map<GumnutData, GumnutData>();
+
+  constructor(
+    private readonly api: DataApiImpl,
+    public readonly name: string,
+  ) {
+    super();
+    this.ino = this.api.gf.byIno(this.name);
+    this.refreshCache();
+
+    // FIXME: too much refreshing
+    // FIXME: when do we "stop" refreshing?
+    this.api.gf.addListener(todoSignal, (c) => {
+      if (c.has(this.name)) {
+        this.refreshCache();
+      }
+    });
+  }
+
+  private refreshCache() {
+    const d = this.ino.data();
+    this.cache.clear();
+
+    for (let i = 0; i < d.length; i += 2) {
+      const k = d[i];
+      const v = d[i + 1];
+
+      if (k instanceof InoPosRef) {
+        // TODO: some other way to avoid this?
+        throw new Error(`can't use InoPosRef as key`);
+      }
+
+      this.cache.set(k, v);
+    }
+  }
+
+  length(): number {
+    // datamap is stored as k,v pairs
+    return this.ino.length() >> 1;
+  }
+
+  get(key: DataType): DataType {
+    const res = this.cache.get(this.api.fromDataType(key));
+    return this.api.convertToDataType(res);
+  }
+
+  set(key: DataType, value: DataType): void {
+    const outer = this;
+
+    this.api.gf.perform(function* (t) {
+      const k = outer.api.fromDataType(key);
+      const v = outer.api.fromDataType(value);
+
+      const ino = t.byIno(outer.name);
+      for (;;) {
+        const d = ino.data();
+
+        for (let i = 0; i < d.length; i += 2) {
+          if (k === d[i]) {
+            // found it
+            ino.set(i + 2, v);
+            outer.refreshCache(); // TODO: we can be more surgical?
+            return;
+          }
+        }
+
+        // not found, insert
+        ino.dataUpdate(d.length, 0, k, v);
+        outer.refreshCache(); // TODO: we can be more surgical?
+        yield [outer.name];
+      }
+    });
+  }
+}
+
+export abstract class DataArray {
+  abstract length(): number;
+  abstract get(index: number): DataType;
+  abstract set(index: number, value: DataType): void;
+}
+
+export abstract class DataString {
+  abstract length(): number;
+  abstract slice(start?: number, end?: number): string;
+}
diff --git a/tests/integration/data.test.ts b/tests/integration/data.test.ts
new file mode 100644
index 0000000..5e5e2d5
--- /dev/null
+++ b/tests/integration/data.test.ts
@@ -0,0 +1,24 @@
+import { test } from 'node:test';
+import * as assert from 'node:assert';
+import { connectToGumnutLow } from './util.ts';
+import { buildDataApi } from '../../client/lib/gumnut/data.ts';
+import { checkCond } from 'thorish';
+
+test('data', async (t) => {
+  const doc1 = await connectToGumnutLow(t.signal);
+  const doc2 = await connectToGumnutLow(t.signal, { docId: doc1.docId });
+
+  const data1 = buildDataApi(doc1.floor);
+  const data2 = buildDataApi(doc2.floor);
+
+  data1.root().set('abc', 'hello');
+  data1.root().set('abc', 'hello123');
+  // data1.root().set('def', data1.root()); // FIXME: this is failing to apply over itself - seq3
+
+  await checkCond(() => {
+    const root = data2.root();
+
+    assert.strictEqual(root.get('abc'), 'hello123');
+    // assert.strictEqual(root.get('def'), root);
+  });
+});