From 58184b8047362a603283c672c4fce82adac0ca8b Mon Sep 17 00:00:00 2001
From: Siddhartha Sahu <siddhartha.sahu@materialize.com>
Date: Thu, 19 Sep 2024 13:54:11 -0400
Subject: [PATCH] compute: add new dyncfg to set replica expiration time

---
 src/compute-types/src/dyncfgs.rs |  9 +++++++++
 src/compute/src/compute_state.rs | 20 ++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/src/compute-types/src/dyncfgs.rs b/src/compute-types/src/dyncfgs.rs
index f37aaec0991a3..878f85585a9e0 100644
--- a/src/compute-types/src/dyncfgs.rs
+++ b/src/compute-types/src/dyncfgs.rs
@@ -132,6 +132,14 @@ pub const PERSIST_SINK_OBEY_READ_ONLY: Config<bool> = Config::new(
     "Whether the compute persist_sink obeys read-only mode.",
 );
 
+/// Sets the max lifetime for replicas configured as an offset to the replica start time.
+/// Diffs generated at timestamps beyond the expiration time are dropped.
+pub const COMPUTE_REPLICA_EXPIRATION: Config<Duration> = Config::new(
+    "compute_replica_expiration",
+    Duration::ZERO,
+    "The expiration time for replicas. Zero disables expiration.",
+);
+
 /// Adds the full set of all compute `Config`s.
 pub fn all_dyncfgs(configs: ConfigSet) -> ConfigSet {
     configs
@@ -150,4 +158,5 @@ pub fn all_dyncfgs(configs: ConfigSet) -> ConfigSet {
         .add(&COPY_TO_S3_ARROW_BUILDER_BUFFER_RATIO)
         .add(&COPY_TO_S3_MULTIPART_PART_SIZE_BYTES)
         .add(&PERSIST_SINK_OBEY_READ_ONLY)
+        .add(&COMPUTE_REPLICA_EXPIRATION)
 }
diff --git a/src/compute/src/compute_state.rs b/src/compute/src/compute_state.rs
index 782232e125fe5..306498ba21b57 100644
--- a/src/compute/src/compute_state.rs
+++ b/src/compute/src/compute_state.rs
@@ -34,6 +34,7 @@ use mz_dyncfg::ConfigSet;
 use mz_expr::SafeMfpPlan;
 use mz_ore::cast::CastFrom;
 use mz_ore::metrics::UIntGauge;
+use mz_ore::now::EpochMillis;
 use mz_ore::task::AbortOnDropHandle;
 use mz_ore::tracing::{OpenTelemetryContext, TracingHandle};
 use mz_persist_client::cache::PersistClientCache;
@@ -162,6 +163,9 @@ pub struct ComputeState {
     /// Interval at which to perform server maintenance tasks. Set to a zero interval to
     /// perform maintenance with every `step_or_park` invocation.
     pub server_maintenance_interval: Duration,
+
+    /// The time at which to expire replicas.
+    pub replica_expiration: Option<Timestamp>,
 }
 
 impl ComputeState {
@@ -208,6 +212,7 @@ impl ComputeState {
             read_only_tx,
             read_only_rx,
             server_maintenance_interval: Duration::ZERO,
+            replica_expiration: None,
         }
     }
 
@@ -294,6 +299,21 @@ impl ComputeState {
         // Remember the maintenance interval locally to avoid reading it from the config set on
         // every server iteration.
         self.server_maintenance_interval = COMPUTE_SERVER_MAINTENANCE_INTERVAL.get(config);
+
+        if self.replica_expiration.is_none() {
+            let offset = COMPUTE_REPLICA_EXPIRATION.get(&self.worker_config);
+            if !offset.is_zero() {
+                // TODO(sdht0): consistently send now() from environmentd.
+                let now = mz_ore::now::SYSTEM_TIME.clone()();
+                let offset: EpochMillis = offset
+                    .as_millis()
+                    .try_into()
+                    .expect("Duration did not fit within u64");
+                let replica_expiration = Timestamp::new(now + offset);
+                info!("Replica frontier expires at {:?}", replica_expiration);
+                self.replica_expiration = Some(replica_expiration)
+            }
+        }
     }
 
     /// Returns the cc or non-cc version of "dataflow_max_inflight_bytes", as