diff --git a/tensorboard/data/server/BUILD b/tensorboard/data/server/BUILD
index a72c6da28a..0eb5e6e8b1 100644
--- a/tensorboard/data/server/BUILD
+++ b/tensorboard/data/server/BUILD
@@ -27,6 +27,7 @@ rust_library(
     name = "rustboard_core",
     srcs = [
         "lib.rs",
+        "commit.rs",
         "data_compat.rs",
         "event_file.rs",
         "masked_crc.rs",
diff --git a/tensorboard/data/server/commit.rs b/tensorboard/data/server/commit.rs
new file mode 100644
index 0000000000..9b0104d21b
--- /dev/null
+++ b/tensorboard/data/server/commit.rs
@@ -0,0 +1,138 @@
+/* Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+==============================================================================*/
+
+//! Shared state for sampled data available to readers.
+
+use std::collections::HashMap;
+use std::sync::RwLock;
+
+use crate::proto::tensorboard as pb;
+use crate::reservoir::Basin;
+use crate::types::{Run, Step, Tag, WallTime};
+
+/// Current state of in-memory sampled data.
+///
+/// A commit is an internally mutable structure. All readers and writers should keep a shared
+/// reference to a single commit. When writers need to update it, they grab an exclusive lock to
+/// the contents.
+///
+/// Deadlock safety: any thread should obtain the outer lock (around the hash map) before an inner
+/// lock (around the run data), and should obtain at most one `RunData` lock at once.
+#[derive(Debug, Default)]
+pub struct Commit {
+    pub runs: RwLock<HashMap<Run, RwLock<RunData>>>,
+}
+
+impl Commit {
+    /// Creates a new, empty commit.
+    pub fn new() -> Self {
+        Commit::default()
+    }
+}
+
+/// Data for a single run.
+///
+/// This contains all data and metadata for a run. For now, that data includes only scalars;
+/// tensors and blob sequences will come soon.
+#[derive(Debug, Default)]
+pub struct RunData {
+    /// The time of the first event recorded for this run.
+    ///
+    /// Used to define an ordering on runs that is stable as new runs are added, so that existing
+    /// runs aren't constantly changing color.
+    pub start_time: Option<WallTime>,
+
+    /// Scalar time series for this run.
+    pub scalars: TagStore<ScalarValue>,
+}
+
+pub type TagStore<V> = HashMap<Tag, TimeSeries<V>>;
+
+#[derive(Debug)]
+pub struct TimeSeries<V> {
+    /// Summary metadata for this time series.
+    pub metadata: Box<pb::SummaryMetadata>,
+
+    /// Reservoir basin for data points in this time series.
+    ///
+    /// See [`TimeSeries::valid_values`] for a client-friendly view that omits `DataLoss` points
+    /// and transposes `Step`s into the tuple.
+    pub basin: Basin<(WallTime, Result<V, DataLoss>)>,
+}
+
+impl<V> TimeSeries<V> {
+    /// Creates a new time series from the given summary metadata.
+    pub fn new(metadata: Box<pb::SummaryMetadata>) -> Self {
+        TimeSeries {
+            metadata,
+            basin: Basin::new(),
+        }
+    }
+
+    /// Gets an iterator over `self.values` that omits `DataLoss` points.
+    pub fn valid_values(&self) -> impl Iterator<Item = (Step, WallTime, &V)> {
+        self.basin
+            .as_slice()
+            .iter()
+            .filter_map(|(step, (wall_time, v))| Some((*step, *wall_time, v.as_ref().ok()?)))
+    }
+}
+
+/// A value in a time series is corrupt and should be ignored.
+///
+/// This is used when a point looks superficially reasonable when it's offered to the reservoir,
+/// but at commit time we realize that it can't be enriched into a valid point. This might happen
+/// if, for instance, a point in a scalar time series has a tensor value containing a string. We
+/// don't care too much about what happens to these invalid values. Keeping them in the commit as
+/// `DataLoss` tombstones is convenient, and [`TimeSeries::valid_values`] offers a view that
+/// abstracts over this detail by only showing valid data.
+#[derive(Debug)]
+pub struct DataLoss;
+
+/// The value of a scalar time series at a single point.
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub struct ScalarValue(pub f64);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_valid_values() {
+        let mut ts = TimeSeries::<&str>::new(Box::new(pb::SummaryMetadata::default()));
+
+        let mut rsv = crate::reservoir::StageReservoir::new(10);
+        let wall_time = WallTime::new(0.0).unwrap(); // don't really care
+        rsv.offer(Step(0), "zero");
+        rsv.offer(Step(1), "one");
+        rsv.offer(Step(2), "two");
+        rsv.offer(Step(3), "three");
+        rsv.offer(Step(5), "five");
+        rsv.commit_map(&mut ts.basin, |s| {
+            (wall_time, if s == "three" { Err(DataLoss) } else { Ok(s) })
+        });
+
+        assert_eq!(
+            ts.valid_values().collect::<Vec<_>>(),
+            vec![
+                (Step(0), wall_time, &"zero"),
+                (Step(1), wall_time, &"one"),
+                (Step(2), wall_time, &"two"),
+                // missing: Step(3)
+                (Step(5), wall_time, &"five")
+            ]
+        );
+    }
+}
diff --git a/tensorboard/data/server/lib.rs b/tensorboard/data/server/lib.rs
index 5511250820..8d3f144423 100644
--- a/tensorboard/data/server/lib.rs
+++ b/tensorboard/data/server/lib.rs
@@ -15,6 +15,7 @@ limitations under the License.
 
 //! Core functionality for TensorBoard data loading.
 
+pub mod commit;
 pub mod data_compat;
 pub mod event_file;
 pub mod masked_crc;
diff --git a/tensorboard/data/server/types.rs b/tensorboard/data/server/types.rs
index 59a764504a..bb50972f3b 100644
--- a/tensorboard/data/server/types.rs
+++ b/tensorboard/data/server/types.rs
@@ -70,6 +70,19 @@ impl Borrow<str> for Tag {
     }
 }
 
+/// The name of a TensorBoard run.
+///
+/// Run names are derived from directory names relative to the logdir, but are lossily converted to
+/// valid Unicode strings.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
+pub struct Run(pub String);
+
+impl Borrow<str> for Run {
+    fn borrow(&self) -> &str {
+        &self.0
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -85,6 +98,17 @@ mod tests {
         assert_eq!(m.get("xent"), None);
     }
 
+    #[test]
+    fn test_run_hash_map_str_access() {
+        use std::collections::HashMap;
+        let mut m: HashMap<Run, i32> = HashMap::new();
+        m.insert(Run("train".to_string()), 1);
+        m.insert(Run("test".to_string()), 2);
+        // We can call `get` given only a `&str`, not an owned `Run`.
+        assert_eq!(m.get("train"), Some(&1));
+        assert_eq!(m.get("val"), None);
+    }
+
     #[test]
     fn test_wall_time() {
         assert_eq!(WallTime::new(f64::INFINITY), None);